1. Introduction
Deep moist convective storms (surface-based or elevated) in western and central Europe (including the United Kingdom) often occur within a synoptic pattern called the Spanish plume (e.g., Carlson and Ludlam 1968; Morris 1986). The preconvective environment is characterized by a midlevel layer of dry air with steep lapse rates overtop a more humid surface layer as much as 1 km thick, resembling a loaded-gun preconvective sounding similar to that in the central United States (e.g., Fawbush and Miller 1954; Miller 1959; Newton 1963). This convectively unstable profile is produced by a long fetch of deep southerly or southwesterly flow associated with a large-amplitude trough or cutoff low in the jet stream extending to low latitudes and a high-amplitude ridge downstream that draws air from the Iberian Peninsula poleward to France and the United Kingdom.
The term Spanish plume consists of two words. Spanish refers broadly to the Iberian Peninsula, which consists of Spain and Portugal. The principal terrain feature on the peninsula is the Meseta Central, a plateau with a maximum elevation of 3482 m, an average elevation of 660 m, and large regions greater than 1000 m above sea level (Fig. 1). In spring and summer, heating of the Meseta Central produces a surface-based mixed layer that is transported northward and becomes an elevated mixed layer with steep lapse rates called the Spanish plume airstream. The word plume is used to describe a buoyant jet where the buoyancy is supplied by a point source (American Meteorological Society 2024b).
Terrain elevation at 15-km grid spacing (m; colored according to the scale) and geographical and political features used in the text. The white line across the Iberian Peninsula is the cross section of terrain elevation used for the idealized model simulations described in section 7 and illustrated in Fig. 9.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
Or so the story goes.
As we will show in this review of the meteorological literature, this seemingly simple and clear picture is more complicated and murky than it first appears. The literature on the Spanish plume sometimes is unevidenced, inconsistent, unclear, and inaccurate. The purpose of this review is to identify these issues, with the goal to provide recommendations to improve the clarity in our terminology, usage, and communication. We also identify opportunities for future research to better understand the Spanish plume synoptic pattern, why it creates an environment favorable for convective storms, and the mechanisms by which convection initiates.
The structure of this review is as follows. Section 2 presents an overview of the Spanish plume. Section 3 describes the systematic method to construct the database of literature on the Spanish plume. Section 4 provides an overview of that literature. Section 5 reveals inconsistencies and inaccuracies in the use of the term Spanish plume and its characteristics as described in the literature. Section 6 investigates the origin of the air in the Spanish plume and the appropriateness of its name. Section 7 examines how its characteristics have been modified by the terrain through both sensible heat fluxes and orographic flow. Section 8 discusses the synoptic and mesoscale characteristics associated with the Spanish plume pattern. Section 9 examines the convective environment over the United Kingdom, including the instability, inhibition, and mechanisms for convection initiation and organization of the convective storms. Section 10 provides recommendations to facilitate clear and accurate scientific communication, as well as a list of future research opportunities. Section 11 concludes this review.
2. Overview of the Spanish plume
in a southerly airstream a “plume” of potentially very warm air from Spain is found aloft over southwest France, and acts like a “lid” to confine the small-scale convection there to a layer only 1 or 2 km deep.
Carlson and Ludlam (1968) showed that the potential temperature in the plume was consistent with the surface temperatures over eastern Spain and was transported northward along an isentropic surface to the United Kingdom (Fig. 2). They demonstrated the consistency of this dry-adiabatic layer across these four cases and showed its similarity to the elevated mixed layer from Mexico responsible for producing the lid (also called a capping inversion) for convective-storm environments in the central United States (e.g., Carlson et al. 1983; Carlson 1991, chapter 16).
Isentropic relative flowchart for θ = 33°C and isobars (pecked lines labeled in tens of hPa), 1200 UTC 5 Sep 1958. During the flow over Spain and into the confluence zone farther north, ascent and condensation remove air to higher isentropic surfaces, especially in the belt of extensive altocumulus (with castellanus; stipped area, terminated arbitrarily over Scotland). Figure from Fig. 6 in Carlson and Ludlam (1968); caption is modified to meet AMS style.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
Although Carlson and Ludlam (1968, pp. 210, 213) used the phrase “the Spanish plume,” it is unclear whether they intended to introduce a new scientific term. The article most responsible for popularizing the term was Morris (1986): “The Spanish plume—testing the forecaster’s nerve.” Young and Grahame (2023) wrote that Morris (1986) had “resurrected” the term, although it is fair to say that he had formally coined the term from its more informal use by Carlson and Ludlam (1968)—despite Morris (1986) not citing the original source! In fact, Morris (1986) also changed the definition of Spanish plume from the airstream to the “weather pattern” responsible for “the development over Iberia and western France of widespread thunderstorms which spread to the United Kingdom.” Morris (1986) emphasized the dynamical processes associated with the Spanish plume pattern, drawing the first conceptual model of its structure and evolution (Fig. 3; discussed further in section 8). In that way, Morris (1986) redefined and expanded the concept of the Spanish plume from its original definition. Since then, the term has been applied to both the particular airstream responsible for producing the elevated mixed layer and the synoptic pattern responsible for producing the airstream, as noted by Dahl and Fischer (2016, p. 1420). Thus, the term Spanish plume provides a concise term encapsulating the synoptic pattern and conceptual model of processes creating a favorable condition for convective storms.
The development of the Spanish plume. Solid lines indicate surface flow, and dashed lines indicate thermal wind. Figure from Fig. 1 in Morris (1986); caption is edited for clarity by changing “thermal flow” to “thermal wind” and dropping the unnecessary sentence “For description of development see text.”
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
The Spanish Plume by Sir Oswald Birley, signed and dated 1912. The sitter is Izme Vickers and is painted in the Spanish style (Black et al. 2017, 58–59). Permission to reproduce this portrait is granted by Museums Sheffield.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
Although Morris (1986) did not cite Carlson and Ludlam (1968) for reasons that remain unclear, it is fair to say that Morris’s big contribution was to introduce awareness of the Spanish plume to the U.K. forecasting community—this occurred over a number of years after 1986. Prior to Morris (1986), there is no evidence of Spanish plume being used by Met Office forecasters (M. E. Hardman and E. McCallum 2024, personal communication). Interestingly, the Spanish plume did not appear in the updated Forecasters’ Reference Book issued to Met Office forecasters (even as late as 1993) and Bader et al.’s (1995) book Images in Weather Forecasting: A Practical Guide for Interpreting Satellite and Radar Imagery (despite techniques for forecasting severe convection being included). So, even though Morris’s redefinition of the term Spanish plume as a synoptic pattern rather than a specific airstream changed the way the term became used, his contribution in making U.K. forecasters aware of this important phenomenon deserves to be recognized.
As will be shown in section 4, much of the work on the Spanish plume has been done in the United Kingdom. Spanish plume synoptic patterns occur less than 5% of the time during the summer (Wilkinson and Neal 2021) but have a 70% chance of producing thunderstorms over the United Kingdom (e.g., Mohr et al. 2019; Hayward et al. 2022, 2023). The Spanish plume pattern is associated with the highest convective available potential energy (CAPE) over the United Kingdom (Holley et al. 2014) and is one of two synoptic patterns in which most supercells and tornadoes form over the United Kingdom (Clark and Smart 2016). Nearly 80% of mesoscale convective systems in the United Kingdom are associated with the Spanish plume synoptic pattern (Gray and Marshall 1998; Lewis and Gray 2010).1 In fact, the synoptic pattern favoring thunderstorms in England was well known before the Spanish plume (Douglas and Harding 1946; Douglas 1952), as noted by Field (1999) and Young and Grahame (2023). Although the synoptic pattern of a large-amplitude upper-level trough or cutoff low extending to low latitudes and high-amplitude blocking ridge downstream with associated airstreams crossing the Iberian Peninsula and reaching the United Kingdom can occur at any time of the year, mesoscale convective systems occur almost exclusively in May through September (e.g., Fig. 2 in Gray and Marshall 1998; Fig. 1b in Lewis and Gray 2010). To our knowledge, although studies in other European countries have mentioned the importance of the Spanish plume pattern to the climatology or case studies of convective storms, quantifying the frequency of the Spanish plume synoptic pattern on their climatologies has not been performed.
Given the importance of this synoptic pattern to severe convective storms, various research articles have studied the Spanish plume, and it has been cited in many more (e.g., sections 3 and 4). The term has even been popularized beyond forecasters and researchers, with an explainer by the Met Office (2016), as well as news stories by Yachting World (2016) and The Guardian (2014, 2019). Like many meteorological terms that have been embraced and popularized in the media (e.g., clash of air masses, polar vortex, sting jet), the success of the term Spanish plume may play off the term’s nonmeteorological association.2 Yet, its success also means the potential for its misuse and misapplication, as happened in these other cases (Schultz et al. 2014; Waugh et al. 2017; Schultz and Browning 2017). For example, the term Spanish plume has sometimes been applied to general periods of warm and humid weather that favor convective storms over the United Kingdom, regardless of whether the synoptic pattern was similar or not (M. Lehnert né Lewis 2023, personal communication). Further examples of misuse will follow in this review and will be summarized in section 10.
3. Method: Building the database of Spanish plume literature
We chose our method of systematically selecting the literature to ensure a complete record of all peer-reviewed meteorological journal articles in English that mention the Spanish plume. A systematic search is critical to this review for four reasons. First, a systematic search creates a complete record—within the selection criteria—without the subjectivity in selection as could occur in a narrative review article. Second, a systematic search ensures reproducibility by future researchers wishing to replicate or update the present study. Third, because the record is complete, we can be more confident that the conclusions derived from our critical evaluation of the literature are without bias. Finally, a systematic search allows quantitative analysis of the resulting database, which for our study is essential to demonstrate the extent to which the lack of evidence, consistency, clarity, and accuracy exists in the literature.
The first author searched the phrase “Spanish plume” in four search engines on 13 August 2023. Google Scholar returned 177 results, Royal Meteorological Society/Wiley returned 77 results, AMS Journals Online returned 15 results, and Web of Science returned 8 results. Duplicates were removed. Non-English texts, books, and nonpeer-reviewed items were excluded. Nonmeteorological sources and sources about weather patterns outside Europe were excluded. Sources that did not use the phrase “Spanish plume” in the text were also excluded (e.g., “Spanish plume” appeared in a title in the reference list within a meteorological article on a topic unrelated to Spanish plumes). After excluding articles that did not meet these eligibility criteria, 102 articles remained.
4. Overview of the literature
A histogram of the publication year of the 102-article database illustrates how the number of articles mentioning Spanish plume has changed over time (Fig. 5). Carlson and Ludlam (1968) coined the term in 1968 (section 2). That same year, Salter (1968) was studying a heavy-rainfall event on 11 July 1968 in the United Kingdom and referred to the 850–700-hPa layer of air of potentially warm air as the Spanish plume, citing Carlson and Ludlam (1968). No new articles appeared until Morris (1986) (section 2). Starting in 1993, some articles in Weather began using the term Spanish plume in case studies of convective storms, averaging about 2 per year during 1993–2010. In 2010, Lewis and Gray (2010) performed a climatology of mesoscale convective systems over the United Kingdom during 1998–2008, adding to an earlier climatology during 1981–97 by Gray and Marshall (1998). Lewis and Gray (2010) identified two different categories of Spanish plume events favorable for mesoscale convective systems: a classical Spanish plume and a modified Spanish plume (Fig. 6; further discussion of these categories occurs in section 8). Since then, the number of articles increased to about 5–6 per year (Fig. 5), with most articles pertaining to convective storms in the United Kingdom, although about a third described convective storms under the same synoptic pattern on the European continent (e.g., Belgium, the Netherlands, France, Germany, Switzerland). This trend represented an increasing operational and research interest in convective storms in Europe around this time (e.g., Antonescu et al. 2017; Groenemeijer et al. 2017).
Number of articles mentioning “Spanish plume” as a function of year of publication. Three articles are labeled and described in the text.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
Schematics of the large-scale features associated with the (a) classic Spanish plume and (b) modified Spanish plume synoptic environments at the time of convection initiation. Mean sea level low pressure centers (L), low-level cold fronts (blue with filled triangles), low-level warm fronts (red with filled semicircles), upper-level cold front (blue with open triangles), representative contours of 500-hPa geopotential height (thick black contours marked Z500), locations of the main jet axes (solid green arrows), axes of cold (CA) and warm (WA) advection (thick blue and red arrows, respectively), and the region of low-level ascent forced by upper-level positive vorticity advection near the warm advection (marked by a black circle labeled PVA). Figure modified from Lewis and Gray (2010, their Fig. 15) by only showing the first two of three environments as these pertain to the Spanish plume synoptic patterns; caption is modified to meet AMS style and includes the content of the first two panels only.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
Table 1 lists the corresponding journals. Over a third of the 102 articles mentioning the Spanish plume appeared in Weather (35 articles, 34%), followed by 26 (26%) in the Quarterly Journal of the Royal Meteorological Society, Atmospheric Research, and International Journal of Climatology. In total, journals published by European professional societies accounted for 73 (72%) of the articles, including 60 (59%) within Royal Meteorological Society journals. American Meteorological Society journals published just 11 (11%) of all articles.
Journals and publishers of the 102 articles mentioning the Spanish plume.
Mentions of the term Spanish plume within previous articles ranged from mere passing mentions to more substantial mentions that strongly motivated the new work and were integral to it. In this database, most articles mentioned the Spanish plume in passing (84 of 102 articles, or 82%), often writing that their case study fit the conceptual model of the Spanish plume. Usually, the term Spanish plume was mentioned only once within each of these 84 articles. For those 18 articles with more substantial mentions to the Spanish plume, four articles (4%) were the most prominent: Carlson and Ludlam (1968), Morris (1986), Gray and Marshall (1998), and Lewis and Gray (2010). Here, the concept of the Spanish plume pervaded each article. The remaining 14 articles (14%) made more substantial mention to the Spanish plume, but the article did not revolve around the concept. Of these 18 articles, 10 were case studies of Spanish plume events, 4 were composite analyses in which one or more composites were Spanish plume events, 2 were climatologies, and 2 were reviews.
Articles mentioning the Spanish plume sometimes did not cite the relevant literature. Specifically, 30 of the 102 articles (29%)—and 26 of the 84 articles that mentioned the Spanish plume in passing—mentioned the Spanish plume but cited no relevant literature. These articles clustered during two periods: 1995–99 and 2012–23 (Fig. 7). Perhaps, these periods indicated how commonly accepted and prevalent the term had become when authors felt that no citation to the scientific literature was necessary. A curious related finding was how infrequently Carlson and Ludlam (1968) was cited: It was cited 245 times on Google Scholar (as of 31 May 2024), yet only 24 times in the 102 articles that cited Spanish plume (Fig. 7). In contrast, Morris (1986) was cited 61 times on Google Scholar and 38 times in the 102-article database (Fig. 7). There are two likely explanations. First, the title of Morris (1986) contained “Spanish plume,” the first article to do so. Second, Morris (1986) did not cite Carlson and Ludlam (1968), which meant that many subsequent authors—likely unfamiliar with the origin of the term—did not cite Carlson and Ludlam (1968) either. Before 2012, only four articles in the 102-article database cited Carlson and Ludlam (1968), Salter (1968), van Delden (1998), Bennett et al. (2006), and Lewis and Gray (2010). Since being cited in Lewis and Gray (2010), Carlson and Ludlam (1968) has been cited more regularly, but not completely. Given the history of the term and its origin by Carlson and Ludlam (1968), we advocate for its proper citation in future work.
Stacked bar chart by year showing the number of articles citing two key articles (Carlson and Ludlam 1968; Morris 1986) or not citing any article when introducing the Spanish plume definition of the 102 articles in the database. “Zero citations” means “cited no relevant Spanish plume literature.” The black squares represent the total number of articles published that year. If each published article in a given year only had one citation, the black square would equal the length of the stacked bar.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
5. Use of the term Spanish plume and the characteristics of the Spanish plume airstream
In this section, we examine the various meanings applied to the term Spanish plume in the literature. This analysis then is used to quantify how these articles have described the characteristics of the Spanish plume airstream, showing the errors and inconsistencies in the literature and thus leading to recommendations for its proper use.
As Dahl and Fischer (2016, p. 1420) recognized, articles have used the term Spanish plume in two principal ways: the elevated mixed-layer airstream created while passing over the Meseta Central and the corresponding synoptic pattern. Table 2 shows that 57 out of 102 articles (56%) used the term Spanish plume to describe the synoptic pattern only, 18 (18%) used it to describe the airstream only, and 15 (15%) used it to describe both, illustrating the influence that Morris (1986) has had in its redefinition. In two cases, the context of the citation made it difficult to understand how the term was used (Searson 1996, p. 577; Galvin 2003, p. 59).
How articles have used the term Spanish plume in the 102 articles mentioning the Spanish plume.
In another, the text read “This warm advection is referred to as the Spanish plume” (Steeneveld and Peerlings 2020, p. 3), and three other articles defined the Spanish plume as the airstream, the synoptic pattern, and a region of warm advection (e.g., van Delden 1998, p. 113; van Delden 2001, p. 101; de Villiers 2020, p. 228). In a similar vein, Lewis and Gray (2010) drew arrows representing warm advection from Spain northward (Figs. 6a,b). As advection could be defined either mathematically as “warm-air advection” (−V ⋅ ∇T) or as warm-air transport (V ⋅ T; although this mathematical expression makes little sense for temperature) (American Meteorological Society 2024a), greater clarity would have been beneficial here. For example, a field of constant temperature moving poleward would have zero temperature gradients by the mathematical definition of warm advection. Although the whole Spanish plume airstream may be transported poleward, temperature gradients would tend to occur along the northern edge. Thus, we recommend greater clarity using the term transport rather than advection.
In German, the word “Konstellation” is used in various contexts (planets, family, furniture, NWP [numerical weather prediction], football, …), as it has a very general meaning comparable with “situation,” “conditions” or “compilation.” For example, it is common to use “Druckkonstellation” for the “spatial arrangement of (high- and low-) pressure systems.” We were not aware that this is not the case in English language. …you could best substitute “constellation” with “situation” or “conditions,” with “flow conditions” meaning the general synoptic-scale pattern of the tropospheric flow over Europe.
In total, 79 of the 102 articles (77%) used the term Spanish plume for the synoptic pattern (including “constellation”), 36 (35%) for the airstream, and 6 (6%) for incorrect uses. (Percentages added up to more than 100% because some articles used the term in more than one way.) Although 94% of articles used the term correctly, there was still a minority using the term incorrectly. In our view, scientific precision requires clear communication and definitions, and the synoptic pattern and airstream should be the only two acceptable uses of the term Spanish plume. In the present article and where such a distinction is necessary, we try to make it clear whether we refer to the synoptic pattern or the airstream.
Of the 36 articles that used the term as an airstream, 32 (89%) described the characteristics of that airstream (Table 3). Only 12 (33%) described the airstream in a similar way to how Carlson and Ludlam (1968) originally described the Spanish plume, as an elevated mixed layer with steep lapse rates. Close but not precise enough, 4 (11%) just referred to it as “warm” and 2 (6%) as “warm/hot and dry.” Most problematically, 8 (22%) referred to it as “warm/hot and humid/moist” and 1 (3%) referred to it as simply “moist.” Three (8%) referred to the airstream as air with high θw (without additional descriptors such as hot, dry, or humid) and 2 (6%) characterized the airstream as “warm advection” with no other descriptors. This variety of descriptors, especially those 25% writing “humid” or “moist,” are concerning, indicating that the original definition of the Spanish plume by Carlson and Ludlam (1968) had not been followed in later articles. In fact, all articles where the Spanish plume was described as humid or a surface airstream did not cite Carlson and Ludlam (1968)—the same articles that failed to cite Carlson and Ludlam (1968) also misapplied the term to the wrong airstream.
Characteristics of the Spanish plume airstream from the 36 articles that used the term Spanish plume as an airstream.
The analysis of these 36 articles identified differences in the altitude of the Spanish plume airstream as stated within the articles: descriptions of it as elevated, low level, surface, 850 hPa, or 800–850 hPa. One of the difficulties of pinning down the altitude more precisely in the articles was that the location mentioned was often unclear, whether it was over the region where the airstream originated or the region the airstream arrived before convection initiation. Further confusion may have arisen because of the often weak directional wind shear with height in the Spanish plume synoptic pattern (to be discussed further in section 9c), in part associated with the well-mixed layer and in part associated with the often high-amplitude blocking ridge downstream (e.g., Mohr et al. 2019). Because of this weak shear, air from different levels would have taken similar paths. Descriptions of southerly flow without further characterization may have led to confusion among readers about the altitude of the Spanish plume airstream and thus propagated misunderstanding through the literature.
What are the best thermodynamic quantities for identifying and tracking the Spanish plume? In most cases, nominally conserved quantities (e.g., θw) are ideal metrics for tracking airstreams as they undergo large horizontal (and possibly also vertical) displacements. In the United Kingdom, θw is commonly used to track air masses, so it also has been the most commonly used metric to identify the Spanish plume airstream (eight articles). A value of θw > 18°C at 850 hPa is typical (e.g., Young and Grahame 2023). Another metric that has been used is a surface dewpoint temperature of 20°C or greater (Webb and Pike 1998), again incorrectly conflating the sometimes-humid surface air with the Spanish plume airstream aloft. Operational experience shows that instability indices can be useful in identifying Spanish plumes (e.g., lifted index, θw difference between 500 and 850 hPa even though the height of elevated mixed layers will vary from case to case). By means of such indices, the movement of Spanish plumes can often be traced at successive forecast times through an individual model run or in a sequence of model analyses over a 12–24-h period (e.g., Dahl and Fischer 2016, their Figs. 4a,c). For example, Holley et al. (2014, p. 3819) argued that, because much of the buoyancy aloft is created by the elevated mixed layer, overnight most unstable convective available potential energy (MUCAPE) is less sensitive to daytime heating than surface-based convective available potential energy (SBCAPE). Thus, MUCAPE is more useful for distinguishing Spanish plume synoptic conditions from more typical episodes of CAPE originating from daytime heating. This idea presaged the concept of a CAPE ratio in situations with nonzero MUCAPE, 1 − SBCAPE/MUCAPE, discussed in Flack et al. (2023, p. 1082), where a CAPE ratio of zero would be more favorable for surface-based convective storms and 1 would be more favorable for elevated convective storms. Such elevated deep moist convection often spread north to the southern United Kingdom overnight into the morning, prior to any fresh surface-based convection.
How the thermodynamic instability is described also varies. Rarely do the articles specify what kind of instability they used. For example, Lewis and Gray (2010, p. 195) described “a plume of convectively unstable air,” but other articles did not, simply writing “unstable air” (e.g., Hamid 2012, p. 87; Sibley 2012, p. 143) or “unstable airflows” (Allan et al. 2020, p. 3864). Because instability is generally not conserved with air motion, “unstable air/airflows” is misleading. As conditional instability is determined by the lapse rate of temperature (or of saturated equivalent potential temperature), convective (or potential) instability is determined by the lapse rate of equivalent or wet-bulb potential temperature (e.g., Schultz et al. 2000; Bohren and Albrecht 2023, p. 444), and CAPE is determined by parcels possessing buoyant energy (e.g., Sherwood 2000). Greater precision and clarity could be achieved by describing a conditionally unstable layer, convectively unstable layer, or conditionally unstable sounding.
6. Origin of the air and appropriateness of the name “Spanish plume”
What does the literature say about the origin of the airstream called the Spanish plume? This section addresses this question.
a. Is it Spanish?
The concept of the origin of an airstream can be misleading. Unless the airstream originated in a region of airmass formation such as a subtropical anticyclone or Arctic regions (e.g., Bergeron 1928, 32–38; Namias 1936; Schultz et al. 2020), then it would only pass over a region at a particular time, not “originate” there. Nevertheless, the concept of origin may be useful in thinking about where the airstream acquired its characteristic or defining properties versus where modification occurred.
Fifteen of the 102 articles offered a location of the origin of the airstream (Table 4). The origin of the air was mostly (10 out of 15 articles) said to be the Iberian Peninsula, Spain, or the Spanish Plateau. (Of course, Portugal is also part of the Iberian Peninsula, although the highest terrain is in Spain. For this reason, use of the term Spanish plume may be problematic.) However, five articles also referred to the origin in Africa. Five articles referred to an origin over the Mediterranean Sea or Atlantic Ocean, but these were usually those that described the Spanish plume as the near-surface warm humid airstream. One article referred to the origin as France or the Bay of Biscay.
Articles describing the origin of the Spanish plume airstream of the 15 articles that offer a location to the origin of the airstream.
Of these articles that described the origin of the airstream, only four supported their claim with evidence.
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Carlson and Ludlam (1968) calculated isentropic relative-flow charts to show that the Spanish plume originated over the North Atlantic, passing over the Iberian Peninsula (e.g., Fig. 2) but recognized that the flow may have earlier traveled over northern Africa in some cases.
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Sibley (2012, their Fig. 5) calculated 5-day backward trajectories from two different forecasting models for a day with thunderstorms over southeastern England and showed that lower-tropospheric air from northern Africa traveled over or to the west of the Iberian Peninsula.
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Lewis and Silkstone (2017, their Fig. 2) calculated 66-h backward trajectories during a record-setting heat wave over England with severe thunderstorms, showing the path of lower-tropospheric air across the Iberian Peninsula.
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de Villiers (2020, their Fig. 6) calculated 5-day backward trajectories during a western Europe heat wave, showing the Spanish plume airstream took a path over western Africa, across the Iberian Peninsula and Bay of Biscay to England (850–700 hPa).
The other 11 articles contained unevidenced statements about the origin of the airstream, presumably inferring its origin from the geopotential height and wind fields. Naturally, earlier research studies would not have had the computational resources to calculate trajectories, and operational forecasters may not have had access to software to calculate trajectories, although nowadays resources for calculating trajectories in real time are more common (e.g., Stein et al. 2015). Given that trajectories often yield surprising results compared to otherwise smooth geopotential height and wind fields (e.g., Saucier 1955, p. 312; Stohl 1998, p. 948), authors should be cautious about identifying the origin of air without accurate trajectory calculations.
Although not included within the 102 articles in this review because it is a book chapter, Clark and Smart (2016, their Fig. 3.10d) illustrated the origin of a Spanish plume airstream, which they described as “a layer of relatively warm air at mid-levels that originates over the elevated terrain of the Iberian Plateau or North Africa” (Clark and Smart 2016, p. 41). In support of this statement, they calculated trajectories for a Spanish plume case on 28 June 2012 (Fig. 8). They found that the blue trajectory ending at 4500 m originated over northern Africa, detoured around the Spanish Plateau by heading west over the Atlantic and Bay of Biscay, and arrived in the United Kingdom (Fig. 8), providing further support for the origin of the airstream farther south in some cases. However, none of the displayed trajectories were in the steep lapse-rate air of the Spanish plume airstream in the layer 870–720 hPa (Clark and Smart 2016, their Fig. 3.10c).
Schematic illustrating the Spanish plume synoptic setup over the United Kingdom on 28 Jun 2012. Trajectories of air at 1500 (red), 4500 (blue), and 6000 m (green) above ground level over the 5-day period ending 0600 UTC 28 Jun 2012 in southern Wales. Arrows indicate idealized trajectories of airflows at these heights in a typical Spanish plume setup. Courtesy of NOAA Air Resources Laboratory. Figure from Clark and Smart (2016), their Fig. 3.10d; caption is modified to correct the height of the trajectory from 3000 to 4500 m and the ending time of the trajectory to 0600 UTC instead of 0900 UTC (M. Clark and D. Smart 2024, personal communications), to add the ending location, and to meet AMS style.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
Support for the Spanish plume airstream originating over northern Africa in some cases also comes from the co-occurrence of widespread dust-fall events and severe storms (e.g., Stevenson 1969; Ryall et al. 2002; Sibley 2012). The steep lapse rates can be traced back to the Saharan air layer, which is a layer of warm, dry, and dusty air that forms over the Sahara Desert (e.g., Carlson and Prospero 1972; Prospero and Carlson 1972; Tsamalis et al. 2013; Prospero et al. 2021). Up to 3 km deep, this layer is often transported away from Africa, sometimes westward across the Atlantic Ocean, sometimes northeastward over the Mediterranean Sea to central and eastern Europe, and sometimes northward over Spain, France, and the United Kingdom. The transport of Saharan dust to the United Kingdom occurs several times per year (e.g., Ryall et al. 2002) and may be associated with electrification in thunderstorms.
Recognizing this case-to-case variability in the path of the airstream, the Met Office has recently been using the term continental plume operationally instead of Spanish plume to indicate a more southeasterly or easterly path that may or may not pass over the Iberian Peninsula (M. Lehnert né Lewis 2023, personal communication). This term expands the Spanish plume synoptic pattern to those that may source steep lapse-rate air from locations other than the Iberian Peninsula, as well as distinguishes this Spanish plume synoptic pattern from other common warm-season synoptic patterns that favor thunderstorms, such as a vertically stacked low center over or near the United Kingdom (M. Lehnert né Lewis 2023, personal communication). The term continental plume also acknowledges that a variety of different weather conditions may be associated with different flavors of the Spanish plume synoptic pattern, as differences between events (including differences in surface fluxes, seasonal variability, airflow, etc.) lead to different properties of the airstreams. Using the term continental plume is recognition that this variety could not be accommodated by the overly specific term Spanish plume.
b. Is it a plume?
The term plume indicates an elongated feather-shaped region of air, enhanced by the surface heating over the Iberian Peninsula, as Carlson and Ludlam (1968) intended. The Glossary of Meteorology defines it as a “buoyant jet in which the buoyancy is supplied from a point source; the buoyant region is continuous” (American Meteorological Society 2024b). Although this definition likely suggests a vertically oriented plume from a smokestack or from a volcano, the buoyant air and point source also evoke a horizontally oriented plume generated by an areal source such as the Meseta Central. Plume is used loosely and broadly in operational forecasting at the Met Office to describe almost any elongated tongue of high wet-bulb potential temperature θw (e.g., a prefrontal warm conveyor belt or the smaller scale, less well-defined tongue of higher θw, often occurring with positive vorticity advection) (e.g., Bader et al. 1995, their section 6.5.2.3). When the synoptic pattern is favorable, lower-tropospheric air may travel across the Meseta Central, regardless of the time of the day and whether or not surface sensible heat fluxes occur. The literature on the Spanish plume rarely discussed the diurnal variations of the plume, so whether the airstream passing over the Meseta Central qualifies as a plume by this definition may need to be reconsidered. These issues are discussed further in section 7.
7. Interactions with the terrain: Sensible heat fluxes and orographic flow
Terrain has two effects on the Spanish plume airstream: surface sensible heat fluxes modify the airstream and orographic flow modifies its path. We address these two points within this section.
First is the sensible heat flux. Carlson and Ludlam (1968) showed that the isentropic-relative flow that crossed the boundary layer over the Meseta Central continued onto the United Kingdom (Fig. 2). Since then, no article has tested whether the heat flux from the high terrain was sufficient to explain this temperature profile. Consider that the Iberian Peninsula is about 800 km long in the north–south direction. Not all is high terrain, however, so this would be an upper limit. Thus, a near-surface wind of 10 m s−1 would take 22 h to cross the Iberian Peninsula, with even less time spent over the Meseta Central. Is that sufficient time to produce the thermal profile of a well-mixed layer 100–200 hPa thick, even if the air passes at a favorable time in the diurnal heating cycle?
Second is the flow over the terrain. Over tall mountains like the Meseta Central and Pyrenees, elevated layers may decouple from lower layers through windward-side blocking, leeside flow separation, or both (e.g., Scorer 1955; Jiang et al. 2007). Both blocking and flow separation create large-amplitude mountain-wave horizontal pressure gradients that can induce flow reversal and decoupling between higher and lower air layers. The higher air layers may also be heated by the terrain forming a plume of warm air aloft, undercut by the lower air layers having a different origin on the lee side. But even if no flow separation occurs, any warmed near-surface air flowing off the terrain may be subsequently cooled and moistened over the ocean to produce a cooler marine layer beneath an inversion and an elevated mixed layer aloft. Moreover, even in situations where low-level flow detours around the Meseta Central and mid-to-upper-level flow crosses the terrain, elevated wave breaking in the lee of the Meseta Central may further warm and dry the elevated boundary layer and also allow moister leeside maritime air to flow underneath it. Such situations might be favorable for convective storms even without direct flow of heated air over the Meseta Central.
In neither case—the effect of surface sensible heating nor the flow modifications by terrain—has a quantitative approach been taken in the literature to understanding the influence of the terrain on the Spanish plume. As a starting point to illustrate how these two effects of heating and flow over terrain interact to produce the Spanish plume airstream, we construct an idealized dry 2D simulation of flow crossing the Meseta Central along the cross section in Fig. 1. Only the terrain of the Iberian Peninsula is included in the simulation, and it is surrounded by oceans extending to the lateral domain edges. The Cloud Model 1 (CM1) (Bryan and Fritsch 2002), version 20.3, is used to create the simulation. The grid spacings are 2 km in the horizontal and 200 m in the vertical; the domain is 5000 km in the horizontal and 20 km in the vertical. The simulation is initialized at 0600 local solar time (LT) and integrated for 12 h to capture the net heating phase of the diurnal cycle. The surface air temperature and sea surface temperature are initialized to 293 K, and the land skin temperature is initialized to 286 K at sea level, with a dry-adiabatic variation with height over terrain, to represent morning conditions. The land surface is set to a cropland/pasture mosaic, and the terrain is smoothed to 20 km to minimize forcing at poorly resolved scales, the latter using a Fourier transform to eliminate all wavelengths below 20 km. Parameterizations include the NASA/Goddard shortwave and longwave radiation schemes (Chou and Suarez 1999; Chou et al. 2001), a surface layer based on Monin–Obukhov similarity theory, and the planetary boundary layer scheme of Hong and Pan (1996). The initial horizontal flow is represented by a uniform wind of 10 m s−1 and a four-layer stability profile: a mixed layer with N = 0 s−1 (0–0.5 km), an inversion with N = 0.03 s−1 (0.5–0.7 km), a free troposphere with N = 0.01 s−1 (0.7–12 km), and a stratosphere with N = 0.02 s−1 (12–20 km). This experiment is not intended to capture the complexities of real-world Spanish plume events, but rather to illustrate whether a Spanish plume–like airstream can be created within a simple but plausible flow.
Cross sections of horizontal winds and perturbation potential temperature along the terrain transect at 1200 LT show a low-level flow decelerated by friction (Fig. 9a) and a developing convective boundary layer over land reaching depths of around 2 km (Fig. 9c). Leeside descent lowers the marine-layer inversion almost to the surface, beneath which lies a shallow and narrow sea-breeze circulation at about x = 400 km and above which lies the air heated by the terrain at x = 400–500 km (Fig. 9c). Six hours later at 1800 LT, the convective boundary layer over land deepens to nearly 3 km and the warm leeside flow remains connected to the surface (Figs. 9b,d). Nevertheless, the mountain wake contains an elevated mixed layer above the shallow marine inversion that progresses downstream (Fig. 9d). Although mountain waves develop above the elevated mixed layer and penetrate downward into it (at around x = 40 and 400 km and z > 3 km), they are weak and only minimally impact the thermal anomalies within the elevated mixed layer.
Potential temperature (solid lines) from a 2D idealized simulation of a multilayer flow of 10 m s−1 traversing the Meseta Central of Spain at (a),(c) 1200 LT and at (b),(d) 1800 LT. (a),(b) winds along the plane of the cross section (m s−1; colored) with the colors showing anomalies relative to the background flow of 10 m s−1. (c),(d) potential temperature anomalies relative to the background temperature profile (K; colored).
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
One other notable result is that strong cooling occurs within and above the inversion layer over the upstream ocean (x < −500 km). This cooling, which results in a deepening of the upstream mixed layer, stems from longwave cooling being unbalanced by heating associated with moist convection or subsidence. Thus, this feature is a consequence of the neglect of moist convection or large-scale forcing in the simulations.
This section reveals three crucial issues about the Spanish plume synoptic pattern and airstream that perhaps have not been well appreciated previously.
First, the diurnal cycle of heating over the Meseta Central produces not a continuous warm plume downstream but a periodic, diurnally varying, warm anomaly downstream. Preliminary results from an extended simulation, while suffering from problems related to the radiative disequilibrium of the configuration, suggest that the daytime heating leads to a diurnal cycle in Spanish Plume properties that can persist far downstream of the elevated source (not shown). The anomaly length is constrained by the wind speed and geometry of the Meseta Central: UT + Lx, where U is the low-level cross-terrain wind speed (10 m s−1), T is the net diurnal heating period (12 h), and Lx is the terrain length (800 km). These values give an anomaly length of around 1250 km. If the low-level wind were stronger (say, 20 m s−1), the anomaly would be longer but also weaker, varying directly with the sensible heat flux and Lx and inversely with U. Such periodicity could be important for convective storms in the United Kingdom, because intense deep convective storms would be favored after the warm phase of the anomaly reaches the United Kingdom. The distance from northern Spain to the United Kingdom is 800 km, so an anomaly with U = 10 m s−1 would take 22 h to travel that distance. This periodicity is well timed to arrive during the following diurnal heating period over the United Kingdom. If the low-level winds were 20 m s−1 instead of 10 m s−1, however, the anomaly would be weaker and arrive earlier, poorly timed with the diurnal heating over the United Kingdom. This analysis also does not consider whether the Saharan air layer is already a source of well-mixed air in the lower troposphere before reaching the Meseta Central. Hence, this discussion only relates to the importance of heating over the Iberian Peninsula for Spanish plume events. (From this present analysis, there is reason to believe that it is, but there is also substantial uncertainty.) The previous literature on the Spanish plume does not address this diurnal periodicity in the warm anomaly in general and these length scales specifically, let alone its structure and its impact on convective storm environments downstream.
Second, due to the deceleration of wind in the lee of the Meseta Central and the slowly moving boundary, the near-surface air north of the Meseta Central is likely not associated with the Spanish plume airstream aloft. This near-surface air will likely have different properties and trajectories than the air aloft. On many occasions, the low-level air has relatively low relative humidity (even if it has relatively high θw by virtue of its warmth). This issue is further evidence that a clear distinction between the Spanish plume airstream aloft and the near-surface air needs to be made. In addition, the range of possible flow dynamics (e.g., blocked vs unblocked, separated vs attached) over and downwind of the Meseta Central and Pyrenees may be partly responsible for the variety of near-surface humidities in the United Kingdom reported during Spanish plume events, which may in part cause the confusion over which airstream is the Spanish plume (section 5). For example, Hand (1984, their Figs. 6–8) showed the abrupt descent in the lee of the Pyrenees and the forecast sensitivity that the height of the Pyrenees had on resulting convective storms over the United Kingdom. Whether this descent occurs and is relevant to the motion and static stability of the synoptic pattern has not apparently been discussed further in the literature. The importance of knowing whether boundary layer separation occurs or not is likely determined by the synoptic configuration of the different airstreams that overlay each other as they approach the United Kingdom. The lack of directional shear of the wind with height in Spanish plume patterns (to be discussed further in section 9c) is one challenge in trying to understand this stacking of air masses in the lee of the Iberian Peninsula, especially in the context of case-to-case variability.
Finally, little or nothing has been mentioned in the literature about how the elevated mixed layer is moistened by the development of storms over Iberia, possibly through the initiation of storms along sea-breeze convergence zones or through surface fluxes from evapotranspiration and soil moisture over the Meseta Central. This moisture may precondition the vertical thermodynamic profiles for further storm development in the plume downstream toward and into western and central Europe. Also, the upstream development of storms over Spain could lessen the lapse rates and eliminate the elevated mixed layer through vertical moisture and latent heating fluxes within clusters of thunderstorms. Such convection might explain why soundings over the United Kingdom often contain several separate shallow residual dry adiabatic layers (rather than one deep one) when convection has been occurring upstream for several hours, rendering the profile more moist and with a lapse rate closer to the moist adiabatic lapse rate. Indeed, such profiles would be more amenable to surface-based convection given some insolation. On the other hand, the lack of upstream thunderstorm development keeps the elevated mixed layer intact with the steep lapse rates generated by the sensible heating of the elevated terrain areas.
8. Synoptic and mesoscale characteristics
A key forecasting challenge of the Spanish plume is that the convective storms do not necessarily occur along a landfalling Atlantic cold front, but ahead of it (e.g., Dahl and Fischer 2016; Pacey et al. 2023; Young and Grahame 2023). Because this synoptic pattern does not fit easily into the Norwegian cyclone model (e.g., Bjerknes 1919; Bjerknes and Solberg 1922), analysts (e.g., at the Met Office) may variously analyze a surface pressure trough, convergence line, or an upper front. If no sensible weather is associated with the Spanish plume airstream, generally no symbol is depicted.
A feedback loop was then established whereby the tightening thermal gradient strengthened the upper-level winds ahead of the upper trough and in consequence increased the positive vorticity advection and hence ascent over the warm plume. In time the plume effectively became the main frontal zone and the original cold front weakened under the influence of adjusting dynamical forces.
Young (1995), van Delden (1998), Lewis and Gray (2010), and Dahl and Fischer (2016) also recognized the frontogenetical pattern associated with the prefrontal trough and convergence line. Dahl and Fischer (2016) showed that these prefrontal convergence lines over western Europe were associated with ascent along the western edge of the elevated mixed layer airstream, serving as a potential mechanism for lifting unstable air parcels through the inhibition layer (Fig. 10). The Spanish plume airstream has also been characterized in the literature as a warm conveyor belt (Galvin et al. 1995; Lionetti 1996; Gray and Marshall 1998; Peyraud 2013). Furthermore, this kind of structure where the elevated mixed layer is drawn poleward east of an approaching low center has similarities to the eastern and western boundaries of high θw air in the warm sector of an extratropical cyclone (Fig. 10), known as warm conveyor belt fronts (Hewson and Titley 2010, p. 362), and may also explain the double fronts described in Mulqueen and Schultz (2015).
Conceptual model of the evolution of the prefrontal convergence line. The shaded region represents the elevated mixed layer plume and the frontal symbols have their standard meanings. The convergence line is depicted as the dashed line. (a)–(d) Each of the panels roughly corresponds to a 12-h progression in time (the exact shape and location of the elevated mixed layer plume may vary from case to case). Figure from Fig. 15 in Dahl and Fischer (2016); caption is modified by spelling out “elevated mixed layer.”
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
One of the common ways that the Spanish plume is mentioned in articles is through comparison with prefrontal troughs in other regions of the world. Such troughs have been noted by Hanstrum et al. (1990) for Australia specifically and by Schultz (2005) more generally. Even if not using the term Spanish plume in the text of the article, some articles mentioned the Spanish plume literature in support of prefrontal troughs (e.g., Speer and Geerts 1994; Charney and Fritsch 1999; Bryan and Fritsch 2000). Carlson and Ludlam (1968) and Bennett et al. (2006) have noted the similarities between the Spanish plume synoptic pattern and the synoptic pattern that creates the dryline of the central United States (e.g., Fujita 1958; Beebe 1958; Rhea 1966; Schaefer 1974, 1986), and such strong confluence and synoptic-scale forcing is integral to the formation of a dryline (Schultz et al. 2007) and subsequent convective storms along it (Mitchell and Schultz 2020), just as with the Spanish plume prefrontal trough (Morris 1986; McCallum and Waters 1993; Dahl and Fischer 2016).
To capture the principal features on the synoptic and mesoscale, various conceptual models have been presented: Fig. 3 (Morris 1986) and Fig. 10 (Dahl and Fischer 2016), as well as two others: Peyraud (2013, their Fig. 1) and Holley et al. (2014, their Fig. 8a). Lewis and Gray (2010) depicted two varieties of Spanish plume synoptic patterns associated with U.K. mesoscale convective systems, what they call “classical” and “modified” Spanish plumes (Table 5; Fig. 6), as well as a third pattern associated with easterly flow from continental Europe (not discussed here). Lewis and Gray (2010) argued that these two patterns were consistent with previously published conceptual models of cyclone life cycles by Thorncroft et al. (1993) and Schultz et al. (1998) with the classical Spanish plume events associated with LC1 cyclone life cycles in confluent jet-entrance environments and modified Spanish plume events associated with LC2 life cycles in diffluent jet-exit environments (Fig. 6).
Synoptic environments associated with U.K. mesoscale convective systems [modified from Lewis and Gray (2010) by only showing the first two of three environments as these pertain to the Spanish plume synoptic patterns].
Common features such as the mid- to upper-level flow pattern, surface frontal features, and low-level plumes of warm air were key determinants in categorising each case. Pattern matching of features at the upper-, mid-, and low-levels was used to the [sic.] group events according to their synoptic configuration. For example, the presence of a cutoff low feature at 500 hPa was used to distinguish a “modified” Spanish plume event from a “classic[al]” plume.
In the table we wanted to focus on the tilted trough structure as being the key distinguishing feature and so leave open that there might not be a fully fledged cut-off cyclone for this synoptic environment (i.e., we didn’t want to make the presence of a cut-off an essential requirement). However, to generate the composites we wanted to cleanly distinguish the different environments and so required a cutoff.
Other inconsistencies exist between their table (Table 5) and their schematic (Fig. 6). Table 5 describes a cyclone centered over the Bay of Biscay, but the modified Spanish plume schematic depicted a cyclone centered over the North Atlantic Ocean (Fig. 6b). There were also a number of features that were described in the table but were omitted in the schematic to simplify the plot (M. Lehnert, né Lewis, and S. Gray, 2024, personal communications). For example, Table 5 describes a low-level “trough extension over Ireland/United Kingdom” that was not depicted in their classical Spanish plume schematic (Fig. 6a). Also, Table 5 describes “occluded fronts” not depicted (Fig. 6b). Finally, regarding word choice, “classical” means a style of literature or art characterized by conformity to established standards, whereas “classic” means traditional or typical. The latter definition (i.e., “traditional”) has a more precise meaning in this context and was used in Lewis and Silkstone (2017, p. 92). Thus, “classical Spanish plume” should be more properly called “classic Spanish plume.”
A further point is that published articles tend to be about cases of Spanish plume patterns that produce convective storms over the United Kingdom, which may lead to a selection bias where warm and humid low-level air would generally be required in such patterns. But, there may be events where the low-level air is insufficiently warm or humid to support convective storms, and these events probably receive little or no attention. Thus, the Spanish plume synoptic pattern may occur, but without producing convective storms. A future research topic would be to study the full range of Spanish plume events, not just the ones that develop deep convective storms over the United Kingdom.
To summarize this section, we construct a conceptual model of a Spanish plume event based on the main or consistent features identified in our literature review that relate synoptic and mesoscale features of the Spanish plume pattern with convective storms (Fig. 11). Case-to-case variability means the intensities and exact relationships between these various features may not be represented exactly as depicted here. Nevertheless, this model focuses on the relationship between the surface cold front, the surface air, the elevated mixed layer, and the prefrontal trough. Representative soundings are presented in three sectors and locations where convection initiates are identified. This model is further developed in the next section.
Conceptual model of surface-based cold front (blue line with triangles), lower-tropospheric prefrontal trough (black dashed line), elevated mixed layer (EML) and Spanish plume airstream (beige shaded region with thick red outline), and sometimes moist surface (SFC) air (red shaded region, stippled when under the elevated mixed layer) based on figures and text from the literature reviewed in this manuscript. Locations of possible underrunning surface-based convection initiation labeled A and B and possible elevated and/or surface-based convection initiation along the prefrontal trough labeled C. Skew T–logp soundings taken at locations 1, 2, and 3 are represented along the bottom. The soundings were based on a synthesis of actual observed cases and operational experience over many years by the second author [e.g., Young (1995) examined 6-h observed soundings over southern England]. The detail in such soundings is representative of the kinds of structures observed, but cannot be taken literally, being a blend of many cases. The underlying geography has intentionally been omitted to avoid overgeneralization of this schematic. The region could be over the United Kingdom or western Europe.
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
9. Instability, inhibition, initiation, and organization of convective storms
To describe the environmental factors for convective storms, we follow the framework articulated by Doswell (1987) and Doswell and Bosart (2001): Synoptic-scale (i.e., quasigeostrophic) processes generally produce the thermodynamic environment favorable for convective storms and mesoscale processes generally initiate the convection. In this section, we ask the following questions. How is the instability created? How is the convective inhibition removed or overcome to allow storms to initiate? What is the role of wind shear in organizing the convective storms? As we show, nearly all articles that describe the origin of the instability or the mechanisms of convection initiation attribute these uncritically, without evidence.
a. Formation of instability
Most articles recognized that the warm near-surface air being overlain by hot dry air with steep lapse rates creates a statically unstable environment for convection above the inhibition layer, as first articulated by Carlson and Ludlam (1968). Yet, five other articles described cold advection aloft ahead of the advancing trough being responsible for creating the instability (McCallum and Waters 1993; Galvin et al. 1995; Gray and Marshall 1998; Galvin 2003; Hamid 2012). In one case, the preconvective hodograph had veering winds below 600 hPa and backing winds to 400 hPa, implying geostrophic warm advection lying below geostrophic cold advection, hence destabilization (McCallum and Waters 1993). In another case of convective storms over central Europe attributed to a Spanish plume synoptic pattern (Agustí-Panareda et al. 2005, p. 2), the creation of the CAPE was attributed to “cold advection by the upper-level trough . . . between 450 and 350 hPa” and “moistening of the boundary layer.” Otherwise, none of the articles provided evidence that their proposed processes were responsible for decreasing the static stability. Summarizing, all studies considered an elevated mixed layer overlying a low-level moist and warm layer to be an important contributor to the instability. But, some studies also pointed to cold advection, seemingly higher aloft, as an additional factor. Thus, one potential research question is how important the cold advection aloft is to the moist instability.
b. Inhibition and initiation
Convective inhibition (CIN) is typically present in Spanish plume patterns, and this inhibition needs to be overcome to initiate surface-based convection (Fig. 11). For the United Kingdom in particular, sometimes convective storms initiate over the United Kingdom, whereas other times they may originate over continental Europe and travel across the English Channel to the United Kingdom. Although Bennett et al. (2006) reviewed convection initiation over the United Kingdom, in our review, we focus specifically on explanations in the literature of processes that eliminate the inhibition and initiate convection in Spanish plume cases. We find such explanations were often uncritically presented, mentioning broad-scale processes that had not been evidenced within the individual articles or were unsupported by the data. We distinguish between processes that overcome existing inhibition and those that initiate convection (Table 6). We also avoid the use of “trigger” as a synonym for the lift that initiates storms because its meaning is imprecise (e.g., Schultz 2009, 363–364).
Processes that overcome CIN and initiate convection for Spanish plume synoptic patterns.
There are different ways for surface-based convection to overcome existing inhibition. First is to remove the lid. For example, Young (1995) hypothesized a combination of differential thermal advection, cooling by precipitation, and airmass ascent to remove the lid. In another example, Agustí-Panareda et al. (2005) hypothesized the removal of a deep layer of inhibition extending from the surface to 600 hPa from “cooling, presumably by turbulent fluxes from the ground.” In a further example, Lewis and Gray (2010) described the removal of the cap “by the onset of dynamical forcing and subsequent cooling by the eastward propagating shortwave trough.” Second is to overcome the inhibition by reaching the convective temperature. For example, Brown and Buchanan (2019) proposed that daytime heating of the near-surface air raises its temperature to a point where, if such parcels were lifted, those parcels would overcome the inhibition, allowing surface-based convection to proceed. Third is for surface-based convective storms to form around the inhibition layer through a process called underrunning (Fig. 11). Underrunning occurs when storms initiate where the near-surface humid air emerges from underneath the layer of convective inhibition at the base of the elevated mixed layer into a region with positive CAPE (e.g., Carlson and Ludlam 1968; Salter 1968; Carlson et al. 1983). Initiation can be at the northern end or western end of the layer of inhibition, as illustrated by convective clouds A and B in Fig. 11.
Other articles proposed synoptic-scale or quasigeostrophic processes that initiate convection. For example, Galvin et al. (1995) attributed initiation to “overrunning cool dry air,” without specifying how this initiated convection. In another example, low-level warm advection associated with the southerly or southwesterly flow and upper-level positive vorticity advection associated with the eastward-advancing trough have been argued to be associated with ascent that released the CAPE (Gray and Marshall 1998). Carlson and Ludlam (1968) recognized castellanus clouds in the Spanish plume layer above 700 hPa, consistent with destabilization and large-scale forcing for ascent aloft (e.g., Corfidi et al. 2008), before deep surface-based convection initiated. These mechanisms, however, seem at odds with the framework that synoptic-scale processes generally produce the thermodynamic environment favorable for the convective storms, and mesoscale processes generally initiate the convection, suggesting that the framework needs further refinement or the proposed mechanisms may not explain initiation.
Even if inhibition were absent, convection initiation does not spontaneously occur. Mesoscale forcing or inhomogeneities in the boundary layer circulations (e.g., open cellular convection, horizontal convective rolls) remains essential to produce convective clouds even in an uninhibited environment due to the suppressive effects of entrainment and adverse vertical pressure perturbations, for example (e.g., Khairoutdinov and Randall 2006; Kirshbaum 2011; Kang and Bryan 2011; Hohenegger and Stevens 2013). Thus, the lifting ingredient for deep moist surface-based convection is still required in an uninhibited environment.
Yet other articles proposed mesoscale mechanisms to initiate convection. We acknowledge that some overlap with quasigeostrophic processes may occur, as discussed by Dahl and Fischer (2016) who showed that quasigeostrophic processes contributed to ascent along the prefrontal trough caused by the thermal low. The prefrontal trough is associated with low-level convergence and has been argued as one locus for initiation (e.g., Carlson and Ludlam 1968; Morris 1986; van Delden 2001; Dahl and Fischer 2016), illustrated as location C in Fig. 11. Sea breezes, outflow boundaries, and surface fronts can also be focal points for surface-based convection initiation due to the shallow convergence and low-level ascent at such boundaries that can lift parcels to their level of free convection (e.g., McCallum and Waters 1993; Young 1995; Galvin et al. 1995). Although not described by the original authors as a Spanish plume event, Browning and Hill (1984) analyzed the case of a mesoscale convective system that initiated over the English Channel and then came onshore over southwestern England which bears many similarities to the Spanish plume synoptic pattern. They attributed the initiation to high θw flow from the south being lifted over cooler easterlies in the boundary layer. Although nocturnal jets and gravity waves may initiate storms over the central United States in synoptic environments with elevated mixed layers (e.g., Geerts et al. 2017), they have not been demonstrated in Spanish plume cases.
One relevant question is whether the initiation mechanism in Spanish plume events differs from non-Spanish-plume events in the United Kingdom. One hypothesis is that typical U.K. convective storms are associated with both small CIN and small CAPE over shallow layers, and only weak disturbances are needed to initiate convection (e.g., Holley et al. 2014). In contrast, the typical Spanish plume environment likely has larger CIN and CAPE, which might change the effectiveness of weaker initiation mechanisms like boundary layer turbulence and sea breezes.
c. Wind shear and organization of convective storms
Some early articles asserted that the vertical shear of the horizontal wind in the Spanish plume synoptic pattern was necessary for convective storms to organize upscale into long-duration mesoscale convective systems (e.g., Carlson and Ludlam 1968; McCallum and Waters 1993; Gray and Marshall 1998; Lewis and Gray 2010) and to produce hail (e.g., Webb and Pike 1998). Wind speed increasing with height in the southerly or southwesterly flow was conducive to such convective organization. Too much shear, however, could delay or even suppress initiation (e.g., Soderholm et al. 2014; Peters et al. 2022a,b). Preconvective soundings from various events adhering to the Spanish plume pattern showed the relatively weak directional shear (typically less than about 30° of turning) above the boundary layer, but moderate speed shear, throughout the troposphere (Table 7). Given the relatively straight hodographs indicating moderate speed shear, multicells or splitting supercells would have been the favored convective mode (e.g., Weisman and Klemp 1982, 1986). Indeed, these were the typical storm morphologies observed in Spanish plume events (e.g., Gray and Marshall 1998; Lewis and Gray 2010; Clark and Smart 2016). Furthermore, although the directional shear may not be large in an absolute sense, what shear there was may be sufficient to cause the airstreams at different levels to separate, which is important for the separation of the elevated mixed layer from the lower-level air that may arrive from a different region, such as central France.
Figures from previously published sources showing preconvective soundings from Spanish plume events indicating relatively weak directional shear above the boundary layer (typically less than about 30° and mostly within the boundary layer, if at all) but strong speed shear throughout the troposphere.
10. Recommendations and outstanding research questions
This critical literature review reveals the following principal recommendations and opportunities for future research (Fig. 12).
Schematic synthesis showing the recommendations and research opportunities arising from this review. Image credits: Carlson (Penn State University), Ludlam (Mason 1997), synoptic pattern (excerpted from Dahl and Fischer 2016, their Fig. 15b), mountain (pngtree.com), and storm cloud (pearlyarts.com).
Citation: Monthly Weather Review 153, 5; 10.1175/MWR-D-24-0139.1
a. Recommendations
1) Citations for Carlson and Ludlam (1968)
Although Carlson and Ludlam (1968) first used the expression “Spanish plume,” Morris (1986) gets more citations for it. When authors use the Spanish plume in their articles, they should cite the primary source, as well as subsequent work that built upon it (section 4).
2) Use of the term Spanish plume
The term Spanish plume is used either as the airstream that passes over the Iberian Peninsula or the synoptic pattern producing that airstream, or both. For maximum clarity, authors may wish to add “Spanish plume airstream” or “Spanish plume synoptic pattern.” Other uses of Spanish plume such as a zone of warm-air advection, front, prefrontal trough, or general period of warm weather should be avoided as incorrect, imprecise, or both (sections 2 and 5). The Spanish plume synoptic pattern may be present within a larger pattern that features such processes as warm-air advection, fronts, prefrontal troughs, and other phenomena, but specificity should be applied to what the Spanish plume actually refers to.
3) Definition of the Spanish plume airstream
Carlson and Ludlam (1968) defined the Spanish plume airstream as the elevated mixed layer, but some later articles defined other near-surface, sometimes-humid airstreams as the Spanish plume. We recommend that researchers stick with Carlson and Ludlam’s (1968) original definition (section 5).
4) Origin and variability of the Spanish plume
Carlson and Ludlam (1968) originally described the elevated mixed layer (i.e., the Spanish plume airstream) as originating over the Iberian Peninsula, but others have said that the flow during such Spanish plume events may have had a different origin (e.g., northern Africa, Mediterranean Sea). Most articles uncritically stated the origin without any explicit calculations of relative-flow isentropic maps or kinematic trajectories. Those that have calculated the path of the airstream found a range of paths, consistent with the case-to-case variability within the Spanish plume synoptic pattern. Open-source software to calculate trajectories has made this option quite feasible for future research projects (e.g., Stein et al. 2015). Thus, we recommend that future research studies perform explicit calculation of trajectories (sections 6a and 7).
5) Use of the term continental plume
Recognizing this case-to-case variability, the Met Office has recently been using the term continental plume to describe synoptic patterns that transport elevated mixed layers from continental Europe to the United Kingdom. Using this term would alleviate the concerns about whether the Meseta Central is responsible for the heating and destabilization of the elevated mixed layer, but it would not alleviate concern about the vagueness of the term plume. We also recommend that authors recognize that a range of near-surface conditions is possible in the United Kingdom. associated with the Spanish plume synoptic pattern, dispelling the idea that low-level humid air must be present (sections 6a and 7).
b. Opportunities
1) Importance of the topography in the creation of instability in the Spanish plume airstream
The previous literature has not quantified the importance of the diurnal variation in sensible heating of the airstream and how it impacts downstream convection. The possible contributions of the Saharan air layer to the elevated mixed layer are sometimes discussed, but not quantified. Also, how flow around the topography affects the resulting static stability profiles downstream has not been examined, but it is crucial for understanding convective-storm environments. To what extent does flow separation in the lee of the Meseta Central occur and precondition the near-surface preconvective environment? A further question is what if the southerly or southwesterly flow deflects around, rather than passes over, the Meseta Central? What modification of the near-surface air masses occurs as the air is transported poleward? Answering these questions sheds more light on the range of possible conditions in a Spanish plume synoptic pattern.
2) Convection initiation
Different mechanisms have been proposed for the processes by which the lid in association with the Spanish plume synoptic pattern is breached for convection initiation: underrunning, overrunning, quasigeostrophic ascent, removal of the lid, reaching the convective temperature, prefrontal troughs, nocturnal jets, gravity waves, etc. However, no articles test these ideas that lead to convection initiation. The question from section 9b about the difference between convection initiation in Spanish plume events and non-Spanish plume events also requires further investigation. Thus, there is the opportunity for future research.
Finally, these research topics are relevant beyond the Iberian Peninsula and Europe to other regions of the world where elevated mixed layers form. Climatologies of the elevated mixed layer have been performed across the United States (e.g., Carlson et al. 1983; Lanicci and Warner 1991; Andrews et al. 2024), as well as locations outside the central Plains such as the upper Midwest (Cordeira et al. 2017) and the northeast United States (Banacos and Ekster 2010). Climatologies of elevated mixed layers have been compared across both North America and South America (Ribeiro and Bosart 2018). The occurrence of drylines in South Africa may suggest the presence of elevated mixed layers in South Africa (van Schalkwyk et al. 2023). Documentation of midlevel dry air in convective storms downstream of the Tibetan Plateau suggests similar processes there (e.g., Das et al. 2015; Parker et al. 2016). For example, section 8 already discussed the relevance of the prefrontal trough to research in Australia and the dryline in the central United States. Thus, we anticipate that research into the Spanish plume airstream holds broader and more general insights into the role of orography on convective storms and their environments.
11. Conclusions
This review examines the Spanish plume, a 57-yr-old concept developed by Carlson and Ludlam (1968) to explain the conditions for the occurrence of severe convective storms in the United Kingdom. They argued that the Spanish plume was a southerly airstream heated by the high terrain of the Iberian Peninsula, creating an elevated mixed layer that led to high lapse rates and CAPE. Such airstreams are created during spring and summer when a large-amplitude trough or cutoff low in the jet stream extends to low latitudes with a strong or even blocking ridge downstream that produces a long fetch of southerly or southwesterly flow across the Iberian Peninsula and into western Europe. Carlson and Ludlam (1968) showed the similarity of these conditions to those in North America where heating from the elevated terrain of Mexico and the southwest United States creates the elevated mixed layer, capping inversion (or lid), and high CAPE over the central United States. This article was farsighted, not only through the introduction of the Spanish plume but also through its recognition of the global generality of conditions for severe convective storms and introduction of the elevated mixed layer and lid.
The Spanish plume was later popularized by Morris (1986) who extended the concept of the Spanish plume airstream to the Spanish plume synoptic pattern, a conceptual model that illustrated the large-scale conditions favorable for the formation of a prefrontal trough, quasigeostrophic ascent, frontogenesis, and secondary circulations favorable for convection initiation. Morris (1986) did not cite Carlson and Ludlam (1968), which likely led to reduced numbers of citations over time to Carlson and Ludlam (1968).
Later research showed that nearly 80% of mesoscale convective storms (Gray and Marshall 1998; Lewis and Gray 2010) and many occurrences of supercells and tornadoes (Clark and Smart 2016) over the United Kingdom were associated with the Spanish plume synoptic pattern. Air-parcel trajectories showed that lower-tropospheric air over the United Kingdom before convective storms formed arrived from the south, perhaps passing over the Iberian Peninsula, perhaps not (Sibley 2012; Lewis and Silkstone 2017; de Villiers 2020). Thus, the conditions that could be classified as a Spanish plume synoptic pattern do not necessarily involve heating over the high terrain of the Meseta Central. More recently, the Met Office has also recognized that the term Spanish plume is too specific given case-to-case variability, broadening the concept to the term continental plume.
We conducted a literature review that identified 102 articles mentioning the Spanish plume. We compared and contrasted this literature, looking for evidenced versus unevidenced results, points of agreement versus disagreement, points that are clear versus unclear, and facts versus inaccuracies.
Nearly three quarters of those 102 articles were published in journals from European or U.K. professional societies. Eighty-two percent of articles mentioning the Spanish plume merely made passing note of it, with almost 30% of those not citing any relevant literature, perhaps indicating how well accepted the concept is in the meteorological community. However, the term Spanish plume means different things to different people. More than three quarters of articles use the term Spanish plume as the synoptic pattern with only about a third using it as the airstream, its original definition; six percent use the term incorrectly. We recognize the pragmatism of the dual usage of the term Spanish plume and encourage future authors to be precise when using it.
Despite the popularity of conceptual models of the Spanish plume (e.g., Morris 1986; Lewis and Gray 2010; Peyraud 2013; Holley et al. 2014; Dahl and Fischer 2016; Clark and Smart 2016), the concept has been misused in the literature. Only a third of articles describe the Spanish plume consistent with Carlson and Ludlam’s (1968) original definition as the dry elevated mixed layer, a quarter of articles incorrectly describe the Spanish plume airstream as a humid airstream, with another 6% describing it as a region of warm advection. Other studies incorrectly describe the Spanish plume as a low-level airstream, not the elevated mixed layer. Clear definitions are needed for clear communication.
The most basic questions that can be asked are “Is it Spanish?” and “Is it a plume?” Of the few examples that have calculated the path of the air (either through isentropic analysis or air-parcel trajectories), air may not have passed over the Meseta Central and Spain on its poleward journey in all occurrences, despite being classified as a Spanish plume synoptic pattern. The point that the Spanish plume airstream is a buoyant jet continuously supplied from a point source remains unresolved.
We discussed the dual nature of the Meseta Central on the Spanish plume airstream, both through sensible heat fluxes and orographic flow. We discussed the formation of the instability and convection initiation. Based on the uncertainties associated with these aspects of Spanish Plume events, opportunities are ripe for quantitative studies on these topics using modern methods of synoptic meteorology, as well as the generalization of these results to other situations worldwide where elevated mixed layers form.
Based on this review of the literature, we make recommendations to authors, researchers, and forecasters on reaching common ground on the term Spanish plume and its meaning. However, by adopting the Spanish plume as a conceptual model, the risk is that we lose sight of it as a feature in an evolving process rather than being a static “cardboard cutout” schematic figure. Different authors may have captured the Spanish plume scenario at different stages of development or across case-to-case variability, which may have been reflected in their varying interpretations. This variety in the literature also reflects the steady drift over the decades away from the original airstream-based concept proposed by Carlson and Ludlam (1968). Thus, we advocate taking stock of how far we have come and recommitting ourselves to a common language and understanding, as well as identifying opportunities for future research explorations.
These Grays are two different authors: M. E. B. Gray and S. L. Gray.
A Spanish plume is a feathered accessory on a hat or helmet, as in the 1912 painting The Spanish Plume (Fig. 4). This portrait was inspired by Spanish painters such as Goya and Velázquez, and the sitter’s hat featured a feathered accessory called a panache that was manufactured in Spain and became popular in European fashion in the sixteenth century (e.g., Hanß 2021; Rublack 2021).
Acknowledgments.
We thank Bogdan Antonescu, Jonathan Fairman, and Daniel Currie for early discussions on this topic with the first author. We thank Toby Carlson, Matthew Clark, Mervyn Hardman, Michael Kunz, Matthew Lehnert (né Lewis), Ewen McCallum, David Smart, W. James Steenburgh, and Jannick Wilhelm for sharing their informal thoughts about the Spanish plume, plus the editor Ron McTaggart-Cowan and five formal reviewers: Jannick Fischer, Suzanne Gray, and three anonymous reviewers. We thank Richard Atherton for drafting Fig. 11. The base map for Fig. 1 was produced by Francisco Herrerias Azcue and Douglas Lowe. Thanks to Elizabeth Lindley of Sheffield Museums Trust for showing the first author The Spanish Plume by Sir Oswald Birley. Schultz was partially supported over the years working on this research by the Natural Environment Research Council, United Kingdom (Grants NE/I005234/1, NE/I026545/1, NE/N003918/1, NE/W000997/1, and NE/X018539/1). He also performed part of this work at the Aspen Center for Physics, which is supported by National Science Foundation Grant PHY-2210452. Kirshbaum was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through Grant RGPIN 418372-17. His simulations in Fig. 9 were enabled by a computing allocation from Calcul Québec (calculquebec.ca) and the Digital Research Alliance of Canada (alliancecan.ca).
Data availability statement.
The database for the literature review can be found in Schultz et al. (2024).
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