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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
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
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
Meteorological mechanisms affecting the evolution of a persistent wintertime cold air pool that began on 2 January and ended on 7 January 1999 in the Columbia basin of eastern Washington were investigated using a mesoscale numerical model together with limited observations. The mechanisms include surface radiative cooling and heating, large-scale subsidence, temperature advection, downslope warming in the lee of a major mountain barrier, and low-level cloudiness.
The cold pool began when cold air accumulated over the basin floor on a clear night and was maintained by a strong capping inversion resulting from a rapid increase of air temperatures above the cold pool. This increase of temperatures aloft was produced primarily by downslope warming associated with strong westerly winds descending the lee slopes of the north–south-oriented Cascade Mountains that form the western boundary of the Columbia basin. While the inversion cap at the top of the cold pool descended with time as the westerly flow intensified, the air temperature inside the cold pool exhibited little variation because of the fog and stratus accompanying the cold pool. Although the low-level clouds reduced the diurnal temperature oscillations inside the pool, their existence was not critical to maintaining the cold pool because surface radiative heating on a midwinter day was insufficient to completely destroy the temperature deficit in the persistent inversion. The presence of low-level clouds becomes much more critical for the maintenance of persistent cold pools in the spring and, perhaps, the fall seasons when insolation is much stronger than in midwinter. The cold pool was destroyed by cold air advection aloft, which weakened and eventually removed the strong inversion cap, and by an unstable boundary layer that grew upward from the heated ground after the dissipation of low-level clouds. Finally, erosion of the cold pool from above by turbulent mixing produced by vertical wind shear at the interface between quiescent air within the pool and stronger winds aloft was found to be insignificant for this case.
Abstract
Meteorological mechanisms affecting the evolution of a persistent wintertime cold air pool that began on 2 January and ended on 7 January 1999 in the Columbia basin of eastern Washington were investigated using a mesoscale numerical model together with limited observations. The mechanisms include surface radiative cooling and heating, large-scale subsidence, temperature advection, downslope warming in the lee of a major mountain barrier, and low-level cloudiness.
The cold pool began when cold air accumulated over the basin floor on a clear night and was maintained by a strong capping inversion resulting from a rapid increase of air temperatures above the cold pool. This increase of temperatures aloft was produced primarily by downslope warming associated with strong westerly winds descending the lee slopes of the north–south-oriented Cascade Mountains that form the western boundary of the Columbia basin. While the inversion cap at the top of the cold pool descended with time as the westerly flow intensified, the air temperature inside the cold pool exhibited little variation because of the fog and stratus accompanying the cold pool. Although the low-level clouds reduced the diurnal temperature oscillations inside the pool, their existence was not critical to maintaining the cold pool because surface radiative heating on a midwinter day was insufficient to completely destroy the temperature deficit in the persistent inversion. The presence of low-level clouds becomes much more critical for the maintenance of persistent cold pools in the spring and, perhaps, the fall seasons when insolation is much stronger than in midwinter. The cold pool was destroyed by cold air advection aloft, which weakened and eventually removed the strong inversion cap, and by an unstable boundary layer that grew upward from the heated ground after the dissipation of low-level clouds. Finally, erosion of the cold pool from above by turbulent mixing produced by vertical wind shear at the interface between quiescent air within the pool and stronger winds aloft was found to be insignificant for this case.
The Department of Energy Atmospheric Radiation Measurement (ARM) program is a climate research user facility operating stationary ground sites that provide long-term measurements of climate-relevant properties, mobile ground- and ship-based facilities to conduct shorter field campaigns (6–12 months), and the ARM Aerial Facility (AAF). The airborne observations acquired by the AAF enhance the surface-based ARM measurements by providing high-resolution in situ measurements for process understanding, retrieval-algorithm development, and model evaluation that are not possible using surface-or satellite-based techniques.
Several ARM aerial efforts were consolidated to form AAF in 2006. With the exception of a small aircraft used for routine measurements of aerosols and carbon cycle gases, AAF at the time had no dedicated aircraft and only a small number of instruments at its disposal. AAF successfully carried out several missions contracting with organizations and investigators who provided their research aircraft and instrumentation. In 2009, AAF started managing operations of the Battelle-owned Gulfstream I (G-1) large twin-turboprop research aircraft. Furthermore, the American Recovery and Reinvestment Act of 2009 provided funding for the procurement of over twenty new instruments to be used aboard the G-1 and AAF contracted aircraft. Depending on the requested scope, AAF now executes campaigns using the G-1 or contracted aircraft, producing freely available datasets for studying gas, aerosol, cloud, and radiative processes and their interactions in the atmosphere. AAF is also engaged in the maturation and testing of newly developed airborne sensors to help foster the next generation of airborne instruments.
The Department of Energy Atmospheric Radiation Measurement (ARM) program is a climate research user facility operating stationary ground sites that provide long-term measurements of climate-relevant properties, mobile ground- and ship-based facilities to conduct shorter field campaigns (6–12 months), and the ARM Aerial Facility (AAF). The airborne observations acquired by the AAF enhance the surface-based ARM measurements by providing high-resolution in situ measurements for process understanding, retrieval-algorithm development, and model evaluation that are not possible using surface-or satellite-based techniques.
Several ARM aerial efforts were consolidated to form AAF in 2006. With the exception of a small aircraft used for routine measurements of aerosols and carbon cycle gases, AAF at the time had no dedicated aircraft and only a small number of instruments at its disposal. AAF successfully carried out several missions contracting with organizations and investigators who provided their research aircraft and instrumentation. In 2009, AAF started managing operations of the Battelle-owned Gulfstream I (G-1) large twin-turboprop research aircraft. Furthermore, the American Recovery and Reinvestment Act of 2009 provided funding for the procurement of over twenty new instruments to be used aboard the G-1 and AAF contracted aircraft. Depending on the requested scope, AAF now executes campaigns using the G-1 or contracted aircraft, producing freely available datasets for studying gas, aerosol, cloud, and radiative processes and their interactions in the atmosphere. AAF is also engaged in the maturation and testing of newly developed airborne sensors to help foster the next generation of airborne instruments.
Abstract
The Observations and Modeling of the Green Ocean Amazon 2014–2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed the emissions of biogenic volatile organic carbon compounds (BVOCs) from the tropical forest, their transformations by the atmospheric oxidant cycle, alterations of this cycle by the influence of the pollutants, transformations of the chemical products into aerosol particles, the relationship of these particles to cloud condensation nuclei (CCN) activity, and the differences in cloud properties and rainfall for background compared to polluted conditions. The observations of the GoAmazon2014/5 experiment illustrate how the hydrologic cycle, radiation balance, and carbon recycling may be affected by present-day as well as future economic development and pollution over the Amazonian tropical forest.
Abstract
The Observations and Modeling of the Green Ocean Amazon 2014–2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed the emissions of biogenic volatile organic carbon compounds (BVOCs) from the tropical forest, their transformations by the atmospheric oxidant cycle, alterations of this cycle by the influence of the pollutants, transformations of the chemical products into aerosol particles, the relationship of these particles to cloud condensation nuclei (CCN) activity, and the differences in cloud properties and rainfall for background compared to polluted conditions. The observations of the GoAmazon2014/5 experiment illustrate how the hydrologic cycle, radiation balance, and carbon recycling may be affected by present-day as well as future economic development and pollution over the Amazonian tropical forest.
A boundary layer field experiment in the Mexico City basin during the period 24 February–22 March 1997 is described. A total of six sites were instrumented. At four of the sites, 915-MHz radar wind profilers were deployed and radiosondes were released five times per day. Two of these sites also had sodars collocated with the profilers. Radiosondes were released twice per day at a fifth site to the south of the basin, and rawinsondes were flown from another location to the northeast of the city three times per day. Mixed layers grew to depths of 2500–3500 m, with a rapid period of growth beginning shortly before noon and lasting for several hours. Significant differences between the mixed-layer temperatures in the basin and outside the basin were observed. Three thermally and topographically driven flow patterns were observed that are consistent with previously hypothesized topographical and thermal forcing mechanisms. Despite these features, the circulation patterns in the basin important for the transport and diffusion of air pollutants show less day-to-day regularity than had been anticipated on the basis of Mexico City's tropical location, high altitude and strong insolation, and topographical setting.
A boundary layer field experiment in the Mexico City basin during the period 24 February–22 March 1997 is described. A total of six sites were instrumented. At four of the sites, 915-MHz radar wind profilers were deployed and radiosondes were released five times per day. Two of these sites also had sodars collocated with the profilers. Radiosondes were released twice per day at a fifth site to the south of the basin, and rawinsondes were flown from another location to the northeast of the city three times per day. Mixed layers grew to depths of 2500–3500 m, with a rapid period of growth beginning shortly before noon and lasting for several hours. Significant differences between the mixed-layer temperatures in the basin and outside the basin were observed. Three thermally and topographically driven flow patterns were observed that are consistent with previously hypothesized topographical and thermal forcing mechanisms. Despite these features, the circulation patterns in the basin important for the transport and diffusion of air pollutants show less day-to-day regularity than had been anticipated on the basis of Mexico City's tropical location, high altitude and strong insolation, and topographical setting.
A first-of-a-kind, extended-term cloud aircraft campaign was conducted to obtain an in situ statistical characterization of continental boundary layer clouds needed to investigate cloud processes and refine retrieval algorithms. Coordinated by the Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF), the Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign operated over the ARM Southern Great Plains (SGP) site from 22 January to 30 June 2009, collecting 260 h of data during 59 research flights. A comprehensive payload aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft measured cloud microphysics, solar and thermal radiation, physical aerosol properties, and atmospheric state parameters. Proximity to the SGP's extensive complement of surface measurements provides ancillary data that support modeling studies and facilitates evaluation of a variety of surface retrieval algorithms. The five-month duration enabled sampling a range of conditions associated with the seasonal transition from winter to summer. Although about twothirds of the flights during which clouds were sampled occurred in May and June, boundary layer cloud fields were sampled under a variety of environmental and aerosol conditions, with about 77% of the cloud flights occurring in cumulus and stratocumulus. Preliminary analyses illustrate use of these data to analyze aerosol– cloud relationships, characterize the horizontal variability of cloud radiative impacts, and evaluate surface-based retrievals. We discuss how an extended-term campaign requires a simplified operating paradigm that is different from that used for typical, short-term, intensive aircraft field programs.
A first-of-a-kind, extended-term cloud aircraft campaign was conducted to obtain an in situ statistical characterization of continental boundary layer clouds needed to investigate cloud processes and refine retrieval algorithms. Coordinated by the Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF), the Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign operated over the ARM Southern Great Plains (SGP) site from 22 January to 30 June 2009, collecting 260 h of data during 59 research flights. A comprehensive payload aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft measured cloud microphysics, solar and thermal radiation, physical aerosol properties, and atmospheric state parameters. Proximity to the SGP's extensive complement of surface measurements provides ancillary data that support modeling studies and facilitates evaluation of a variety of surface retrieval algorithms. The five-month duration enabled sampling a range of conditions associated with the seasonal transition from winter to summer. Although about twothirds of the flights during which clouds were sampled occurred in May and June, boundary layer cloud fields were sampled under a variety of environmental and aerosol conditions, with about 77% of the cloud flights occurring in cumulus and stratocumulus. Preliminary analyses illustrate use of these data to analyze aerosol– cloud relationships, characterize the horizontal variability of cloud radiative impacts, and evaluate surface-based retrievals. We discuss how an extended-term campaign requires a simplified operating paradigm that is different from that used for typical, short-term, intensive aircraft field programs.
A field campaign was carried out near Boardman, Oregon, to study the effects of subgrid-scale variability of sensible- and latent-heat fluxes on surface boundary-layer properties. The experiment involved three U.S. Department of Energy laboratories, one National Oceanic and Atmospheric Administration laboratory, and several universities. The experiment was conducted in a region of severe contrasts in adjacent surface types that accentuated the response of the atmosphere to variable surface forcing. Large values of sensible-heat flux and low values of latent-heat flux characterized a sagebrush steppe area; significantly smaller sensible-heat fluxes and much larger latent-heat fluxes were associated with extensive tracts of irrigated farmland to the north, east, and west of the steppe. Data were obtained from an array of surface flux stations, remote-sensing devices, an instrumented aircraft, and soil and vegetation measurements. The data will be used to address the problem of extrapolating from a limited number of local measurements to area-averaged values of fluxes suitable for use in global climate models.
A field campaign was carried out near Boardman, Oregon, to study the effects of subgrid-scale variability of sensible- and latent-heat fluxes on surface boundary-layer properties. The experiment involved three U.S. Department of Energy laboratories, one National Oceanic and Atmospheric Administration laboratory, and several universities. The experiment was conducted in a region of severe contrasts in adjacent surface types that accentuated the response of the atmosphere to variable surface forcing. Large values of sensible-heat flux and low values of latent-heat flux characterized a sagebrush steppe area; significantly smaller sensible-heat fluxes and much larger latent-heat fluxes were associated with extensive tracts of irrigated farmland to the north, east, and west of the steppe. Data were obtained from an array of surface flux stations, remote-sensing devices, an instrumented aircraft, and soil and vegetation measurements. The data will be used to address the problem of extrapolating from a limited number of local measurements to area-averaged values of fluxes suitable for use in global climate models.
