Search Results
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
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 7–10 m s−1 and one with lighter winds of 4–7 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 7–10 m s−1 and one with lighter winds of 4–7 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
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
A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north–south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s−1 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.
Abstract
A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north–south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s−1 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.
