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Observations of the water-vapor mixing ratio in the lower atmosphere and its temporal evolution have been made with a Raman lidar. Comparison with an independent radiosonde measurement indicated excellent agreement. The moisture structure, observed up to an altitude of 5 km and over an 80-min period during the early morning of 30 April 1985 (the present lidar is limited to night operation), showed temporal variations of several atmospheric features which could not be resolved by balloon soundings. Application of the lidar should provide the opportunity to study details of atmospheric moisture, its structure, and its evolution in a manner never before realized.
Observations of the water-vapor mixing ratio in the lower atmosphere and its temporal evolution have been made with a Raman lidar. Comparison with an independent radiosonde measurement indicated excellent agreement. The moisture structure, observed up to an altitude of 5 km and over an 80-min period during the early morning of 30 April 1985 (the present lidar is limited to night operation), showed temporal variations of several atmospheric features which could not be resolved by balloon soundings. Application of the lidar should provide the opportunity to study details of atmospheric moisture, its structure, and its evolution in a manner never before realized.
Abstract
This paper presents the results of a field program using a ground-based Raman lidar system to observe changes in moisture profiles as a cold and a warm front passed over the NASA/Goddard Space Flight Center in Greenbelt, Maryland. The lidar operating only during darkness is capable of providing continuous high vertical resolution profiles of water vapor mixing ratio and aerosol scattering ratio from near the surface to about 7 km altitude. The lidar data acquired on three consecutive nights from shortly after sunset to shortly before sunrise, along with upper air data from specially launched rawinsondes, have provided a unique visualization of the detailed structure of the two fronts.
Abstract
This paper presents the results of a field program using a ground-based Raman lidar system to observe changes in moisture profiles as a cold and a warm front passed over the NASA/Goddard Space Flight Center in Greenbelt, Maryland. The lidar operating only during darkness is capable of providing continuous high vertical resolution profiles of water vapor mixing ratio and aerosol scattering ratio from near the surface to about 7 km altitude. The lidar data acquired on three consecutive nights from shortly after sunset to shortly before sunrise, along with upper air data from specially launched rawinsondes, have provided a unique visualization of the detailed structure of the two fronts.
Abstract
The Water Vapor Variability-Satellite/Sondes (WAVES) 2006 field campaign provided a contiguous 5-day period of concentrated high-resolution measurements to examine finescale boundary layer phenomena under the influence of a summertime subtropical high over the mid-Atlantic region that is characterized by complex geography. A holistic analytical approach to low-level wind observations was adopted to identify the low-level flow structures and patterns of evolution on the basis of airmass properties and origination. An analysis of the measurements and the other available observations is consistent with the classic depiction of the daytime boundary layer development but revealed a pronounced diurnal cycle that was categorized into three stages: (i) daytime growth of the convective boundary layer, (ii) flow intensification into a low-level jet regime after dusk, and (iii) interruption by a downslope wind regime after midnight. The use of the field campaign data allows for the differentiation of the latter two flow regimes by their directions with respect to the orientation of the Appalachian Mountains and their airmass origins. Previous studies that have investigated mountain flows and low-level jet circulations have focused on regions with overt geographic prominence, stark gradients, or frequent reoccurrences, whereby such meteorological phenomena exhibit a clear signature and can be easily isolated and diagnosed. The results of this study provide evidence that similar circulation patterns operate in nonclassic locations with milder topography and atmospheric gradients, such as the mid-Atlantic region. The new results have important implications for the understanding of the mountain-forced flows and some air quality problems during the nocturnal period.
Abstract
The Water Vapor Variability-Satellite/Sondes (WAVES) 2006 field campaign provided a contiguous 5-day period of concentrated high-resolution measurements to examine finescale boundary layer phenomena under the influence of a summertime subtropical high over the mid-Atlantic region that is characterized by complex geography. A holistic analytical approach to low-level wind observations was adopted to identify the low-level flow structures and patterns of evolution on the basis of airmass properties and origination. An analysis of the measurements and the other available observations is consistent with the classic depiction of the daytime boundary layer development but revealed a pronounced diurnal cycle that was categorized into three stages: (i) daytime growth of the convective boundary layer, (ii) flow intensification into a low-level jet regime after dusk, and (iii) interruption by a downslope wind regime after midnight. The use of the field campaign data allows for the differentiation of the latter two flow regimes by their directions with respect to the orientation of the Appalachian Mountains and their airmass origins. Previous studies that have investigated mountain flows and low-level jet circulations have focused on regions with overt geographic prominence, stark gradients, or frequent reoccurrences, whereby such meteorological phenomena exhibit a clear signature and can be easily isolated and diagnosed. The results of this study provide evidence that similar circulation patterns operate in nonclassic locations with milder topography and atmospheric gradients, such as the mid-Atlantic region. The new results have important implications for the understanding of the mountain-forced flows and some air quality problems during the nocturnal period.
Abstract
Recent advances in electronics miniaturization have allowed the commercial development of sensor/datalogger combinations that are sufficiently inexpensive and appear to be sufficiently accurate to deploy in measurement arrays to resolve local atmospheric structure over periods of weeks to months. As part of an extended wintertime field experiment in the Columbia Basin of south-central Washington, laboratory and field tests were performed on one such set of battery-powered temperature dataloggers (HOBO H8 Pro from Onset Computer, Bourne, Massachusetts). Five loggers were selected for laboratory calibration. These were accurate to within 0.26°C over the range from −5° to +50°C with a resolution of 0.04°C or better. Sensor time constants were 122 ± 6 s. Sampling intervals can be varied over a wide range, with onboard data storage of more than 21 000 data points. Field experiences with a set of 15 dataloggers are also described. The loggers appear to be suitable for a variety of meteorological applications.
Abstract
Recent advances in electronics miniaturization have allowed the commercial development of sensor/datalogger combinations that are sufficiently inexpensive and appear to be sufficiently accurate to deploy in measurement arrays to resolve local atmospheric structure over periods of weeks to months. As part of an extended wintertime field experiment in the Columbia Basin of south-central Washington, laboratory and field tests were performed on one such set of battery-powered temperature dataloggers (HOBO H8 Pro from Onset Computer, Bourne, Massachusetts). Five loggers were selected for laboratory calibration. These were accurate to within 0.26°C over the range from −5° to +50°C with a resolution of 0.04°C or better. Sensor time constants were 122 ± 6 s. Sampling intervals can be varied over a wide range, with onboard data storage of more than 21 000 data points. Field experiences with a set of 15 dataloggers are also described. The loggers appear to be suitable for a variety of meteorological applications.
Abstract
A mesoscale model is used to simulate the nocturnal evolution of the wind and temperature fields within a small, elliptical basin located in western Colorado that has a drainage area of about 84 km2. The numerical results are compared to observed profiles of wind and potential temperature. The thermal forcing of the basin wind system and the sources of air that support the local circulations are determined. Individual terms of the basin atmospheric heat budget are also calculated from the model results.
The model is able to reproduce key features of the observed potential temperature profiles over the basin floor and winds exiting the basin through the narrow canyon that drains the basin. Complex circulations are produced within the basin atmosphere as a result of the convergence of drainage flows from the basin sidewalls. The strength of the sidewall drainage flow varies around the basin and is a function of the source area above the basin, the local topography, and the ambient winds. Flows on the basin floor are affected primarly by the drainage winds from the northern part of the basin. The near-surface sidewall drainage flows converge within the southern portion of the basin, producing a counterclockwise eddy during most of the evening. Evaluation of the individual terms of the atmospheric heat budget show that the forcing due to advection and turbulent diffusion is significantly larger above the sidewalls than over the basin floor; therefore, measurements made over the basin floor would not be representative of the basin as a whole. The cooling in the center of the basin results from the local radiative flux divergence and the advection of cold air from the sidewalls, and the cooling above the basin sidewalls is due primarily to turbulent sensible heat flux divergence. A high rate of atmospheric cooling occurs within the basin throughout the evening, although the strongest cooling occurs in the early evening hours. Sensitivity tests show that the thermal structure, circulations, and rate of cooling can be significantly affected by ambient wind direction and, to a lesser extent, vegetation coverage.
Abstract
A mesoscale model is used to simulate the nocturnal evolution of the wind and temperature fields within a small, elliptical basin located in western Colorado that has a drainage area of about 84 km2. The numerical results are compared to observed profiles of wind and potential temperature. The thermal forcing of the basin wind system and the sources of air that support the local circulations are determined. Individual terms of the basin atmospheric heat budget are also calculated from the model results.
The model is able to reproduce key features of the observed potential temperature profiles over the basin floor and winds exiting the basin through the narrow canyon that drains the basin. Complex circulations are produced within the basin atmosphere as a result of the convergence of drainage flows from the basin sidewalls. The strength of the sidewall drainage flow varies around the basin and is a function of the source area above the basin, the local topography, and the ambient winds. Flows on the basin floor are affected primarly by the drainage winds from the northern part of the basin. The near-surface sidewall drainage flows converge within the southern portion of the basin, producing a counterclockwise eddy during most of the evening. Evaluation of the individual terms of the atmospheric heat budget show that the forcing due to advection and turbulent diffusion is significantly larger above the sidewalls than over the basin floor; therefore, measurements made over the basin floor would not be representative of the basin as a whole. The cooling in the center of the basin results from the local radiative flux divergence and the advection of cold air from the sidewalls, and the cooling above the basin sidewalls is due primarily to turbulent sensible heat flux divergence. A high rate of atmospheric cooling occurs within the basin throughout the evening, although the strongest cooling occurs in the early evening hours. Sensitivity tests show that the thermal structure, circulations, and rate of cooling can be significantly affected by ambient wind direction and, to a lesser extent, vegetation coverage.
Abstract
The evolution of potential temperature and wind structure during the buildup of nocturnal cold-air pools was investigated during clear, dry, September nights in Utah's Peter Sinks basin, a 1-km-diameter limestone sinkhole that holds the Utah minimum temperature record of −56°C. The evolution of cold-pool characteristics depended on the strength of prevailing flows above the basin. On an undisturbed day, a 30°C diurnal temperature range and a strong nocturnal potential temperature inversion (22 K in 100 m) were observed in the basin. Initially, downslope flows formed on the basin sidewalls. As a very strong potential temperature jump (17 K) developed at the top of the cold pool, however, the winds died within the basin and over the sidewalls. A persistent turbulent sublayer formed below the jump. Turbulent sensible heat flux on the basin floor became negligible shortly after sunset while the basin atmosphere continued to cool. Temperatures over the slopes, except for a 1–2-m-deep layer, became warmer than over the basin center at the same altitude. Cooling rates for the entire basin near sunset were comparable to the 90 W m−2 rate of loss of net longwave radiation at the basin floor, but these rates decreased to only a few watts per square meter by sunrise. This paper compares the observed cold-pool buildup in basins with inversion buildup in valleys.
Abstract
The evolution of potential temperature and wind structure during the buildup of nocturnal cold-air pools was investigated during clear, dry, September nights in Utah's Peter Sinks basin, a 1-km-diameter limestone sinkhole that holds the Utah minimum temperature record of −56°C. The evolution of cold-pool characteristics depended on the strength of prevailing flows above the basin. On an undisturbed day, a 30°C diurnal temperature range and a strong nocturnal potential temperature inversion (22 K in 100 m) were observed in the basin. Initially, downslope flows formed on the basin sidewalls. As a very strong potential temperature jump (17 K) developed at the top of the cold pool, however, the winds died within the basin and over the sidewalls. A persistent turbulent sublayer formed below the jump. Turbulent sensible heat flux on the basin floor became negligible shortly after sunset while the basin atmosphere continued to cool. Temperatures over the slopes, except for a 1–2-m-deep layer, became warmer than over the basin center at the same altitude. Cooling rates for the entire basin near sunset were comparable to the 90 W m−2 rate of loss of net longwave radiation at the basin floor, but these rates decreased to only a few watts per square meter by sunrise. This paper compares the observed cold-pool buildup in basins with inversion buildup in valleys.
Abstract
This paper examines the calibration characteristics of the NASA/GSFC Raman water vapor lidar during three field experiments that occurred between 1991 and 1993. The lidar water vapor profiles are calibrated using relative humidity profiles measured by AIR and Vaisala radiosondes. The lidar calibration computed using the AIR radiosonde, which uses a carbon hygristor to measure relative humidity, was 3%–5% higher than that computed using the Vaisala radiosonde, which uses a thin film capacitive element. These systematic differences were obtained for relative humidities above 30% and so cannot be explained by the known poor low relative humidity measurements associated with the carbon hygristor. The lidar calibration coefficient was found to vary by less than 1% over this period when determined using the Vaisala humidity data and by less than 5% when using the AIR humidity data. The differences between the lidar relative humidity profiles and those measured by these radiosondes are also examined. These lidar–radiosonde comparisons are used in combination with a numerical model of the lidar system to assess the altitude range of the GSFC lidar. The model results as well as the radiosonde comparisons indicate that for a lidar located at sea level measuring a typical midlatitude water vapor profile, the absolute error in relative humidity for a 10-min, 75-m resolution profile is less than 10% for altitudes below 8.5 km. Model results show that this maximum altitude can be extended to 10 km by increasing the averaging time and/or reducing the range resolution.
Abstract
This paper examines the calibration characteristics of the NASA/GSFC Raman water vapor lidar during three field experiments that occurred between 1991 and 1993. The lidar water vapor profiles are calibrated using relative humidity profiles measured by AIR and Vaisala radiosondes. The lidar calibration computed using the AIR radiosonde, which uses a carbon hygristor to measure relative humidity, was 3%–5% higher than that computed using the Vaisala radiosonde, which uses a thin film capacitive element. These systematic differences were obtained for relative humidities above 30% and so cannot be explained by the known poor low relative humidity measurements associated with the carbon hygristor. The lidar calibration coefficient was found to vary by less than 1% over this period when determined using the Vaisala humidity data and by less than 5% when using the AIR humidity data. The differences between the lidar relative humidity profiles and those measured by these radiosondes are also examined. These lidar–radiosonde comparisons are used in combination with a numerical model of the lidar system to assess the altitude range of the GSFC lidar. The model results as well as the radiosonde comparisons indicate that for a lidar located at sea level measuring a typical midlatitude water vapor profile, the absolute error in relative humidity for a 10-min, 75-m resolution profile is less than 10% for altitudes below 8.5 km. Model results show that this maximum altitude can be extended to 10 km by increasing the averaging time and/or reducing the range resolution.
Abstract
Detailed moisture observations from a ground-based Raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented. A synthesis of the lidar data with conventional meteorological data, in conjunction with theoretical calculations and comparison to laboratory studies, leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore. The disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site. Bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current. Analysis suggests that the bore was generated by the collapse of the gravity current into a stable, nocturnal inversion layer, and subsequently propagated along this wave guide at nearly twice the speed of the gravity current.
The Raman lidar provided detailed measurements of the vertical structure of the bore and its parent generation mechanism. A mean bore depth of 1.9 km is revealed by the lidar, whereas a depth of 2.2 km is predicted from hydraulic theory. Observed and calculated bore speeds were also found to agree reasonably well with one another (∼ ±20%). Comparison of these observations with those of internal bores generated by thunderstorms in other studies reveals that this bore was exceedingly strong, being responsible for nearly tripling the height of a surface-based inversion that had existed ahead of the bore and dramatically increasing the depth of the moist layer due to strong vertical mixing. Subsequent appearance of the relatively shallow gravity current underneath this mixed region resulted in the occurrence of an elevated mixed layer, as confirmed with the special radiosonde measurements.
A synthesis of the lidar and radiosonde observations indicates that bore-induced parcel displacements attenuated rapidly at the same height as the level of strongest wave trapping predicted from the theory of Crook. This trapping mechanism, which is due to the existence of a low-level jet, results in a long-lived bore, and seems to he a common phenomenon in the environment of thunderstorm-generated bores and solitary waves. Despite the weakening of a capping inversion by this strong and persistent bore, analysis indicates that the 30-min averaged lifting of 0.7 m s−1 was confined to a too shallow layer near the surface to trigger deep convection, and could only produce scattered low clouds as deduced from the lidar measurements.
Abstract
Detailed moisture observations from a ground-based Raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented. A synthesis of the lidar data with conventional meteorological data, in conjunction with theoretical calculations and comparison to laboratory studies, leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore. The disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site. Bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current. Analysis suggests that the bore was generated by the collapse of the gravity current into a stable, nocturnal inversion layer, and subsequently propagated along this wave guide at nearly twice the speed of the gravity current.
The Raman lidar provided detailed measurements of the vertical structure of the bore and its parent generation mechanism. A mean bore depth of 1.9 km is revealed by the lidar, whereas a depth of 2.2 km is predicted from hydraulic theory. Observed and calculated bore speeds were also found to agree reasonably well with one another (∼ ±20%). Comparison of these observations with those of internal bores generated by thunderstorms in other studies reveals that this bore was exceedingly strong, being responsible for nearly tripling the height of a surface-based inversion that had existed ahead of the bore and dramatically increasing the depth of the moist layer due to strong vertical mixing. Subsequent appearance of the relatively shallow gravity current underneath this mixed region resulted in the occurrence of an elevated mixed layer, as confirmed with the special radiosonde measurements.
A synthesis of the lidar and radiosonde observations indicates that bore-induced parcel displacements attenuated rapidly at the same height as the level of strongest wave trapping predicted from the theory of Crook. This trapping mechanism, which is due to the existence of a low-level jet, results in a long-lived bore, and seems to he a common phenomenon in the environment of thunderstorm-generated bores and solitary waves. Despite the weakening of a capping inversion by this strong and persistent bore, analysis indicates that the 30-min averaged lifting of 0.7 m s−1 was confined to a too shallow layer near the surface to trigger deep convection, and could only produce scattered low clouds as deduced from the lidar measurements.
Abstract
The feasibility of using a multiwavelength Mie–Raman lidar based on a tripled Nd:YAG laser for profiling aerosol physical parameters in the planetary boundary layer (PBL) under varying conditions of relative humidity (RH) is studied. The lidar quantifies three aerosol backscattering and two extinction coefficients and from these optical data the particle parameters such as concentration, size, and complex refractive index are retrieved through inversion with regularization. The column-integrated, lidar-derived parameters are compared with results from the AERONET sun photometer. The lidar and sun photometer agree well in the characterization of the fine-mode parameters, however the lidar shows less sensitivity to coarse mode. The lidar results reveal a strong dependence of particle properties on RH. The height regions with enhanced RH are characterized by an increase of backscattering and extinction coefficient and a decrease in the Ångström exponent coinciding with an increase in the particle size. The hygroscopic growth factor calculated for a select case is consistent with previous literature results despite the lack of collocated radiosonde data. These results demonstrate the potential of the multiwavelength Raman lidar technique for the study of aerosol humidification process.
Abstract
The feasibility of using a multiwavelength Mie–Raman lidar based on a tripled Nd:YAG laser for profiling aerosol physical parameters in the planetary boundary layer (PBL) under varying conditions of relative humidity (RH) is studied. The lidar quantifies three aerosol backscattering and two extinction coefficients and from these optical data the particle parameters such as concentration, size, and complex refractive index are retrieved through inversion with regularization. The column-integrated, lidar-derived parameters are compared with results from the AERONET sun photometer. The lidar and sun photometer agree well in the characterization of the fine-mode parameters, however the lidar shows less sensitivity to coarse mode. The lidar results reveal a strong dependence of particle properties on RH. The height regions with enhanced RH are characterized by an increase of backscattering and extinction coefficient and a decrease in the Ångström exponent coinciding with an increase in the particle size. The hygroscopic growth factor calculated for a select case is consistent with previous literature results despite the lack of collocated radiosonde data. These results demonstrate the potential of the multiwavelength Raman lidar technique for the study of aerosol humidification process.
