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- Author or Editor: A. K. Betts x
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Abstract
Composite maps at levels from 950 to 150 mb of relative wind field (Vτ), mixing ratio (r), equivalent potential temperature (θE), and temperature perturbation (T′) from the growth and decay phases of a mean mesoscale cumulonimbus system (systems used had a maximum radar echo area >400 km2) were constructed using radar and one rawinsonde (experiment VIMHEX) for days having a similar synoptic-scale wind field. Echo area and track were measured from radar film, and relative winds calculated by subtracting a mean echo velocity; positions of radiosonde data points relative to the echo as center were computed, scaled by an echo radius, and plotted with echo motion vectors aligned along one coordinate axis. Mass flows into the mean system at all levels give vertical mass transports for growth and decay phases, and net mass balance. The net convergence of r closely balances a mean surface rainfall per echo, and the net enthalpy source by the cumulonimbus system. Fluxes of θ E , into and out of the system for 5K ranges confirm energy conservation, and give updraft, downdraft transports. The vertical structure of net mass r and θ E fluxes are presented. The mesoscale results are related to the large-scale modification of the mean atmosphere, using a theoretical cumulonimbus model. The large-scale vertical motion is computed as a residual from the temperature and water vapor budgets. Suitably averaged, the synoptic-scale mass transport is similar but not identical to the (life-cycle mean) cumulonimbus vertical mass transport. It is concluded that parametric models of cumulonimbus convection in terms of mass transport are quite realistic for these data above the lowest 150 mb, where the effects of horizontal variations between updraft and downdraft are dominant. The precise relationship between synoptic-scale controls and cumulonimbus-scale mass transport remains unclear.
Abstract
Composite maps at levels from 950 to 150 mb of relative wind field (Vτ), mixing ratio (r), equivalent potential temperature (θE), and temperature perturbation (T′) from the growth and decay phases of a mean mesoscale cumulonimbus system (systems used had a maximum radar echo area >400 km2) were constructed using radar and one rawinsonde (experiment VIMHEX) for days having a similar synoptic-scale wind field. Echo area and track were measured from radar film, and relative winds calculated by subtracting a mean echo velocity; positions of radiosonde data points relative to the echo as center were computed, scaled by an echo radius, and plotted with echo motion vectors aligned along one coordinate axis. Mass flows into the mean system at all levels give vertical mass transports for growth and decay phases, and net mass balance. The net convergence of r closely balances a mean surface rainfall per echo, and the net enthalpy source by the cumulonimbus system. Fluxes of θ E , into and out of the system for 5K ranges confirm energy conservation, and give updraft, downdraft transports. The vertical structure of net mass r and θ E fluxes are presented. The mesoscale results are related to the large-scale modification of the mean atmosphere, using a theoretical cumulonimbus model. The large-scale vertical motion is computed as a residual from the temperature and water vapor budgets. Suitably averaged, the synoptic-scale mass transport is similar but not identical to the (life-cycle mean) cumulonimbus vertical mass transport. It is concluded that parametric models of cumulonimbus convection in terms of mass transport are quite realistic for these data above the lowest 150 mb, where the effects of horizontal variations between updraft and downdraft are dominant. The precise relationship between synoptic-scale controls and cumulonimbus-scale mass transport remains unclear.
Abstract
The low-level atmospheric transformation associated with a class of traveling convective storms observed. over Venezuela is described. A strong low-level cooling is observed, confined mostly to the subcloud layer, and associated with a deeper layer of drying and acceleration of the easterly flow. A density current model is used to stratify the storm travel speeds, peak surface gusts and the accelerated flow at low levels behind the storm, and to relate these to the low-level flow ahead of the storm. There is reasonable agreement between these atmospheric data and laboratory observations of density currents. The updraft and down-draft structure is discussed using an interesting sounding cross section and trajectories from a three-dimensional numerical simulation. It appears that two distinct downdrafts exist: one driven by precipitation within the cumulonimbus cell, and a second mesoscale feature which is dynamically driven, and associated with descent over the spreading cold outflow.
Abstract
The low-level atmospheric transformation associated with a class of traveling convective storms observed. over Venezuela is described. A strong low-level cooling is observed, confined mostly to the subcloud layer, and associated with a deeper layer of drying and acceleration of the easterly flow. A density current model is used to stratify the storm travel speeds, peak surface gusts and the accelerated flow at low levels behind the storm, and to relate these to the low-level flow ahead of the storm. There is reasonable agreement between these atmospheric data and laboratory observations of density currents. The updraft and down-draft structure is discussed using an interesting sounding cross section and trajectories from a three-dimensional numerical simulation. It appears that two distinct downdrafts exist: one driven by precipitation within the cumulonimbus cell, and a second mesoscale feature which is dynamically driven, and associated with descent over the spreading cold outflow.
Abstract
An idealized energy-balance model for a closed circulation is first presented to illustrate the coupling between the net tropospheric radiative cooling, the surface fluxes and the mean subsidence away from the precipitation zones. Then a one-dimensional diagnostic model and a radiation model with boundary layer clouds are combined to explore this coupling for a specific region using mean sounding data over the tropical Pacific. The radiatively driven subsidence rate at the top of the convective boundary layer is approximately 35 mb day−1 (0.04 Pa s−1 and is largely independent of boundary layer cloud fraction. The sensitivity of the corresponding convective heat flux profiles to the mass divergence profile and cloud fraction within the boundary layer is explored. Reasonable assumptions give realistic surface sensible and latent heat fluxes for this region of approximately 10 and 130 W m−1. The paper illustrates the important background climatic control of the radiation field on the tropical surface fluxes.
Abstract
An idealized energy-balance model for a closed circulation is first presented to illustrate the coupling between the net tropospheric radiative cooling, the surface fluxes and the mean subsidence away from the precipitation zones. Then a one-dimensional diagnostic model and a radiation model with boundary layer clouds are combined to explore this coupling for a specific region using mean sounding data over the tropical Pacific. The radiatively driven subsidence rate at the top of the convective boundary layer is approximately 35 mb day−1 (0.04 Pa s−1 and is largely independent of boundary layer cloud fraction. The sensitivity of the corresponding convective heat flux profiles to the mass divergence profile and cloud fraction within the boundary layer is explored. Reasonable assumptions give realistic surface sensible and latent heat fluxes for this region of approximately 10 and 130 W m−1. The paper illustrates the important background climatic control of the radiation field on the tropical surface fluxes.
Abstract
An analysis of FGGE dropwindsonde data using conserved thermodynamic variables shows mixing line structures for the convective boundary layer over the equatorial Pacific. Deeper boundary layers show a double structure. Reversals of the gradients of mixing ratio and equivalent potential temperature above the boundary- layer top are present in all the averages and suggest that the origin of the air sinking into the boundary layer needs further study.
Abstract
An analysis of FGGE dropwindsonde data using conserved thermodynamic variables shows mixing line structures for the convective boundary layer over the equatorial Pacific. Deeper boundary layers show a double structure. Reversals of the gradients of mixing ratio and equivalent potential temperature above the boundary- layer top are present in all the averages and suggest that the origin of the air sinking into the boundary layer needs further study.
The tracking of Hurricane Elena by the ECMWF operational analysis system is compared with reported positions from reconnaissance aircraft and coastal radar. An example forecast is shown for the operational model and also for an experimental version of the model. A strong sensitivity to the parameterization of deep cumulus convection is found.
The tracking of Hurricane Elena by the ECMWF operational analysis system is compared with reported positions from reconnaissance aircraft and coastal radar. An example forecast is shown for the operational model and also for an experimental version of the model. A strong sensitivity to the parameterization of deep cumulus convection is found.
Abstract
During the wet season in the southwestern Amazon region, daytime water transport out of the atmospheric mixed layer into the deeper atmosphere is shown to depend upon cloud amounts and types and synoptic-scale velocity fields. Interactions among clouds, convective conditions, and subcloud-layer properties were estimated for two dominant flow regimes observed during the 1999 Tropical Rainfall Measuring Mission component of the Brazilian Large-Scale Biosphere–Atmosphere (TRMM-LBA) field campaign. During daytime the cloud and subcloud layers were coupled by radiative, convective, and precipitation processes. The properties of cloud and subcloud layers varied according to the different convective influences of easterly versus westerly lower-tropospheric flows. The most pronounced flow-regime effects on composite cloud cycles occurred under persistent lower-tropospheric flows, which produced strong convective cloud growth with a near absence of low-level stratiform clouds, minimal cumulative attenuation of incoming solar irradiance (∼25%), rapid daytime mixed-layer growth (>100 m h−1), and boundary layer drying (0.22 g kg−1 h−1), high convective velocities (>1.5 m s−1), high surface buoyancy flux (>200 W m−2), and high latent heat flux (600 W m−2) into cloud layer. In contrast, persistent westerly flows were less convective, showing a strong morning presence of low-level stratiform genera (>0.9 cloud amount), greater cumulative attenuation of incoming solar irradiance (∼47%), slower mixed-layer growth (<50 m h−1) with a slight tendency for mixed-layer moistening, and a delayed peak in the low-level cumuliform cloud cycle (2000 versus 1700 UTC). The results reported in this article indicate that numerical models need to account for cloud amounts and types when estimating water vapor transport to the cloud layer.
Abstract
During the wet season in the southwestern Amazon region, daytime water transport out of the atmospheric mixed layer into the deeper atmosphere is shown to depend upon cloud amounts and types and synoptic-scale velocity fields. Interactions among clouds, convective conditions, and subcloud-layer properties were estimated for two dominant flow regimes observed during the 1999 Tropical Rainfall Measuring Mission component of the Brazilian Large-Scale Biosphere–Atmosphere (TRMM-LBA) field campaign. During daytime the cloud and subcloud layers were coupled by radiative, convective, and precipitation processes. The properties of cloud and subcloud layers varied according to the different convective influences of easterly versus westerly lower-tropospheric flows. The most pronounced flow-regime effects on composite cloud cycles occurred under persistent lower-tropospheric flows, which produced strong convective cloud growth with a near absence of low-level stratiform clouds, minimal cumulative attenuation of incoming solar irradiance (∼25%), rapid daytime mixed-layer growth (>100 m h−1), and boundary layer drying (0.22 g kg−1 h−1), high convective velocities (>1.5 m s−1), high surface buoyancy flux (>200 W m−2), and high latent heat flux (600 W m−2) into cloud layer. In contrast, persistent westerly flows were less convective, showing a strong morning presence of low-level stratiform genera (>0.9 cloud amount), greater cumulative attenuation of incoming solar irradiance (∼47%), slower mixed-layer growth (<50 m h−1) with a slight tendency for mixed-layer moistening, and a delayed peak in the low-level cumuliform cloud cycle (2000 versus 1700 UTC). The results reported in this article indicate that numerical models need to account for cloud amounts and types when estimating water vapor transport to the cloud layer.
Abstract
A numerical model which predicts the time variation of the thermodynamic structure of the trade-wind boundary layer is developed. Horizontally homogeneous conditions are assumed and the large-scale divergence, sea surface temperature and surface wind speed are specified externally. The model predicts the average value of mixing ratio and moist static energy in the subcloud and cloud layer and the slopes of these quantities in the cloud layer; the model also predicts the height of the transition layer (the layer which defines the boundary between the cloud and subcloud layer) and the height of the trade inversion. Subcloud layer convective fluxes are specified by using the bulk aerodynamic method for specifying the surface fluxes and a mixed-layer parameterization of the fluxes at the top of the subcloud layer. The moist convective processes are parameterized in terms of a mass flux which varies linearly with height and a cloud-environment difference in thermodynamic quantities which also varies linearly with height. Radiative fluxes are parameterized in terms of a specified cloud cover and vertically averaged boundary-layer heating.
The steady-state model solutions are shown to be relatively insensitive to the specification of closure parameters. The thermodynamic structure below the inversion is shown to be sensitive to the specification of surface wind speed, sea surface temperature, radiative heating and cloud cover. The height of the inversion is shown to be sensitive to these parameters and the large-scale divergence.
Abstract
A numerical model which predicts the time variation of the thermodynamic structure of the trade-wind boundary layer is developed. Horizontally homogeneous conditions are assumed and the large-scale divergence, sea surface temperature and surface wind speed are specified externally. The model predicts the average value of mixing ratio and moist static energy in the subcloud and cloud layer and the slopes of these quantities in the cloud layer; the model also predicts the height of the transition layer (the layer which defines the boundary between the cloud and subcloud layer) and the height of the trade inversion. Subcloud layer convective fluxes are specified by using the bulk aerodynamic method for specifying the surface fluxes and a mixed-layer parameterization of the fluxes at the top of the subcloud layer. The moist convective processes are parameterized in terms of a mass flux which varies linearly with height and a cloud-environment difference in thermodynamic quantities which also varies linearly with height. Radiative fluxes are parameterized in terms of a specified cloud cover and vertically averaged boundary-layer heating.
The steady-state model solutions are shown to be relatively insensitive to the specification of closure parameters. The thermodynamic structure below the inversion is shown to be sensitive to the specification of surface wind speed, sea surface temperature, radiative heating and cloud cover. The height of the inversion is shown to be sensitive to these parameters and the large-scale divergence.
Abstract
Cloud fraction is a widely used parameter for estimating the effects of boundary-layer cloud on radiative transfer. During the Atlantic Stratocumulus Transition Experiment (ASTEX) during June 1992, ceilometer and satellite-based measurements of boundary-layer cloud fraction were made in the subtropical North Atlantic, a region typified by a 1–2 km deep marine boundary layer with cumulus clouds rising into a broken stratocumulus layer underneath an inversion. Both the diurnal cycle and day-to-day variations in low-cloud fraction are examined. It is shown that ECMWF low cloudiness analyses do not correlate with the observed variations in cloudiness and substantially underestimate the mean low cloudiness.
In these analyses, the parameterization of low cloud fraction is primarily based on the inversion strength. A comparison of ECMWF analyses and ASTEX soundings (most of which were assimilated into the analyses) shows that the thermodynamic structure of the boundary layer and the inversion strength are well represented (with some small but significant systematic biases) in the analyses and preserved (again with some biases) in 5-day forecasts.
However, even when applied to the actual sounding the ECMWF low cloud scheme cannot predict the observed day-to-day variations or the diurnal cycle in low cloud. Other diagnostic schemes based on lower tropospheric stability, cloud-top entrainment instability, boundary-layer depth, and vertical motion do equally poorly. The only successful predictor of low cloud frontier from sounding information is the relative humidity in the upper part of the boundary layer.
Abstract
Cloud fraction is a widely used parameter for estimating the effects of boundary-layer cloud on radiative transfer. During the Atlantic Stratocumulus Transition Experiment (ASTEX) during June 1992, ceilometer and satellite-based measurements of boundary-layer cloud fraction were made in the subtropical North Atlantic, a region typified by a 1–2 km deep marine boundary layer with cumulus clouds rising into a broken stratocumulus layer underneath an inversion. Both the diurnal cycle and day-to-day variations in low-cloud fraction are examined. It is shown that ECMWF low cloudiness analyses do not correlate with the observed variations in cloudiness and substantially underestimate the mean low cloudiness.
In these analyses, the parameterization of low cloud fraction is primarily based on the inversion strength. A comparison of ECMWF analyses and ASTEX soundings (most of which were assimilated into the analyses) shows that the thermodynamic structure of the boundary layer and the inversion strength are well represented (with some small but significant systematic biases) in the analyses and preserved (again with some biases) in 5-day forecasts.
However, even when applied to the actual sounding the ECMWF low cloud scheme cannot predict the observed day-to-day variations or the diurnal cycle in low cloud. Other diagnostic schemes based on lower tropospheric stability, cloud-top entrainment instability, boundary-layer depth, and vertical motion do equally poorly. The only successful predictor of low cloud frontier from sounding information is the relative humidity in the upper part of the boundary layer.
