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- Author or Editor: William J. Shaw x
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Abstract
During the last three decades, aircraft have assumed an increasingly important role as platforms for atmospheric measurement. Because of the nature of the wind measurement process, however, low-frequency winds remain susceptible to significant errors, which have their origin in the inertial navigation system (INS) of the aircraft. Computations are presented showing that the Electra, a state-of-the-art aircraft for meteorological measurements, experiences low-frequency wind measurement errors typically as large as 3 m s−1 in each component of the horizontal wind. Errors of this magnitude can produce can produce line-integral divergence errors of 10−4 s−1 in typical flight geometries. Use of the LORAN-C navigational information now received on the aircraft offers an order-of-magnitude reduction of errors from the INS.
Abstract
During the last three decades, aircraft have assumed an increasingly important role as platforms for atmospheric measurement. Because of the nature of the wind measurement process, however, low-frequency winds remain susceptible to significant errors, which have their origin in the inertial navigation system (INS) of the aircraft. Computations are presented showing that the Electra, a state-of-the-art aircraft for meteorological measurements, experiences low-frequency wind measurement errors typically as large as 3 m s−1 in each component of the horizontal wind. Errors of this magnitude can produce can produce line-integral divergence errors of 10−4 s−1 in typical flight geometries. Use of the LORAN-C navigational information now received on the aircraft offers an order-of-magnitude reduction of errors from the INS.
Abstract
Calibration of an airborne Lyman-α hygrometer against simultaneous measurements of humidity from a dewpoint hygrometer shows that Lyman-α biases drift with time. Analyses of low-level flight data from four days during the 1986 Frontal Air–Sea Interaction Experiment indicate that Lyman-α gains are constant for each Right (∼ 3 h duration) but different on each day. If not explicitly accounted for, the time-varying bias can introduce a significant error in calculated humidities from the Lyman-α hygrometer.
Abstract
Calibration of an airborne Lyman-α hygrometer against simultaneous measurements of humidity from a dewpoint hygrometer shows that Lyman-α biases drift with time. Analyses of low-level flight data from four days during the 1986 Frontal Air–Sea Interaction Experiment indicate that Lyman-α gains are constant for each Right (∼ 3 h duration) but different on each day. If not explicitly accounted for, the time-varying bias can introduce a significant error in calculated humidities from the Lyman-α hygrometer.
Abstract
Data from several surface meteorological networks in the vicinity of the U.S. Department of Energy’s Southern Great Plains Cloud and Radiation Testbed were used to investigate the relationship between boundary layer circulations, as reflected in composited divergence fields, and variations in vegetation, surface temperature, and topography. The study is unique in using data from a dense collection of surface meteorological instruments that are distributed over a region comparable in size to a GCM grid cell in a region of sharply varying land use. These land use differences provide variations in surface heat flux on a scale O(100 km) that has been postulated to produce the strongest surface-induced mesoscale circulations in the boundary layer. This paper details the first signature in data of a boundary layer circulation that can be attributed to land use differences at this scale. It is found, however, that in the composited fields the majority of the divergence extrema persist over seasons, are present in all observed wind conditions, are geographically fixed, and are more likely related to gentle topographic features rather than to land use differences.
Abstract
Data from several surface meteorological networks in the vicinity of the U.S. Department of Energy’s Southern Great Plains Cloud and Radiation Testbed were used to investigate the relationship between boundary layer circulations, as reflected in composited divergence fields, and variations in vegetation, surface temperature, and topography. The study is unique in using data from a dense collection of surface meteorological instruments that are distributed over a region comparable in size to a GCM grid cell in a region of sharply varying land use. These land use differences provide variations in surface heat flux on a scale O(100 km) that has been postulated to produce the strongest surface-induced mesoscale circulations in the boundary layer. This paper details the first signature in data of a boundary layer circulation that can be attributed to land use differences at this scale. It is found, however, that in the composited fields the majority of the divergence extrema persist over seasons, are present in all observed wind conditions, are geographically fixed, and are more likely related to gentle topographic features rather than to land use differences.
Abstract
Aircraft data from the JASIN Experiment have been used to examine the role that intermittency plays in turbulent transfer in the near-neutral marine atmospheric boundary layer. Conditional sampling, using the time-varying dissipation rate as an indicator, was the technique chosen for studying the dimensions of observed bursts of dissipation and their relation to the turbulent transfer. Burst fractional area coverage, γ, showed significant height variability in the surface layer, from a value of 0.45 near the surface decreasing to a constant value of about 0.30 above Z=0.2Zi
. It was shown that γ is quite sensitive in the surface layer to the height of measurement and to the surface roughness (scaling with
The plume model of Frisch provided an estimate of the physical dimensions of the bursts. Their area varied little with height and corresponded to an average diameter of 140 m, but the number density decreased with height. The regions of high turbulence activity showed an elongation of 10% in the mean wind direction throughout the ABL.
Bursts of dissipation rate were generally coincident with regions of enhanced flux. Conditional statistics showed that 50–60% of the vertical velocity variance, stress, and water vapor fluxes were concentrated in 30% of the area over most of the ABL. The mean vertical velocity difference, Δw, between the bursts and the ambient state was found to reflect buoyant input of energy into the ABL through a dependence on the convective scaling velocity w *. This observation, the roughness height dependence of γ, and various laboratory findings suggest that plumes may be generated by the shear properties of the flow, rather than by thermal instabilities.
The turbulence kinetic energy balance showed that bursts of dissipation are also regions of enhanced turbulent transfer. In the convective case, buoyant production is concentrated in these regions. The transport of turbulence kinetic energy out of the lower ABL by the bursts actually exceeds the net transport, so that the ambient state transports turbulence kinetic energy to the surface.
Abstract
Aircraft data from the JASIN Experiment have been used to examine the role that intermittency plays in turbulent transfer in the near-neutral marine atmospheric boundary layer. Conditional sampling, using the time-varying dissipation rate as an indicator, was the technique chosen for studying the dimensions of observed bursts of dissipation and their relation to the turbulent transfer. Burst fractional area coverage, γ, showed significant height variability in the surface layer, from a value of 0.45 near the surface decreasing to a constant value of about 0.30 above Z=0.2Zi
. It was shown that γ is quite sensitive in the surface layer to the height of measurement and to the surface roughness (scaling with
The plume model of Frisch provided an estimate of the physical dimensions of the bursts. Their area varied little with height and corresponded to an average diameter of 140 m, but the number density decreased with height. The regions of high turbulence activity showed an elongation of 10% in the mean wind direction throughout the ABL.
Bursts of dissipation rate were generally coincident with regions of enhanced flux. Conditional statistics showed that 50–60% of the vertical velocity variance, stress, and water vapor fluxes were concentrated in 30% of the area over most of the ABL. The mean vertical velocity difference, Δw, between the bursts and the ambient state was found to reflect buoyant input of energy into the ABL through a dependence on the convective scaling velocity w *. This observation, the roughness height dependence of γ, and various laboratory findings suggest that plumes may be generated by the shear properties of the flow, rather than by thermal instabilities.
The turbulence kinetic energy balance showed that bursts of dissipation are also regions of enhanced turbulent transfer. In the convective case, buoyant production is concentrated in these regions. The transport of turbulence kinetic energy out of the lower ABL by the bursts actually exceeds the net transport, so that the ambient state transports turbulence kinetic energy to the surface.
Abstract
An expedition to study the stability of the weakly stratified water column in the eastern Weddell Sea was undertaken in the austral winter of 2005. A regional CTD survey around Maud Rise delineated water mass boundaries associated with flow around the seamount and identified areas most susceptible to overturning. A downstream region of the seamount Taylor column was found least stable, with a potential density difference across the pycnocline less than 0.018 kg m−3. Intensive water column measurements, including 1300 profiles of temperature, conductivity, and fast-response microconductivity, were made during a series of 13 drift stations to investigate vertical turbulent transports and the evolution of water column stability. The dependence of pycnocline turbulent diffusivity k T on Froude number Fr (turbulence generated by internal wave shear) and density ratio R ρ (turbulence generated by diffusive layering and possibly diapycnal cabbeling) is investigated. The Fr alone cannot explain completely the observed k T variability. Instead, there is also a strong dependence on R ρ . Turbulent diffusivity is an order of magnitude larger in the weakly stratified Taylor cap over Maud Rise (where R ρ approaches one) than in the surrounding water column that is unaffected by flow around Maud Rise. In terms of water column stability, diffusive heat flux across the pycnocline inhibits winter ice growth and densification of the surface layer. The observed R ρ dependence of k T thus provides a strong negative feedback on the winter evolution of the Maud Rise area water column toward overturning instability.
Abstract
An expedition to study the stability of the weakly stratified water column in the eastern Weddell Sea was undertaken in the austral winter of 2005. A regional CTD survey around Maud Rise delineated water mass boundaries associated with flow around the seamount and identified areas most susceptible to overturning. A downstream region of the seamount Taylor column was found least stable, with a potential density difference across the pycnocline less than 0.018 kg m−3. Intensive water column measurements, including 1300 profiles of temperature, conductivity, and fast-response microconductivity, were made during a series of 13 drift stations to investigate vertical turbulent transports and the evolution of water column stability. The dependence of pycnocline turbulent diffusivity k T on Froude number Fr (turbulence generated by internal wave shear) and density ratio R ρ (turbulence generated by diffusive layering and possibly diapycnal cabbeling) is investigated. The Fr alone cannot explain completely the observed k T variability. Instead, there is also a strong dependence on R ρ . Turbulent diffusivity is an order of magnitude larger in the weakly stratified Taylor cap over Maud Rise (where R ρ approaches one) than in the surrounding water column that is unaffected by flow around Maud Rise. In terms of water column stability, diffusive heat flux across the pycnocline inhibits winter ice growth and densification of the surface layer. The observed R ρ dependence of k T thus provides a strong negative feedback on the winter evolution of the Maud Rise area water column toward overturning instability.
Abstract
A U.S. Environmental Protection Agency (EPA)-approved diagnostic wind model [California Meteorological Model (CALMET)] was evaluated during a typical lake-breeze event under fair weather conditions in the Chicago region. The authors focused on the performance of CALMET in terms of simulating winds that were highly variable in space and time. The reference winds were generated by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) assimilating system, with which CALMET results were compared. Statistical evaluations were conducted to quantify overall model differences in wind speed and direction over the domain. Below 850 m above the surface, relative differences in (layer averaged) wind speed were about 25%–40% during the simulation period; wind direction differences generally ranged from 6° to 20°. Above 850 m, the differences became larger because of the limited number of upper-air stations near the studied domain. Analyses implied that model differences were dependent on time because of time-dependent spatial variability in winds. Trajectory analyses were made to examine the likely spatial dependence of CALMET deviations from the reference winds within the domain. These analyses suggest that the quality of CALMET winds in local areas depended on their proximity to the lake-breeze front position. Large deviations usually occurred near the front area, where observations cannot resolve the spatial variability of wind, or in the fringe of the domain, where observations are lacking. Results simulated using different datasets and model options were also compared. Differences between CALMET and the reference winds tended to be reduced with data sampled from more stations or from more uniformly distributed stations. Suggestions are offered for further improving or interpreting CALMET results under complex wind conditions in the Chicago region, which may also apply to other regions.
Abstract
A U.S. Environmental Protection Agency (EPA)-approved diagnostic wind model [California Meteorological Model (CALMET)] was evaluated during a typical lake-breeze event under fair weather conditions in the Chicago region. The authors focused on the performance of CALMET in terms of simulating winds that were highly variable in space and time. The reference winds were generated by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) assimilating system, with which CALMET results were compared. Statistical evaluations were conducted to quantify overall model differences in wind speed and direction over the domain. Below 850 m above the surface, relative differences in (layer averaged) wind speed were about 25%–40% during the simulation period; wind direction differences generally ranged from 6° to 20°. Above 850 m, the differences became larger because of the limited number of upper-air stations near the studied domain. Analyses implied that model differences were dependent on time because of time-dependent spatial variability in winds. Trajectory analyses were made to examine the likely spatial dependence of CALMET deviations from the reference winds within the domain. These analyses suggest that the quality of CALMET winds in local areas depended on their proximity to the lake-breeze front position. Large deviations usually occurred near the front area, where observations cannot resolve the spatial variability of wind, or in the fringe of the domain, where observations are lacking. Results simulated using different datasets and model options were also compared. Differences between CALMET and the reference winds tended to be reduced with data sampled from more stations or from more uniformly distributed stations. Suggestions are offered for further improving or interpreting CALMET results under complex wind conditions in the Chicago region, which may also apply to other regions.
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.
Abstract
This study examines the interaction of diffusive convection and shear through a series of 2D and 3D direct numerical simulations (DNS). The model employed is based on the Boussinesq equations of motion with oscillating shear represented by a forcing term in the momentum equation. This study calculates thermal diffusivities for a wide range of Froude numbers and density ratios and compares the results with those from the analysis of observational data gathered during a 2005 expedition to the eastern Weddell Sea. The patterns of layering and the strong dependence of thermal diffusivity on the density ratio described here are in agreement with observations. Additionally, the authors evaluate salinity fluxes that are inaccessible from field data and formulate a parameterization of buoyancy transport. The relative significance of double diffusion and shear is quantified through comparison of density fluxes, efficiency factor, and dissipation ratio for the regimes with/without diffusive convection. This study assesses the accuracy of the thermal production dissipation and turbulent kinetic energy balances, commonly used in microstructure-based observational studies, and quantifies the length of the averaging period required for reliable statistics and the spatial variability of heat flux.
Abstract
This study examines the interaction of diffusive convection and shear through a series of 2D and 3D direct numerical simulations (DNS). The model employed is based on the Boussinesq equations of motion with oscillating shear represented by a forcing term in the momentum equation. This study calculates thermal diffusivities for a wide range of Froude numbers and density ratios and compares the results with those from the analysis of observational data gathered during a 2005 expedition to the eastern Weddell Sea. The patterns of layering and the strong dependence of thermal diffusivity on the density ratio described here are in agreement with observations. Additionally, the authors evaluate salinity fluxes that are inaccessible from field data and formulate a parameterization of buoyancy transport. The relative significance of double diffusion and shear is quantified through comparison of density fluxes, efficiency factor, and dissipation ratio for the regimes with/without diffusive convection. This study assesses the accuracy of the thermal production dissipation and turbulent kinetic energy balances, commonly used in microstructure-based observational studies, and quantifies the length of the averaging period required for reliable statistics and the spatial variability of heat flux.
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.
