1. Introduction
Mountainous areas cover 25% of Earth’s land surface and contain 26% of the global population (Meybeck et al. 2001). As a result, phenomena unique to the atmosphere in complex terrain have significant impacts on a large part of the human population. This impact extends beyond relatively rare but potentially damaging events such as downslope windstorms and extreme precipitation events to the daily wind patterns whose variations are significant for air quality (Whiteman et al. 2014; Jerrett et al. 2005; Coen et al. 2013), wind energy generation (Wilczak et al. 2014), wildfire suppression and prediction (Butler et al. 2006), agriculture (Li et al. 2015), and emergency response and military operations (Liu et al. 2008). Accurate forecasts of the surface and near-surface wind in complex terrain are important for these applications. Unfortunately, mesoscale models run at high resolution often fail to produce accurate simulations of the wind field in complex terrain (Hart et al. 2005; Rife et al. 2004) and tend to be worse over complex terrain than elsewhere (Jiménez and Dudhia 2013).
Winds in much of the western United States are heavily influenced by the thermal and dynamic impacts of the complex topography that characterizes the region. The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program was conducted at Dugway Proving Ground (DPG) in northwestern Utah for the purpose of testing and improving mesoscale models and, by extension, operational forecasts (Fernando et al. 2015). To achieve this objective, a better understanding of how flows on multiple scales interact in complex terrain is necessary.
Wind is a dynamic response to pressure gradients in the atmosphere and is forced by mechanisms operating on a wide range of spatial and temporal scales. The passage of synoptic-scale troughs and ridges constitutes the largest scale of control on surface winds. On a smaller scale, there are numerous thermally induced and dynamically induced mesoscale forcings that can modify or even overwhelm the synoptic-scale flow. At the smallest scale, differential heating of small terrain features can induce microscale flows that also may dominate those imposed from larger scales.
Synoptic-scale weather in the western United States is controlled by the seasonal migration of the polar jet stream (Lareau and Horel 2012). While spatial variations of the synoptic-scale flow in the horizontal are usually very gradual, the vertical variations are often significant. In areas of complex terrain with large topographic relief, knowledge of these vertical variations is particularly important to assessing the synoptic forcing on the surface flow.
Whiteman and Doran (1993) posit that synoptically forced flows influence the surface flow in complex terrain in three primary ways. The first is through downward transport of momentum to the surface due to turbulent mixing. This mechanism is common when a well-developed convective boundary layer is present and results in minimal directional shear between the surface and upper levels. In situations where there is vertical wind shear, significant variation of the surface wind can occur as momentum from different levels is turbulently transported to the surface by a growing convective boundary layer.
The two other methods are pressure-driven channeling and forced channeling, when a terrain barrier forces the flow out of geostrophic balance through friction and blockage. Pressure-driven channeling occurs when the surface wind next to a terrain barrier aligns with the component of the pressure gradient parallel to the barrier. Forced channeling, on the other hand, occurs when the surface wind next to a terrain barrier aligns with the component of the flow parallel to the barrier. Depending on the orientation of the wind relative to the terrain, pressure-driven and forced channeling can either reinforce or oppose one another.
In complex terrain, strong local horizontal gradients in temperature exist near the surface because of differential heating of the land surface. These temperature differences hydrostatically produce pressure gradients, which in turn drive winds (Defant 1949). For meso-β and smaller scales, these flows tend to have a low Rossby number and flow up or down terrain under the influence of the buoyancy force (Zardi and Whiteman 2012). Such flows are referred to as thermally driven flows. In locations where these flows are particularly strong or synoptic-scale flows are particularly weak, the thermally driven flows can dominate the surface wind evolution. The largest in scale of these flows, the mountain–plain circulation, is driven by the large contrast in daytime heating and nighttime cooling that occurs over the mountains versus the surrounding free atmosphere (Weissmann et al. 2005).
On a smaller scale, thermally driven valley circulations have been observed during daytime and nighttime in individual valleys (Pinto et al. 2006) including two valleys adjacent to DPG (Stewart et al. 2002; Ludwig et al. 2004; Doran et al. 2002). In some cases, thermally driven nocturnal valley flows can take the form of low-level jets (Banta et al. 2004). Thermally driven slope circulations have also been observed on individual slopes (Doran and Horst 1983; Whiteman and Zhong 2008; Lehner et al. 2015), in closed basins (Steinacker et al. 2007), and on elevated plateaus (Bossert 1997).
Thermally driven flows can also occur as a result of large land surface contrasts. Lake breezes are common to the valleys close to the Great Salt Lake and were studied by Zumpfe and Horel (2007). Rife et al. (2002) identified slope and valley flows as well as lake breezes and playa breezes at DPG. The playa breeze is driven by temperature contrasts between the playa and surrounding terrain. The playa surface remains cooler during the day and warmer during the night, primarily because of a higher thermal conductivity of the playa soil and a higher surface albedo (Hoch et al. 2014).
This research uses both available long-term datasets and high-resolution datasets collected during MATERHORN to understand the climatological evolution of the surface wind field at DPG on diurnal and seasonal time scales. This climatology, combined with atmospheric reanalysis data, will allow us to identify potential forcing mechanisms causing the observed diurnal and seasonal changes. A better understanding of the forcings will allow investigators to target particular aspects of the numerical model for verification and improvement efforts.
2. Data and methods
The region of interest for this work, the Intermountain West (IMW), consists largely of semiarid high-elevation basins punctuated by meridionally elongated mountain ranges and includes the western half of Utah and nearly all of Nevada. Relative to much of the IMW, the area controlled by the U.S. Army’s DPG has an extensive meteorological observation network that hosted two extensive field campaigns as part of the MATERHORN program. The specific focus of this work is the northeastern quadrant of the IMW where DPG is located, an area that includes much of northwestern Utah (Fig. 1). Since the region under examination includes much of the area covered by Pleistocene Lake Bonneville, we will refer to this area as the Bonneville basin (BB).
Terrain of the Bonneville basin. The location of selected automatic weather stations (circles) and terrain features are annotated. The Bonneville basin is roughly outlined by tan colors.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Terrain of the Bonneville basin. The location of selected automatic weather stations (circles) and terrain features are annotated. The Bonneville basin is roughly outlined by tan colors.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Terrain of the Bonneville basin. The location of selected automatic weather stations (circles) and terrain features are annotated. The Bonneville basin is roughly outlined by tan colors.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Average annual precipitation over the valleys of the BB is 150–200 mm. Occupying the lowest elevations of the BB is a seasonally flooded playa that is largely vegetation free. This area is minimally confined by terrain and will be referred to in this paper as the playa. Ground cover off the playa generally consists of 0.5–1-m-high grasses and shrubs. Mountain ranges in the BB rise 200–800 m above the surrounding plain. The basin to the east of the playa, in this paper referred to as the east basin, is surrounded by higher terrain that forms a horseshoe shape. The basin opens to the northwest where it empties onto the playa. All the significant mountain ranges and valleys at DPG are oriented roughly north–south.
Dugway Proving Ground maintains 32 surface automated meteorological stations (SAMS) that are mostly situated on the basin floors. We will only use the 26 stations with the longest periods of record. Several stations are installed on the playa and two are installed on topographic high points. Variables measured at these stations include 2-m temperature, 2- and 10-m wind speed and direction, atmospheric pressure, 2-m humidity, and incoming solar radiation. Our wind analysis will be limited to the 10-m wind field. All data are reported as 5- or 15-min averages. The SAMS chosen for this analysis have long periods of record (between 7 and 17 years), while 6 SAMS with shorter periods of record were excluded from our analysis. Only 3% of the SAMS data is missing because of instrument or data communication failure; 15% is missing for the station with the poorest data coverage.
Quality control of the SAMS observations consisted of plausible value and time-consistency checks. To reduce systematic errors in the dataset, all SAMS sensors are calibrated once per year.
All available data were averaged to 15-min means; 10-m wind speed and direction were calculated as vector means before being converted to polar coordinates. Monthly climatological averages and statistics were then calculated using all available data for each station. Monthly mean diurnal cycles derived from the dataset permit the analysis of climatologically persistent flows that occur on both the seasonal and diurnal time scales.
Three representative SAMS will be referred to intensively in this work to describe the conditions observed at DPG. The first station, Tower Grid SAMS, is located at 1325 m MSL on the floor of the east basin. The second station, Camelback Mountain SAMS (1547 m MSL), sits atop Camels Back Ridge, a steep and narrow isolated ridgeline protruding up to 225 m from the floor of east basin. Camelback Mountain SAMS provides information on the flow field above the basin surface. The third station, Playa SAMS, is located at 1294 m MSL on the playa, 15 km west of the closest large topographic feature. The locations of these three representative SAMS are shown in Fig. 1.
To characterize dynamic and thermodynamic patterns imposed upon the DPG area at the meso-α scale and larger, European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) global reanalysis data from 1995 to 2012 are used. The ERA-Interim runs with 60 vertical levels and a reduced Gaussian grid with 79-km spacing for surface and other gridpoint fields (Dee et al. 2011). The ERA-Interim was chosen because of its improved representation of the atmosphere over the IMW (Jeglum et al. 2010) relative to the NCEP–NCAR reanalysis and the North American Regional Reanalysis. We used 850- and 700-mb levels (1 mb ≡ 1 hPa) to analyze above-surface flows as they were the most effective for characterizing the synoptic-to-mesoscale environment in an elevated area such as the IMW (Jeglum et al. 2010; Stewart et al. 2002; Ludwig et al. 2004; Steenburgh 2003).
With its relatively coarse resolution the ERA-Interim does not resolve any individual mountain ranges in the BB [see Fig. 4 in Jeglum et al. (2010)], instead integrating them into large plateaus. Thus, processes and features on the meso-β scale and smaller, such as slope and small valley circulations, are not resolved. However, the north–south-oriented topography of the BB and the meso-α and larger flows there are resolved.
3. Results
Figure 2 shows surface wind roses for the DPG area for the full period of record (between 7 and 17 years) for all 26 stations. Observations below 1 m s−1 have been excluded (less than 5% of the data at all stations). Most sites show a pronounced bimodality in the wind direction, with winds blowing either from the southern or northern direction. This bimodality could arise from diurnal shifts of the thermally driven valley wind circulation in north–south-oriented valleys (Whiteman 2000). However, the bimodality persists when the data are stratified by hour and month. For instance, at 1600 mountain standard time (MST) in August the wind at most DPG stations is either southerly or northwesterly (Fig. 3). If the afternoon flow at DPG was dominated by local thermally driven up-valley flows, there would be a clear single maximum in the up-valley (northerly or northwesterly) direction. We call this bimodality the nondiurnal bimodality and it refers specifically to the surface wind. To better understand what causes the nondiurnal bimodality, the data were subjectively divided into three regimes, a “northerly” regime, a “southerly” regime, and a “calm” regime.
Wind rose map of all DPG SAMS stations. Each station is represented by a wind rose that includes all hourly winds observed over the full period of record for each station (the period 1995–2012 for most stations). The stations are plotted on a UTM grid with 80-m vertical resolution of the terrain contours.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Wind rose map of all DPG SAMS stations. Each station is represented by a wind rose that includes all hourly winds observed over the full period of record for each station (the period 1995–2012 for most stations). The stations are plotted on a UTM grid with 80-m vertical resolution of the terrain contours.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Wind rose map of all DPG SAMS stations. Each station is represented by a wind rose that includes all hourly winds observed over the full period of record for each station (the period 1995–2012 for most stations). The stations are plotted on a UTM grid with 80-m vertical resolution of the terrain contours.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
As in Fig. 2, but only winds at 1600 MST in August are included.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
As in Fig. 2, but only winds at 1600 MST in August are included.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
As in Fig. 2, but only winds at 1600 MST in August are included.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
To define these regimes, each 24-h period from 0000 to 2400 UTC at each station at DPG was designated as being in either the northerly, southerly, or calm regime based on the direction of the daily mean wind (mean wind for that 24-h period). For each station a given day was considered to be in the northerly regime if the daily mean wind was dominated by a northerly component and the mean wind speed was at least 1 m s−1. Southerly regime days were designated similarly, by having a dominating southerly component and a mean wind speed of at least 1 m s−1. Calm days consisted of the days with a mean wind speed below 1 m s−1. Nearly all the DPG surface stations display wind direction distributions that are extremely elongated in the north–south direction, making this method robust. Finally, each day over the 1995–2012 period was designated as being in the northerly, southerly, or calm regime based on the designated regime of the plurality of stations on that particular day. If more than 25% of the SAMS-wide data were missing on a given day, the day was excluded from the analysis. Since these regimes are very often coherent across the entirety of DPG, there were only few days that were not clearly in one regime or another.
Figure 4 shows the monthly mean frequencies of the days in the northerly, southerly, and calm regimes for the 1995–2012 period. The difference between northerly and southerly regimes frequencies is statistically significant at the 0.05 level in the months of January, May, July, August, and December. The northerly regime is most frequent in May and least frequent in August and January. The southerly regime is most frequent in August and least frequent in May. This produces a tripolar seasonal distribution with poles in May, August, and January.
Relative frequencies of the southerly (solid), northerly (dash), and calm (dot–dash) regimes over the DPG area by month. The sum of the three lines adds up to 1 for each month. Using Pearson’s chi-square test, the frequency differences between the northerly and southerly regimes in January, May, July, August, and December are statistically significant at the 0.05 level.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Relative frequencies of the southerly (solid), northerly (dash), and calm (dot–dash) regimes over the DPG area by month. The sum of the three lines adds up to 1 for each month. Using Pearson’s chi-square test, the frequency differences between the northerly and southerly regimes in January, May, July, August, and December are statistically significant at the 0.05 level.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Relative frequencies of the southerly (solid), northerly (dash), and calm (dot–dash) regimes over the DPG area by month. The sum of the three lines adds up to 1 for each month. Using Pearson’s chi-square test, the frequency differences between the northerly and southerly regimes in January, May, July, August, and December are statistically significant at the 0.05 level.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
The tripolar seasonal changes in the surface wind field are also apparent in the ERA-Interim mean wind vectors at levels above the surface as shown in Fig. 5. Figure 5 displays the monthly mean geostrophic wind at 850 and 700 mb and the thermal wind for the intervening layer for the afternoon period. The thermal wind is proportional to the mean horizontal temperature gradient in the 850–700-mb layer and is identical to the magnitude of the geostrophic wind shear vector between 850 and 700 mb (Wallace and Hobbs 1977). The tripolar distribution is visible in the seasonal variation of the 850-mb geostrophic wind and the thermal wind. The 700-mb geostrophic wind, in contrast, displays a bipolar pattern, indicating that the primary drivers for the tripolarity are likely below 700 mb.
Hodograph plot at 1700 MST of the monthly vector mean 700-mb geostrophic wind (black), 850-mb geostrophic wind (green), and 700–850-mb thermal wind (red) over DPG from the ERA-Interim for 1995–2012. Each number corresponds to the end of a vector starting at the origin representing the mean value for the month indicated by the number.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Hodograph plot at 1700 MST of the monthly vector mean 700-mb geostrophic wind (black), 850-mb geostrophic wind (green), and 700–850-mb thermal wind (red) over DPG from the ERA-Interim for 1995–2012. Each number corresponds to the end of a vector starting at the origin representing the mean value for the month indicated by the number.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Hodograph plot at 1700 MST of the monthly vector mean 700-mb geostrophic wind (black), 850-mb geostrophic wind (green), and 700–850-mb thermal wind (red) over DPG from the ERA-Interim for 1995–2012. Each number corresponds to the end of a vector starting at the origin representing the mean value for the month indicated by the number.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
In May the 850-mb geostrophic wind is primarily easterly and in August it is primarily southwesterly. The 700-mb geostrophic wind, however, is westerly in May and southwesterly in August with a similar magnitude during both months. In May the magnitude of the thermal wind vector is larger than the geostrophic wind at 700 mb. Such a situation requires a reversed pressure gradient at lower levels. January exhibits stronger flow at 700 mb and a moderately strong thermal gradient.
The strong nondiurnal bimodality and the tripolar seasonal variation of the surface wind will be discussed in the following sections through investigating the large-scale patterns that characterize the southerly regime in August, the northerly regime in May, and both regimes in January. This will be followed by a discussion of the surface wind flows common to each regime and the forcing mechanisms behind those flows.
a. Synoptic patterns of the three poles
August is one pole of the tripole distribution because it sees the seasonal maximum in southerly regime conditions. August exemplifies the phenomena that characterize the southerly regime during the warm season, as discussed below.
The synoptic conditions during southerly regime conditions (see Fig. 6) are characterized by southwesterly flow at 700 mb, the positioning of the primary midlatitude baroclinic zone northwest of Utah, and a meridionally oriented trough at 850 mb. At 850 mb a low is centered over the Utah–Nevada–Idaho triple point as part of the larger positively tilted trough over the IMW (Fig. 6b). Strong southerly flow at 850 mb extends from southern California up to the BB. Additionally, a thermal maximum is visible over eastern Nevada at 700 mb (Fig. 6a).
(a) Composite mean 1700 MST 700-mb wind (vectors) and temperature (color contours) for southerly regime days in August taken from the ERA-Interim grid point over DPG for the years 1995–2012. (b) Composite mean 850-mb wind (vectors) and geopotential height (color contours) for southerly regime days in August taken from the ERA-Interim grid point over DPG for the years 1995–2012. (c) As in (a), but for northerly regime days in May; the color scale is the same as in (a). (d) As in (b), but for northerly regime days in May; the color scale is the same as in (b).
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
(a) Composite mean 1700 MST 700-mb wind (vectors) and temperature (color contours) for southerly regime days in August taken from the ERA-Interim grid point over DPG for the years 1995–2012. (b) Composite mean 850-mb wind (vectors) and geopotential height (color contours) for southerly regime days in August taken from the ERA-Interim grid point over DPG for the years 1995–2012. (c) As in (a), but for northerly regime days in May; the color scale is the same as in (a). (d) As in (b), but for northerly regime days in May; the color scale is the same as in (b).
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
(a) Composite mean 1700 MST 700-mb wind (vectors) and temperature (color contours) for southerly regime days in August taken from the ERA-Interim grid point over DPG for the years 1995–2012. (b) Composite mean 850-mb wind (vectors) and geopotential height (color contours) for southerly regime days in August taken from the ERA-Interim grid point over DPG for the years 1995–2012. (c) As in (a), but for northerly regime days in May; the color scale is the same as in (a). (d) As in (b), but for northerly regime days in May; the color scale is the same as in (b).
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
May is the second pole of the tripole and sees the seasonal maximum in northerly regime conditions. While many of the same phenomena that are observed in August are present in May, the relative strength and position of them is sufficiently different such that northerly regime conditions predominate.
In the northerly regime in May, the 700-mb baroclinic zone is now draped across the BB (Fig. 6c) and there is no thermal maximum over eastern Nevada. The 850-mb trough axis is now almost zonally oriented, with northwesterly flow at 850 mb (Fig. 6d) over the BB. Directional shear in the geostrophic wind between 850 and 700 mb is much larger during the northerly regime than the southerly regime.
Evident in the 850-mb wind field in both Figs. 6b and 6d is a pronounced confluence of the wind in the lee of the Sierra Nevada. This confluence has been identified as the Great Basin confluence zone (GBCZ; West and Steenburgh 2010) and is a semipermanent airstream boundary that results from the flow near and below 700 mb splitting around the highest terrain of the Sierra Nevada and converging in their lee. Its position influences whether the BB will be dominated by southerly or northerly flow. The GBCZ often crosses DPG as a coherent surface boundary that may mark the abrupt transition from southerly regime to northerly regime conditions. During southerly regime conditions the GBCZ is located north of the BB, while in northerly regime conditions it lies over the Utah–Arizona border.
The January synoptic patterns present a strong contrast to May and August. Only 850-mb maps are shown, in Fig. 7a for the southerly regime and Fig. 7b for the northerly regime. Across the IMW, 850-mb winds are very weak relative to those during May and June. During southerly regime conditions in January both the 850 and 700 mb (not shown) geostrophic winds are westerly and the atmosphere is relatively stable throughout the day. Under these conditions, pressure-driven channeling in the BB may be responsible for producing the southerly surface winds.
Composite mean 850-mb wind (vectors) and geopotential height (color contours) for (a) southerly and (b) northerly regime days in January from the ERA-Interim over 1995–2012.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Composite mean 850-mb wind (vectors) and geopotential height (color contours) for (a) southerly and (b) northerly regime days in January from the ERA-Interim over 1995–2012.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Composite mean 850-mb wind (vectors) and geopotential height (color contours) for (a) southerly and (b) northerly regime days in January from the ERA-Interim over 1995–2012.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
The most remarkable feature of the mean January 850-mb geopotential height field under the northerly regime is the lack of a pressure gradient over Utah. The northerly surface flow on these days is likely due to forced channeling of the climatological northwesterly 700-mb flow.
b. Surface station evolution
The southerly and northerly regime definitions are based on the dominating daily mean surface wind direction. A more detailed investigation of the diurnal variations of the surface wind direction and wind speed as well as the surface temperature under the different regimes follows. The goal is to highlight the possible forcing mechanisms influencing the surface wind field throughout a full diurnal cycle under both regimes.
The diurnal and seasonal evolution of the surface winds during the northerly and southerly regimes is illustrated in Fig. 8 using composite diurnal hodographs. The three representative locations (Tower Grid, Camelback, and Playa SAMS) are shown for three representative months (January, May, and August) that define the tripolar seasonality.
Temporal hodographs illustrating the diurnal wind evolution at three surface meteorological stations during three different months. Each line traces the composite mean wind vector with a time resolution of 15 min. The lines are labeled every 3 h in MST. Red lines show composite means of southerly regime days at the given station in the given month. Blue lines show composite means of northerly regime days. Each diurnal cycle shown has a sample size of at least 200 days.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Temporal hodographs illustrating the diurnal wind evolution at three surface meteorological stations during three different months. Each line traces the composite mean wind vector with a time resolution of 15 min. The lines are labeled every 3 h in MST. Red lines show composite means of southerly regime days at the given station in the given month. Blue lines show composite means of northerly regime days. Each diurnal cycle shown has a sample size of at least 200 days.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Temporal hodographs illustrating the diurnal wind evolution at three surface meteorological stations during three different months. Each line traces the composite mean wind vector with a time resolution of 15 min. The lines are labeled every 3 h in MST. Red lines show composite means of southerly regime days at the given station in the given month. Blue lines show composite means of northerly regime days. Each diurnal cycle shown has a sample size of at least 200 days.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
In January (Figs. 8a,d,g) the diurnal variations of the surface wind are relatively small at all three stations. The difference between the two regimes is large relative to the diurnal wind variations.
At the Playa SAMS, the diurnal variations of the surface winds in May and August are small relative to the other two sites (Figs. 8g–i). Interestingly, the wind field variations at Playa SAMS in August show a meridional variation under the southerly regime and a zonal diurnal variation under northerly regime conditions.
The diurnal variation of the winds at Tower Grid and Camelback in May and August are shown in Figs. 8b,c and 8e,f, respectively. During both regimes the diurnal patterns follow a preference for westerly flow in the early afternoon and easterly flow in the evening. A preference for southerly flow is observed in the early morning, peaking after sunrise at around 0800 MST. The strength of this early morning southerly component marks the biggest difference between northerly and southerly regime conditions. The difference between the daily average wind vectors in each regime is comparable to those observed in January.
The variations at Camelback and Tower Grid do not occur on a strictly bimodal diurnal time scale, where the variation is controlled by night or daytime conditions. Instead, the extremes of the diurnal variations occur at approximately 0800, 1500, and 2100 MST. We can separate two modes of variations: one that is roughly east–west oriented and one roughly oriented in the north–south directions. The east–west variations are somewhat enhanced at Camelback, and the north–south variation is larger at the Tower Grid site. The north–south-oriented variations could point to an influence of thermal forcing that would lead to a valley circulation in the absence of other forcing mechanisms. However, the large east–west-oriented variations point to forcing mechanisms other than local thermally driven circulations.
To examine the regime differences in more detail, Fig. 9 compares composite median wind speed, wind direction, and temperature at the Tower Grid and Camelback Mountain SAMS sites during southerly regime days in August. The former station is on the floor of the east basin while the latter sits atop a steep and narrow isolated ridgeline just 4 km horizontally and 220 m vertically away. The station locations are shown in Fig. 2. In this analysis we will be using Camelback Mountain as a proxy for the winds in the free air above Tower Grid. Comparison of the winds at Camelback Mountain with free-air winds observed by a tethered balloon 15 km to the west (comparison not shown) indicate that Camelback Mountain winds are not overly sensitive to local terrain effects.
Meteogram showing median daily conditions at Tower Grid (solid black line for wind speed, plus shape for wind direction) and Camelback Mountain (dashed black line for wind speed, diamond shape for wind direction) during the southerly regime in August. Gray lines indicate the interquartile range for each station. Vertical black lines indicate average times of astronomical sunrise and sunset in August.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Meteogram showing median daily conditions at Tower Grid (solid black line for wind speed, plus shape for wind direction) and Camelback Mountain (dashed black line for wind speed, diamond shape for wind direction) during the southerly regime in August. Gray lines indicate the interquartile range for each station. Vertical black lines indicate average times of astronomical sunrise and sunset in August.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Meteogram showing median daily conditions at Tower Grid (solid black line for wind speed, plus shape for wind direction) and Camelback Mountain (dashed black line for wind speed, diamond shape for wind direction) during the southerly regime in August. Gray lines indicate the interquartile range for each station. Vertical black lines indicate average times of astronomical sunrise and sunset in August.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
The composite wind speeds and directions are very similar at both sites during the day. Because of a deep convective boundary layer both stations are aligned with the prevailing southwesterly 700-mb flow and the temperature difference between the two sites follows the dry-adiabatic lapse rate. The surface wind at Tower Grid is not strongly influenced at this time by a thermally driven up-valley wind, as such a wind would be northwesterly.
After sunset the composite temperatures at Tower Grid drop more quickly than at the ridgetop (Camelback) because of the development of the basin cold pool. This cold pool strengthens throughout the night. The winds at Tower Grid decrease as the growing stability in the cold pool reduces the momentum exchange with the free atmosphere. Wind speeds at Camelback decrease more slowly than at Tower Grid until 0200 MST, after which they begin to increase, reaching their diurnal maximum shortly after sunrise.
Shortly after sunrise the composite temperature at Tower Grid increases quickly, indicating the development of a convective boundary layer that destroys the nocturnal stable layer. This development coincides with a wind speed increase at Tower Grid, while wind speeds at Camelback Mountain decrease, suggesting an enhanced turbulent momentum exchange between the surface and the layers above as the convective boundary layer grows. The flow at Tower Grid remains stronger than at Camelback Mountain through 1200 MST. Wind speeds during the morning transition at Tower Grid are the highest of the diurnal period.
The composite median diurnal evolution of winds and temperatures during northerly regime days in August at the two selected stations are shown in Fig. 10. A comparison with Fig. 9 illustrates the differences between the two regimes. On average, temperatures during the northerly regime are lower, mainly because of much lower nighttime temperatures. The stronger nighttime valley cold pools further reflect in lower surface winds and a larger diurnal temperature cycle.
As in Fig. 9, but for the northerly regime.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
As in Fig. 9, but for the northerly regime.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
As in Fig. 9, but for the northerly regime.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
Under northerly regime conditions in August, the composite afternoon surface winds and temperatures are similar at both stations because of the deep convective boundary layer present at that time. The mean afternoon wind direction differs by 90° between the northerly and southerly regimes, reflecting the 70° mean 700-mb wind direction difference between them.
Composite winds show an increase near sunset (roughly 1920 MST) under the northerly regime, especially at the Camelback ridgetop site. The wind speed increase coincides with a directional shift from northwest to northeast and a reduced vertical momentum exchange due to the collapse of the daytime convective boundary layer (CBL). The relatively stronger composite north-northeasterly to easterly surface flow observed at both Tower Grid and Camelback persists from 1900 MST through roughly 2300 MST. A similar shift of the Tower Grid surface flow toward the easterly direction is observed during the southerly regime (see Fig. 8c), although it is superimposed on a preexisting southerly instead of a northerly flow.
The composite northeasterly flow weakens after 2200 MST. With the weakening flow and resulting reduced mixing the surface wind at Tower Grid decouples from the atmosphere above and temperatures and wind speeds drop. Further, the wind direction at Tower Grid switches from the mesoscale flow direction of roughly 70° to the local down-valley flow direction of approximately 145°. DPG SAMS south and east of Tower Grid observe the strongest down-valley flows (not shown). Stations to the north and west of Tower Grid are both further removed from the terrain and on a flatter part of the east basin floor and therefore experience a very weak or nonexistent down-valley flow (not shown). Camelback Mountain, elevated above the basin floor, remains above the down-valley flow layer and in easterly flow.
Under the northerly regime, the composite nighttime temperatures at Tower Grid fall more rapidly than temperatures at Camelback Mountain. The maximum nighttime temperature difference between the two sites reaches 7°C, almost doubling the 4°C maximum difference seen during the southerly regime.
Composite wind speeds reach their diurnal minimum during the morning transition under northerly regime conditions. Unlike during the southerly regime, both stations see only a slow increase in wind speeds in the northerly regime during the morning transition. However, there is a marked switch from southeasterly nighttime flow at both stations to a northwesterly daytime flow. Wind directions change minimally through the morning transition on southerly regime days.
4. Discussion and conclusions
We demonstrated that two surface wind regimes dominate at DPG: the southerly regime and the northerly regime. The resulting bimodal distribution of wind direction was shown to be poorly related to the diurnal variations of the thermally driven flow directions. In addition to lacking the expected diurnal changes from up-valley to down-valley flows, the absence of wind speed minima around the evening and morning transition times illustrates the importance of other forcing mechanisms. At DPG, wind speed maxima are instead observed during the morning (southerly regime) and evening transition times (northerly regime).
We found that the largest differences between the northerly and southerly regimes in the temperature and wind field evolution occur during the morning transition, when a median daily maximum in wind speed is observed during the southerly regime. The median daily maximum in wind speed during the northerly regime is actually observed in the evening. The regime differences were illustrated for two representative sites in DPG’s east basin: one on the basin floor, the other on top of an isolated 220-m-high, steep-sided ridgeline (see Fig. 1). The pronounced maximum in wind speed during southerly regime mornings at Tower Grid may be due to the turbulent mix out of a nocturnal southerly mesoscale flow under southerly regime conditions. This mesoscale flow can include periods with a low-level jet structure, as observed with tethered balloons and radiosondes during MATERHORN under southerly regime conditions (not shown). The southerly mesoscale flow likely arises from the strong synoptic-scale 850-mb pressure gradient over Utah during the southerly regime (see Fig. 6b).
Under northerly regime conditions, a strong northeasterly flow was observed throughout DPG during the evening transition. This flow is observed over terrain that would induce a southerly or southeasterly thermally driven down-valley flow and despite a mean westerly 700-mb wind. This points to the importance of the 850-mb pressure gradient during the evening transition period, when the decay of the convective boundary layer decouples the near-surface flow from the westerly 700-mb wind field aloft. The 850-mb pressure gradient is likely the result of strong synoptic-scale and mesoscale thermal gradients over northern Utah.
We found that the two regimes correspond to distinct synoptic patterns that differ substantially from one another. This illustrates that the different surface wind forcing is a result of variations in the large-scale flow as well as mesoscale processes that modify the large-scale flow, such as thermal low pressure and lee troughing.
The zonally oriented 850-mb trough that exists under southerly regime conditions (Fig. 6b) likely includes contributions from the Nevada thermal low and the dynamic effects of lee troughing. The thermal low is an important feature of the warm season synoptic pattern over the IMW that develops via strong heating of the arid, elevated surface. According to Tang and Reiter (1984), there are two primary lobes of the thermal low in the IMW: one over southern Nevada and another over northwestern Utah. The former is present from March to September with a broad peak in strength from April to August. The latter is present from June to August and helps to develop southerly flow at low levels over the BB, contributing to the preponderance of southerly regime conditions in July and August. Southerly regime days in August display a 700-mb-height minimum over Nevada and northwest Utah (not shown) similar to that seen by Tang and Reiter (1984).
Lee troughs in the IMW may contribute to the strengthening of the meridional pressure gradient over the BB under southerly regime conditions. While they are generally considered a spring phenomenon in the IMW (Jeglum et al. 2010), there is evidence to support lee trough occurrence even in July and August. The primary ingredient for lee troughing is flow impinging upon a large topographic barrier, and the distribution of 700-mb winds during the southerly regime in August (Fig. 11a) shows a clear bias toward southwesterly flow crossing the Sierra Nevada during those conditions. When the 700-mb wind is directly out of the southwest, the flow passes over the high terrain of the Sierra Nevada and eastern Nevada directly upstream of the BB. It is possible that the integrated effect of the basin-and-range topography of eastern Nevada combine to produce a broad plateau that slopes downward toward the northeast into the BB. Under southwesterly flow, a lee trough could be generated north and east of this plateau, directly over the BB, as seen in Fig. 6b. In this case, the lee trough likely results from a combination of adiabatic descent-induced warming and column stretching.
(a) Wind rose showing the distribution of 700-mb winds during the southerly regime in August at the ERA-Interim grid point over DPG. Note dramatic maximum from the southwest; 325 of 558 total August days fall into this category. (b) Wind rose showing the distribution of 700-mb winds during the northerly regime in August. Note dramatic minimum from the southwest; 151 of 558 total August days fall into this category.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
(a) Wind rose showing the distribution of 700-mb winds during the southerly regime in August at the ERA-Interim grid point over DPG. Note dramatic maximum from the southwest; 325 of 558 total August days fall into this category. (b) Wind rose showing the distribution of 700-mb winds during the northerly regime in August. Note dramatic minimum from the southwest; 151 of 558 total August days fall into this category.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
(a) Wind rose showing the distribution of 700-mb winds during the southerly regime in August at the ERA-Interim grid point over DPG. Note dramatic maximum from the southwest; 325 of 558 total August days fall into this category. (b) Wind rose showing the distribution of 700-mb winds during the northerly regime in August. Note dramatic minimum from the southwest; 151 of 558 total August days fall into this category.
Citation: Journal of Applied Meteorology and Climatology 55, 7; 10.1175/JAMC-D-15-0313.1
The mean synoptic pattern under southerly regime conditions bears a strong similarity to that identified by Adams and Comrie (1997) as the prototypical pattern of the North American monsoon (NAM) with relatively low 850-mb geopotential heights oriented southwest–northeast over Nevada and Utah. The 700-mb flow during the southerly regime is southerly or even southeasterly over northern Mexico, New Mexico, and Arizona and resembles the typical NAM pattern at this level.
With the trough axis oriented zonally, and relatively strong southwesterly 700-mb flow across Mexico, New Mexico, and Arizona, the northerly regime differs from the flow typical of the NAM. It does, however, resemble the prototypical precursor pattern of the West Coast thermal trough (WCTT) identified by Brewer et al. (2012). In this situation the primary IMW baroclinic zone has been pushed southeast over Utah and the GBCZ is southeast of the BB, and there is a positively tilted trough at 700 mb (not shown). Correspondingly, the southerly regime is similar to the post-WCTT period shown in Brewer et al. (2012) where both the IMW baroclinic zone and GBCZ are north of Utah and the 700-mb trough has moved west and acquired a more negative tilt.
We have identified several likely forcing mechanisms of the surface wind at DPG and discussed their potential contributions to the observed diurnal and seasonal cycles. While persistent (climatological) flow patterns such as the northerly and southerly regime can be readily identified when decadal datasets are available, quantifying the contribution of different forcing mechanisms to a given surface wind pattern is exceedingly difficult.
During MATERHORN one objective was to capture the flows at DPG under “synoptically quiescent” conditions when surface flows are purely thermally driven, while other observations targeted moderately to strongly synoptically forced conditions. The strength of the 700-mb flow was used to distinguish between these three forcing categories. However, periods with weak 700-mb flow aloft still displayed surface flows that could not be explained by thermally driven processes alone. The use of other proxies such as mountaintop wind speed thresholds or synoptic-scale pressure gradients from the ERA-Interim were similarly unsuccessful in limiting the observations to purely thermally driven conditions.
Instead of trying to remove certain scales of forcing with simple proxies and a priori assumptions, in this research we first searched for the most coherent surface flow regimes at DPG. By analyzing the composite synoptic-scale pattern and the composite surface wind evolution of these regimes we can illustrate the potential forcing mechanisms.
To illustrate the two different approaches, we can compare our findings with those of Rife et al. (2002). In their study, well-established synoptic-scale anticylonic conditions with clear skies were used as the criteria to define days with dominating thermally driven circulations. As a result, an evening northeasterly flow in the east basin of DPG was attributed to the propagation of the lake breeze from the Great Salt Lake into the east basin, across a distance of 80 km.
The time of the case study presented by Rife et al. (2002) is characteristic for northerly regime conditions (not shown). From our analysis of northerly regime conditions, we learned that northeasterly surface winds are extremely common across all of DPG during the evening hours. These flows frequently persist until 2200 MST or later, extend into areas more than 120 km from the lake, and are also observed at a mountaintop station 800 m above the valley floor (Cedar Mountain SAMS). Our analysis suggests that this flow may be related to the low-level easterly synoptic-scale pressure gradient that exists despite a westerly pressure gradient aloft. This pressure gradient reversal in the vertical is due to the relatively strong baroclinic zone over Utah under northerly regime conditions.
Based on our analysis of northerly regime conditions, we speculate that the thermal contrast between the Great Salt Lake and the elevated terrain to the south likely strengthens the zonal low-level pressure gradient that drives the northeasterly flow. During the day and late afternoon on northerly regime days, the terrain contrast results in a north–south temperature gradient that can magnify the synoptically imposed pressure gradient that encourages northerly or northeasterly flow. At night, the temperature contrast reverses, opposing the synoptically imposed gradient.
When using long-term datasets it is common to combine observations from several months to calculate mean persistent or climatological wind patterns. For example, data collected during June, July, and August are averaged to indicate mean summer conditions. The large, high-quality dataset used here to examine the surface wind forcing mechanisms allowed us to identify the climatologically persistent flow patterns on smaller than seasonal (i.e., monthly) time scales. We found a tripolar seasonal evolution of the surface wind fields at DPG, with large changes in the forcing mechanisms between June and August. Our work thus demonstrates that a traditional seasonal averaging can lead to results that may disguise the forcing mechanisms governing the wind field evolution.
Operational forecasters at DPG are frequently required to produce high-resolution forecasts in both time and space. In many cases this leaves the forecaster dependent on high-resolution mesoscale model forecasts (Liu et al. 2008). However, these forecasts are only available at short lead times because of high computational expense. Being able to identify the large-scale patterns unique to each regime would allow a forecaster to anticipate surface flows characteristic of the southerly regime or northerly regime and lengthen the lead time in anticipation of weather events ideal for DPG operations.
Observations collected during MATERHORN have been (Massey et al. 2014; Serafin et al. 2015; Dimitrova et al. 2016) and will continue to be used to test and improve NWP model parameterizations. We hope to have provided sufficient insight into the variety of forcing mechanisms of the surface wind field at DPG that modelers can better pinpoint model deficiencies and validate their results based on resolved flow patterns instead of being limited to a statistical evaluation. For example, if a particular model run underestimates the speed of the nocturnal southeasterly flow in the east basin, our analysis would tell the investigator that the error could lie in an underestimation of the mesoscale southerly flow, an underprediction of the strength of the thermally driven down-valley flows, or a combination of both.
Because of the multiscalar nature of the forcing mechanisms, deficiencies in numerical model forecasts of surface wind may arise from several sources. An accurate prediction of the placement and strength of boundaries such as the GBCZ is critical for avoiding large errors in surface wind predictions. Synoptic-scale features such as the GBCZ and lee troughing require an accurate representation of both the terrain and the large-scale incident flow to be forecast well. The ability of global or mesoscale models to accurately represent the GBCZ is not known, although anecdotal evidence from the MATERHORN field campaigns indicated that forecasts of the GBCZ suffered from phase and amplitude errors. Mesoscale models are likely sensitive to errors passed from the initial and boundary conditions in these cases (Etherton and Santos 2008). Increased horizontal and vertical resolution of global and mesoscale models should be beneficial for predicting such features (McGinley and Goerss 1986).
The correct forecast of the strength and temporal evolution of mesoscale thermal circulations such as the Nevada low is expected to strongly depend on an accurate representation of sensible heating and therefore of the surface radiation and energy budgets. Massey et al. (2016) showed how poor representation of soil moisture in the IMW in global models has led to errors in surface temperature forecasts. Addressing model deficiencies affecting surface temperature forecasts over large areas such as these are expected to improve the model representation of mesoscale thermal circulations.
On a smaller scale, accurate representations of land surface contrasts and of energy exchange processes at the land–atmosphere interface are necessary in NWP models to successfully model the forcing mechanisms of local thermally driven flows. Massey et al. (2014) demonstrated an improvement in surface temperature forecasts over certain soil types at DPG by modifications to the parameterization of soil thermal properties and of soil moisture initial conditions. Future validation and improvements to the land surface model components are expected to lead to a better representation of surface temperature contrasts and thus the forcing mechanisms of thermally driven flows.
Our analysis has primarily been focused on the area of DPG, but a preliminary look at observations throughout northern Utah revealed a bimodal surface wind pattern very similar to those of the southerly and northerly regime identified over DPG. Diagnosing the pertinent wind regimes and their forcing mechanisms could be especially useful in the Salt Lake City basin in context of the significant air-quality issues experienced by its 2 million residents (Whiteman et al. 2014).
Acknowledgments
This research was funded by Office of Naval Research Award N00014-11-1-0709 through the MATERHORN Program. The authors thank Dugway Proving Ground for providing the surface meteorological station data and Jim Steenburgh, Dave Whiteman, and John Horel for discussions and feedback on the paper. Comments and suggestions by three anonymous reviewers were crucial for improving the article.
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