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  • View in gallery

    Meteosat-5 0.6-μm visible channel imagery of the October 1996 medicane at (a) 1000 UTC 7 Oct, (b) 1030 UTC 8 Oct, (c) 1030 UTC 9 Oct, and (d) 1030 UTC 10 Oct.

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    (a) Elevation map and schematic example of the domain shifting set up employed on the downscaling of reanalysis. The central domain is denoted by the black square. The arrows indicate the direction of the shifting. (b) The nested domain is shown. Gray dots indicate the position of each cyclone at the MWCT. The black line denotes the track of one of the cyclones.

  • View in gallery

    Cyclone-phase-space diagrams at the MWCT: (a) vs B and (b) vs . Each point represents a composite member. Red indicates the MED composite members, blue indicates the NONMED composite members, and gray indicates the remaining ensemble members. (c) Composite time series of SLP minimum (no markers), (triangular markers), and (circular markers). The phase space metrics refer to the right-hand axis, and the SLP minimum refers to the left-hand axis.

  • View in gallery

    Composite 300-hPa geopotential height [dam (contours)], 400-hPa PV [PVU (colors)], and divergent wind (vectors) for (a) MED and (b) NONMED.

  • View in gallery

    Composite 900-hPa potential temperature (colors) and SLP (contours) for (a) MED and (b) NONMED. The x and y axes indicate the distance from the composite center (850-hPa circulation max: star symbol).

  • View in gallery

    Composite 900-hPa vorticity (colors), 700–900-hPa vertical velocity sum (contours), and 1000-hPa wind for (a) MED and (b) NONMED. The x and y axes indicate the distance from the composite center (850-hPa circulation max: star symbol).

  • View in gallery

    Composite 500-hPa potential temperature (colors) and 300-hPa geopotential height (contours) for (a) MED and (b) NONMED. The x and y axes indicate the distance from the composite center (850-hPa circulation max: star symbol).

  • View in gallery

    The W–E cross sections of equivalent potential temperature (colors) and geopotential height anomaly (contours every 1 dam) for (a) MED and (b) NONMED. The x axis indicates the distance from the composite center.

  • View in gallery

    The W–E cross sections of potential temperature anomaly (K in gray contours when >0.5 K) and vertical velocity (m s−1 in black contours if >0.1 m s−1) and diabatic heating (K h−1 in colors) for (a) MED and (b) NONMED. The x axis indicates the distance from the composite center.

  • View in gallery

    Time series of composite (a) 300–850-hPa wind shear, (b) 600–700-hPa mean relative humidity, (c) 300–400-hPa mean wind divergence, (d) 850–100-hPa wind convergence, (e) sensible heat flux, and (f) latent heat flux. MED is shown in red, NONMED is shown in blue, and the composite difference is shown in black. The circular markers indicate where the difference is statistically significant at the 95% confidence level.

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    Scatterplots of vs composite (a) 300–850-hPa wind shear, (b) 600–700-hPa mean relative humidity, (c) 300–400-hPa mean wind divergence, (d) 850–100-hPa wind convergence, (e) sensible heat flux, and (f) latent heat flux. MED is shown in red, and NONMED is shown in blue. The black line represents the least squares fit line. The Pearson’s correlation coefficient is indicated in the text boxes.

  • View in gallery

    Composites difference in latent heat flux [colors (W m−2)], 1000-hPa wind [gray vectors (m s−1)], and 500–850-hPa vertical velocity sum (black contours every 0.5 m s−1 if >0.3 m s−1). Differences shown are significant at the 95% confidence interval. The x and y axes indicate the distance from the composite center (star symbol).

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The Tropical Transition of the October 1996 Medicane in the Western Mediterranean Sea: A Warm Seclusion Event

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  • 1 Institute of Meteorology, Freie Universität, Berlin, Germany
  • 2 Institute of Mathematics, Freie Universität, Berlin, Germany
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Abstract

The processes leading to the tropical transition of the October 1996 medicane in the western Mediterranean are investigated on the basis of a 50-member ensemble of regional climate model (RCM) simulations. By comparing the composites of transitioning and nontransitioning cyclones it is shown that standard extratropical dynamics are responsible for the cyclogenesis and that the transition results from a warm seclusion process. As the initial thermal asymmetries and vertical tilt of the cyclones are reduced, a warm core forms in the lower troposphere. It is demonstrated that in the transitioning cyclones, the upper-tropospheric warm core is also a result of the seclusion process. Conversely, the warm core remains confined below 600 hPa in the nontransitioning systems. In the baroclinic stage, the transitioning cyclones are characterized by larger vertical wind shear and intensification rates. The resulting stronger low-level circulation in turn is responsible for significantly larger latent and sensible heat fluxes throughout the seclusion process.

Denotes content that is immediately available upon publication as open access.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Publisher’s Note: This article was revised on 11 July 2017 to include missing information in the Acknowledgments section.

Corresponding author: Edoardo Mazza, edoardo.mazza@met.fu-berlin.de

Abstract

The processes leading to the tropical transition of the October 1996 medicane in the western Mediterranean are investigated on the basis of a 50-member ensemble of regional climate model (RCM) simulations. By comparing the composites of transitioning and nontransitioning cyclones it is shown that standard extratropical dynamics are responsible for the cyclogenesis and that the transition results from a warm seclusion process. As the initial thermal asymmetries and vertical tilt of the cyclones are reduced, a warm core forms in the lower troposphere. It is demonstrated that in the transitioning cyclones, the upper-tropospheric warm core is also a result of the seclusion process. Conversely, the warm core remains confined below 600 hPa in the nontransitioning systems. In the baroclinic stage, the transitioning cyclones are characterized by larger vertical wind shear and intensification rates. The resulting stronger low-level circulation in turn is responsible for significantly larger latent and sensible heat fluxes throughout the seclusion process.

Denotes content that is immediately available upon publication as open access.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Publisher’s Note: This article was revised on 11 July 2017 to include missing information in the Acknowledgments section.

Corresponding author: Edoardo Mazza, edoardo.mazza@met.fu-berlin.de

1. Introduction

Over the last several years our understanding of the connection between midlatitude and tropical cyclones (TCs) has significantly evolved. Different pathways have been identified along which a cyclone can acquire tropical or extratropical characteristics (Hart 2003; Yanase et al. 2014). During extratropical transitions TCs usually move to higher latitudes and interact with upper-level jets and low-level baroclinic zones, resulting in their transformation into midlatitude systems (Jones et al. 2003). Conversely, a tropical transition refers to the formation of a TC from a well-defined baroclinic precursor or remnant baroclinic structure (Bosart and Bartlo 1991; Davis and Bosart 2003, 2004). This precursor disturbance can be a well-defined frontal wave (Hulme and Martin 2009a), also referred to as a strong extratropical cyclone (SEC; Davis and Bosart 2004), or a remnant frontal zone [i.e., a weak extratropical cyclone (WEC); Davis and Bosart 2004] that moved into an area favorable for tropical development. McTaggart-Cowan et al. (2013) estimated that baroclinically influenced cyclogenesis accounts for nearly 30% of the global TC formations. Tropical transitions have been documented both in the North Atlantic (Hulme and Martin 2009a,b) and in the northeast Pacific (Bentley and Metz 2016) oceans. There is increasing evidence that the genesis of several medicanes or tropical-like cyclones in the Mediterranean basin can be interpreted as the result of the tropical transition of originally baroclinic cyclones (McTaggart-Cowan et al. 2010a,b; Chaboureau et al. 2012; Cioni et al. 2016).

Medicanes form in an environment generally not favorable for standard TC genesis, as observed in the tropics. They are usually subsynoptic in scale following a contraction of the precursor cyclone and are associated with strong surface wind gusts, along with intense precipitation. The Mediterranean Sea is a highly cyclogenetic area, the vast majority of the observed cyclones being extratropical in nature (Trigo et al. 1999; Ulbrich et al. 2012). It has been estimated that only 1.6 ± 1.3 medicanes form on average every year (Cavicchia et al. 2014). Two areas appear to be the favored locations for medicane genesis (Cavicchia et al. 2014): the western Mediterranean (see Reale and Atlas 2001; Cioni et al. 2016) and the central Mediterranean–Ionian Sea (see Moscatello et al. 2008b; Pytharoulis et al. 2000; Reed et al. 2001). Medicane climatology exhibits a remarkable year-to-year variability; however, late fall and winter emerges as the preferred period for their formation (Cavicchia et al. 2014). Moreover, their geographical and seasonal distribution is compatible with that of the combination of low wind shear, a large temperature differential between the upper troposphere and the sea surface, high column-integrated relative humidity and large low-level vorticity, according to Cavicchia et al. (2014).

The synoptic setting associated with the formation of these subsynoptic, warm-core cyclones is often characterized by an upper-level feature (Claud et al. 2010), either a fully isolated cutoff (Reale and Atlas 2001) or an elongated trough (Pantillon et al. 2013), responsible for the destabilization of the atmospheric column (Emanuel 2005; Fita et al. 2007; Cavicchia et al. 2014) and the quasigeostrophic forcing on vertical motions (Chaboureau et al. 2012).

On the cyclone scale, instead, enhanced vorticity is present (Cavicchia et al. 2014) along with substantial heat fluxes from the sea surface (Pytharoulis et al. 2000; Reale and Atlas 2001). Medicanes are capable of self-amplifying in absence of standard baroclinic processes (Emanuel 2005). Surface heat fluxes appear to be involved not only in maintaining the systems (Fita et al. 2007; Davolio et al. 2009; Lagouvardos et al. 1999; Pytharoulis et al. 2000; Moscatello et al. 2008b) but also in promoting the scale reduction of the vortex (Reed et al. 2001) and in modifying the stability of the boundary layer (Moscatello et al. 2008a). The intensification of medicanes is also dependent on diabatic effects, such as latent heat release by condensation in the midtroposphere (Pytharoulis et al. 2000; Moscatello et al. 2008b). In fact, according to Chaboureau et al. (2012) and Miglietta et al. (2013) the strongest convective activity is found before the storms reach their maturity.

Different dynamical mechanisms have been proposed to explain the genesis of medicanes. McTaggart-Cowan et al. (2010a,b) identified the interplay between a coherent tropopause disturbance, a diabatically generated potential vorticity (PV) anomaly, and an orographically generated PV banner as the key factor in the cyclogenesis of a medicane in the Gulf of Genoa. Chaboureau et al. (2012) suggested that the enhancement in convection due to the surface cyclone crossing an upper-level jet was responsible for the tropical transition of the September 2006 medicane.

Using reanalysis data, Tous and Romero (2011, 2013) compared the meteorological environment associated with 12 medicanes against that of the bulk of the Mediterranean cyclones. Among the parameters examined, only heat fluxes, expressed as a diabatic contribution to surface equivalent potential temperature, and an empirically defined genesis index proved to be moderately distinctive. An axisymmetric, cloud-resolving model was instead employed by Fita et al. (2007) to show that medicanes are highly sensitive to the relative humidity (RH) profile while less so to the sea surface temperature (SST). Similar conclusions were reached by Tous et al. (2013), even though a lower limit of 15°C is deemed to be necessary for their genesis (Tous and Romero 2013).

Our present study aims to document the processes involved in the genesis, transition, and maturity of the October 1996 medicane. To do so, an ensemble of model realizations is obtained through a dynamical downscaling of reanalysis data. This paper offers an original perspective on the development of medicanes in that it is based on the direct comparison of composites of transitioning and nontransitioning cyclones. The synoptic environment is analyzed by means of full-domain composites. Cyclone-centered composites are instead presented to identify the smaller-scale processes involved in the initial cyclogenesis, warm seclusion, and consequent warm-core formation. Finally, we analyze the differences in the composite environments of the transitioning and nontransitioning cyclones.

The remainder of the paper is organized as follows: section 2 provides the case overview and section 3 describes the model setup and the methodology. The results are presented in section 4, while the discussion and conclusions are included in section 5.

2. Case description

The October 1996 medicane is among the 12 events detected by Tous and Romero (2011). The precursor baroclinic cyclone originated off the Algerian coast in the afternoon of 6 October and was located beneath an upper-level cutoff low that previously moved from France to the Catalan Coast. On the southern edge of the cutoff low a strong geopotential height gradient was present along with it a sharp upper-level jet and a vorticity maximum (Reale and Atlas 2001).

As shown by the 0.6-μm visible channel imagery in Fig. 1, the cyclone progressed northward to a position between Sardinia and the Balearic Islands (Fig. 1a). At this stage, it still featured frontal structures and a broad circulation indicative of its extratropical origin. By 1200 UTC 7 October, its sea level pressure (SLP) minimum measured 999 hPa and incipient warm core was present along with a well-defined, eyelike structure (Reale and Atlas 2001). The system made its first landfall over Sardinia (Cavicchia and von Storch 2012), temporarily weakening and partially losing its tropical-like structure. Soon after 0000 UTC 8 October, the cyclone moved over the Tyrrhenian Sea. As the system reintensified, the SLP minimum dropped and a distinctive eyelike structure formed again. Figures 1b and 1c show the vortex-scale contraction as well as the presence of deep convection close the eye. According to Reale and Atlas (2001) and Cavicchia and von Storch (2012) a ship located around 100 km off the center recorded winds up to 25 m s−1 at 1200 UTC 8 October. The system subsequently moved south-eastward: on 9 October wind speeds of 22.5 m s−1 were recorder on the island of Ustica at 1500 and 1800 UTC. Having traveled almost 3000 km (Cavicchia and von Storch 2012), the medicane dissipated after making a final landfall over Calabria on 10 October (Fig. 1d).

Fig. 1.
Fig. 1.

Meteosat-5 0.6-μm visible channel imagery of the October 1996 medicane at (a) 1000 UTC 7 Oct, (b) 1030 UTC 8 Oct, (c) 1030 UTC 9 Oct, and (d) 1030 UTC 10 Oct.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

3. Data and methodology

a. Experimental setup

The numerical simulations are performed with the full physics, nonhydrostatic COSMO Climate Limited-Area Model (CLM; Rockel et al. 2008) version . The two-step downscaling configuration consists of a 257 × 271 grid point, 0.165° resolution parent domain (black square in Fig. 2a) in which a 288 × 192 grid point, 0.0625° resolution inner domain is nested (Fig. 2b). For both downscaling the model setup includes an extended microphysics scheme accounting for cloud water and cloud ice for grid-scale precipitation based on Kessler (1969), the Ritter and Geleyn (1992) radiation scheme, and the Tiedtke parameterization scheme for convection (Tiedtke 1989). Both domains feature 40 vertical levels and a 6-h update for boundary conditions. Two ensembles are performed with initial and boundary conditions provided by the 1.125° resolution version of the ERA-40 (Uppala et al. 2005) and the 0.7° resolution ERA-Interim (Dee et al. 2011), respectively. Both reanalyses have been previously employed as driving data in two numerical simulation studies of medicanes by Picornell et al. (2014) and Akhtar et al. (2014).

Fig. 2.
Fig. 2.

(a) Elevation map and schematic example of the domain shifting set up employed on the downscaling of reanalysis. The central domain is denoted by the black square. The arrows indicate the direction of the shifting. (b) The nested domain is shown. Gray dots indicate the position of each cyclone at the MWCT. The black line denotes the track of one of the cyclones.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

b. Ensemble generation

Different studies (see Davolio et al. 2009; Chaboureau et al. 2012; Cioni et al. 2016) provided independent evidence that numerical forecasts of medicanes are highly sensitive to differences in the initial conditions, both in terms of their track and intensity.

In this experiment a straight-forward technique, referred to as domain shifting (DS), is employed to generate an ensemble of model simulations. DS consists in performing the numerical integrations over multiple domains that are shifted relative to each other. This technique has been effectively employed in generating ensembles for climate applications (Pardowitz et al. 2016) as well as short-range, convection-permitting numerical simulations (Rezacova et al. 2009).

A simplified representation of the modeling scheme is provided in Fig. 2a. The procedure is applied to the first downscaling step from reanalysis data to the parent domain as follows:

  • Locate a central domain (black domain in Fig. 2a).
  • Shift the central domain in longitude and latitude in eight directions [north (N), south (S), east (E), west (W), northeast (NE), northwest (NW), southeast (SE), southwest (SW), see Fig. 2a].
  • Apply the shifting in steps of 0.25°, 0.50°, and 0.75° obtaining in 24 additional domains.
  • Run the first downscaling step for each of the domains.

This results in two sets of 25 simulations that constitute the 50-member ensemble. Each simulation is then downscaled on a unique, nested domain spanning the western and central Mediterranean basin (Fig. 2b). The initialization for the parent simulations is 0000 UTC 1 October 1996 while the inner domains are initialized at 0000 UTC 4 October 1996. A 72-h lag is found to be a satisfactory compromise between introducing sufficient spread in the ensemble and the ability to correctly simulate the event.

A comparison is performed with a corresponding ensemble obtained with a time-lagging approach, where the initialization date has been lagged back in time from 4 October to 1 October at 24-h intervals. The two methods provide a similar amount of SLP ensemble spread (not shown) with comparable temporal and spatial local maxima.

c. Cyclone phase space

The tropical transition of the simulated cyclones is assessed by means of a modified cyclone phase space, following Hart (2003). This three-dimensional diagnostic methodology has already been successfully applied to the study of medicanes (see Davolio et al. 2009; Cavicchia and von Storch 2012; Miglietta et al. 2013). Following the works of Picornell et al. (2014), Miglietta et al. (2013), and Cioni et al. (2016), a variation of the original phase space is applied. In particular, in order to account for the limited vertical extent of medicanes the upper bound is reduced from 300 to 400 hPa. However, the middle level is kept at 600 hPa as in Cioni et al. (2016). This level better approximates the height at which the vertical derivative of the height perturbation changes sign.

The parameters that define the phase space are as follows:

  • The thermal symmetry in the lower troposphere (B): the difference in storm-relative 600–900-hPa thickness between left and right semicircles with respect to the cyclone’s trajectory.
  • The lower-tropospheric thermal wind (): the vertical derivative of the cyclone’s height perturbation between 900 and 600 hPa.
  • The upper-tropospheric thermal wind (): the vertical derivative of the cyclone’s height perturbation between 600 and 400 hPa.

Because of the smaller spatial scale of medicanes compared to TCs, a radius smaller than the 500 km one used by Hart (2003) has to be employed for the phase space calculation. Several authors investigated the choice of an optimal radius: generally, the values range between 100 and 200 km. Miglietta et al. (2013) and Picornell et al. (2014) suggested that a variable radius might be more appropriate to capture the differences in size and structure among different storms. In our application, a 150-km radius around the cyclone center is selected. This represents a compromise between a larger radius required to account for the initial baroclinic tilt of the cyclones (about 300 km in scale) and the identification of the subsequent warm core at a much smaller scale (50–70 km). A cyclone is classified as a medicane if and when it meets simultaneously all of three requirements suggested by Hart (2003): B < 10 m, 0, and 0. Hence, according to this definition medicanes are axisymmetric cyclones with a tropospheric warm-core structure extending from 900 to 400 hPa.

d. Cyclone tracking

The simulated cyclones are tracked using both the SLP and the 850-hPa circulation fields. Before the tracking algorithm is applied, the 850-hPa wind components are filtered with a 2D convolution filter of size 10 × 10 grid points in order to smooth smaller-scale structures. At each grid point the circulation is calculated by considering the eight neighboring grid points. The algorithm works as follows:

  • First, a SLP-based tracking is applied: the location of the cyclone is recorded if the associated SLP minimum is deeper than 1013 hPa.
  • The subsequent positions of the SLP minimum are determined using a nearest-neighbor algorithm, applied in a circular area within 0.9° from the previous SLP minimum.
  • The position of the SLP minimum at the final landfall (Sardinia is not considered as a landfall) or dissipation (minimum > 1013 hPa) is recorded.
  • At the last point on the track, the 850-hPa circulation maximum associated with the cyclone is searched within 0.9° from SLP minimum position.
  • The final track is obtained by backtracking the 850-hPa circulation maximum using the same nearest-neighbor algorithm employed for the forward tracking.
  • Two requirements are imposed: at each step, the circulation maximum must exceed 0.5 m2 s−1 and must be located over the sea (Sardinia is not considered land in order to maintain the integrity of the tracks).

There are several motivations behind this methodology. First, the circulation renders a smoother version of the vorticity field and allows for an easier feature tracking by discarding small-scale, transient secondary vorticity maxima. The backtracking is started from the landfall as it is much easier to identify the circulation maximum of a mature cyclone relative to a developing one. By backtracking the circulation maximum, longer and more coherent tracks can be obtained, due to the limited effects of topography and the presence of a defined circulation maximum even in the stages preceding the appearance of a closed SLP minimum. One example track is shown in Fig. 2b.

e. Cyclone compositing

Cyclone compositing has been frequently applied to the study of extratropical and tropical cyclones (see Frank 1977; Bracken and Bosart 2000; Bengtsson et al. 2007; Catto et al. 2010). It essentially relies on simple arithmetic averaging to extract the signal from fields that are contaminated by noise. In this study it is applied to obtain the mean thermal structure as well as the average environment of transitioning and nontransitioning cyclones. To align the temporal evolution of the simulated cyclones to a common reference time, for each track we identify the time step when is maximum, B < 10 m and 0. This instant is meant to reflect to stage of maximum warm-core strength, hence it is referred to as the maximum warm-core time (MWCT). This study focuses solely on the genesis and tropical transition of the simulated cyclones. Their evolution after the temporary transition over Sardinia is not considered. The geographical location of all cyclones at their respective MWCT is shown in Fig. 2b. The composites are built for the time range from MCWT − 30 h to MCWT + 30 h.

Two composites are built: MED that includes the strongest 15 medicanes ( values exceeding the 70th percentile) and NONMED that includes the weakest 15 nonmedicanes ( values within the 30th percentile). Two types of hourly composites are built with respect to the MWCT: a full-domain, 1800 km × 1200 km composite and a cyclone-centered (relative to the 850-hPa circulation maximum) composite over a 420 km × 420 km regular grid.

For a selection of atmospheric parameters the MED-NONMED difference is calculated. Its statistical significance is tested at the 95% confidence interval by means of a bootstrapping by resampling approach, similar to that of Rios-Berrios et al. (2016). However, in this study the null hypothesis is that there is no difference between the composites. The method allows the test to be performed without assuming a specific probability distribution for the quantities of interest. The statistical significance is computed for both individual grid points and area-averaged quantities.

4. Results

a. Phase space evolution

According to the cyclone phase space metrics, all the 50 simulated cyclones attain a state characterized by negligible thermal asymmetry (B < 10 m) and a low-level warm core ( 0) (Fig. 3a). In 28 of them the warm-core structure extends to the upper troposphere as indicated by positive values: these are identified as medicanes (upper-right quadrant in Fig. 3b). In the 22 nontransitioning cyclones, never exceeds 0 (lower-right quadrant in Fig. 3b) and are referred to as nonmedicanes. Of those, 10 are ensemble members driven with ERA-40 and 12 with ERA-Interim. MED and NONMED composite members are denoted in Figs. 3a and 3b by red and blue markers, respectively. Gray markers indicate the ensemble members left out of the composite analysis (i.e., whose values at the MWCT were between the 30th and the 70th percentile).

Fig. 3.
Fig. 3.

Cyclone-phase-space diagrams at the MWCT: (a) vs B and (b) vs . Each point represents a composite member. Red indicates the MED composite members, blue indicates the NONMED composite members, and gray indicates the remaining ensemble members. (c) Composite time series of SLP minimum (no markers), (triangular markers), and (circular markers). The phase space metrics refer to the right-hand axis, and the SLP minimum refers to the left-hand axis.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

Figure 3c displays the time series of the composite SLP minimum (solid line), (solid line with triangles) and (solid line with circles) for MED and NONMED. Both composites are characterized by an early intensification phase denoted by a steady deepening of the SLP minimum. Until 15 h before the MWCT, the transitioning cyclones intensify on average at a rate of approximately 0.5 hPa h−1. The pressure minimum of nontransitioning cyclones deepens instead at a lower rate of nearly 0.25 hPa h−1. Later, in both composites SLP deepens at a smaller rate. As a result at the MWCT, the MED composite is 6 hPa deeper than NONMED, with a minimum of 999 hPa.

The evolution of the phase space metrics offers further insight into the thermal characteristics of the cyclones. As the pressure drops the structure of both composites progressively changes from that of a full-tropospheric cold-core system to that of an axisymmetric, warm-core cyclone. A low-level warm core develops around MWCT − 16 h and MWCT − 13 h in MED and NONMED, respectively, as indicated by positive in Fig. 3c. The formation of an upper-level warm core is evident from the steady increase of (solid lines with circular markers in Fig. 3c) toward and, in the case of MED, beyond 0. At approximately MWCT − 6 h, the MED composite exhibits all the characteristics of a fully developed tropical system. In NONMED, instead, the warm-core structure remains confined to the lower troposphere, as denoted by the coexistence of positive value and negative values at the MWCT (Fig. 3c).

b. Synoptic analysis

In Fig. 4, the composites of 300-hPa geopotential height, 400-hPa potential vorticity, and divergent wind for MED (Fig. 4a) and NONMED (Fig. 4b) are presented. In both composites, 30 h before the MWCT, an upper-level trough is located between the Balearic Islands and the Gulf of Lion. Associated with it there is a PV maximum, partly due to a lowering of the tropopause (not shown), a significant height gradient on the western and southern edges, and enhanced upper-level divergence. The upper-level trough becomes a cutoff low as it moves eastward toward Corsica and Sardinia while gradually filling up from 902 to 906 dam at the MWCT. As indicated by the vectors in Figs. 4a and 4b, in both composites local divergence maxima are present off the coast of Algeria, the area where the cyclogenesis takes place, 30 h prior to the MWCT. This agrees very well with the scenario described by Reale and Atlas (2001) although it does not provide a clear distinction between the composites. However, in MED the role of the divergent flow in restructuring the PV field becomes more evident. Between MWCT − 18 h and MWCT − 10 h, there is a nonnegligible southward component in the divergent wind vectors over the western Mediterranean Sea that acts almost perpendicular to the PV gradient. The negative advection of PV maxima away from the surface cyclones by the upper-level outflow is a recurrent feature of tropical transition (Davis and Bosart 2003, 2004) and is generally associated with an overall reduction in vertical wind shear (Hulme and Martin 2009a,b). The overall evolution of both composites is qualitatively consistent with this paradigm, even in the case of NONMED.

Fig. 4.
Fig. 4.

Composite 300-hPa geopotential height [dam (contours)], 400-hPa PV [PVU (colors)], and divergent wind (vectors) for (a) MED and (b) NONMED.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

c. Thermal structure of the cyclones

In the lower troposphere, the evolution of the cyclone and its associated thermal structure is initially similar in both composites and exhibits many of the characteristics of extratropical cyclones. As Fig. 5 shows, the cyclone develops along an area of strong 900-hPa potential temperature gradient, with colder air to the northwest and warmer air to the southeast. In response to the upper-tropospheric dynamics previously discussed, a center of low pressure develops. At MWCT − 16 h the potential temperature field of both composites features a well-defined warm sector to the northeast of the SLP minimum. Hence, the precursor cyclone of this tropical transition can be classified as SEC, following Davis and Bosart (2004). The subsequent evolution resembles that of warm seclusions (Shapiro and Keyser 1990). An elongated tongue of warm air extends from the warm sector to the center of the composites. Between MWCT − 12 h and MWCT − 8 h, the low-level warm core is separated from the parent warm sector and becomes collocated with the SLP minimum. In the MED composite, this low-level warm core is more robust and symmetric. Its temperature difference from the surrounding environment approaches 3 K at the MWCT.

Fig. 5.
Fig. 5.

Composite 900-hPa potential temperature (colors) and SLP (contours) for (a) MED and (b) NONMED. The x and y axes indicate the distance from the composite center (850-hPa circulation max: star symbol).

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

In the lower troposphere, the cyclogenesis is also revealed by examining the 900-hPa vorticity and 1000-hPa wind vectors (Fig. 6). Similar to the low-level thermal evolution, there are significant similarities between MED and NONMED. Until MWCT − 25 h, the area of cyclonic vorticity tends to be stretched SW–NE, along the thermal gradient. As the central pressure falls, the near-surface cyclonic circulation intensifies, the vorticity maximum amplifies and the circulation becomes more axisymmetric. At the MWCT the vorticity maximum in MED exceeds that of NONMED by 2.6 × 10−4 s−1. The black contours in Fig. 6 indicate the 700–900-hPa ascent. During the early stages of the warm seclusion, ascent is confined to the north of the SLP minimum and gradually progresses cyclonically. It is worth noting that toward the final stages of the seclusion, between MWCT − 12 h and MCWT − 8 h the ascent is oriented along the advancing edge of the warm air tongue shown in Fig. 5. This is consistent with previous studies indicating that a bent-back warm front often accompanies warm seclusions where frontogenesis and ascent are enhanced (Hulme and Martin 2006, 2009a,b). After the seclusion is completed, the composites show little signal in vertical velocities. This reflects both a temporary reduction in convection as well as a less coherent distribution of convection around the core in the absence of frontal structures.

Fig. 6.
Fig. 6.

Composite 900-hPa vorticity (colors), 700–900-hPa vertical velocity sum (contours), and 1000-hPa wind for (a) MED and (b) NONMED. The x and y axes indicate the distance from the composite center (850-hPa circulation max: star symbol).

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

The 300-hPa geopotential height fields and 500-hPa potential temperature fields in Fig. 7 provide indications of the baroclinic dynamics responsible for the cyclogenesis. In both composites, the low-level cyclone forms under a tight geopotential height gradient. The upper troposphere is, in fact, characterized by a strong thermal contrast. The trough is located to the northwest of the low-level cyclone and gradually progresses cyclonically around it. The axisymmetrization of the vertical structure is completed around MWCT − 8 h, when both composites show the geopotential height minimum of 904 dam directly above the low-level cyclone, indicated by the star in Fig. 7.

Fig. 7.
Fig. 7.

Composite 500-hPa potential temperature (colors) and 300-hPa geopotential height (contours) for (a) MED and (b) NONMED. The x and y axes indicate the distance from the composite center (850-hPa circulation max: star symbol).

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

Most importantly, by examining the evolution of the 500-hPa potential temperature, it is possible to identify the mechanism leading to the formation of an upper-tropospheric warm core in MED. In this case, the difference with respect to NONMED is evident. In MED a warm anomaly is separated from the warm sector around MWCT − 8 h. Initially the warm core is approximately 60 km in diameter; at the MWCT the diameter of the warm core exceeds 100 km. In MED the seclusion of a warm core by colder air extends from the lower troposphere to its upper portion. Conversely, in NONMED the warm seclusion dynamics are confined in the lower troposphere. In fact, its upper-level thermal structure remains largely asymmetric. Under the geopotential height minimum instead of a smaller-scale warm core as in MED, there is a widespread area of colder air that was advected cyclonically from the western quadrants.

d. Warm-core formation

The formation of the warm core is further analyzed by looking at longitudinal (west–east) cross sections of equivalent potential temperature and geopotential height anomaly (calculated as a departure from the composite area average) through the centers of the composites (Fig. 8). Both in MED and NONMED, the upper-level cutoff appears as a negative geopotential height anomaly to the west of the surface cyclone, encroaching on a near-surface horizontal equivalent potential temperature gradient. The two composites display a significant westward vertical tilt of the geopotential height anomaly.

Fig. 8.
Fig. 8.

The W–E cross sections of equivalent potential temperature (colors) and geopotential height anomaly (contours every 1 dam) for (a) MED and (b) NONMED. The x axis indicates the distance from the composite center.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

As already indicated in Fig. 7, during the warm seclusion the thermal asymmetry and the vertical tilt are reduced as the upper-level height anomaly weakens. Conversely, the height anomaly in low levels amplifies as denoted by a growing negative geopotential height anomaly. At MWCT − 12 h, MED has already acquired a vertically stacked structure, with the largest geopotential height anomaly located close to the surface (Fig. 8a). Moreover, the surface cyclone amplification is much stronger than in NONMED, with an associated height anomaly twice as large. A symmetric, lower-tropospheric warm core also becomes evident in MED around MWCT − 12 h. In NONMED the overall geopotential height anomaly associated with the cyclone is weaker. Its vertical tilt shifts from westward to eastward at MWCT − 8 h. Only at the MWCT the structure is vertically aligned. While the warm-core structure is evident at all levels below 400 hPa in MED, the upper-tropospheric layers in NONMED exhibit a nearly constant equivalent potential temperature.

Figure 9 displays the west–east cross sections of potential temperature anomaly (calculated as a departure from the area-averaged mean and contoured only when exceeding 0.5 K) superimposed on the composite vertical velocity and diabatic heating fields. Diabatic effects associated with deep ascent can contribute to the formation of an upper-tropospheric warm core, as well as accelerate the occlusion of cyclones (Posselt and Martin 2004). As already indicated by Fig. 6, in both composites ascent is initially weak and scattered around the developing 850-hPa vorticity maximum. As the seclusion progresses, however, the areas of positive vertical velocities focus in the NW and NE quadrants. These areas of ascent are associated with the release of latent heat by condensation. The resulting diabatic warming is largely concentrated in the midtroposphere between 400 and 700 hPa. Figure 9 suggests that MED during the seclusion (between MWCT − 16 h and MWCT − 8 h) is associated with relatively larger diabatic heating west of the cyclone center. It is likely that this warming contributed to bridging the gap between the low-level and the upper-level warm anomalies at MWCT − 12 h. In contrast, NONMED features weaker ascent and diabatic heating. The two positive potential temperature anomalies observed, one beneath 700 hPa and one around 300 hPa, remain separated, hence the warm-core structure remains confined to the lower troposphere. This is consistent with what the evolution of the cyclone phase space metrics also suggested (Fig. 3).

Fig. 9.
Fig. 9.

The W–E cross sections of potential temperature anomaly (K in gray contours when >0.5 K) and vertical velocity (m s−1 in black contours if >0.1 m s−1) and diabatic heating (K h−1 in colors) for (a) MED and (b) NONMED. The x axis indicates the distance from the composite center.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

e. Subsynoptic environment

Most of the meteorological parameters analyzed by Tous and Romero (2013) to describe the environment that supports the genesis of medicanes are presented in Fig. 10, along with others that have been investigated in the context of tropical cyclones. Vertical wind shear is calculated as the difference between the mean wind vectors at 300 and 850 hPa over the composite area, in order to reduce any contribution from the vortex circulation.

Fig. 10.
Fig. 10.

Time series of composite (a) 300–850-hPa wind shear, (b) 600–700-hPa mean relative humidity, (c) 300–400-hPa mean wind divergence, (d) 850–100-hPa wind convergence, (e) sensible heat flux, and (f) latent heat flux. MED is shown in red, NONMED is shown in blue, and the composite difference is shown in black. The circular markers indicate where the difference is statistically significant at the 95% confidence level.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

In the early stages, MED exhibits stronger vertical wind shear along with larger upper-level wind divergence (Figs. 10a,c). In particular, 20 h before MWCT, MED has a 300–850-hPa wind shear nearly 5 m s−1 stronger than NONMED. Although rarely statistically significant, these differences suggest that the enhanced baroclinicity in the early stages likely contributed to the stronger intensification observed in MED. It is worth noting that the temporal trend of vertical wind shear is consistent with PV evolution in Fig. 4: in fact, the largest reduction is observed between MWCT − 25 h and MWCT − 10 h. During this period the area-averaged upper-level wind divergence reaches its maximum value (Fig. 10c) and acts to advect high-PV air away from the western Mediterranean Sea.

Area-averaged midtropospheric RH also peaks before the seclusion takes place, with values larger than 80% (Fig. 10b). In both composites, it later decreases between 65% and 70%. MED are characterized by slightly higher relative humidity, on the order of 1%–2%, throughout much of the period preceding MWCT. However, these differences are not statistically significant.

Low-level convergence exhibits a similar evolution to that of upper-level divergence: it is maximized during the baroclinic phase of both composites and decreases during the seclusion (Fig. 10d). It follows a different behavior only after MED completed the transition to a full-tropospheric warm-core system. It is in fact characterized by a significantly larger wind convergence between 850 and 1000 hPa, almost 100% larger than in NONMED, from MWCT − 3 h to MWCT + 9 h.

The temporal evolution of both latent and sensible heat fluxes in MED and NONMED is also very similar (Figs. 10e,f). Both composites start off at values close to 150 and 50 W m−2, respectively. In the case of latent heat, in MED it reaches its peak at MWCT − 7 h, while in NONMED it does so at MWCT − 4 h. Similarly, the sensible heat flux peaks at MCWT − 8 h in MED and MWCT − 5 h in NONMED. Most importantly, both heat fluxes exhibit also remarkable differences, especially with respect to their magnitudes. In particular, there is a prolonged period of time, approximately from MWCT − 18 h to MWCT − 5 h, where MED is characterized by significantly larger latent and sensible heat fluxes, with maxima exceeding 50 and 20 W m−2, respectively. The latent heat flux is also significantly larger for a 13-h period from MWCT + 7 h to MWCT + 20 h.

Figure 11 displays the scatterplots for the area-averaged quantities examined in Fig. 10 and the phase space parameter . Both wind shear and divergence are negatively correlated with . Wind shear, in particular, exhibits the highest linear correlation coefficient, −0.78, while a coefficient −0.59 is obtained in the case of upper-level wind divergence (Figs. 10a,c). For midtropospheric relative humidity the linear correlation between the two variables is extremely small () (Fig. 10b). Low-level convergence is negatively correlated with the upper-tropospheric warm-core metric, although only weakly () (Fig. 10d). Conversely, latent and sensible fluxes show a strong positive correlation with , with coefficients of 0.67 and 0.57, respectively (Figs. 10e and 10f).

Fig. 11.
Fig. 11.

Scatterplots of vs composite (a) 300–850-hPa wind shear, (b) 600–700-hPa mean relative humidity, (c) 300–400-hPa mean wind divergence, (d) 850–100-hPa wind convergence, (e) sensible heat flux, and (f) latent heat flux. MED is shown in red, and NONMED is shown in blue. The black line represents the least squares fit line. The Pearson’s correlation coefficient is indicated in the text boxes.

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

Both the time series and the correlation analysis suggest a further investigation of the temporal and spatial evolution of surface fluxes. Figure 12 shows the evolution of the difference between MED and NONMED in the case of latent heat flux; in this figure, nonstatistically significant values are masked out. As expected from the time series in Fig. 10, the largest difference appear after WMCT − 18 h. Well-defined areas of larger heat fluxes are present during the seclusion process and gradually progress cyclonically around the composite center. In particular, at MWCT − 8 h, a large positive anomaly is located to the south and to the west of the center. In the subsequent hours, this anomaly elongates and rotates to the NW quadrant.

Fig. 12.
Fig. 12.

Composites difference in latent heat flux [colors (W m−2)], 1000-hPa wind [gray vectors (m s−1)], and 500–850-hPa vertical velocity sum (black contours every 0.5 m s−1 if >0.3 m s−1). Differences shown are significant at the 95% confidence interval. The x and y axes indicate the distance from the composite center (star symbol).

Citation: Monthly Weather Review 145, 7; 10.1175/MWR-D-16-0474.1

The influence of the SST on the heat fluxes is marginal. Given that the SST is prescribed at the lower boundary the only differences can be due to the cyclones having slightly different tracks. The time series of composite area-averaged SST for MED and NONMED do not show any substantive or significant difference (not shown). The model parameterization of the surface latent heat flux depends instead on the wind speed at the lowest model level as well as on the specific humidity difference between the sea surface and the lowest model level. The vectors in Fig. 12 represent the difference in 1000-hPa winds between MED and NONMED. The results indicate that MED has stronger low-level winds over a large part of the composite area. Throughout the 30 h prior to the MWCT, it can be seen that the areas of enhanced latent heat flux are collocated with the largest differences in 1000-hPa winds. The evidence presented suggests that the stronger low-level circulation resulting from the larger intensification of MED results in enhanced heat fluxes from the sea over a prolonged period of time preceding the MWCT.

Latent and sensible heat fluxes are also known to affect convection, in particular they can contribute to substantial moistening and warming of the boundary layer. In the case of the genesis of tropical cyclones under westerly shear, Rappin and Nolan (2012) suggested that enhanced heat fluxes from the sea favor the downwind propagation of convection in a scenario where the mean surface flow and the vertical wind shear are aligned. During most of the seclusion process, the shear is westerly in both MED and NONMED. In MED the mean surface flow has a stronger westerly component (see Fig. 6a) than in NONMED. In agreement with Rappin and Nolan (2012), this results in an area of enhanced latent heat flux upwind of the region of stronger ascent (Fig. 12) where the dry air from the downdrafts cannot offset the warming and moistening produced by sensible and latent heat fluxes. However, Rappin and Nolan (2012) found that the interplay between surface heat fluxes and convection is more conductive to rapid intensification of tropical cyclones when the mean surface flow and the wind shear are counteraligned.

5. Discussion and conclusions

The present study describes the tropical transition of a medicane in the western Mediterranean Sea using a 50-member ensemble of full-physics, nonhydrostatic COSMO-CLM simulations at a resolution of 0.0625°. The cyclones are analyzed and classified according to a modified cyclone phase space following Hart (2003). Out of the 28 transitioning cyclones, the 15 cyclones featuring the strongest upper-level warm cores are composited. This is compared to a composite of the 15 nontransitioning storms featuring the strongest upper-level cold cores.

Many similarities between the two composites arise in terms of the synoptic environment associated with the cyclogenesis. Both feature a well-defined upper-tropospheric cutoff over the western Mediterranean basin, with a strong jet and substantial upper-level wind divergence. In the low levels, a pressure minimum forms along an area of enhanced potential temperature gradient. The cyclogenesis results in a structure that exhibits many of the characteristics of strong extratropical cyclones such as a vertical tilt, a warm sector, and frontal structures. The tropical transition takes place as the cyclones undergo a warm seclusion. In fact, a warm anomaly collocated with the surface pressure minimum is secluded as colder and drier air wraps around the center from the western quadrants. Most of the ascent is focused along an area reminiscent of a bent-back warm front located to the NW of the cyclone center. From the genesis to the seclusion the cyclones span a lifetime generally shorter than 35 h. The evolution of the composites is akin to that of the Shapiro–Keyser model of marine cyclones (Shapiro and Keyser 1990).

There are, however, significant differences between the two composites. At the MWCT, the transitioning cyclones are remarkably stronger than the nontransitioning ones. Their structure is characterized by a vertical warm core that extends from the surface up to the tropopause, with its maximum below 700 hPa. The dynamics involved in the warm-core formation constitute a major contribution of this study. Specifically, the composite analysis shows that in the transitioning cyclones the upper-tropospheric warm core is also a result of the warm seclusion. In fact, 8 h prior to the MWCT an upper-tropospheric warm core of approximately 60 km in diameter is separated from the warm sector and becomes collocated above the SLP minimum. After the seclusion the core warms up and expands to a diameter larger than 100 km at the MWCT. Conversely, in the nontransitioning cyclones the seclusion renders a warm core that is confined below 600 hPa. In the upper troposphere, in fact, the cyclones remain asymmetric and cold core. This is the first time that such a difference is documented in the case of medicanes.

The transition is accompanied by a strong reduction in vertical shear, as previously documented in several tropical transitions of Atlantic hurricanes (see Davis and Bosart 2004; Hulme and Martin 2009a,b). In both composites shear is reduced from values exceeding 25 m s−1 to values below 5 m s−1 at MWCT − 5 h. However, MED is characterized by substantially larger wind shear over a period of time longer than 15 h preceding the seclusion. This led us to two findings: first, in proximity of the MWCT the wind shear is smaller than the thresholds above which TC genesis is prevented (see DeMaria et al. 2001; Gallina and Velden 2002); second, in contrast with what previous studies of tropical cyclones suggested (see Davis and Bosart 2003; Kaplan et al. 2010), wind shear does not seem to discriminate between transitioning and nontransitioning systems.

Relative humidity has often been investigated in relation to TC genesis (see Hendricks et al. 2010; Wu et al. 2012; Brown and Hakim 2015). Different measures of RH have been used to assess its role in medicanes formation: Tous and Romero (2013) focused on its value at the 600-hPa level while Fita et al. (2007) and Cavicchia et al. (2014) considered vertically integrated RH. While Fita et al. (2007) found that changing the RH profile, in particular reducing its value across the troposphere, drastically influenced its axisymmetric simulation of medicane, Tous and Romero (2013) concluded that midtropospheric RH alone does not differentiate the environments supporting the genesis of medicanes from those supporting extratropical cyclones. Our study qualitatively agrees with the latter given that the RH composite difference is consistently small and not significant.

Surface heat fluxes appear to be the environmental parameters that better discriminate between transitioning cyclones and the nontransitioning ones. They show a large positive correlation with the upper-tropospheric warm-core metric and the corresponding composites difference is consistently large and statistically significant. The importance of heat fluxes for the intensification of medicanes has been previously suggested by several authors (see Homar et al. 2003; Reed et al. 2001; Emanuel 2005; Tous et al. 2013). When medicanes are numerically simulated without surface heat fluxes, they are broader, less intense and with shallower warm cores. Emanuel (2005) and Fita et al. (2007) argued that a feedback mechanism comparable to the Wind Induced Surface Heat Exchange (WISHE; Emanuel 1986; Rotunno and Emanuel 1987) could sustain mature medicanes. Emanuel (2005) also argued that baroclinic processes might be required to produce a vortex capable of self-amplification through WISHE. Here we documented such a scenario. In fact, the observed positive anomaly in latent and sensible heat flux is associated with stronger low-level winds resulting from a more pronounced intensification in the baroclinic stages. WISHE, however, relies heavily on convection as a mean as a transfer of heat extracted from the ocean. The analysis of the simulations does not provide sufficient evidence to sustain that a cooperative process similar to WISHE is in place. On the one hand, because the numerical model employed in this study does not explicitly resolve convection. On the other hand, because the composites suggest that ascent is temporarily reduced after the seclusion has taken place.

The difficulty in proving the statistical significance of the observed composites differences might be attributable to the small sample sizes employed in this study. However, within the limitations posed by a single case study, these findings provide a more detailed characterization of processes leading to the tropical transition of a Mediterranean tropical-like cyclone. Further investigations are required in order to assess the causal relationships between the observed differences and the transition process as well as to abstract these findings to the general problem of medicane genesis. Moreover, it would be interesting to investigate further the role of moist processes involved in the seclusion dynamics by means of convection-resolving numerical simulations.

Acknowledgments

The computational resources were made available by the German Climate Computing Center (DKRZ). The authors would like to acknowledge the Helmholtz graduate research school GeoSim for funding this research. U.U.’s and R.K.’s research have been partially funded by Deutsche Forschungsgemeinschaft (DFG) through CRC 1114 “Scaling Cascades in Complex Systems,” projects A01 and C06. E.M. would like to thank Daniel Befort for his help in setting up the RCM; Bijan Fallah, Emmanuele Russo, Nico Becker, and Walter Acevedo for their fruitful comments; and Beatrice Magistro for her constant support. E.M. would also like to thank Prof. Gregory Hakim and the Dept. of Atmospheric Sciences at the University of Washington for providing invaluable scientific, technical, and personal guidance during the revision of this manuscript.

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