1. Introduction
The West Coast thermal trough (WCTT) plays a crucial role in western U.S. weather during the warm season (approximately May–October). The WCTT is an inverted pressure trough near the surface that extends northward out of an area of low pressure over the southwestern United States and into the Pacific Northwest (Fig. 1). Although this phenomenon has been referred to as a thermal trough in the literature for over a half century, its name is perhaps misleading since it is characterized by a thermal ridge of higher temperatures in the lower troposphere. Though the contributions of mesoscale adiabatic warming, surface-based diabatic heating, and horizontal advection in producing this localized warming are uncertain, the hydrostatic impact of mesoscale temperature variations, combined with the overall synoptic pressure field, results in the characteristic inverted pressure trough (Mass et al. 1986).
A WCTT at 1200 UTC 14 May 2007 showing 925-hPa geopotential height (m), temperature (°C, color shading), and wind (kt; 1 kt = 0.5144 m s−1) from the 36-km WRF model (12-h forecast).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
The WCTT has profound impacts on regional temperature, wind, and cloudiness, particularly during the warm season. Most WCTT events are associated with offshore winds and downslope flow that result in adiabatically warmed continental air to the west of the Cascade, Coastal, or Siskiyou/Klamath Mountains. This scenario, which also suppresses cloud development and precipitation on the western slopes, brings abnormally hot and dry conditions to the highly populated regions west of the Cascade crest. When this situation is coupled with strong east winds, wildland fire danger is greatly enhanced (Joy 1923). The development of WCTTs can result in poor air quality in the Pacific Northwest since the synoptic conditions associated with the WCTT result in thermally enhanced photochemical reactions and a weakening of regional sea-breeze circulations (McKendry 1994).
As the synoptic-scale flow evolves, warm-season WCTTs often move eastward across the Cascades. Their passage brings a shift to westerly winds, increased cloud cover and wind speed, and a drop in temperature, thus signaling the end of the warm period (Mass et al. 1986). A WCTT can have a large impact on wind energy generation since associated wind speed changes can result in sudden increases and subsequent declines in wind energy (known in the industry as the ramp up/ramp down problem). These rapid wind changes can also cause a substantial modification of wildfire behavior. As a WCTT moves eastward across the Cascades, the associated instability, strong surface convergence, rising motion, and variable low-level winds can enhance fire danger (Saltenberger and Barker 1993).
Although WCTTs are best defined during the warm season, similar structures are apparent throughout the year. Earlier studies (e.g., Ferber et al. 1993) have documented the development of elongated inverted coastal troughs as high pressure builds in the interior during the winter. Such wintertime troughs are closely associated with offshore flow and adiabatic warming along the lee sides of regional terrain barriers and can serve as the focal points for coastal cyclogenesis.
In summary, the WCTT is the key mesoscale/synoptic feature of the U.S. west coast during the warm season. Its development and movement have a large impact on a range of meteorological parameters that influence major weather-related issues from energy to wildfires. Although some papers have mentioned the WCTT, none have provided a comprehensive examination of this important phenomenon, a gap partially remedied by this work.
2. Background and prior research
Terrain and land surface variations interacting with the synoptic-scale flow dominate the development and evolution of WCTT events. Figure 2 presents the complex topography of the northwest United States. The Cascade Mountains, running north–south with a general crest level of approximately 1500–2000 m and several peaks exceeding 3000 m, divide Washington and Oregon into two climatic regions. The west side, including the Puget Sound, the Willamette Valley, and the Coast Range, is greatly influenced by the Pacific Ocean, making this region moderate in temperature, with higher humidity and greater precipitation than the rain-shadowed regions to the east of the Cascade crest. The eastern side of Oregon is characterized by a plateau with heights around 1000 m, while eastern Washington is dominated by the topographic bowl of the Columbia Basin and the Rocky Mountains to the east. A lower coastal range extends from the Olympic Mountains southward down the coast, while the Klamath/Siskiyou Mountains extend from the Cascades to the coast, representing the highest coastal terrain. Farther south, the Cascades transition to the Sierra Mountains of California, and to the north, the Coast Range of British Columbia.
Topography of the northwest United States.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
During the summer months, the dominant synoptic feature over the eastern Pacific Ocean is the east Pacific high, which often extends over the coastal zone of the northwest United States and southwestern Canada. As a result of the circulation around this high, there is typically northerly flow along the coastal zone from Washington southward to central California, producing upwelling and cool sea surface temperatures in the immediate offshore waters. Atmospheric subsidence associated with the east Pacific high contributes to the formation of a low-level inversion that traps a cool marine layer near the surface. The top of this inversion is typically below the crests of the coastal mountains (approximately 1000 m), which prevents the marine layer from spreading into the interior (Neiburger et al. 1961).
Another important feature during the warm season is an area of low pressure over the southwest United States, where intense solar heating over arid land results in high temperatures and lower pressure near the surface (Rowson and Colucci 1992). This thermal low is one of several around the world, including the Iberian low (Gaertner et al. 1993; Hoinka and Castro 2003), the Australian thermal low (Leslie 1980), the India–Pakistan low (Joshi and Desai 1985), and the Saudi Arabian low (Blake et al. 1983). Between the persistent southwest U.S. thermal low and the nearby Pacific high, there is often a large pressure gradient over the Pacific coast that is typically most intense over northern California and southern Oregon (Mass and Albright 1987). As a result, summertime northerly surface winds can be strong over the Pacific coastal waters near the California–Oregon border, often reaching 10–15 m s−1.
Although WCTTs are best known for their impacts during the warm season, they occur throughout the year. Seasonal composites, using the National Centers for Environmental Prediction (NCEP) North American Regional Analysis (NARR) of 925-hPa geopotential height and temperature, document the seasonally varying synoptic environments in which WCTTs form (Fig. 3). The summer [June–August (JJA)] composite reveals a high over the eastern Pacific and a strong thermal low over the southwest U.S. interior. The inland low greatly weakens during the fall [September–November (SON)] as the interior cools and the Pacific high recedes equatorward and weakens. During the winter [December–February (DJF)], the inland thermal low gives way to high pressure and cool temperatures over the continental interior, and the Pacific high recedes and weakens further. In the spring [March–May (MAM)], the Pacific high begins to build northward, and the inland high is replaced by weak troughing as temperatures warm over the southwest United States. Importantly, the WCTT is not apparent in the seasonal climatological composites at 925 hPa for any season, probably because of its transient nature.
Seasonal composites of 925-hPa geopotential height (m) and temperature (°C) for 1979–2009 from the NARR analyses.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
Persistent northerly flow along the West Coast during the warm season is sometimes interrupted by northward-propagating coastally trapped wind reversals (CTWR) that often follow behind a WCTT (Bond et al. 1996). Behind the reversals, there are mesoscale coastal pressure ridges. These wind reversals bring a change from clear skies to low clouds and a decrease in temperature. A tongue of coastal stratus follows the extension of the thermal trough northward (Jackson 1983). Mass and Albright (1987) suggested that synoptically forced offshore and downslope flow drives the northward extension of the WCTT, and results in a reversal of the climatological alongshore gradient south of the trough, thus causing the southerly flow.
Chien et al. (1997) demonstrated that a CTWR/WCTT can be simulated realistically using a high-resolution mesoscale model, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Based on sensitivity studies in which the mountains and surface diabatic heating were removed, they proposed that surface diabatic heating is the primary thermal driver of the hydrostatic reduction in pressure associated with the WCTT, and that adiabatic warming from downslope flow is of secondary importance. They suggested that horizontal advection is of minimal influence. However, since this study was mainly concerned with an onshore push, which occurs at the end of a WCTT event, only the terminating portion the WCTT evolution was simulated.
The evolution of WCTTs is intimately connected with the transition from warm, dry continental air in offshore flow to cool, moist onshore flow, this change being known as an onshore or marine push. Onshore pushes are usually manifest as a drop in temperature, a rise in dewpoint, a change in wind direction to onshore, increased wind speed, and enhanced cloud cover (Mass et al. 1986). Mass et al. (1986) noted that as an upper-level ridge builds over the Pacific Northwest and surface high pressure extends inland, offshore flow develops over the Cascade Mountains, producing warming and low-level pressure falls. As a result, the California thermal trough moves northward, jumping the Klamath/Siskiyou Mountains near the Oregon–California border, and extending northward along the Oregon and Washington coasts. As the synoptic flow evolves and high pressure moves inland, downslope flow weakens over the western slopes of the Cascade Mountains, and marine air spreads over the coastal regions. At the same time, downslope flow develops on the western slopes of the Rocky Mountains and the eastern slopes of the Cascades, and a thermal low over the Columbia Basin becomes more prominent. Movement of the trough axis across the Cascades can manifest as abrupt changes in wind direction and speed, as well as surface air temperature and humidity.
Several studies have found that the WCTT influences air pollution in the Pacific Northwest (McKendry 1994; Barna et al. 2000; Ainslie and Steyn 2007). McKendry (1994) found that days exceeding the Canadian 1-h ambient O3 air quality objective of 82 ppb at Vancouver, British Columbia, coincide with an upper-level ridge overhead and an associated low-level thermal trough to the west of Vancouver. McKendry suggested that the WCTT inhibits the sea breeze, creating stagnation and reducing ventilation, and thus hindering the dispersal of photochemical pollutants such as ozone. Furthermore, the warm temperatures and sunny conditions associated with WCTT events enhance the photochemical production of ozone.
Research by the wildland fire community has shown that WCTTs have a significant influence on wildfires. Joy (1923) noted that forest fires along the western slopes of the Cascades become particularly dangerous and unpredictable when hot, dry air floods in from the east, as is often the case during thermal trough development west of the crest. Under these conditions, winds can fan fires out of control, as they did during the Great Tillamook Fire of August 1933 (Dague 1934) and the Bandon Fire of 1936 (Cramer 1957). Many studies describe the WCTT as an important meteorological factor favoring wildfire growth in the Pacific Northwest (Gisborne 1927; Cramer 1954; Colson 1956; Cramer 1957; Werth and Ochoa 1993; Rorig and Ferguson 1999). Of particular interest is when a WCTT moves eastward across the Cascade Mountains. Jannuzzi (1993) argued that the higher wind speeds following a WCTT initially enhance fire danger before lowering the fire threat as temperature declines and relative humidity rises.
The California heat trough was analyzed by Gilliland (1980), who defined this feature as a region of low sea level pressure that extends across the northern Mexico deserts into the Central Valley of California during the summer. He found that surface heating plays a dominant role in the formation and maintenance of the heat trough, with temperature advection, vertical motion, and vorticity advection being of secondary importance.
Closely related to the WCTT are two warm-season thermal troughs in Australia (Fig. 4). The west side feature is known as the Australian west coast thermal trough and is the stronger of the two (Fandry and Leslie 1984). The location of the trough has a significant regional impact, particularly around the city of Perth, Australia (Gaffney 1955). If the trough moves to the east of Perth before the afternoon hours, the city can expect an onshore influence and temperatures in the 20s (°C). However, if the trough stays west of the city, Perth experiences offshore flow and temperatures in the 30s (°C). Watson (1980) suggests that both divergence aloft and warm air advection make important contributions to the west-side trough.
Composite of 10-yr-average mean sea level pressure at 0900 UTC in January (from Fandry and Leslie 1984).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
The thermal trough on the eastern side of Australia extends southward from a thermal low over the northeastern part of the country. Adams (1986) states that a marked deepening of the trough takes place in the afternoon, suggesting that surface heating is important. The trough separates dry continental air to the west from moist marine air to the east. Such typical summer conditions on the eastern side of Australia are sometimes interrupted by cold fronts pushing equatorward from the southwest. These cold fronts interact with the Great Dividing Range of southeastern Australia to produce a coastally trapped disturbance known as a Southerly Buster. This is very similar to the CTWR, with a shift in wind direction from offshore to onshore, a drop in temperature, an increase in relative humidity, and enhanced cloud cover (Baines 1980; Colquhoun et al. 1985).
Coastally trapped disturbances and coastal troughing similar to the Southerly Buster, the CTWR, and the WCTT also occur in Chile and South Africa. Garreaud et al. (2002) described the South American coastal low of Chile; its passage is associated with a transition from sunny, clear weather and offshore flow to overcast, cool, and moist conditions. Another related phenomenon is the South African coastal low, which propagates eastward around the coastal mountains of South Africa and is associated with offshore, downslope flow (Bannon 1981).
Although the WCTT is the most important mesoscale and forecasting feature of the West Coast during the summer, there are still many outstanding questions regarding its structure, evolution, and development. What is its diurnal and seasonal variability? What is the typical synoptic and mesoscale evolution of the WCTT from its initiation to dissipation, and how do these conditions change throughout the year? What are the synoptic conditions that cause the WCTT to move across the Cascades? How does the nature of WCTTs vary between cool and warm seasons? This paper will attempt to address these and other questions through synoptic compositing and analysis, as well as a climatological study of WCTT frequency and development.
3. Pressure reduction issues
Since low-level pressure or geopotential height variations are used to describe the evolution of thermal troughs, the validity of pressure reduction methodologies is important. Examining the composite summer (JJA) sea level pressure distribution at 0000 UTC using the NARR (Fig. 5, left side) reveals a number of mesoscale pressure features, such as troughing in the interior valley of California and a closed low within the basin of eastern Washington. The idea that a thermal trough exists in the Central Valley of California throughout the duration of the warm season is often discussed in the literature (e.g., Mass et al. 1986; Mass and Albright 1987; Mass and Steenburgh 2000) and is prevalent in the operational community. However, considering that many of these features closely resemble the terrain distribution, one might ask whether they are real or the result of the pressure reduction approach.
Composites of sea level pressure (solid lines, hPa) and 1000-hPa temperature (color shading, °C) using the (left) Shuell and (right) Mesinger methods for JJA at 0000 UTC.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
NARR analyses use the Shuell pressure reduction method, an approach commonly applied to models because of its similarity to the way in which individual stations calculate sea level pressure. The Shuell method assumes a constant lapse rate (6.5°C km−1) to vertically extrapolate “surface” temperature (the temperature of a layer immediately above the surface) in order to calculate an average column temperature below ground level. The hypsometric equation is then used to calculate sea level pressure or subsurface geopotential heights using this assumed column temperature (Mohr 2004; Mesinger and Treadon 1995). In the case of the elongated area of low pressure over the Central Valley California in Fig. 5 (left side), the observed summer lapse rate over this area during the day is usually close to dry adiabatic (9.8°C km−1). However, below the surface of the nearby Sierra Nevada Mountains, the Shuell algorithm assumes a lapse rate of 6.5°C km−1, thus producing cooler temperatures within the terrain compared to similar elevations above the Central Valley. As a result, the Shuell algorithm results in higher sea level pressure for the high terrain than over the Central Valley, irrespective of the actual meteorological situation.
Another pressure reduction method, described by Mesinger and Treadon (1995), horizontally extrapolates free air temperatures into the volume of terrain. This method (Fig. 5, right side) produces a large thermal low over the Great Basin region and much smoother isobars that less closely follow the terrain. The Mesinger reduction does not indicate a mesoscale pressure trough in the interior valley of California, a closed low in eastern Washington, or pressure ridging over high terrain. Studies suggest that this method is superior to the Shuell method in mountainous regions (Mesinger and Treadon 1995; Pauley 1998).
These pressure reduction issues imply that mesoscale structures, such as the narrow thermal trough of the Central Valley of California and the Columbia Basin low, may not reflect real physical features but rather the effects of unphysical pressure reduction to sea level. Therefore, as an initial step in this study, several cases were examined to understand the vertical structure of the WCTT, to evaluate the impacts of various pressure reduction algorithms, and to determine a level that most clearly identifies the WCTT.
Analyzing a number of events reveals that the signature of a WCTT becomes evident between 700 and 800 hPa when descending from above. WCTTs intensify further at lower elevations, indicated by a better defined trough and larger geopotential height or pressure gradients. For example, at 1500 UTC 13 May 2007 (first column, Fig. 6), the WCTT extends northward into southern Oregon at sea level and 925 hPa. At 850 hPa, the trough is weaker and remains in California, while at 725 hPa, the signature of the WCTT is not evident. Fifteen hours later, at 0600 UTC 14 May (middle column, Fig. 6), the WCTT extends farther north at sea level, as well as at 925 and 850 hPa, with little evidence at 725 hPa. Finally, at 1200 UTC 14 May (third column, Fig. 6), the WCTT extends all the way into British Columbia at the three lower levels, is weakly evident at 725 hPa, and is not apparent at 700 hPa (not shown).
The vertical structure of a WCTT on 13–14 May 2007 showing sea level pressure and geopotential heights at various levels. The sea level pressure analyses use the Shuell reduction method.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
The sea level pressure analysis (Shuell method) at 1200 UTC 14 May indicates a lobe of low pressure over the Oregon Cascades. Is this real or a reflection of pressure reduction problems? During the morning of 14 May, the observed lapse rate of the nearby Salem, Oregon, sounding (not shown) was nearly isothermal in the lower troposphere. Therefore, the low-level temperatures outside the terrain were much colder than the values produced by the assumed 6.5°C km−1 lapse rate in the Shuell approach, thus explaining lower sea level pressure below the terrain surface compared to the nearby lowlands.
To deal with the pressure reduction issue, a constant pressure level in the lower troposphere, but above a significant amount of low-level terrain, was selected. An examination of the May 2007 case and others reveal that the 925-hPa surface (roughly 770 m) is less affected by the pressure reduction problem than sea level pressure in this region, but still shows a strong signature of the WCTT. Thus, the 925-hPa level was used for the climatological studies described below. Though 925 hPa was used for this analysis, both sea level and 850-hPa fields are valuable surfaces for analyzing WCTT cases. For example, sea level pressure is optimal for analyzing the CTWR and its interaction with the thermal trough, and 850 hPa is useful east of the Cascades and Sierra Mountains where the elevation is higher.
4. WCTT definition and climatology
A WCTT climatology was prepared using NCEP’s NARR dataset, which uses the Eta Model (32-km grid spacing) coupled with a frozen assimilation system (Mesinger et al. 2006). The NARR grids span from 1979 to 2009 in 3-h increments. As a first step, a program was written to inspect NARR grids over the northwest United States from 129° to 116°W and from 41° to 53°N at 925 hPa every 3 h over 1979–2009. The following criteria were used to identify inverted troughs in the NARR data:
An inverted trough must extend northward beyond 42°N, starting from the southern end of the domain (41°N).
Grid points within the axis of the inverted trough must be within five grid points to the east or west of the axis grid points to the north and south of it. This ensures trough continuity in the meridional direction.
The south–north line of grid points that makes up the axis of the WCTT must all have a lower geopotential height than the adjacent grid points to the east and west.
Geopotential heights increase northward along the trough axis, thus confirming an inverted trough.
Figure 7 presents the seasonal frequency of WCTT occurrence by location, based on 3-hourly NARR data at 925 hPa from 1979 to 2009. For the 31 years considered, there are roughly 22 500 three-hour increments for each season. For all seasons, the highest frequency of WCTTs is along the southern coast of Oregon–northern coast of California, where over 2000 of the increments for each season (a frequency of approximately 10%) have a WCTT. This area is west of the highest coastal terrain, the Siskiyou/Klamath Mountains, and is a region known for strong downslope warming (Mass 1987). Since the highest WCTT occurrence for any given region is only ~10%, WCTTs do not show up in the seasonal West Coast climatology shown in Fig. 3.
Seasonal variation in WCTT frequency based on 3-hourly NARR grids for the period 1979–2009.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
High frequencies are also found along the western slopes of the Cascades, adjacent to the Willamette Valley, and along the remaining coast. These maxima to the west of major mountain ranges or along their western slopes are consistent with past studies (Mass et al. 1986; Mass and Bond 1996; Chien et al. 1997), suggesting the importance of downslope flow in forming the WCTT. WCTT frequency decreases east of the Cascades, where most occur over southeastern Oregon, northeastern California, and northwestern Nevada during the warm season. Eastern Washington experiences few cases any time of the year.
These seasonal variations in WCTT occurrence are consistent with the seasonal synoptic pressure variations (see Fig. 3). High pressure dominates inland during the winter, reflecting the cooling of the continental interior, and works against troughing. Also, warm-season conditions, with inland warming and pressure declines, are conducive to WCTT development on the east side of the Cascade and Sierra Mountains. Coastal troughing appears much more likely during fall and winter as high pressure builds in the interior, fostering offshore flow that allows WCTTs to extend northward.
Figure 8 shows the seasonal frequency of WCTT occurrence over the northwest United States. The region was divided east–west along 46°N (the Oregon–Washington border) and north–south along the Cascade crest to create four quadrants (eastern Oregon, western Oregon, eastern Washington, and western Washington). Since WCTTs must extend through Oregon to reach Washington, the Oregon quadrants have larger frequencies than those in Washington. Only a small number of WCTTs extend into eastern Oregon and Washington during the winter months, with the summer months experiencing the highest frequency in those locations. West of the Cascade crest, the autumn months have the highest WCTT frequency, with winter being second.
Number of WCTTs that extend into each quadrant by season during 1979–2009.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
The diurnal frequency of WCTT events possesses significant seasonal and spatial variations (Fig. 9). The winter months (DJF) have relatively little diurnal variability for all four quadrants. However, during the summer (JJA) and east of the Cascades, WCTTs are more likely to occur during the warmest portion of the day, particularly from 1800 to 0300 UTC [1100 to 2000 Pacific daylight time (PDT)]. West of the Cascade Mountains, WCTTs are much more common during the morning hours (1200 and 1500 UTC, 0500 and 0800 PDT). The spring (MAM) and fall (SON) months have a similar modulation to the summer, but with less diurnal variability.
Number of WCTTs that extend into each quadrant by season and hour from 1979–2009.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
To explain the morning maximum in WCTT frequency west of the Cascades, the summer 850-hPa geopotential heights for 1200 and 0000 UTC, as well as their difference, are shown in Fig. 10. These hours are close to the times of minimum and maximum temperatures, respectively. Some locations in the Great Basin have diurnal swings of over 20 m, with lower heights during the afternoon near the time of maximum temperature. In the afternoon, the large inland pressure/height declines can overwhelm the WCTT signature west of the Cascade crest. As the interior low weakens and recedes during the night, the WCTT signature again becomes apparent if synoptic conditions support the WCTT thermal and pressure structures.
Composites of 850-hPa geopotential height for JJA at (left) 1200, (middle) 0000, and (right) 0000 UTC minus 1200 UTC composites. Dashed lines indicate negative differences.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
An example of the diurnal variations in the Great Basin low and its impact on a thermal trough is provided in Fig. 11, which shows 925-hPa geopotential heights and temperatures for one case using NARR data. A WCTT was evident during the morning [1200 UTC (0500 LST)] along the West Coast on 11 July 2008. However, daytime heating resulted in falling heights and a strengthening of the thermal low over the Great Basin. By 0000 UTC, the WCTT definition as outlined above was not met since the lowest pressure was then east of the Cascades, even though offshore flow and above-normal temperatures were still in place west of the Cascades. By 1200 UTC the next morning, the thermal trough was again apparent in the 925-hPa heights.
925-hPa geopotential heights (m) and temperatures (K) using NARR data for (left) 1200 UTC 11 Jul 2008, (middle) 0000 UTC 12 Jul 2008, and (right) 1200 UTC 12 Jul 2008.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
5. Composite WCTT evolution
Creating synoptic composites of WCTT evolution requires an event definition. For that purpose, a WCTT event day was defined as when at least one of the eight 3-h periods in a day meets the following criteria:
The WCTT axis intersects both 42° and 46°N on the west side of the Cascades.
Along 42° and 46°N and between 125° and 117°W, the lowest geopotential height at 925 hPa must be found west of the Cascade crest.
The start (end) of the event was defined as the first (last) time on the first (last) event day when the aforementioned criteria were met. Synoptic composites using NARR data were completed for 925, 850, 775, and 500 hPa for several meteorological fields at the start and end of the event as well as periods before and after.
Using the above definition for a WCTT event, a climatology of the frequency of such WCTT events by month and duration is shown in Fig. 12. The number of WCTT events is generally similar (50–70) for most months, except for September and October when frequency is considerably higher (100–120). One-day events, followed by two-day events, are the most common for all seasons, with a tendency for longer events during the fall. Interestingly, the summer months of June and July have the lowest occurrence of WCTT events. This period is characterized by strong onshore flow and weak synoptic forcing. During the autumn, higher pressure builds inland and synoptic variability strengthens and shifts southward, resulting in periods of enhanced offshore flow.
Number of WCTT events by month based on the NARR data from 1979–2009. Colors indicate the duration of events.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
Figure 13 shows the histograms of frequency versus hour of the day for the beginning and ending of WCTT events using the definitions noted above for both the winter and summer seasons. During the summer, the WCTT events commonly begin and end during the morning hours, usually 1200 to 1500 UTC for the beginning, and 1500 to 1800 UTC for the end. In contrast, during the winter season there is little diurnal variation in the start and end times, suggesting the dominance of synoptic controls. The implication of these frequencies is that, when viewing the composites for the summer, one must also take into account their diurnal modulation.
Diurnal variations of the start and end times of WCTT events.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
Figures 14 and 15 present the composite and anomaly evolutions of WCTT events at 500 hPa from 36 h before event initiation to 36 h after the event ends. Figures 16 and 17 present composites and anomalies for the same period at 925 hPa. Student’s t test was used to determine which areas in the composite anomalies from climatology were statistically significant at the 95% level. It was found that all major anomalies in Figs. 15 and 17 are statistically significant at or above this level.
WCTT event composites of 500-hPa geopotential height (m).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
WCTT event anomalies of 500-hPa geopotential height (m).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
WCTT event composites of 925-hPa geopotential height (m).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
WCTT event anomalies of 925-hPa geopotential height (m).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
Although there are some seasonal variations, the composites reveal common aspects of WCTT evolution for all seasons. A 500-hPa ridge is evident off the West Coast 36 h before the beginning of the composite WCTT event, and this ridge amplifies and moves eastward during the subsequent 36 h. By the beginning of a WCTT event, the 500-hPa ridge reaches its maximum amplitude while positioned near the coast. In the following days, the ridge moves eastward and weakens, except during the winter.
The composites and anomalies at 500 hPa indicate that the winter 500-hPa synoptic evolution is different from other seasons, starting with the development of anomalous troughing over the southwest United States at the beginning of the event. The winter-event ridge develops farther offshore than during other seasons, and at the end of the event, it weakens. The anomalous high pressure retrogresses westward, in contrast to other seasons in which the high pressure anomaly moves eastward.
The 925-hPa composites (Fig. 16) and associated anomalies from climatology (Fig. 17) describe the lower-tropospheric evolution of WCTT events. The summer composite, and to a great degree the transition season composites as well, begins (−36 h) with high pressure over the eastern Pacific that extends into the Pacific Northwest, and lower pressure over the southwest interior. During the next 36 h, the high pressure spreads eastward into Washington and British Columbia, then into Idaho. At the same time, the WCTT develops and extends northward up the West Coast. The anomalies at 925 hPa reveal that most of the western United States experiences above-average geopotential heights during WCTT events, with the exception of the trough where heights remain near average. After the event, the trough moves east over the Cascades and into eastern Oregon and Washington, as the synoptic high pressure anomaly moves into the central United States.
The winter evolution of the WCTT at 925 hPa is considerably different than that of summer. Between −36 h and the start of the event, high pressure builds over southwestern Canada and merges with preexisting high pressure over the Intermountain West, while troughing over the southwest United States builds northward up the coast. Unlike other seasons, a closed low appears off the coast of northern California during the composite WCTT event. This feature is associated with the trough aloft over the southwest United States. Rather than moving eastward over the Cascades as during JJA, the wintertime WCTT recedes back into California and eventually dissipates. The 925-hPa anomalies highlight the progressive nature of the high pressure during the summer and transitional months, and the tendency for the high pressure anomalies to retrogress after the event during the winter.
Figure 18 presents a more detailed analysis of the DJF composite event. Twenty-four hours before the start of the event, there is little evidence of coastal troughing at 925 and 850 hPa, with higher heights over the Intermountain West. At −12 h, an inverted pressure trough is apparent at 925 hPa, while at 850 and 775 hPa, troughing intensifies off the coast of northern California. Approaching the start time, offshore flow increases, thermal ridging becomes more apparent, and the coastal trough strengthens and extends northward into Washington. Since easterly flow is generally associated with cold air advection during the winter in this region, and diabatic heating is at a minimum during this season, it is likely that easterly downslope flow causes mesoscale warming and the associated narrow inverted trough. From the start to the end of the event, temperatures warm by a few degrees everywhere in the domain, with a tongue of warmer temperatures associated with the trough. Subsequently, high pressure over the upper-right section of the domain moves southeastward, and winds near the Cascades turn from easterly to southerly. This results in a weakening of the coastal low pressure at all levels. Though WCTTs along the West Coast are associated with summer heat waves, comparing the DJF composites to climatology (Fig. 3) reveals that temperatures are very near climatology during winter WCTT events.
WCTT winter (DJF) composites of heights (m), temperature (°C), and wind (full barb = 5 m s−1).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
Because the largest impacts of WCTTs occur during the summer months, the remainder of this paper will focus on the WCTTs during June–August. Twelve hours before the beginning of a summer WCTT event (Fig. 19) there is northerly flow over the Pacific Ocean, weak offshore flow in the lower troposphere near the coast, and a lower-tropospheric thermal low centered over southern Nevada. By the start of the event, geopotential heights increase over British Columbia, eastern Oregon, eastern Washington, and Idaho at 925, 850, and 775 hPa. At this time, a WCTT extends all the way up to British Columbia near the surface and weakens aloft. There is moderate easterly flow crossing the Cascades at the lower two levels, while at 775 hPa, there is weak southerly flow over Oregon, Washington, and northern California. A tongue of warm air at 850 and 775 hPa west of the Cascades corresponds with the WCTT axis. Warm air advection occurs over Oregon and Washington at 775 hPa and, with adiabatic warming from downslope easterly flow at the lower levels, contributes to pressure falls associated with the WCTT.
WCTT summer (JJA) composites of heights (m), temperature (°C), and wind (full barb = 5 m s−1).
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
At the end of the event (typically at 1500–1800 UTC), heights fall over British Columbia as the high pressure area moves southeast. Downslope flow is weaker as winds become more southerly at all three levels. By 6 h after the event, downslope continues to weaken over the mountains, and winds along the coast are northerly. The interior experiences strong surface heating and weak warm air advection aloft, particularly over central and eastern Oregon and Washington, contributing to pressure falls. In response to these changes, the WCTT broadens and moves over the Cascades, where it combines with the thermal low at the lowest two levels.
Finally, 12 h after the end of the event, there is a strong thermal low over the Great Basin with an inverted trough extending northward over eastern Oregon, Washington, and Idaho at low levels, while at 775 hPa, a deepening trough is found off the coast. Winds over Oregon and Washington appear to be very weak at this time, though this may be deceptive due to the smoothing effects of compositing on mobile features.
Figure 20 shows the composites and anomalies at 925 hPa from 36 h before to 36 h after a summer WCTT event. Since diurnal variations are important, and to reduce the smearing effects of averaging different times, compositing for the beginning (end) of the event, as well as times before (after) it, were done only if the event start (end) time was 1500 UTC.
Event composites and anomalies for JJA of heights (m), temperature (°C), and wind at 925 hPa.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
Thirty-six hours before the beginning of the event, the anomalies indicate above-normal geopotential heights and stronger-than-normal northerly flow, reflecting the effects of the building ridge. Twelve hours before the event, a tongue of above-average temperatures extends northward along the California and Oregon coasts, an easterly (offshore) wind anomaly has developed, and enhanced heights built over most of the domain with largest positive anomalies inland. Only along the California coast are height anomalies negative. In the compositing, a WCTT is not evident in the height composites at −12 h.
From −12 h to the end of the event, there is dramatic warming along the West Coast, where positive temperature anomalies increase by over 4°C. Easterly flow anomalies extend from the Idaho border to offshore, and extend northward up the coast as the event develops, as do the negative height anomalies. During the 36 h after the end of the event, negative geopotential height anomalies spread inland, with a similar shift in the WCTT. As the synoptic high retreats eastward, the easterly wind anomalies move eastward to the Rockies, resulting in a weakening of the downslope flow over the Cascades. The temperature anomalies also shift eastward so that by +36 h, the warmest anomalies are over eastern Washington and Oregon.
The evolution of the thermal trough has a large influence on the moisture distributions and lapse rates over the region, which is of substantial interest to those forecasting wildfires. Anomalies of dewpoint depression at 850 and 700 hPa, as well temperature differences between 850–700 hPa and 700–500 hPa are shown in Fig. 21.1 These specific levels were chosen because they are components of the Haines Index (Haines 1988), which is used to evaluate the potential for large or erratic fires. Twelve hours before the beginning of the event, during the prior evening, the atmosphere is much drier than normal (positive dewpoint depression anomalies) off the southwest Oregon and northern California coasts, with enhanced lapse rates over the coast and offshore. At the start of the event (1500 UTC), both the dewpoint depression/lapse rate anomalies increase northward, west of the Cascade Mountains, both effects conducive to fire initiation and spread. The coastal drying and large lapse rates are most intense at low levels during the end of the event. During the ensuing 12 h, positive dewpoint depression anomalies and large lapse rates spread eastward to the Cascade crest. By +36 h, the positive lapse rate anomalies and strongest drying are east of the Cascades. In summary, during the course of the event, drier air and higher lapse rates first develop over the western side of the region, and then propagate over the Cascades into eastern Washington–Oregon as the event terminates.
Anomalies of WCTT events showing dewpoint depression (black contours) at (left) 850 and (right) 700 hPa and vertical temperature difference (colors) between (left) 850–700 and (right) 700–500 hPa.
Citation: Monthly Weather Review 140, 12; 10.1175/MWR-D-12-00078.1
6. Discussion and conclusions
The West Coast thermal trough (WCTT) is the most important mesoscale weather feature of the coastal western United States, with large impacts on temperature, wind, and humidity. WCTTs greatly influence a wide range of phenomena of societal importance, including air quality, wildland fire behavior, and wind energy generation. A WCTT is defined as an elongated inverted pressure trough extending out of the southwest United States and into the Pacific Northwest. This study describes the WCTT climatology using data from NCEP’s NARR gridded dataset. By applying compositing, the synoptic evolution of typical events are examined and analyzed.
There are significant problems with using sea level pressure to analyze WCTTs. The Shuell method for sea level pressure reduction is commonly used in numerical models. However, this approach can create unrealistic sea level pressure features if the actual lapse rate differs significantly from the assumed value inside terrain, as is often the case. To deal with this issue, this analysis uses the 925-hPa level for most applications since this level is high enough to eliminate much of the reduction problem in the northwest United States, but low enough to effectively capture the signature of the WCTT.
For all seasons, the highest frequency of WCTTs is along the southern coast of Oregon and the northern coast of California, a region where high terrain extends to the coast. High frequencies are also found on the western slopes of the Cascades, as well as the remaining coastline. WCTTs west of the Cascades are most common during the fall months. East of the Cascades, WCTTs are more frequent during summer and relatively rare during winter.
During the cold season, there is little diurnal modulation of WCTT events on either side of the Cascades. In contrast, during the warm season, WCTTs are more likely in the morning hours on the west side of the Cascade Mountains, while on the east side they are more common in the afternoon. WCTT occurrence is intimately connected to the diurnal variation of the heat low over the Great Basin. This interior thermal low strengthens during the day, often merging with west-side thermal troughs. As the inland thermal low weakens at night, the WCTT again becomes evident on the west side of the Cascades.
Compositing reveals that before a WCTT extends up the West Coast during the warm season, a ridge builds aloft over the eastern Pacific. As the amplifying ridge moves toward the coast, low-level high pressure strengthens inland over British Columbia and Washington. The low-level wind turns from northerly to easterly over Oregon and Washington as the WCTT extends northward along the West Coast. As the upper-level ridge continues eastward, low-level high pressure moves east and south, weakening the downslope flow off the Cascades. Because of the diminishing downslope flow, and the subsequent rush of marine air inland, the WCTT shifts eastward, jumping the Cascades, and merging with the thermal low residing over the Intermountain West. Summer WCTT events are associated with drier and warmer conditions than normal, with lowered stabilities, especially near the WCTT axis.
Before a cold-season WCTT event, an upper-level ridge strengthens over the eastern Pacific. In contrast to summer events, as the ridge approaches the coast a trough aloft deepens over the southwest United States. Surface high pressure associated with the synoptic ridge aloft builds over the northwest United States and southwestern Canada, enhancing high pressure over the Great Basin. At the same time, a low-level closed low associated with the upper-level trough develops off the coast of northern California. As easterly downslope flow develops, an inverted trough extends northward out of this closed low, forming the WCTT. As the ridge aloft and the downstream trough weaken and progress eastward, downslope flow lessens over the western slopes of the Cascades. Rather than jump the Cascades as in the summer, the WCTT recedes back into California and eventually dissipates. During winter, the timing of the formation of the WCTT has no diurnal preference due to the dominance of synoptic control and the weak diabatic forcing of the season.
Previous literature has suggested that diabatic heating, adiabatic warming, and advection all play a role in the initiation and evolution of the WCTT. This paper does not quantitatively evaluate the magnitude of these physical processes, but does provide insight into WCTT development. Since WCTTs are most frequent west of the major regional mountain ranges and are observed during winter when diabatic heating is minimal, it appears that adiabatic warming through downslope flow is the most important mechanism for WCTT development. While diabatic heating does not seem to drive the development of the WCTT, it clearly has a role in modulating WCTT diurnal variability. Since downslope flow often develops simultaneously with the northward extension of the WCTT, there is a question whether downslope flow is caused by synoptic forcing, the cross-mountain pressure gradient, or if both are factors.
Horizontal temperature advection may have some impact on WCTT development, though it may not be essential. During winter, cold air advection occurs through the full duration of the WCTT event and appears to work against trough development. During the summer, modest warm air advection occurs in the lower troposphere during the latter half of most events. A quantitative diagnosis of realistic WCTT simulations is necessary for a clear determination of the relative role of various forcing mechanisms during a range of events.
Though this study has examined the synoptic aspects of WCTT development, case studies are needed to better define the associated mesoscale structures and evolution. While composite analysis is an important first step in understanding WCTTs, compositing smoothes the details of individual events. High-resolution models such as the Weather Research and Forecasting (WRF) model can aid in the analysis of case studies. Comprehensive sensitivity studies are necessary to understand the roles of diabatic heating, adiabatic warming, and advection in the formation and dissipation of the WCTT. In addition, comparisons of WCTT simulations to observations are required to evaluate the realism of model fields. Such work is under way and will be reported on in subsequent publications.
Acknowledgments
This research was supported by the USDA Forest Service through Agreement PNW-10-JV-11261987-033. We wish to thank Mr. John Saltenberger and Ms. Julia Ruthford for numerous helpful conversations regarding the fire impacts of thermal troughs, and Jeff Baars and Rick Steed for technical support in downloading, analyzing, and displaying the NARR data.
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