1. Introduction
Knowledge of the spatial variability of convective boundary layer (CBL) height over mountainous terrain is limited. Observational studies show a variety of behaviors, from CBLs that follow the underlying terrain to CBLs that seem unaffected by terrain irregularities (De Wekker et al. 2004, 1997; De Wekker 1995; Kalthoff et al. 1998; Kossmann et al. 1998). The processes affecting these behaviors and their relative importance are still poorly understood. Knowledge of the variability of the CBL height over mountainous terrain, however, is required in many boundary layer and air quality studies and should be considered, for example, in the meteorology of ozone episodes in regions of complex terrain (Steyn et al. 1997). Therefore, an enhancement of this understanding is desired and will occur gradually as the analysis of existing datasets progresses and new datasets become available. In comparison to the CBL structure in a valley, CBL morphology near a mountain base (or similarly, near the base of extensive mountain ridges) has received relatively little research attention. Some similarities may be expected since thermally driven slope flows, for example, occur on both the slopes of individual mountains and on valley sidewalls. While it is generally accepted that processes occurring on the slopes have a profound influence on atmospheric processes in a valley (Whiteman 1990), the effects of slope processes near a mountain base are relatively unknown and poorly documented.
In this paper, CBL height variability near a mountain base is investigated by focusing on a specific phenomenon, the so-called CBL height depression, using observations from Pacific’93 and a mesoscale numerical model. Observational evidence for the phenomenon is presented, and it is shown that this phenomenon also occurs in numerical simulations over idealized topography. The existence of a CBL height depression has been noted in previous studies (Cai et al. 2000; Hayden et al. 1997; De Wekker et al. 1998) but there has been no attempt to explain the phenomenon. In this paper, the observations from Pacific’93 and idealized simulations are combined to investigate the mechanisms underlying the phenomenon. This is done by analyzing heating rates near the mountain base and away from it. The correspondence between the observed and modeled heating rates is encouraging, and allows an in-depth numerical investigation of the processes causing the phenomenon. A better insight into atmospheric processes near a mountain base is thus obtained.
2. Observations of a “CBL height depression”
a. Observational evidence from Pacific’93
In this section, characteristics of the CBL height depression will be presented using data from the Pacific’93 field campaign that was carried out in July and August 1993 in the Lower Fraser Valley of British Columbia, Canada. The general objective of Pacific’93 was to develop a better understanding of the mechanisms leading to high ground-level ozone concentrations in the Lower Fraser Valley (Steyn et al. 1997). Figure 1a shows a map of the Lower Fraser Valley, which is bounded by the Coast Mountains to the north and the Cascade Mountains to the southeast. The maximum height of the Coast Mountains is 1000 to 1400 m in this region and the terrain is characterized by steep slopes (>15°). A complex coastline is present to the west and south. The densely populated valley floor extends more than 60 km inland, having a wide range of land use: urban areas including the city of Vancouver, agricultural and horticultural fields, parks and forests, bogs and swamps, rivers and lakes. The region of interest in this study is indicated by the rectangle in Fig. 1a, which is magnified in Fig. 1b. Surface-based and airborne measurements were conducted under convective weather conditions. A longwave ridge became established over western British Columbia on 1 August 1993 and produced a weak ozone episode during the following five days. Synoptic winds were light to moderate and generally from the west. Afternoon CBL heights were 500 to 1000 m, which is typical in this region under these conditions (Steyn and Oke 1982; Hayden et al. 1997). A detailed description of the Pacific’93 field study, the synoptic conditions, and the various measurement systems was presented by Steyn et al. (1997) and Pottier et al. (1997).
Airborne lidar data have proven very useful in field campaigns to examine the spatial variability of the CBL height (e.g., Hoff et al. 1997; Nyeki et al. 2000). As part of Pacific’93, a 1.064-μm downward-looking Nd-Yag lidar was flown on an aircraft (Convair 580) at approximately 4200 m MSL. The horizontal and vertical resolutions of the lidar measurements were approximately 200 and 10 m, respectively. Lidar measurements are described in detail by Hoff et al. (1997). Figures 2a and 2b show airborne lidar data in the flight tracks T1 and L7, respectively, on 4 August 1993. The locations of these tracks are depicted in Fig. 1. Flight track T1 was heading west from 1351 to 1406 Pacific standard time (PST) while L7 was heading north from 1445 to 1458 PST. The shading on the cross sections represents the intensity of backscattered radiation. Aerosol backscatter is proportional to aerosol concentration so that dark shading represents clean air and light shading represents aerosol-laden air. The terrain surface is indicated in black. The aerosol layer (AL) height is clearly defined by the boundary between high and low backscatter intensities. A feature that can be seen in both Figs. 2a and 2b is the small and irregularly spaced peaks indicating individual thermal plumes penetrating the inversion that caps the CBL, a well-known feature of the CBL (e.g., Stull 1988). The backscatter field during track L7 (Fig. 2a) shows that the AL is typically 500 to 600 m deep in the southern part of this region of the Lower Fraser Valley. Farther to the north, the AL first increases slightly in height but then, rather suddenly, starts to decrease at about 5 km from the base of the Coast Mountains, eventually reaching a height of only 300 m at the mountain base. The dotted line in Fig. 2a indicates this depression in the AL height. The backscatter field during flight track T1 (Fig. 2b) shows AL heights of 700 to 800 m on the eastern and western part of the flight track with smaller AL heights in between. The region where the AL is smaller (indicated by the arrow) corresponds with the location where flight tracks L7 and T1 intersect, just south of the steep southeast-facing slope of the Coast Mountains. Flight track L7 was about 1 h later than flight track T1, which can explain the somewhat larger AL heights at the intersection point (49.25°N, 122.7°W) in Fig. 2a.
Tethered balloon profiles up to about 1 km above ground level (AGL) were obtained at Harris Road, roughly 10 km south of the base of the Coast Mountains as depicted by the “H” in Fig. 1b. A tethered balloon sounding took about 45 min (up and down) with measurements taken at a height interval of about 10 m. Additional sounding data are available from Langley in the center of the valley, roughly 20 km south of Harris Road (“L” in Fig. 1b). Harris Road is located near sea level while Langley’s elevation is roughly 80 m above sea level (ASL). To facilitate further analysis of the sounding data, they are interpolated to regular height intervals of 25 m (with respect to mean sea level). Shown in Fig. 3 are temperature soundings at Harris Road at 1452 PST and Langley at 1600 PST. A CBL height of 500 to 600 m can be detected, comparable with what is derived from the lidar backscatter field. This provides evidence that the AL height and CBL height are of comparable magnitude over the plains. This equivalence of CBL and AL heights is further supported by the thermal plumes visible in the lidar backscatter field. The equivalence of CBL and AL heights that occurs over flat terrain breaks down over mountain ranges (De Wekker et al. 2004).
In the Fraser Valley case, CBL heights at Langley and Harris Road were of comparable magnitude and the depression in CBL height appears as a localized feature occurring over a horizontal scale of roughly 5 km just north of Harris Road but before the terrain elevation starts to increase (see horizontal dotted line in Fig. 1b). It was noted by Hayden et al. (1997), who did a thorough analysis of CBL heights in the Lower Fraser Valley during Pacific’93, that CBL heights at Harris Road were generally lower than those farther to the south. This could indicate that the horizontal scale of the depression in CBL height varies and is not necessarily as confined to the mountain base as is seen in the example above. Although only one example of the CBL height depression near a mountain base is presented, several other lidar cross sections on other days during Pacific’93 show a similar feature (Hoff et al. 1997).
b. Other observational evidence
Relatively low CBL heights near a mountain base compared to the nearby plains were also observed during the Transport of Air Pollutants over Complex Terrain (TRACT) field study carried out in the Rhine Valley, Germany, in 1992 (Fiedler 1992). The slopes characterizing the transition from the Rhine Valley (approximately 50 km wide here) to the mountainous Black Forest region are less steep than those characterizing the transition from the Lower Fraser Valley to the Coast Mountains. Another difference is the absence of a coastline in the Rhine Valley. Figure 4 shows potential temperature profiles obtained from radiosondes along a northwest–southeast cross section in the Rhine Valley on 22 September 1992. In the northwestern part of the region, CBLs extend up to 1000 m AGL while farther to the southeast near the mountain base, CBLs only extend up to roughly 500 m. Except for foggy conditions in the morning hours in the Rhine Valley, convective weather conditions prevailed during the main part of this day and winds in the CBL were light (<5 m s−1). These synoptic conditions, conducive to an ozone episode, were similar to those on 4 August during Pacific’93. Comparing the depressions in the CBL heights in Pacific’93 and TRACT, the horizontal scale of the phenomenon appears larger in the Rhine Valley (note the different horizontal scales in Figs. 2a and 4). In the following sections, the depression of the CBL heights in the Pacific’93 field study will be examined further.
3. Modeling of a “CBL height depression”
An idealized numerical study of the plain-to-basin flow by De Wekker et al. (1998) indicated that modeled CBL heights near a mountain base show a pronounced depression in the afternoon (see their Fig. 10). Cai et al. (2000) also remarked on the presence of anomalously low CBL heights near a mountain base in their realistic three-dimensional simulations of the meteorological conditions during Pacific’93. The question now is whether the depressed CBL height also occurs in a numerical model if it is initialized with the atmospheric conditions encountered on 4 August 1993. An attempt is made to keep the approach as simple as possible. Two-dimensional simulations rather than three-dimensional simulations are performed so that there are no complicating effects as a consequence of valley flows or flows around mountain ridges, for example. The behavior of CBL heights in mountainous terrain can become very complex, and isolating the processes associated with this behavior becomes difficult or impossible. Idealized two-dimensional simulations can help to reduce the complexity and facilitate the isolation of atmospheric processes producing a certain phenomenon. The idea is that if simple two-dimensional simulations are able to simulate a depression in CBL heights, the model can be used further to investigate this phenomenon.
a. Model setup
The idealized two-dimensional simulations are carried out with the Regional Atmospheric Modeling System (RAMS). RAMS (Pielke et al. 1992) is a nonhydrostatic model with a terrain-following coordinate system employing a Mellor–Yamada level 2.5 turbulence closure scheme. The turbulent exchange at the surface is determined with the so-called Louis scheme (Pielke et al. 1992), which is based on Monin–Obukhov similarity theory. The computed surface fluxes for the soil and vegetation must be averaged to provide the grid-averaged surface flux. These fluxes serve as the lower boundary for the subgrid diffusion scheme for the atmosphere. Short- and longwave radiation are parameterized with a radiation scheme described by Chen and Cotton (1983). A basic radiative condition is used for the lateral boundary conditions. For more detailed descriptions of the treatment of physics in RAMS, see Pielke et al. (1992).
The vertical resolution used in the simulations ranges from 25 m near the surface to 1000 m near the upper boundary. The upper boundary at about 16 km is a rigid lid. A damping scheme is applied to the three vertical layers below the upper boundary to suppress the amplitudes of vertically propagating gravity waves and to reduce wave reflection from the rigid lid. The two-dimensional simulations are performed in a north–south cross section (resembling flight leg L7; see Fig. 1b) with a horizontal grid spacing of 1 km. The RAMS cumulus and microphysical parameterizations are not activated, so water vapor is treated as a passive scalar.
The model is initialized on 4 August 1993 at 1200 UTC (0400 PST) and is run for 16 h. Since there are no clouds in the model, the model is forced by maximum possible solar radiation for this date at the geographical latitude of the investigation area (49°N). The initial temperature field is horizontally homogeneous and is based on a sounding at Langley (indicated by “L” in Fig. 1b) in the center of the Lower Fraser Valley on 4 August 1993, at 0400 PST. Further initial conditions used in this study are calm winds (0.01 m s−1), invariant soil (sandy loam), and vegetation (short grass) properties, and a relative humidity of 20% that is constant with height. The volumetric soil moisture content is set to 0.3.
An idealized triangular shaped mountain ridge surrounded by flat terrain on both sides is used for the simulations. The mountain height is 1 km and the simulations are carried out for slope angles varying between 3° and 12°. To minimize numerical errors and to keep from changing the horizontal and vertical grid spacing, simulations with slopes steeper than 12° are not carried out. Also, at slopes steeper than about 15°, the model blew up in this particular configuration. The lateral boundaries are set 50 km away from both sides of the mountain base.
b. CBL heights from the numerical model
CBL heights are determined from simulations by analyzing vertical profiles of turbulent kinetic energy (TKE). The CBL height is taken as the height where the TKE has dropped to a value of 0.03 m2 s−2 following Cai and Steyn (1995). The TKE drops sharply at the top of the CBL, and a value of 0.03 m2 s−2 has been found in previous studies (Cai and Steyn 1995; De Wekker 2002) to give similar CBL heights as the more traditional methods using profiles of wind and temperature (both over flat and complex terrain). Also for the current study, the various methods resulted in similar CBL heights (De Wekker 2002).
The CBL height as a function of the distance to the mountain base at 1200, 1500, and 1700 PST and as a function of slope steepness at 1600 PST is shown in Figs. 5a and 5b, respectively. Figure 5a shows that the model produces CBLs over the plains far away from the mountain base that evolve gradually and eventually reach a height of about 600 m. This is of comparable magnitude to the CBL heights in the Lower Fraser Valley on 4 August (recall that the model was initialized with the 0400 PST temperature sounding at Langley on 4 August). Near the mountain base, CBL growth is significantly reduced. There, CBLs remain relatively low over the day and even decrease in height late in the afternoon. The horizontal extent of the depression increases as time progresses. As shown in Fig. 5b, the steep slope simulations produce a CBL that is about 200 m lower near the mountain base than the shallow slope simulations. Furthermore, the horizontal scale of the depression increases as slope steepness increases, although there still is a confined region near the mountain base where the depression is more pronounced on a small horizontal scale. Several additional simulations were performed to investigate the sensitivity of the CBL height depression to various factors, such as horizontal and vertical resolution. CBL height depressions in these simulations were not significantly different from those in the main simulations performed in the study, showing that the phenomenon is consistent.
Comparing the simulations with the lidar observations, it can be concluded that the simple two-dimensional idealized simulations capture the observed phenomenon, even though the observed depression in CBL height is larger than the modeled one. This is an encouraging result that stimulates further use of both observational data and the numerical model to investigate the phenomenon.
4. Comparison between observations and numerical model
In the previous two sections, evidence was provided of the presence of depressed CBL heights near a mountain base from observations and numerical modeling. Additional analyses using Pacific’93 data will now be presented in an attempt to clarify the mechanisms that produce this depression and to determine whether the phenomenon is produced by similar mechanisms in the model and in the observations.
Unfortunately, temperature soundings are not available at the location where the depression in CBL heights occurred (see Fig. 2a). Harris Road is located just south of the observed depression (see horizontal dotted line in Fig. 1b). However, boundary layer processes occurring at Harris Road can be expected to be affected by the proximity of the mountains in a similar though less enhanced way than in the region directly adjacent to the mountain base. To isolate the mountain proximity effect, the following approach is taken. First, observed vertical profiles of potential temperature at Harris Road and Langley are used to calculate the vertical profiles of heating rate at both locations between approximately 1000 and 1300 PST and between 1300 and 1600 PST. Then the heating rate at Langley between 1000 and 1300 PST is subtracted from the heating rate at Harris Road between 1000 and 1300 PST. The same is done for the time period between 1300 and 1600 PST. The result of this analysis is seen in Fig. 6 for (a) 1000–1300 PST (“morning”) and (b) 1300–1600 PST (“afternoon”). Clearly, the heating rate at Harris Road is larger than at Langley at virtually every height both in the morning and afternoon. The difference in heating rate is a minimum at about 300 m ASL in the morning and 400 m ASL in the afternoon (shown by the lower arrows). Above this minimum, the difference increases significantly and shows a maximum at 500 m ASL in the morning and 600 m ASL in the afternoon (shown by the upper arrows). Thus, the atmospheric heating rate near the mountain base is not only larger inside the CBL but also above, when compared to the heating rate far away from the mountain base (recall that the maximum CBL height was 500–600 m ASL at Harris Road on 4 August).
In an attempt to understand this behavior, the model output is analyzed in a similar way. The approach is depicted in Fig. 7. The heating rates in a column of air near the mountain base, column B, and a column of air near the edge of the modeling domain, column A, are compared. Column-averaged values are obtained by averaging over a horizontal distance of 5 km (this distance was chosen rather arbitrarily; any distance between 1 and about 10 km gave very similar results). Subsequently, the heating rates in column A are subtracted from the heating rates in column B for the same time periods as the observations, that is, between 1000 and 1300 PST and between 1300 and 1600 PST. The result of this analysis is presented in Fig. 8 as a function of height. It can be seen that there is a good qualitative agreement between the model and the observations. Noticeable in the vertical profile of total heating rate are the minimum and maximum (indicated by arrows) that were also seen in the observations.
Several points should be considered when comparing the results of the idealized numerical simulations with the observations. For example, the larger values of the heating rates in the CBL at Harris Road might be explained by differences in surface properties, which are not considered in the numerical simulations. A simple calculation indicates that the sensible heat flux would have to be more than ∼50 W m−2 larger at Harris Road continuously to account for the difference in heating rate in the CBL. Given the similar surface properties between Harris Road and Langley (mixed, but mostly short grass), it is unlikely that the difference in heating rate can be attributed to this effect. Three-dimensional channeling effects, valley flow effects, and sea-breeze effects are also not accounted for in the idealized simulations, and the simple topography used in the simulations is only a very rough approximation of the real topography. Despite these considerations the agreement is encouraging and implies that the idealized two-dimensional model simulations capture some important process affecting CBL growth near a mountain base.
If the depression in CBL height near a mountain base is established by sinking motions associated with the upslope flows, then the phenomenon might only occur along mountain bases with strongly developed upslope flows. The mountain base where the phenomenon was observed on 4 August is indeed characterized by strong upslope flows, as indicated by the presence of vigorous mountain venting. This can be seen in Fig. 2 (notice the light shaded area over the slope and above mountain top level) and is further demonstrated by Rucker et al. (1998) who describe mountain venting characteristics during Pacific’93. Additional numerical simulations in which the strength of the upslope flow was regulated by changing the incoming solar radiation indicate that weaker upslope flows and related weaker sinking motions made the CBL height depression smaller.
Sinking motions associated with upslope flows have also been shown to play an important role in inversion breakup and the subsequent development of the CBL in narrow mountain valleys (Whiteman 1990). Until now, the importance of these sinking motions in the vicinity of mountain bases in very wide valleys or mountain bases of single ridges has not been considered. The subsidence in the narrow valleys has been attributed to mass compensation for upslope flows that transport mass out of the valley. These compensatory flows certainly also happen in the model and are associated with the depressed CBL height. The reduced CBL height at the mountain base might be enhanced by subsidence generated there by horizontal wind divergence at the interface between upslope flows over the slopes and weak horizontal winds over the adjacent plain. Wind and temperature data in a much higher spatial resolution than obtained in Pacific’93 is needed to investigate this. The lack of data also hinders the examination of other processes that may contribute to the spatial variability of the CBL height in regions of complex terrain. Examples include sea-breeze and valley wind effects, and mechanical effects generated as air flows over and around mountain ranges.
The same analysis as presented above was carried out for an additional day 1 August 1993 and is shown in Fig. 11. The only difference with the 4 August 1993 case is the initial temperature profile. The agreement between the observations and numerical model results is not as good with regard to the heights where minimum and maximum differences in heating rates occur. The overall shape of the vertical profile, however, corresponds well. Interestingly, CBLs were deeper on this day and, correspondingly, the maximum heating rate difference was found at higher elevations than on 4 August. The enhanced heating below the mountain height that was present on 4 August cannot be seen clearly on 1 August when atmospheric stability was lower. The CBL height depression was not observed on this day, implying that ambient stability and the CBL height relative to the average mountain height play a role.
5. Conclusions
CBL height variability near a mountain base was investigated using data from the Pacific’93 field study and a mesoscale numerical model. In particular, it was shown from observations and mesoscale numerical simulations that the proximity of a mountain base sometimes affects the CBL in such a way that a CBL height depression is produced near a mountain base. Idealized simulations with a single mountain ridge suggest that the depression evolves in time and is most pronounced in the late afternoon. Also, the depression becomes larger as slope steepness increases. Observations and modeling results show that the atmosphere near the mountain base is heated more intensely, not only inside the CBL but also above. An analysis of the different terms in the temperature tendency equation indicates that vertical and horizontal advection of warm air, associated with the thermally driven circulation along the mountain slope, play a role in the enhanced heating aloft. Factors that affect this heating include the ambient stability and the height of the CBL relative to mountain height. The enhanced heating aloft and modeled sinking motions near the mountain base are associated with the CBL height depression. These modeling results suggest that this phenomenon only occurs near mountain slopes with a well-developed slope wind system.
Acknowledgments
Most of this work was done while the author (SFJD) was a graduate student at the University of British Columbia (UBC). SFJD thanks UBC for a two-year University Graduate Fellowship, the Isaac Walton Killam Memorial Trust for a two-year predoctoral fellowship, and the UBC geography department for several teaching assistantships. Doctors Douw Steyn and Dave Whiteman are thanked for their comments on previous versions of the manuscript, and Dr. Fiedler is acknowledged for making TRACT data available to SFJD.
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Map of the Lower Fraser Valley of British Columbia. Contour intervals are (a) 200 and (b) 100 m. The region of interest in this study is indicated by the rectangle in (a), which is magnified in (b). The lidar paths are indicated by L7 and T1, while H and L denote the Harris Road tethered balloon station and the Langley radiosonde station, respectively. The horizontal dotted line near H denotes the approximate location at which the aerosol layer starts to decrease in height in Fig. 2a. Darker shades of gray represent higher elevations in (a). Elevation higher than 700 m is shaded in (b).
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Map of the Lower Fraser Valley of British Columbia. Contour intervals are (a) 200 and (b) 100 m. The region of interest in this study is indicated by the rectangle in (a), which is magnified in (b). The lidar paths are indicated by L7 and T1, while H and L denote the Harris Road tethered balloon station and the Langley radiosonde station, respectively. The horizontal dotted line near H denotes the approximate location at which the aerosol layer starts to decrease in height in Fig. 2a. Darker shades of gray represent higher elevations in (a). Elevation higher than 700 m is shaded in (b).
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Map of the Lower Fraser Valley of British Columbia. Contour intervals are (a) 200 and (b) 100 m. The region of interest in this study is indicated by the rectangle in (a), which is magnified in (b). The lidar paths are indicated by L7 and T1, while H and L denote the Harris Road tethered balloon station and the Langley radiosonde station, respectively. The horizontal dotted line near H denotes the approximate location at which the aerosol layer starts to decrease in height in Fig. 2a. Darker shades of gray represent higher elevations in (a). Elevation higher than 700 m is shaded in (b).
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Flight tracks (a) L7 and (b) T1 on 4 Aug 1993. The grayscale is proportional to lidar backscatter with light shades of gray indicating high backscatter intensities. The depression in the AL height is indicated by the dotted line in (a). The arrow in (b) indicates a region with relatively shallow aerosol layer heights, corresponding to the location at which flight tracks L7 and T1 intersect.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Flight tracks (a) L7 and (b) T1 on 4 Aug 1993. The grayscale is proportional to lidar backscatter with light shades of gray indicating high backscatter intensities. The depression in the AL height is indicated by the dotted line in (a). The arrow in (b) indicates a region with relatively shallow aerosol layer heights, corresponding to the location at which flight tracks L7 and T1 intersect.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Flight tracks (a) L7 and (b) T1 on 4 Aug 1993. The grayscale is proportional to lidar backscatter with light shades of gray indicating high backscatter intensities. The depression in the AL height is indicated by the dotted line in (a). The arrow in (b) indicates a region with relatively shallow aerosol layer heights, corresponding to the location at which flight tracks L7 and T1 intersect.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Vertical profiles of potential temperature at Harris Road and Langley on 4 Aug 1993 at 1452 and 1600 PST. CBL height is indicated with a horizontal dashed line. The black circle near the surface is the surface temperature measured at Harris Road at 1500 PST.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Vertical profiles of potential temperature at Harris Road and Langley on 4 Aug 1993 at 1452 and 1600 PST. CBL height is indicated with a horizontal dashed line. The black circle near the surface is the surface temperature measured at Harris Road at 1500 PST.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Vertical profiles of potential temperature at Harris Road and Langley on 4 Aug 1993 at 1452 and 1600 PST. CBL height is indicated with a horizontal dashed line. The black circle near the surface is the surface temperature measured at Harris Road at 1500 PST.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Vertical profiles of potential temperature on a cross section of the Rhine Valley at 1300 central European standard time 22 Sep 1992 during the TRACT field study (adapted from De Wekker 1995).
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Vertical profiles of potential temperature on a cross section of the Rhine Valley at 1300 central European standard time 22 Sep 1992 during the TRACT field study (adapted from De Wekker 1995).
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Vertical profiles of potential temperature on a cross section of the Rhine Valley at 1300 central European standard time 22 Sep 1992 during the TRACT field study (adapted from De Wekker 1995).
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Variation of CBL heights with horizontal distance from the mountain base (a) as a function of time for an 11.3° slope and (b) as a function of slope steepness. The shaded mountain in (b) is given for an 11.3° slope for illustration purposes only. CBL heights are determined by the TKE method.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Variation of CBL heights with horizontal distance from the mountain base (a) as a function of time for an 11.3° slope and (b) as a function of slope steepness. The shaded mountain in (b) is given for an 11.3° slope for illustration purposes only. CBL heights are determined by the TKE method.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Variation of CBL heights with horizontal distance from the mountain base (a) as a function of time for an 11.3° slope and (b) as a function of slope steepness. The shaded mountain in (b) is given for an 11.3° slope for illustration purposes only. CBL heights are determined by the TKE method.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Differences in heating rates as a function of height between Harris Road and Langley averaged between roughly (a) 1000 and 1300 PST (“morning”) and (b) 1300 and 1600 PST (“afternoon”) for 4 Aug 1993. Arrows indicate the minimum and maximum differences in heating rates. Error bars indicate a measurement error of 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Differences in heating rates as a function of height between Harris Road and Langley averaged between roughly (a) 1000 and 1300 PST (“morning”) and (b) 1300 and 1600 PST (“afternoon”) for 4 Aug 1993. Arrows indicate the minimum and maximum differences in heating rates. Error bars indicate a measurement error of 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Differences in heating rates as a function of height between Harris Road and Langley averaged between roughly (a) 1000 and 1300 PST (“morning”) and (b) 1300 and 1600 PST (“afternoon”) for 4 Aug 1993. Arrows indicate the minimum and maximum differences in heating rates. Error bars indicate a measurement error of 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Idealized topography used in the model simulations showing the columns A and B whose heat budget terms were compared.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Idealized topography used in the model simulations showing the columns A and B whose heat budget terms were compared.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Idealized topography used in the model simulations showing the columns A and B whose heat budget terms were compared.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Differences in heating rates as a function of height between the atmospheric columns B and A (depicted in Fig. 7), as averaged between (a) 1000 and 1300 PST (“morning”) and (b) 1300 and 1600 PST (“afternoon”). Arrows indicate the minimum and maximum differences in heating rates.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Differences in heating rates as a function of height between the atmospheric columns B and A (depicted in Fig. 7), as averaged between (a) 1000 and 1300 PST (“morning”) and (b) 1300 and 1600 PST (“afternoon”). Arrows indicate the minimum and maximum differences in heating rates.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Differences in heating rates as a function of height between the atmospheric columns B and A (depicted in Fig. 7), as averaged between (a) 1000 and 1300 PST (“morning”) and (b) 1300 and 1600 PST (“afternoon”). Arrows indicate the minimum and maximum differences in heating rates.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
As in Fig. 8 but with the contribution of the individual terms to the difference in total heating rate also shown.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
As in Fig. 8 but with the contribution of the individual terms to the difference in total heating rate also shown.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
As in Fig. 8 but with the contribution of the individual terms to the difference in total heating rate also shown.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Cross section of potential temperature and wind field at (a) 1100 and (b) 1600 PST. The CBL height determined by the TKE method from model output is indicated with the heavy black line. In both panels the second contour from the top is 302 K, and contour lines are drawn every 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Cross section of potential temperature and wind field at (a) 1100 and (b) 1600 PST. The CBL height determined by the TKE method from model output is indicated with the heavy black line. In both panels the second contour from the top is 302 K, and contour lines are drawn every 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
Cross section of potential temperature and wind field at (a) 1100 and (b) 1600 PST. The CBL height determined by the TKE method from model output is indicated with the heavy black line. In both panels the second contour from the top is 302 K, and contour lines are drawn every 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
(a) Observed differences in heating rates as a function of height between Harris Road and Langley averaged between roughly 1300 and 1600 PST for 1 Aug 1993. (b) The corresponding modeled difference in heating rate as a function of height is shown by the bold line. The contribution of the individual terms to the total heating rate is also shown. Arrows indicate the corresponding locations of minimum and maximum differences in heating rates. Error bars in (a) indicate a measurement error of 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
(a) Observed differences in heating rates as a function of height between Harris Road and Langley averaged between roughly 1300 and 1600 PST for 1 Aug 1993. (b) The corresponding modeled difference in heating rate as a function of height is shown by the bold line. The contribution of the individual terms to the total heating rate is also shown. Arrows indicate the corresponding locations of minimum and maximum differences in heating rates. Error bars in (a) indicate a measurement error of 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
(a) Observed differences in heating rates as a function of height between Harris Road and Langley averaged between roughly 1300 and 1600 PST for 1 Aug 1993. (b) The corresponding modeled difference in heating rate as a function of height is shown by the bold line. The contribution of the individual terms to the total heating rate is also shown. Arrows indicate the corresponding locations of minimum and maximum differences in heating rates. Error bars in (a) indicate a measurement error of 1 K.
Citation: Journal of Applied Meteorology and Climatology 47, 4; 10.1175/2007JAMC1651.1
