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Abstract
A near-real-time integrated temperature and water vapor sounding system has been designed and in operation since June 1993. It combines hourly data from the ground-based radio acoustic sounding system (RASS), a two-channel microwave radiometer, standard surface meteorological instruments, a lidar ceilometer, and the Aerodynamic Research Incorporated Communication, Addressing and Reporting System aboard commercial airlines with space-based data from the TIROS-N Operational Vertical Sounder (TOVS). The physical retrieval algorithm provided by the International TOVS Processing Package is used for combining the ground- and space-based temperature and humidity profiles. The first-guess profiles of temperature and humidity required by the physical retrieval algorithm arc obtained by using a statistical inversion technique and the ground-based remote sensors measurements.
Statistical error estimates are presented for the hourly. near-real-time, ground-, and space-based retrieved temperature and humidity profiles based on 119 soundings collected during a two-month-long experiment conducted at Platteville, Colorado, during February and March 1994. Radiosonde data collected by the Environmental Technology Laboratory and the Winter Icing and Storms Program in Platteville and the National Weather Service in Denver, Colorado, are used for comparison. The comparison showed excellent agreement between retrieved and radiosonde soundings. Retrieved temperature profiles show better performance than the retrieved humidity profiles because of the high vertical resolution of the RASS measurements. It is suggested that adding more information from the new individual remote sensors as they develop, through the technique used here, would lead to further profiling improvements.
Abstract
A near-real-time integrated temperature and water vapor sounding system has been designed and in operation since June 1993. It combines hourly data from the ground-based radio acoustic sounding system (RASS), a two-channel microwave radiometer, standard surface meteorological instruments, a lidar ceilometer, and the Aerodynamic Research Incorporated Communication, Addressing and Reporting System aboard commercial airlines with space-based data from the TIROS-N Operational Vertical Sounder (TOVS). The physical retrieval algorithm provided by the International TOVS Processing Package is used for combining the ground- and space-based temperature and humidity profiles. The first-guess profiles of temperature and humidity required by the physical retrieval algorithm arc obtained by using a statistical inversion technique and the ground-based remote sensors measurements.
Statistical error estimates are presented for the hourly. near-real-time, ground-, and space-based retrieved temperature and humidity profiles based on 119 soundings collected during a two-month-long experiment conducted at Platteville, Colorado, during February and March 1994. Radiosonde data collected by the Environmental Technology Laboratory and the Winter Icing and Storms Program in Platteville and the National Weather Service in Denver, Colorado, are used for comparison. The comparison showed excellent agreement between retrieved and radiosonde soundings. Retrieved temperature profiles show better performance than the retrieved humidity profiles because of the high vertical resolution of the RASS measurements. It is suggested that adding more information from the new individual remote sensors as they develop, through the technique used here, would lead to further profiling improvements.
Abstract
We calculated integral scales for horizontal and vertical velocity components, temperature, humidity and ozone concentration, as well as for their variances and covariances from aircraft measurements in the convective atmospheric boundary layer over both ocean and land surfaces. We found that the integral scales of the second-order moment quantities are 0.67± 0.09 that of the variables themselves. Consequently, only the second-order moment integral scales are presented here. These results are used to calculate the averaging lengths necessary to measure second-order moment quantities to a given accuracy. We found that a measurement length of 10 to 100 times the boundary-layer height is required to measure variances to 10% accuracy, while scalar fluxes require a measurement length of 102 to 104 and stress a measurement length of 103 to 105 times the boundary layer height. We also show that the ratio of the wavelength of the spectral peak to the integral scale can be used to estimate the sharpness of the spectral peak.
Abstract
We calculated integral scales for horizontal and vertical velocity components, temperature, humidity and ozone concentration, as well as for their variances and covariances from aircraft measurements in the convective atmospheric boundary layer over both ocean and land surfaces. We found that the integral scales of the second-order moment quantities are 0.67± 0.09 that of the variables themselves. Consequently, only the second-order moment integral scales are presented here. These results are used to calculate the averaging lengths necessary to measure second-order moment quantities to a given accuracy. We found that a measurement length of 10 to 100 times the boundary-layer height is required to measure variances to 10% accuracy, while scalar fluxes require a measurement length of 102 to 104 and stress a measurement length of 103 to 105 times the boundary layer height. We also show that the ratio of the wavelength of the spectral peak to the integral scale can be used to estimate the sharpness of the spectral peak.
Abstract
A new method for deriving profiles of tropospheric water vapor and liquid water from a combination of ground-based remote sensors was applied and tested under winter conditions in Colorado. The method is an extension of physical retrieval techniques used to derive coarse profiles from passive microwave radiometer measurements. Unlike an earlier method, it does not depend on climatological data for first-guess profile inputs. Instead, information about current cloud conditions aloft, obtained with active remote sensors, is used to determine physically realistic, first-guess vertical distributions of the radiometer's integrated vapor and liquid measurements. In preliminary tests, the retrieved profiles were compared with in situ measurements by aircraft and radiosondes during the Winter Icing and Storms Project. The shape of the retrieved liquid profiles agreed well with the aircraft measurements, but heights, thicknesses, and amplitudes differed considerably in some cases. The derived vapor profiles agreed better with radiosonde measurements than the traditional climatological retrievals, but standard deviations of the dewpoint differences wore still quite large (5°C). In an integrated, unattended instrument design, the new method has the potential to provide continuous real-lime profiles of temperature, wind, humidity, liquid water, and pressure.
Abstract
A new method for deriving profiles of tropospheric water vapor and liquid water from a combination of ground-based remote sensors was applied and tested under winter conditions in Colorado. The method is an extension of physical retrieval techniques used to derive coarse profiles from passive microwave radiometer measurements. Unlike an earlier method, it does not depend on climatological data for first-guess profile inputs. Instead, information about current cloud conditions aloft, obtained with active remote sensors, is used to determine physically realistic, first-guess vertical distributions of the radiometer's integrated vapor and liquid measurements. In preliminary tests, the retrieved profiles were compared with in situ measurements by aircraft and radiosondes during the Winter Icing and Storms Project. The shape of the retrieved liquid profiles agreed well with the aircraft measurements, but heights, thicknesses, and amplitudes differed considerably in some cases. The derived vapor profiles agreed better with radiosonde measurements than the traditional climatological retrievals, but standard deviations of the dewpoint differences wore still quite large (5°C). In an integrated, unattended instrument design, the new method has the potential to provide continuous real-lime profiles of temperature, wind, humidity, liquid water, and pressure.
Abstract
Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s−1 respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites.
A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.
The observed transition in temperature typically precedes the velocity transition by 20–40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.
Abstract
Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s−1 respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites.
A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.
The observed transition in temperature typically precedes the velocity transition by 20–40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.
Abstract
This study of a 5-yr continuous record of midtropospheric horizontal wind components from a radar wind profiler operating at Fleming, Colorado, shows a broad spectral peak centered around a period of 1 week and a minimum at about 4 months, in addition to the expected 1-yr peak. However, when the records are separated according to seasons, the pattern becomes more complicated, with several distinct peaks and clear differences between the summer and winter behavior emerging. In this paper the different spectral patterns observed are presented and the synoptic-scale features in the weather that could produce them are speculated on.
Abstract
This study of a 5-yr continuous record of midtropospheric horizontal wind components from a radar wind profiler operating at Fleming, Colorado, shows a broad spectral peak centered around a period of 1 week and a minimum at about 4 months, in addition to the expected 1-yr peak. However, when the records are separated according to seasons, the pattern becomes more complicated, with several distinct peaks and clear differences between the summer and winter behavior emerging. In this paper the different spectral patterns observed are presented and the synoptic-scale features in the weather that could produce them are speculated on.
Abstract
Methods by which attitude ranges of supercooled cloud liquid water in the atmosphere may be estimated are explored using measurements from a combination of ground-based remote sensors. The tests were conducted as part of the Winter Icing and Storms Project that took place in eastern Colorado during the winters of 1990, 1991, and 1993. The basic method augments microwave radiometer measurements of path-integrated liquid water with observations from additional remote sensors to establish height limits for the supercooled liquid. One variation uses a simple adiabatic parcel lifting model initiated at a cloud-base height determined from a ecilometer, temperature and pressure from a radio acoustic sounding system or rawinsonde, and combines these with the radiometers total liquid measurement to obtain an estimate of the liquid cloud-top height. Since it does not account for liquid loss by entrainment or ice-liquid interaction processes this method tends to underestimate the true liquid cloud top; for two cases examined in detail, 54% of icing pilot reports in the area were from above this estimated height. Some error is introduced due to differences in sampling locations and from horizontal variability in liquid water content. Vertical cloud boundaries from a Ka-band radar were also used in the study; these often indicated thicker clouds than the liquid-layer depths observed from research aircraft, possibly due to the ambiguity of the ice-liquid phase distinction.
Comparisons of liquid vertical profiles are presented, using normalized profile shapes based an uniform, adiabatic, and aircraft-derived composite assumptions. The adiabatic and climatological profile shapes generally agreed well with measurements from a research aircraft and were more realistic than the uniform profile. Suggestions for applications of these results toward a red-time aviation hazard identification system are presented.
Abstract
Methods by which attitude ranges of supercooled cloud liquid water in the atmosphere may be estimated are explored using measurements from a combination of ground-based remote sensors. The tests were conducted as part of the Winter Icing and Storms Project that took place in eastern Colorado during the winters of 1990, 1991, and 1993. The basic method augments microwave radiometer measurements of path-integrated liquid water with observations from additional remote sensors to establish height limits for the supercooled liquid. One variation uses a simple adiabatic parcel lifting model initiated at a cloud-base height determined from a ecilometer, temperature and pressure from a radio acoustic sounding system or rawinsonde, and combines these with the radiometers total liquid measurement to obtain an estimate of the liquid cloud-top height. Since it does not account for liquid loss by entrainment or ice-liquid interaction processes this method tends to underestimate the true liquid cloud top; for two cases examined in detail, 54% of icing pilot reports in the area were from above this estimated height. Some error is introduced due to differences in sampling locations and from horizontal variability in liquid water content. Vertical cloud boundaries from a Ka-band radar were also used in the study; these often indicated thicker clouds than the liquid-layer depths observed from research aircraft, possibly due to the ambiguity of the ice-liquid phase distinction.
Comparisons of liquid vertical profiles are presented, using normalized profile shapes based an uniform, adiabatic, and aircraft-derived composite assumptions. The adiabatic and climatological profile shapes generally agreed well with measurements from a research aircraft and were more realistic than the uniform profile. Suggestions for applications of these results toward a red-time aviation hazard identification system are presented.
Abstract
A radio acoustic sounding system (RASS), coupled with the NOAA/Wave Propagation Laboratory 915-MHz wind profiler, observed an arctic front and arctic air mass that passed over Denver, Colorado, between 1 and 5 February 1989. The RASS temperature measurements extended to approximately 1.5 km above ground level and were taken at 15-min intervals during the frontal passage and at 1-h intervals thereafter. During the frontal passage on 1 February, the RASS documented a temperature decrease of >15°C. The succeeding cold air (∼−20° to −40°C) over Denver never exceeded 1.3 km in depth. The frontal inversion at the top of the cold air mass was 300 m in depth and possessed large static stability [−∂θ/∂p ∼ 80 K (100 mb)−1] and vertical wind shear [∂V/∂p ∼ 30 m s−1 (100 mb)−1]. Temporal fluctuations (∼3 h) in the depth of the cold air were observed by the RASS between the operational 12-h rawinsonde observing periods. Simultaneous RASS and rawinsonde measurements showed good agreement with regard to key thermal features.
Abstract
A radio acoustic sounding system (RASS), coupled with the NOAA/Wave Propagation Laboratory 915-MHz wind profiler, observed an arctic front and arctic air mass that passed over Denver, Colorado, between 1 and 5 February 1989. The RASS temperature measurements extended to approximately 1.5 km above ground level and were taken at 15-min intervals during the frontal passage and at 1-h intervals thereafter. During the frontal passage on 1 February, the RASS documented a temperature decrease of >15°C. The succeeding cold air (∼−20° to −40°C) over Denver never exceeded 1.3 km in depth. The frontal inversion at the top of the cold air mass was 300 m in depth and possessed large static stability [−∂θ/∂p ∼ 80 K (100 mb)−1] and vertical wind shear [∂V/∂p ∼ 30 m s−1 (100 mb)−1]. Temporal fluctuations (∼3 h) in the depth of the cold air were observed by the RASS between the operational 12-h rawinsonde observing periods. Simultaneous RASS and rawinsonde measurements showed good agreement with regard to key thermal features.
Abstract
Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which is about twice that estimated in a previous experiment. The wavelength is up to 5.5 times the mixed-layer height above the top of the mixed layer. Within the mixed layer, terms in the turbulence kinetic energy equation are similar to previous studies. Above the mixed layer, shear production becomes large, and is approximately balanced by the sum of the buoyancy, dissipation and transport terms. The temperature variance and flux budgets also have large terms and significant residuals in the overlying inversion.
Abstract
Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which is about twice that estimated in a previous experiment. The wavelength is up to 5.5 times the mixed-layer height above the top of the mixed layer. Within the mixed layer, terms in the turbulence kinetic energy equation are similar to previous studies. Above the mixed layer, shear production becomes large, and is approximately balanced by the sum of the buoyancy, dissipation and transport terms. The temperature variance and flux budgets also have large terms and significant residuals in the overlying inversion.
Abstract
On 2 July 1987 a nonmesocyclone tornado was observed in northeastern Colorado during the Convection Initiation and Downburst Experiment (CINDE). This tornado, reaching FI–F2 intensity, developed under a rapidly growing convective cell, without a preceding supercell or midlevel mesocyclone being present.
The pretornado environment on 2 July is described, including observations from a triangle of wind profilers, a dense surface mesonet array, and a special balloon sounding network. Important features contributing to tornado generation include the passage of a 700-mb short-wave trough; the formation of an ∼70-km diameter, terrain-induced mesoscale vortex (the Denver Cyclone) and its associated baroclinic zone; the presence of a stationary low-level convergence boundary; and the presence of low-level azimuthal sheer maxima (misovortices) along the boundary.
Vorticity budget terms are calculated in the lowest 2 km AGL using a multiple-Doppler radar analysis. These terms and their spatial distributions are compared with observations of mesocyclone-associated supercell tornadoes. Results show that vorticity associated with the 2 July nonsupercell tornado was generated in a more complicated manner than that proposed by previous nonsupercell tornadogenesis theory. In particular, tilting of baroclinically generated streamwise horizontal vorticity into the vertical was important for the formation of low-level rotation, in a manner similar to that previously proposed for supercell tornadic storms.
Abstract
On 2 July 1987 a nonmesocyclone tornado was observed in northeastern Colorado during the Convection Initiation and Downburst Experiment (CINDE). This tornado, reaching FI–F2 intensity, developed under a rapidly growing convective cell, without a preceding supercell or midlevel mesocyclone being present.
The pretornado environment on 2 July is described, including observations from a triangle of wind profilers, a dense surface mesonet array, and a special balloon sounding network. Important features contributing to tornado generation include the passage of a 700-mb short-wave trough; the formation of an ∼70-km diameter, terrain-induced mesoscale vortex (the Denver Cyclone) and its associated baroclinic zone; the presence of a stationary low-level convergence boundary; and the presence of low-level azimuthal sheer maxima (misovortices) along the boundary.
Vorticity budget terms are calculated in the lowest 2 km AGL using a multiple-Doppler radar analysis. These terms and their spatial distributions are compared with observations of mesocyclone-associated supercell tornadoes. Results show that vorticity associated with the 2 July nonsupercell tornado was generated in a more complicated manner than that proposed by previous nonsupercell tornadogenesis theory. In particular, tilting of baroclinically generated streamwise horizontal vorticity into the vertical was important for the formation of low-level rotation, in a manner similar to that previously proposed for supercell tornadic storms.
Abstract
Remote soundings of precipitable water vapor from three systems are compared with each other and with ground truth from radiosondes. Ancillary data from a mesoscale network of surface observing stations and from wind-profiling radars are also used in the analysis. The three remote-sounding techniques are: (a) a reflectance technique using spectral data collected by the Airborne Visible-Infrared Imaging Spectrometer (AVIRIS); (b) an emission technique using Visible-Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) data acquired from the National Oceanic and Atmospheric Administration's (NOAA) Geostationary Operational Environmental Satellite (GOES); and (c) a microwave technique using data from a limited network of three ground-based dual-channel microwave radiometers. The data were taken over the Front Range of eastern Colorado on 22–23 March 1990. The generally small differences between the three types of rernote-sounding measurements are consistent with the horizontal and temporal resolutions of the instruments. The microwave and optical reflectance measurements agreed to within 0. 1 cm; comparisons of the microwave data with radiosondes were also either that good or explainable. The largest differences between the VAS and the microwave radiometer at Elbert were between 0.4 and 0.5 cm and appear to he due to variable terrain within the satellite footprint.
Abstract
Remote soundings of precipitable water vapor from three systems are compared with each other and with ground truth from radiosondes. Ancillary data from a mesoscale network of surface observing stations and from wind-profiling radars are also used in the analysis. The three remote-sounding techniques are: (a) a reflectance technique using spectral data collected by the Airborne Visible-Infrared Imaging Spectrometer (AVIRIS); (b) an emission technique using Visible-Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) data acquired from the National Oceanic and Atmospheric Administration's (NOAA) Geostationary Operational Environmental Satellite (GOES); and (c) a microwave technique using data from a limited network of three ground-based dual-channel microwave radiometers. The data were taken over the Front Range of eastern Colorado on 22–23 March 1990. The generally small differences between the three types of rernote-sounding measurements are consistent with the horizontal and temporal resolutions of the instruments. The microwave and optical reflectance measurements agreed to within 0. 1 cm; comparisons of the microwave data with radiosondes were also either that good or explainable. The largest differences between the VAS and the microwave radiometer at Elbert were between 0.4 and 0.5 cm and appear to he due to variable terrain within the satellite footprint.
