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Abstract
Fast-responding thermocouple psychrometers are often used in atmospheric boundary-layer turbulence measurements for the computation of heat and moisture fluxes. Small size, low cost, ease of interchange-ability and the use of the familiar psychrometric equations make this an ideal sensor for many applications at temperatures above freezing. However, a feature of these instruments that is frequently disregarded is that, due to wicking, the wet-bulb sensor has a frequency response that is an order of magnitude slower than the dry-bulb sensor. This difference in response time between the wet and dry sensors causes errors in the variances of humidity in one set of data as large as a factor of 5 and major errors in the shape of the humidity spectrum at high frequencies. We present a known but infrequently applied solution to this problem of sensor response differences in the hope that its simplicity, together with the reminder that a problem exists, will serve to encourage its use in the computation of humidity from fast-response psychrometric sensors.
Abstract
Fast-responding thermocouple psychrometers are often used in atmospheric boundary-layer turbulence measurements for the computation of heat and moisture fluxes. Small size, low cost, ease of interchange-ability and the use of the familiar psychrometric equations make this an ideal sensor for many applications at temperatures above freezing. However, a feature of these instruments that is frequently disregarded is that, due to wicking, the wet-bulb sensor has a frequency response that is an order of magnitude slower than the dry-bulb sensor. This difference in response time between the wet and dry sensors causes errors in the variances of humidity in one set of data as large as a factor of 5 and major errors in the shape of the humidity spectrum at high frequencies. We present a known but infrequently applied solution to this problem of sensor response differences in the hope that its simplicity, together with the reminder that a problem exists, will serve to encourage its use in the computation of humidity from fast-response psychrometric sensors.
Abstract
Velocities produced by energetic waves can contaminate direct covariance estimates of near-bottom turbulent shear stress and turbulent heat flux. A new adaptive filtering technique is introduced to minimize the contribution of wave-induced motions to measured covariances. The technique requires the use of two sensors separated in space and assumes that the spatial coherence scale of the waves is much longer than the spatial coherence scale of the turbulence. The proposed technique is applied to an extensive set of data collected in the bottom boundary layer of the New England shelf. Results from the oceanic test demonstrate that the technique succeeds at removing surface-wave contamination from shear stress and heat flux estimates using pairs of sensors separated in the vertical dimension by a distance of approximately 5 times the height of the lower sensor, even during the close passage of hurricanes. However, the technique fails at removing contamination caused by internal motions that occur occasionally in the dataset. The internal case is complicated by the facts that the motions are highly intermittent; the internal-wave period is comparable to the Reynolds-averaging period; the height of the internal-wave boundary layer is on the order of the height of measurement; and, specifically for heat flux estimates, nonlinear effects are large. The presence of internal motions does not pose a significant problem for estimating turbulent shear stress, because contamination caused by them is limited to frequencies lower than those of the stress-carrying eddies. In contrast, the presence of internal motions does pose a problem for estimating turbulent heat flux, because the contamination extends into the range of the heat flux–carrying eddies.
Abstract
Velocities produced by energetic waves can contaminate direct covariance estimates of near-bottom turbulent shear stress and turbulent heat flux. A new adaptive filtering technique is introduced to minimize the contribution of wave-induced motions to measured covariances. The technique requires the use of two sensors separated in space and assumes that the spatial coherence scale of the waves is much longer than the spatial coherence scale of the turbulence. The proposed technique is applied to an extensive set of data collected in the bottom boundary layer of the New England shelf. Results from the oceanic test demonstrate that the technique succeeds at removing surface-wave contamination from shear stress and heat flux estimates using pairs of sensors separated in the vertical dimension by a distance of approximately 5 times the height of the lower sensor, even during the close passage of hurricanes. However, the technique fails at removing contamination caused by internal motions that occur occasionally in the dataset. The internal case is complicated by the facts that the motions are highly intermittent; the internal-wave period is comparable to the Reynolds-averaging period; the height of the internal-wave boundary layer is on the order of the height of measurement; and, specifically for heat flux estimates, nonlinear effects are large. The presence of internal motions does not pose a significant problem for estimating turbulent shear stress, because contamination caused by them is limited to frequencies lower than those of the stress-carrying eddies. In contrast, the presence of internal motions does pose a problem for estimating turbulent heat flux, because the contamination extends into the range of the heat flux–carrying eddies.
Abstract
This paper describes results from a June 1992 field program to study the response of the boundary layer over a site with well-defined extreme differences in sensible and latent heat fluxes over clearly separated areas, each with characteristic length scales of 10 km or more. The experiment region consisted of semiarid grassland steppe and irrigated farmland. Sensible heat flux maxima over the steppe regularly reached values in excess of 300 W m−2 and were typically a factor of 4 or more greater than those over the farmland. Two days were selected for analysis: one with moderate winds of 710 m s−1 and one with lighter winds of 47 m s−1 over the steppe. In both cases the wind directions were nearly perpendicular to the boundary between the steppe and farm. An analysis of potential temperature soundings showed that mixed-layer characteristics over both the farm and the steppe were largely determined by heating over the steppe, with advection from the steppe to the farm playing a significant role. On the day with the lighter winds, a secondary circulation related to the thermal contrasts between the two areas was observed. A simple conceptual model is described that predicts the extent of the cooler area required to generate such circulations. The observations illustrate how predictions of boundary layer structure in terms of local surface sensible heat fluxes may be compromised by advective effects. Such difficulties complicate efforts to obtain accurate representations of surface fluxes over inhomogeneous surfaces even if parameterizations of mesoscale contributions to the heat flux are included.
Abstract
This paper describes results from a June 1992 field program to study the response of the boundary layer over a site with well-defined extreme differences in sensible and latent heat fluxes over clearly separated areas, each with characteristic length scales of 10 km or more. The experiment region consisted of semiarid grassland steppe and irrigated farmland. Sensible heat flux maxima over the steppe regularly reached values in excess of 300 W m−2 and were typically a factor of 4 or more greater than those over the farmland. Two days were selected for analysis: one with moderate winds of 710 m s−1 and one with lighter winds of 47 m s−1 over the steppe. In both cases the wind directions were nearly perpendicular to the boundary between the steppe and farm. An analysis of potential temperature soundings showed that mixed-layer characteristics over both the farm and the steppe were largely determined by heating over the steppe, with advection from the steppe to the farm playing a significant role. On the day with the lighter winds, a secondary circulation related to the thermal contrasts between the two areas was observed. A simple conceptual model is described that predicts the extent of the cooler area required to generate such circulations. The observations illustrate how predictions of boundary layer structure in terms of local surface sensible heat fluxes may be compromised by advective effects. Such difficulties complicate efforts to obtain accurate representations of surface fluxes over inhomogeneous surfaces even if parameterizations of mesoscale contributions to the heat flux are included.
Abstract
Cloud characteristics at two sites on the North Slope of Alaska separated by ∼100 km have been examined for the warmer months of 2001–03 using data collected from microwave radiometers, ceilometers, rotating shadowband radiometers, and pyranometers. Clouds at the inland site, Atqasuk, were found to have approximately 26% greater optical depths than those at the coastal site, Barrow, and the ratio of measured irradiance to clear-sky irradiance was nearly 20% larger at Barrow under cloudy conditions. It is hypothesized that a significant factor contributing to these differences is the upward fluxes of heat and water vapor over the wet tundra and lakes. Support for this hypothesis is found from the behavior of the liquid water paths for low clouds, which tend to be higher at Atqasuk than at Barrow for onshore winds but not for offshore ones, from differences in sensible heat fluxes, which are small but significant over the tundra but are nearly zero over the ocean adjacent to Barrow, and from the mixing ratios, which are significantly higher at Atqasuk than at Barrow. Results from a simple model further indicate that latent heat fluxes over the tundra and lakes can account for a significant fraction of the differences in the estimated boundary layer water content between Barrow and Atqasuk.
Abstract
Cloud characteristics at two sites on the North Slope of Alaska separated by ∼100 km have been examined for the warmer months of 2001–03 using data collected from microwave radiometers, ceilometers, rotating shadowband radiometers, and pyranometers. Clouds at the inland site, Atqasuk, were found to have approximately 26% greater optical depths than those at the coastal site, Barrow, and the ratio of measured irradiance to clear-sky irradiance was nearly 20% larger at Barrow under cloudy conditions. It is hypothesized that a significant factor contributing to these differences is the upward fluxes of heat and water vapor over the wet tundra and lakes. Support for this hypothesis is found from the behavior of the liquid water paths for low clouds, which tend to be higher at Atqasuk than at Barrow for onshore winds but not for offshore ones, from differences in sensible heat fluxes, which are small but significant over the tundra but are nearly zero over the ocean adjacent to Barrow, and from the mixing ratios, which are significantly higher at Atqasuk than at Barrow. Results from a simple model further indicate that latent heat fluxes over the tundra and lakes can account for a significant fraction of the differences in the estimated boundary layer water content between Barrow and Atqasuk.
Abstract
A case study of ocean radar backscatter dependence on near-surface wind and wind stress is presented using the data obtained on 18 February 1986 during the Frontal Air-Sea Interaction Experiment. Our interest in this case stems from the particular wind-wave conditions and their variations across a sharp sea surface temperature front. These are described. Most importantly, the small change in wind speed across the front cannot account for the large change in wind stress implying significant changes in the drag coefficient and surface roughness length. When compared with previous results, the corresponding changes in radar backscatter cross-section at 50° and 20° angles of incidence were consistent with the observed variations in wind stress, but inconsistent with both the mean wind and the equivalent neutral wind. Although not definitive, the results strengthen the hypothesis that radar backscatter is closely correlated to wind stress, and therefore, could be used for remote sensing of the wind stress itself over the global oceans.
Abstract
A case study of ocean radar backscatter dependence on near-surface wind and wind stress is presented using the data obtained on 18 February 1986 during the Frontal Air-Sea Interaction Experiment. Our interest in this case stems from the particular wind-wave conditions and their variations across a sharp sea surface temperature front. These are described. Most importantly, the small change in wind speed across the front cannot account for the large change in wind stress implying significant changes in the drag coefficient and surface roughness length. When compared with previous results, the corresponding changes in radar backscatter cross-section at 50° and 20° angles of incidence were consistent with the observed variations in wind stress, but inconsistent with both the mean wind and the equivalent neutral wind. Although not definitive, the results strengthen the hypothesis that radar backscatter is closely correlated to wind stress, and therefore, could be used for remote sensing of the wind stress itself over the global oceans.
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 6.25-km resolution dataset of meteorology, vegetation type, and soil type for a domain covering a typical global climate model grid cell is used to drive a land surface physics model for a period of 6 months. Additional simulations are performed driving the land surface physics model by spatially averaged meteorology, spatially averaged vegetation characteristics, spatially averaged soil properties, and spatially averaged meteorology, vegetation characteristics, and soil properties. By comparing the simulated water balance for the whole domain for each simulation, the relative influence of subgrid variability in meteorology, vegetation, and soil are assessed. Subgrid variability in summertime precipitation is found to have the largest effect on the surface hydrology, with a nearly twofold increase on surface runoff and a 15% increase in evapotranspiration. Subgrid variations in vegetation and soil properties also increase surface runoff and reduce evapotranspiration, so that surface runoff is 2.75 times as great with subgrid variability than without and evapotranspiration is 19% higher with subgrid variability than without.
Abstract
A 6.25-km resolution dataset of meteorology, vegetation type, and soil type for a domain covering a typical global climate model grid cell is used to drive a land surface physics model for a period of 6 months. Additional simulations are performed driving the land surface physics model by spatially averaged meteorology, spatially averaged vegetation characteristics, spatially averaged soil properties, and spatially averaged meteorology, vegetation characteristics, and soil properties. By comparing the simulated water balance for the whole domain for each simulation, the relative influence of subgrid variability in meteorology, vegetation, and soil are assessed. Subgrid variability in summertime precipitation is found to have the largest effect on the surface hydrology, with a nearly twofold increase on surface runoff and a 15% increase in evapotranspiration. Subgrid variations in vegetation and soil properties also increase surface runoff and reduce evapotranspiration, so that surface runoff is 2.75 times as great with subgrid variability than without and evapotranspiration is 19% higher with subgrid variability than without.
Abstract
Results from a field campaign to study the response of the planetary boundary layer to spatially varying surface conditions are presented. Radiosondes released at four locations with contrasting land use characteristics in the U.S. Department of Energy’s Cloud and Radiation Testbed (CART) in Kansas and Oklahoma showed significant variations in mixed-layer depth, temperature, and water vapor mixing ratios over distances of 100–200 km. Using CART and radiosonde data, estimates of the surface sensible and latent heat fluxes are derived; the results from several methods are compared and a discussion of the similarities and differences in the values is given. Although substantial flux differences among the sites account for some of the variations in the boundary layer behavior, other features of the ambient meteorology and initial conditions appear to be equally important. Despite large changes in mixed-layer and surface-layer temperatures over scales of approximately 100 km, no evidence for temperature-induced secondary circulations was found. A simple scaling argument is presented that gives a possible reason for this absence.
Abstract
Results from a field campaign to study the response of the planetary boundary layer to spatially varying surface conditions are presented. Radiosondes released at four locations with contrasting land use characteristics in the U.S. Department of Energy’s Cloud and Radiation Testbed (CART) in Kansas and Oklahoma showed significant variations in mixed-layer depth, temperature, and water vapor mixing ratios over distances of 100–200 km. Using CART and radiosonde data, estimates of the surface sensible and latent heat fluxes are derived; the results from several methods are compared and a discussion of the similarities and differences in the values is given. Although substantial flux differences among the sites account for some of the variations in the boundary layer behavior, other features of the ambient meteorology and initial conditions appear to be equally important. Despite large changes in mixed-layer and surface-layer temperatures over scales of approximately 100 km, no evidence for temperature-induced secondary circulations was found. A simple scaling argument is presented that gives a possible reason for this absence.
Abstract
Persistent midwinter cold air pools produce multiday periods of cold, dreary weather in basins and valleys. Persistent stable stratification leads to the buildup of pollutants and moisture in the pool. Because the pool sometimes has temperatures below freezing while the air above is warmer, freezing precipitation often occurs, with consequent effects on transportation and safety. Forecasting the buildup and breakdown of these cold pools is difficult because the interacting physical mechanisms leading to their formation, maintenance, and destruction have received little study.
In this paper, persistent wintertime cold pools in the Columbia River basin of eastern Washington are studied. First a succinct meteorological definition of a cold pool is provided and then a 10-yr database is used to develop a cold pool climatology. This is followed by a detailed examination of two cold pool episodes that were accompanied by fog and stratus using remote and in situ temperature and wind sounding data. The two episodes illustrate many of the physical mechanisms that affect cold pool evolution. In one case, the cold pool was formed by warm air advection above the basin and was destroyed by downslope winds that descended into the southern edge of the basin and progressively displaced the cold air in the basin. In the second case, the cold pool began with a basin temperature inversion on a clear night and strengthened when warm air was advected above the basin by a westerly flow that descended from the Cascade Mountains. The cold pool was nearly destroyed one afternoon by cold air advection aloft and by the growth of a convective boundary layer (CBL) following the partial breakup of the basin stratus. The cold pool restrengthened, however, with nighttime cooling and was destroyed the next afternoon by a growing CBL.
Abstract
Persistent midwinter cold air pools produce multiday periods of cold, dreary weather in basins and valleys. Persistent stable stratification leads to the buildup of pollutants and moisture in the pool. Because the pool sometimes has temperatures below freezing while the air above is warmer, freezing precipitation often occurs, with consequent effects on transportation and safety. Forecasting the buildup and breakdown of these cold pools is difficult because the interacting physical mechanisms leading to their formation, maintenance, and destruction have received little study.
In this paper, persistent wintertime cold pools in the Columbia River basin of eastern Washington are studied. First a succinct meteorological definition of a cold pool is provided and then a 10-yr database is used to develop a cold pool climatology. This is followed by a detailed examination of two cold pool episodes that were accompanied by fog and stratus using remote and in situ temperature and wind sounding data. The two episodes illustrate many of the physical mechanisms that affect cold pool evolution. In one case, the cold pool was formed by warm air advection above the basin and was destroyed by downslope winds that descended into the southern edge of the basin and progressively displaced the cold air in the basin. In the second case, the cold pool began with a basin temperature inversion on a clear night and strengthened when warm air was advected above the basin by a westerly flow that descended from the Cascade Mountains. The cold pool was nearly destroyed one afternoon by cold air advection aloft and by the growth of a convective boundary layer (CBL) following the partial breakup of the basin stratus. The cold pool restrengthened, however, with nighttime cooling and was destroyed the next afternoon by a growing CBL.
Abstract
Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.
Abstract
Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.