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Abstract
The breakup of temperature inversions in the deep mountain valleys of western Colorado has been studied by means of tethered balloon observations of wind and temperature structure on clear weather days in different seasons. Vertical potential temperature structure profiles evolve following one of three patterns. Two of the patterns are special cases of the third pattern, in which inversions are destroyed by two continuous processes-upward growth of a convective boundary layer (CBL) into the base of the valley inversion, and descent of the inversion top. The three idealized patterns are described and 21 case studies of inversion breakup following the patterns are summarized. Inversion breakup begins at sunrise and is generally completed in 3½–5 h, unless the valley is snow covered or the ground is wet. Warming of the inversion layer is consistent with subsidence heating. An hypothesis is offered to explain the observations, stressing the role of the sensible heat flux in causing the CBL to grow and an upslope flow to develop over the sidewalls. As mass is removed from the base of the inversion layer in the upslope flows, the inversion sinks and warms.
Abstract
The breakup of temperature inversions in the deep mountain valleys of western Colorado has been studied by means of tethered balloon observations of wind and temperature structure on clear weather days in different seasons. Vertical potential temperature structure profiles evolve following one of three patterns. Two of the patterns are special cases of the third pattern, in which inversions are destroyed by two continuous processes-upward growth of a convective boundary layer (CBL) into the base of the valley inversion, and descent of the inversion top. The three idealized patterns are described and 21 case studies of inversion breakup following the patterns are summarized. Inversion breakup begins at sunrise and is generally completed in 3½–5 h, unless the valley is snow covered or the ground is wet. Warming of the inversion layer is consistent with subsidence heating. An hypothesis is offered to explain the observations, stressing the role of the sensible heat flux in causing the CBL to grow and an upslope flow to develop over the sidewalls. As mass is removed from the base of the inversion layer in the upslope flows, the inversion sinks and warms.
Abstract
Three sulfur hexafluoride atmospheric tracer experiments were conducted during the post-sunrise temperature inversion breakup period in the deep, narrow Brush Creek Valley of Colorado. Experiments were conducted under clear, undisturbed weather conditions.
A continuous elevated tracer plume was produced along the axis of the valley before sunrise and the behavior of the plume during the inversion breakup period was detected down-valley from the release point using an array of radio-controlled sequential bag samplers, a vertical SF6 profiling system carried on a tethered balloon, two portable gas chromatographs operated on a sidewall of the valley, and a continuous real-time SF6 monitor operated from a research aircraft. Supporting meteorological data came primarily from tethered balloon profilers. The nocturnal elevated plume was carried and diffused in down-valley flows. After sunrise, convective boundary layers grew upward from the sunlit valley surfaces, fumigating the elevated plume onto the valley floor and sidewalls. Upslope flow developed in the growing convective boundary layers, carrying fumigated SF6 up the sidewalls and causing a compensating subsidence over the valley center. High post-sunrise SF6 concentrations were experienced on the northeast-facing sidewall of the northwest–southeast oriented valley as a result of cross-valley flow, which developed due to differential solar heating of the sidewalls. Reversal of the down-valley wind system brought air with lower SF6 concentrations into the lower valley.
Abstract
Three sulfur hexafluoride atmospheric tracer experiments were conducted during the post-sunrise temperature inversion breakup period in the deep, narrow Brush Creek Valley of Colorado. Experiments were conducted under clear, undisturbed weather conditions.
A continuous elevated tracer plume was produced along the axis of the valley before sunrise and the behavior of the plume during the inversion breakup period was detected down-valley from the release point using an array of radio-controlled sequential bag samplers, a vertical SF6 profiling system carried on a tethered balloon, two portable gas chromatographs operated on a sidewall of the valley, and a continuous real-time SF6 monitor operated from a research aircraft. Supporting meteorological data came primarily from tethered balloon profilers. The nocturnal elevated plume was carried and diffused in down-valley flows. After sunrise, convective boundary layers grew upward from the sunlit valley surfaces, fumigating the elevated plume onto the valley floor and sidewalls. Upslope flow developed in the growing convective boundary layers, carrying fumigated SF6 up the sidewalls and causing a compensating subsidence over the valley center. High post-sunrise SF6 concentrations were experienced on the northeast-facing sidewall of the northwest–southeast oriented valley as a result of cross-valley flow, which developed due to differential solar heating of the sidewalls. Reversal of the down-valley wind system brought air with lower SF6 concentrations into the lower valley.
Abstract
A two-dimensional dynamical model was used to simulate the daytime boundary-layer evolution and resulting plume dispersion in a cross-valley section of a northwest–southeast oriented narrow valley in the first 4 h after sunrise. Two cases were simulated, one using a summertime heating distribution and a second with a wintertime heating distribution. In each case, additional conservation equations were added to simulate the dispersion of two plumes released 150 m and 650 m above the valley floor. In the summer case, the lower plume migrated to the more strongly heated southwest sidewall in the first 90 min after sunrise, and was then advected up the sidewall in the slope flow for the remainder of the simulation. This result is consistent with observations. The upper plume diffused slowly in the remnants of the nocturnal inversion layer until it was entrained by the growing convective boundary layer 3 h after sunrise. The boundary layer's thermodynamic structure remained nearly symmetric about the valley axis throughout the transition period. The asymmetric dispersion characteristics seen in the summer case were not found in the winter simulation. The seasonal change in solar illumination reduced the differences in surface heat flux between the two sidewalls that gave rise to the asymmetry observed in the summer case.
Abstract
A two-dimensional dynamical model was used to simulate the daytime boundary-layer evolution and resulting plume dispersion in a cross-valley section of a northwest–southeast oriented narrow valley in the first 4 h after sunrise. Two cases were simulated, one using a summertime heating distribution and a second with a wintertime heating distribution. In each case, additional conservation equations were added to simulate the dispersion of two plumes released 150 m and 650 m above the valley floor. In the summer case, the lower plume migrated to the more strongly heated southwest sidewall in the first 90 min after sunrise, and was then advected up the sidewall in the slope flow for the remainder of the simulation. This result is consistent with observations. The upper plume diffused slowly in the remnants of the nocturnal inversion layer until it was entrained by the growing convective boundary layer 3 h after sunrise. The boundary layer's thermodynamic structure remained nearly symmetric about the valley axis throughout the transition period. The asymmetric dispersion characteristics seen in the summer case were not found in the winter simulation. The seasonal change in solar illumination reduced the differences in surface heat flux between the two sidewalls that gave rise to the asymmetry observed in the summer case.
Abstract
A thermodynamic model is developed to simulate the evolution of vertical temperature structure during the breakup of nocturnal temperature inversions in mountain valleys. The primary inputs to the model are the valley floor width, sidewall inclination angles, characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent height of a convective boundary layer that grows upward from the valley floor after sunrise, the height of the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model can simulate the three patterns of temperature structure evolution observed in deep valleys of western Colorado. The well-known inversion breakup over flat terrain is a special case of the model, for which valley floor width becomes infinite. The characteristics of the model equations are investigated for several limiting conditions using the topography of a reference valley and typical inversion and solar radiation characteristics. The model is applied to simulate observations of inversion breakup taken in Colorado's Eagle and Yampa Valleys in different seasons. Simulations are obtained by fitting two constants in the model, relating to the surface energy budget and energy partitioning, to the data. The model accurately simulates the evolution of vertical potential temperature profiles and predicts the time of inversion destruction.
Abstract
A thermodynamic model is developed to simulate the evolution of vertical temperature structure during the breakup of nocturnal temperature inversions in mountain valleys. The primary inputs to the model are the valley floor width, sidewall inclination angles, characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent height of a convective boundary layer that grows upward from the valley floor after sunrise, the height of the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model can simulate the three patterns of temperature structure evolution observed in deep valleys of western Colorado. The well-known inversion breakup over flat terrain is a special case of the model, for which valley floor width becomes infinite. The characteristics of the model equations are investigated for several limiting conditions using the topography of a reference valley and typical inversion and solar radiation characteristics. The model is applied to simulate observations of inversion breakup taken in Colorado's Eagle and Yampa Valleys in different seasons. Simulations are obtained by fitting two constants in the model, relating to the surface energy budget and energy partitioning, to the data. The model accurately simulates the evolution of vertical potential temperature profiles and predicts the time of inversion destruction.
Abstract
Hourly tethered-balloon wind soundings from the 650-m deep, narrow, Brush Creek Valley of Colorado are analyzed to determine the nocturnal atmospheric mass (or volume) budget of the valley. Under the assumption that the volume flux on an entire valley cross section can be approximated from balloon soundings over the valley center, volume fluxes are calculated from tethered balloon profiles taken on 30–31 July 1982 at several points along the valley's longitudinal axis in a 7-km long segment of the valley.
Down-valley volume fluxes increased in the 3 h following sunset to levels that were basically maintained through the night. Down-valley volume fluxes increased with distance down the valley axis from 0.9 million m3 s−1 at the upper end of the segment to 2.8 million m3 s−1 at the lower end, producing an average volume flux divergence of 271 m2 s−1. If we assume that the volume flux divergence is supported entirely by subsidence of air into the valley, a peak sinking rate of 0.10 m s−1 is obtained at the level of the valley's rim. Mean vertical velocity profiles through the valley's depth are calculated, and an error analysis is performed.
Abstract
Hourly tethered-balloon wind soundings from the 650-m deep, narrow, Brush Creek Valley of Colorado are analyzed to determine the nocturnal atmospheric mass (or volume) budget of the valley. Under the assumption that the volume flux on an entire valley cross section can be approximated from balloon soundings over the valley center, volume fluxes are calculated from tethered balloon profiles taken on 30–31 July 1982 at several points along the valley's longitudinal axis in a 7-km long segment of the valley.
Down-valley volume fluxes increased in the 3 h following sunset to levels that were basically maintained through the night. Down-valley volume fluxes increased with distance down the valley axis from 0.9 million m3 s−1 at the upper end of the segment to 2.8 million m3 s−1 at the lower end, producing an average volume flux divergence of 271 m2 s−1. If we assume that the volume flux divergence is supported entirely by subsidence of air into the valley, a peak sinking rate of 0.10 m s−1 is obtained at the level of the valley's rim. Mean vertical velocity profiles through the valley's depth are calculated, and an error analysis is performed.
Abstract
The nocturnal potential temperature inversion in Switzerland's Dischma Valley on 11 August 1980 was destroyed during a 4½-h period following sunrise. The temperature inversion breakup was accomplished primarily by descent of the inversion top rather than upward growth of a convective boundary layer from the valley floor. The thermodynamic model of Whiteman and McKee, as extended with Steinacker's concept of valley area-height relationships, simulated inversion breakup well when sensible heat flux was assumed to be about 6% of the extraterrestrial solar flux. Observations in the valley support this value of sensible heat flux, which is lower than values observed in the drier Colorado valleys where the model was initially tested.
Abstract
The nocturnal potential temperature inversion in Switzerland's Dischma Valley on 11 August 1980 was destroyed during a 4½-h period following sunrise. The temperature inversion breakup was accomplished primarily by descent of the inversion top rather than upward growth of a convective boundary layer from the valley floor. The thermodynamic model of Whiteman and McKee, as extended with Steinacker's concept of valley area-height relationships, simulated inversion breakup well when sensible heat flux was assumed to be about 6% of the extraterrestrial solar flux. Observations in the valley support this value of sensible heat flux, which is lower than values observed in the drier Colorado valleys where the model was initially tested.
Abstract
The Weather Research and Forecasting model is used to perform large-eddy simulations of thermally driven cross-basin winds in idealized, closed basins. A spatially and temporally varying heat flux is prescribed at the surface as a function of slope inclination and orientation to produce a horizontal temperature gradient across the basin. The thermal asymmetry leads to the formation of a closed circulation cell flowing toward the more strongly heated sidewall, with a return flow in the upper part of the basin. In the presence of background winds above the basin, a second circulation cell forms in the upper part of the basin, resulting in one basin-sized cell, two counterrotating cells, or two cells with perpendicular rotation axes, depending on the background-wind direction with respect to the temperature gradient. The thermal cell near the basin floor and the background-wind-induced cell interact with each other either to enhance or to reduce the thermal cross-basin flow and return flow. It is shown that in 5–10-km-wide basins cross-basin temperature differences that are representative of east- and west-facing slopes are insufficient to maintain perceptible cross-basin winds because of reduced horizontal temperature and pressure gradients, particularly in a neutrally stratified atmosphere.
Abstract
The Weather Research and Forecasting model is used to perform large-eddy simulations of thermally driven cross-basin winds in idealized, closed basins. A spatially and temporally varying heat flux is prescribed at the surface as a function of slope inclination and orientation to produce a horizontal temperature gradient across the basin. The thermal asymmetry leads to the formation of a closed circulation cell flowing toward the more strongly heated sidewall, with a return flow in the upper part of the basin. In the presence of background winds above the basin, a second circulation cell forms in the upper part of the basin, resulting in one basin-sized cell, two counterrotating cells, or two cells with perpendicular rotation axes, depending on the background-wind direction with respect to the temperature gradient. The thermal cell near the basin floor and the background-wind-induced cell interact with each other either to enhance or to reduce the thermal cross-basin flow and return flow. It is shown that in 5–10-km-wide basins cross-basin temperature differences that are representative of east- and west-facing slopes are insufficient to maintain perceptible cross-basin winds because of reduced horizontal temperature and pressure gradients, particularly in a neutrally stratified atmosphere.
Abstract
As part of the winter 2010/11 Persistent Cold-Air Pool Study in Utah’s Salt Lake Valley, a laser ceilometer was used to continuously measure aerosol-layer characteristics in support of an investigation of the meteorological processes producing the cold-air pools. A surface-based aerosol layer was present during much of the winter. Comparisons were made between ceilometer-measured and visual characteristics of the aerosol layers. A 3–4 January 2011 case study illustrated the meteorological value of time–height backscatter cross sections when used as a base map for meteorological analyses. A variety of meteorological mixing processes were illustrated using ceilometer backscatter data. The mean altitude of the top of the aerosol layer during undisturbed subperiods of the 1 December–7 February experimental period was 1811 m MSL, with a standard deviation of 185 m. The mean aerosol depth was ~500 m AGL in the 1200-m-deep valley. There was surprisingly little variation in the wintertime aerosol layer depth despite large variations in bulk atmospheric stability and ground-based fine particulate matter concentrations.
Abstract
As part of the winter 2010/11 Persistent Cold-Air Pool Study in Utah’s Salt Lake Valley, a laser ceilometer was used to continuously measure aerosol-layer characteristics in support of an investigation of the meteorological processes producing the cold-air pools. A surface-based aerosol layer was present during much of the winter. Comparisons were made between ceilometer-measured and visual characteristics of the aerosol layers. A 3–4 January 2011 case study illustrated the meteorological value of time–height backscatter cross sections when used as a base map for meteorological analyses. A variety of meteorological mixing processes were illustrated using ceilometer backscatter data. The mean altitude of the top of the aerosol layer during undisturbed subperiods of the 1 December–7 February experimental period was 1811 m MSL, with a standard deviation of 185 m. The mean aerosol depth was ~500 m AGL in the 1200-m-deep valley. There was surprisingly little variation in the wintertime aerosol layer depth despite large variations in bulk atmospheric stability and ground-based fine particulate matter concentrations.
Abstract
Pseudovertical temperature “soundings” from lines of inexpensive temperature sensors on the sidewalls of Utah’s Salt Lake valley are compared with contemporaneous radiosonde soundings from the north, open end of the valley. Morning [0415 mountain standard time (MST)] soundings are colder, and afternoon (1615 MST) soundings are warmer than radiosonde soundings because of warm and cold boundary layers that form over the slopes. Cross-valley temperature differences occur between east- and west-facing sidewalls because of differing insolation. Differences in vertically averaged pseudovertical and radiosonde temperatures are generally within 1°C, with a standard deviation of 2°–3°C. The pseudovertical soundings are especially good proxies for radiosondes in winter. The sounding comparisons identified along-valley differences in temperature, inversion depth, and lapse rate that have led to hypotheses concerning their causes, to be evaluated with future research. The low cost and much better time resolution of the pseudovertical soundings suggest that such lines will be a useful supplement to valley radiosondes and will have significant operational advantages if available in real time. Lines of surface-based sensors will prove useful in identifying intravalley meteorological differences and may be used to estimate free-air temperature structure in other valleys where radiosondes are unavailable.
Abstract
Pseudovertical temperature “soundings” from lines of inexpensive temperature sensors on the sidewalls of Utah’s Salt Lake valley are compared with contemporaneous radiosonde soundings from the north, open end of the valley. Morning [0415 mountain standard time (MST)] soundings are colder, and afternoon (1615 MST) soundings are warmer than radiosonde soundings because of warm and cold boundary layers that form over the slopes. Cross-valley temperature differences occur between east- and west-facing sidewalls because of differing insolation. Differences in vertically averaged pseudovertical and radiosonde temperatures are generally within 1°C, with a standard deviation of 2°–3°C. The pseudovertical soundings are especially good proxies for radiosondes in winter. The sounding comparisons identified along-valley differences in temperature, inversion depth, and lapse rate that have led to hypotheses concerning their causes, to be evaluated with future research. The low cost and much better time resolution of the pseudovertical soundings suggest that such lines will be a useful supplement to valley radiosondes and will have significant operational advantages if available in real time. Lines of surface-based sensors will prove useful in identifying intravalley meteorological differences and may be used to estimate free-air temperature structure in other valleys where radiosondes are unavailable.
