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Abstract
Measurements of air velocity, temperature and humidity were made from an aircraft in the fair-weather trade wind boundary layer. On the day of the experiment, the region studied was characterized by north-south bands of cloud-free and moderately clouded areas roughly 40 km in width. Mean winds, turbulence quantities, and thermodynamic variables were measured in both a clear and a partly cloudy region. Production of turbulence energy in the subcloud layer of both regions was mainly from wind shear. In the cloud-free region, the turbulence energy and momentum flux budgets were measured. One of the most striking features was the large region (covering almost two-thirds of the depth of the mixed layer) of negative production of turbulence energy by wind shear. Notwithstanding this unusual feature, the terms of the turbulence energy budget agree quite well with a model developed by Lenschow.
In the cloudy area, a layer of strong wind shear was observed near cloud base. This, coupled with corresponding minima in the turbulence quantities, suggests a weak coupling, on the turbulence scale, between the cloud and subcloud layers.
Abstract
Measurements of air velocity, temperature and humidity were made from an aircraft in the fair-weather trade wind boundary layer. On the day of the experiment, the region studied was characterized by north-south bands of cloud-free and moderately clouded areas roughly 40 km in width. Mean winds, turbulence quantities, and thermodynamic variables were measured in both a clear and a partly cloudy region. Production of turbulence energy in the subcloud layer of both regions was mainly from wind shear. In the cloud-free region, the turbulence energy and momentum flux budgets were measured. One of the most striking features was the large region (covering almost two-thirds of the depth of the mixed layer) of negative production of turbulence energy by wind shear. Notwithstanding this unusual feature, the terms of the turbulence energy budget agree quite well with a model developed by Lenschow.
In the cloudy area, a layer of strong wind shear was observed near cloud base. This, coupled with corresponding minima in the turbulence quantities, suggests a weak coupling, on the turbulence scale, between the cloud and subcloud layers.
Abstract
The refractive index structure parameter CN 2 has contributions from the temperature and humidity structure parameters Cr 2 and CQ 2 and from the joint structure parameter CTQ . We briefly review the behavior of these structure parameters in the surface layer. We show that the surface-layer similarity expressions for Cr 2, CTQ and CQ 2 yield, in the unstable limit, mixed-layer scaling laws which are in good agreement with data at small z/zi , where zi is the mixed-layer depth. However, we show that entrainment effects cause large departures from these laws in mid and upper regions of the convective boundary layer.
Using Deardorff's idealization of the structure of the interfacial region at the top of a convective boundary layer, we use a “mean-field closure” approach to develop scaling expressions for the structure parameters generated by the entrainment process there. The available data on CT 2, CTQ and CQ 2 near the convective boundary-layer top, from both steady and evolving cases, are shown to be consistent with these new scaling expressions.
Abstract
The refractive index structure parameter CN 2 has contributions from the temperature and humidity structure parameters Cr 2 and CQ 2 and from the joint structure parameter CTQ . We briefly review the behavior of these structure parameters in the surface layer. We show that the surface-layer similarity expressions for Cr 2, CTQ and CQ 2 yield, in the unstable limit, mixed-layer scaling laws which are in good agreement with data at small z/zi , where zi is the mixed-layer depth. However, we show that entrainment effects cause large departures from these laws in mid and upper regions of the convective boundary layer.
Using Deardorff's idealization of the structure of the interfacial region at the top of a convective boundary layer, we use a “mean-field closure” approach to develop scaling expressions for the structure parameters generated by the entrainment process there. The available data on CT 2, CTQ and CQ 2 near the convective boundary-layer top, from both steady and evolving cases, are shown to be consistent with these new scaling expressions.
Abstract
The properties of convective drafts and cores are presented in Part I. By our definition a convective updraft must have a positive vertical velocity for 0.5 km, and exceed 0.5 m s−1 for 1 s; a convective updraft core must exceed 1 m s−1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. Here the properties of the drafts and cores are compared to results of previous work. In addition, the implications of the results in Part I are discussed.
GATE cores and drafts are comparable in size and intensity to those measured in hurricanes but weaker than those measured in continental thunderstorms. The lesser intensity seems related to the nearly moist adiabatic GATE sounding. The mass flux by GATE cores is consistent with large-scale requirements. It is fairly evenly distributed over a range of core size and intensity. Updraft core vertical velocity and diameter are positively correlated, primarily the result of a few large strong events.
The vast majority of GATE convective cores are sufficiently weak, with mean vertical velocities < 3–5 m s−1, that the time scale for air starting at cloud base to reach the upper troposphere can be in excess of 1 h. The microphysical implications of such long time scales are discussed. They include large fractional rainout from the warm part of the cloud, the presence of ice at relatively warm temperatures, and rapid decrease of radar reflectivity with height above the 0°C level.
Usually the clouds in GATE were part of a larger, organized mesoscale system. The typical distribution of cumulonimbus clouds, cores and drafts in such a system is synthesized by combining our results with other GATE results. A schematic updraft core and downdraft core in the middle troposphere are presented, emphasizing that these entities were rather narrow and weak in GATE clouds.
Abstract
The properties of convective drafts and cores are presented in Part I. By our definition a convective updraft must have a positive vertical velocity for 0.5 km, and exceed 0.5 m s−1 for 1 s; a convective updraft core must exceed 1 m s−1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. Here the properties of the drafts and cores are compared to results of previous work. In addition, the implications of the results in Part I are discussed.
GATE cores and drafts are comparable in size and intensity to those measured in hurricanes but weaker than those measured in continental thunderstorms. The lesser intensity seems related to the nearly moist adiabatic GATE sounding. The mass flux by GATE cores is consistent with large-scale requirements. It is fairly evenly distributed over a range of core size and intensity. Updraft core vertical velocity and diameter are positively correlated, primarily the result of a few large strong events.
The vast majority of GATE convective cores are sufficiently weak, with mean vertical velocities < 3–5 m s−1, that the time scale for air starting at cloud base to reach the upper troposphere can be in excess of 1 h. The microphysical implications of such long time scales are discussed. They include large fractional rainout from the warm part of the cloud, the presence of ice at relatively warm temperatures, and rapid decrease of radar reflectivity with height above the 0°C level.
Usually the clouds in GATE were part of a larger, organized mesoscale system. The typical distribution of cumulonimbus clouds, cores and drafts in such a system is synthesized by combining our results with other GATE results. A schematic updraft core and downdraft core in the middle troposphere are presented, emphasizing that these entities were rather narrow and weak in GATE clouds.
Abstract
No Abstract available.
Abstract
No Abstract available.
Statistics regarding the fractional participation of women in meteorology/atmospheric sciences gathered by the AMS are quite similar to those based on annual National Science Foundation (NSF) surveys. The absolute numbers in the biennial AMS/UCAR survey of academic departments for the Curricula series ceased being useful by around 2005, when many departments stopped participating fully, but numbers from less-frequent direct AMS membership surveys have been increasing. Despite the limitations of the AMS data, the NSF statistics confirm conclusions from an earlier analysis of AMS data. Both numbers and percentages are required to tell the evolving story of the atmospheric sciences' “pipeline.” Furthermore, after correction of an error regarding the AMS statistics in our 2010 paper, both NSF and AMS data show the same increase in the proportion of women graduate students in the field over the last four decades, as well as an apparent leveling off at approximately one-third.
Statistics regarding the fractional participation of women in meteorology/atmospheric sciences gathered by the AMS are quite similar to those based on annual National Science Foundation (NSF) surveys. The absolute numbers in the biennial AMS/UCAR survey of academic departments for the Curricula series ceased being useful by around 2005, when many departments stopped participating fully, but numbers from less-frequent direct AMS membership surveys have been increasing. Despite the limitations of the AMS data, the NSF statistics confirm conclusions from an earlier analysis of AMS data. Both numbers and percentages are required to tell the evolving story of the atmospheric sciences' “pipeline.” Furthermore, after correction of an error regarding the AMS statistics in our 2010 paper, both NSF and AMS data show the same increase in the proportion of women graduate students in the field over the last four decades, as well as an apparent leveling off at approximately one-third.
Abstract
Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s−1 km−1 through the troposphere, a convective available potential energy of 600 m2 s−2, and a convective Richardson number of 10, based on the wind in the lowest 6 km.
The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel to the shear.
Trajectory analyses based on the Doppler-derived wind field show that three-dimensional airflow is crucial to the maintenance of the squall line. Boundary-layer air directly ahead of the strongest reflectivity centers fed the associated updrafts while air on their flanks rose slightly, was cooled by evaporation of rain, and then descended to become the primary source of air in the subcloud cold dome. In contrast to typical midlatitude squall lines, there was no evidence of organized rear-to-front system-relative airflow in the subcloud air. This is explained in terms of the initial front-to-rear momentum of the cold-dome source air, with frictional effects also playing a role for air near the surface. Since the ground is traveling rearward relative to the storm, frictional effects oppose the pressure gradient ahead of the cold-dome pressure maximum and keep the near-surface air moving rearward throughout the cold dome. Only a small fraction of the subcloud air originated at midcloud levels, probably because evaporation above cloud base was inhibited by high relative humidities in the environment and because comparatively weak updrafts produced only modest amounts of condensate for water loading.
The persistence of squall-line elements is discussed in light of (a) their resemblance to supercells as represented in numerical simulations, and (b) recent theories involving the balance of vorticity between vertical shear in the low-level environment and the cold dome in the subcloud layer. The squall line is representative of that part of the spectrum of mesoscale convective systems that does not have a rear inflow jet, does not produce a trailing stratiform precipitation region, and does not rely upon penetrative downdrafts to sustain the air mass within the subcloud cold dome.
Abstract
Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s−1 km−1 through the troposphere, a convective available potential energy of 600 m2 s−2, and a convective Richardson number of 10, based on the wind in the lowest 6 km.
The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel to the shear.
Trajectory analyses based on the Doppler-derived wind field show that three-dimensional airflow is crucial to the maintenance of the squall line. Boundary-layer air directly ahead of the strongest reflectivity centers fed the associated updrafts while air on their flanks rose slightly, was cooled by evaporation of rain, and then descended to become the primary source of air in the subcloud cold dome. In contrast to typical midlatitude squall lines, there was no evidence of organized rear-to-front system-relative airflow in the subcloud air. This is explained in terms of the initial front-to-rear momentum of the cold-dome source air, with frictional effects also playing a role for air near the surface. Since the ground is traveling rearward relative to the storm, frictional effects oppose the pressure gradient ahead of the cold-dome pressure maximum and keep the near-surface air moving rearward throughout the cold dome. Only a small fraction of the subcloud air originated at midcloud levels, probably because evaporation above cloud base was inhibited by high relative humidities in the environment and because comparatively weak updrafts produced only modest amounts of condensate for water loading.
The persistence of squall-line elements is discussed in light of (a) their resemblance to supercells as represented in numerical simulations, and (b) recent theories involving the balance of vorticity between vertical shear in the low-level environment and the cold dome in the subcloud layer. The squall line is representative of that part of the spectrum of mesoscale convective systems that does not have a rear inflow jet, does not produce a trailing stratiform precipitation region, and does not rely upon penetrative downdrafts to sustain the air mass within the subcloud cold dome.
Abstract
We examine the pressure fields wound the cloud-base updraft of three cumulus clouds observed in environments with low vertical shear of the horizontal wind near cloud base. These fields are compared to the corresponding pressure fields beneath convective clouds embedded in moderate to large shear. All of the pressure fields are derived from aircraft measurements taken during the 1981 Cooperative Convective Experiment, CCOPE.
The pressure fields associated with these low-shear clouds are weaker than those for the clouds in higher shear. Furthermore, the low-shear fields are not consistently dominated by the dynamic pressure created by the interaction of the cloud-base updraft with the vertical shear of the horizontal wind. The weaker dynamic pressure is due to the smaller size and intensity of the cloud-base updraft as well as the smaller vertical shear of the horizontal wind. The reduction of the dynamic Pressure allows buoyancy effects on the pressure field to become more apparent.
Abstract
We examine the pressure fields wound the cloud-base updraft of three cumulus clouds observed in environments with low vertical shear of the horizontal wind near cloud base. These fields are compared to the corresponding pressure fields beneath convective clouds embedded in moderate to large shear. All of the pressure fields are derived from aircraft measurements taken during the 1981 Cooperative Convective Experiment, CCOPE.
The pressure fields associated with these low-shear clouds are weaker than those for the clouds in higher shear. Furthermore, the low-shear fields are not consistently dominated by the dynamic pressure created by the interaction of the cloud-base updraft with the vertical shear of the horizontal wind. The weaker dynamic pressure is due to the smaller size and intensity of the cloud-base updraft as well as the smaller vertical shear of the horizontal wind. The reduction of the dynamic Pressure allows buoyancy effects on the pressure field to become more apparent.
Abstract
Examination of aircraft and rawinsonde data gathered in nine tropical mesoscale convective line cases indicates that all but two lines systematically increased front-to-rear momentum at heights greater than about 4 km, and rear-to-front momentum at lower levels, where “front” is defined as the direction toward which the line is moving. The convective lines were characterized by a leading 10–20 km wide band of convective clouds, and a trailing region of stratiform cloudiness. Most wore “propagating” lines, moving into the wind at all levels. Consistent with mixing-length theory, the vertical transport of the horizontal wind component parallel to the lines was down the vertical gradient of the component, resulting in a decrease of its vertical shear. Smaller, more random cloud groups and the upper portions of a convective line with isolated towers transported both components of horizontal momentum downgradient.
Normalization of the vertical flux of horizontal momentum normal to the line (u′¯w′¯) suggests that it is achieved mainly by updraft cores which could be traced to the undisturbed mixed layer ahead of the line. The air in the cores is accelerated upward and backward into a mesoscale area of low pressure located in the rear portion of the line's leading convective region. The low pressure is primarily hydrostatic, its intensity proportional to the depth and average buoyancy of the cloudy air overhead. However, dynamic pressure effects are important where convective cores are particularly concentrated. From the aircraft data, the momentum transport by the trailing, stratiform region appears small, but this conclusion needs confirmation by sensing platforms more suited to gathering mesoscale wind field data. The failure to account for the momentum transport properties of two-dimensional convective lines might explain the lack of success in parameterizing the effects of cumulus clouds on the mean wind profile.
Abstract
Examination of aircraft and rawinsonde data gathered in nine tropical mesoscale convective line cases indicates that all but two lines systematically increased front-to-rear momentum at heights greater than about 4 km, and rear-to-front momentum at lower levels, where “front” is defined as the direction toward which the line is moving. The convective lines were characterized by a leading 10–20 km wide band of convective clouds, and a trailing region of stratiform cloudiness. Most wore “propagating” lines, moving into the wind at all levels. Consistent with mixing-length theory, the vertical transport of the horizontal wind component parallel to the lines was down the vertical gradient of the component, resulting in a decrease of its vertical shear. Smaller, more random cloud groups and the upper portions of a convective line with isolated towers transported both components of horizontal momentum downgradient.
Normalization of the vertical flux of horizontal momentum normal to the line (u′¯w′¯) suggests that it is achieved mainly by updraft cores which could be traced to the undisturbed mixed layer ahead of the line. The air in the cores is accelerated upward and backward into a mesoscale area of low pressure located in the rear portion of the line's leading convective region. The low pressure is primarily hydrostatic, its intensity proportional to the depth and average buoyancy of the cloudy air overhead. However, dynamic pressure effects are important where convective cores are particularly concentrated. From the aircraft data, the momentum transport by the trailing, stratiform region appears small, but this conclusion needs confirmation by sensing platforms more suited to gathering mesoscale wind field data. The failure to account for the momentum transport properties of two-dimensional convective lines might explain the lack of success in parameterizing the effects of cumulus clouds on the mean wind profile.
Abstract
The second part of the parameterization of subgrid-scale surface fluxes model (PASS2) has been developed to estimate long-term evapotranspiration rates over extended areas at a high spatial resolution by using satellite remote sensing data and limited, but continuous, surface meteorological measurements. Other required inputs include data on initial root-zone available moisture (RAM) content computed by PASS1 for each pixel at the time of clear-sky satellite overpasses, normalized difference vegetation index (NDVI) from the overpasses, and databases on available water capacity and land-use classes. Site-specific PASS2 parameterizations evaluate surface albedo, roughness length, and ground heat flux for each pixel, and special functions distribute areally representative observations of wind speed, temperature, and water vapor pressure to individual pixels. The surface temperature for each pixel and each time increment is computed with an approximation involving the surface energy budget, and the evapotranspiration rates are computed via a bulk aerodynamic formulation. Results from PASS2 were compared with observations made during the 1997 Cooperative Atmosphere–Surface Exchange Study field campaign in Kansas. The modeled diurnal variation of RAM content, latent heat flux, and daily evapotranspiration rate were realistic in comparison to measurements at eight surface sites. With the limited resolution of the NDVI data, however, model results deviated from the observations at locations where the measurement sites were in fields with surface vegetative conditions notably different than surrounding fields. Comparisons with aircraft-based flux measurements suggested that the evapotranspiration rates over distances of tens of kilometers were modeled without significant bias.
Abstract
The second part of the parameterization of subgrid-scale surface fluxes model (PASS2) has been developed to estimate long-term evapotranspiration rates over extended areas at a high spatial resolution by using satellite remote sensing data and limited, but continuous, surface meteorological measurements. Other required inputs include data on initial root-zone available moisture (RAM) content computed by PASS1 for each pixel at the time of clear-sky satellite overpasses, normalized difference vegetation index (NDVI) from the overpasses, and databases on available water capacity and land-use classes. Site-specific PASS2 parameterizations evaluate surface albedo, roughness length, and ground heat flux for each pixel, and special functions distribute areally representative observations of wind speed, temperature, and water vapor pressure to individual pixels. The surface temperature for each pixel and each time increment is computed with an approximation involving the surface energy budget, and the evapotranspiration rates are computed via a bulk aerodynamic formulation. Results from PASS2 were compared with observations made during the 1997 Cooperative Atmosphere–Surface Exchange Study field campaign in Kansas. The modeled diurnal variation of RAM content, latent heat flux, and daily evapotranspiration rate were realistic in comparison to measurements at eight surface sites. With the limited resolution of the NDVI data, however, model results deviated from the observations at locations where the measurement sites were in fields with surface vegetative conditions notably different than surrounding fields. Comparisons with aircraft-based flux measurements suggested that the evapotranspiration rates over distances of tens of kilometers were modeled without significant bias.
Abstract
On 10 October 1983 the two NOAA WP-3D aircraft completed a mission designed to provide airborne Doppler radar data for a convective cell embedded in a weak rainband on the trailing side of Hurricane Raymond. Comparisons of the wind field produced from the pseudo-dual-Doppler radar technique with in situ wind measurements suggest that the larger convective-scale feature may be resolved if the sampling time is kept to a minimum. The convective cell was found to move downband faster than any environmental winds but slightly slower than the winds found in the reflectivity core that delineates the cell. In the core of the cell the tangential wind is increased and the radial inflow turns to outflow with respect to the circulation center. The flow field demonstrates that the downband stratiform portion of a rainband is not from cells currently active since the updraft detrains upwind relative to the cell but rather it is due to the fallout from ice particles placed into the upper troposphere by clouds that have since dissipated. The mass flux of this cell is estimated to be 5%–10% of the mass flux accomplished by an eyewall of a moderate tropical cyclone. This finding supports the concept that large, convectively active rainbands have a major effect on the subcloud layer air flowing toward the eyewall.
Abstract
On 10 October 1983 the two NOAA WP-3D aircraft completed a mission designed to provide airborne Doppler radar data for a convective cell embedded in a weak rainband on the trailing side of Hurricane Raymond. Comparisons of the wind field produced from the pseudo-dual-Doppler radar technique with in situ wind measurements suggest that the larger convective-scale feature may be resolved if the sampling time is kept to a minimum. The convective cell was found to move downband faster than any environmental winds but slightly slower than the winds found in the reflectivity core that delineates the cell. In the core of the cell the tangential wind is increased and the radial inflow turns to outflow with respect to the circulation center. The flow field demonstrates that the downband stratiform portion of a rainband is not from cells currently active since the updraft detrains upwind relative to the cell but rather it is due to the fallout from ice particles placed into the upper troposphere by clouds that have since dissipated. The mass flux of this cell is estimated to be 5%–10% of the mass flux accomplished by an eyewall of a moderate tropical cyclone. This finding supports the concept that large, convectively active rainbands have a major effect on the subcloud layer air flowing toward the eyewall.
