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Abstract
The role of the surface velocity fields in the formation, maintenance and decay of convective storms is examined using approximately 90 days of measurements in a densely instrumented network (660 km2) in south Florida. The results show statistically strong cause and effect relationships between surface convergence and onset of rain, storm intensity and duration. Short-term prediction of the onset of rain and the amount of rain produced proves possible.
The surface fields of divergence provide an estimate of storm mass and moisture transports. The size of the surface area of convergence, by governing the supply of moisture, plays a controlling role in storm intensity. Large storms are efficient (72%), in terms of moisture supplied to rain produced, compared to smaller storms (37%). Within the confines of the experiment network, weak storms are in near mass balance, while inflow greatly exceeds outflow in the intense storm. The near mass balance of the weak storm suggests cloud-to-subcloud layer interaction as a further control in storm intensity.
Abstract
The role of the surface velocity fields in the formation, maintenance and decay of convective storms is examined using approximately 90 days of measurements in a densely instrumented network (660 km2) in south Florida. The results show statistically strong cause and effect relationships between surface convergence and onset of rain, storm intensity and duration. Short-term prediction of the onset of rain and the amount of rain produced proves possible.
The surface fields of divergence provide an estimate of storm mass and moisture transports. The size of the surface area of convergence, by governing the supply of moisture, plays a controlling role in storm intensity. Large storms are efficient (72%), in terms of moisture supplied to rain produced, compared to smaller storms (37%). Within the confines of the experiment network, weak storms are in near mass balance, while inflow greatly exceeds outflow in the intense storm. The near mass balance of the weak storm suggests cloud-to-subcloud layer interaction as a further control in storm intensity.
Abstract
Three stages, prerain, mature and decay, in the life cycle of a convective storm are shown by compiling surface observations from 12 well-organized Florida summertime thunderstorms. In the mature stage, a distinction is made between stationary and moving storms. The prerain phase is characterized by cyclonic inflow, the mature phase shows cyclonic inflow and anticyclonic outflow simultaneously, while in the dissipating stage only anticyclonic outflow exists. Numerical values for the updraft and downdraft areas, upward flux of water vapor, maximum divergence and vorticity, and rainfall are presented. Moving storms last longer (by 25 min), transport roughly four times the moisture of a stationary storm and produce more rain over a larger area. These numbers provide valuable information not previously available for the surface layers of the convective storm environments.
Abstract
Three stages, prerain, mature and decay, in the life cycle of a convective storm are shown by compiling surface observations from 12 well-organized Florida summertime thunderstorms. In the mature stage, a distinction is made between stationary and moving storms. The prerain phase is characterized by cyclonic inflow, the mature phase shows cyclonic inflow and anticyclonic outflow simultaneously, while in the dissipating stage only anticyclonic outflow exists. Numerical values for the updraft and downdraft areas, upward flux of water vapor, maximum divergence and vorticity, and rainfall are presented. Moving storms last longer (by 25 min), transport roughly four times the moisture of a stationary storm and produce more rain over a larger area. These numbers provide valuable information not previously available for the surface layers of the convective storm environments.
Recent advances in our understanding of the boundary layer and cumulus convection in the tropics are reviewed. The review reflects the interactive nature of the atmosphere-ocean system. It discusses the observational picture of the atmosphere that is emerging from tropical field experiments, results of diagnostic studies of convective transports and structure, and the progress in both modeling and parameterizing convection and the tropical boundary layer.
Recent advances in our understanding of the boundary layer and cumulus convection in the tropics are reviewed. The review reflects the interactive nature of the atmosphere-ocean system. It discusses the observational picture of the atmosphere that is emerging from tropical field experiments, results of diagnostic studies of convective transports and structure, and the progress in both modeling and parameterizing convection and the tropical boundary layer.
An intensive experiment in tropical meteorology with emphasis upon the interaction of the atmosphere, the ocean and the land will be centered upon the island of Barbados, West Indies, during the summer of 1968. The scientific objectives of this program are outlined, and the means by which these objectives are expected to be attained are described in terms of instrumentation and systems development that has been carried out over the past two years. The role and importance of such experiments in the tropical atmosphere is emphasized.
An intensive experiment in tropical meteorology with emphasis upon the interaction of the atmosphere, the ocean and the land will be centered upon the island of Barbados, West Indies, during the summer of 1968. The scientific objectives of this program are outlined, and the means by which these objectives are expected to be attained are described in terms of instrumentation and systems development that has been carried out over the past two years. The role and importance of such experiments in the tropical atmosphere is emphasized.
Abstract
Obsemations presented show that the undisturbed subcloud layer near the ITCZ resembles that of the trades. Mixed and transition layers are also seen between cloud drafts during disturbed periods when shallow mixed layers (∼200 m) can persist for several hours. Modification of the lower atmosphere during disturbed periods is most marked in the lowest 1200 m. Mixed layers are observed during prolonged disturbed periods; disturbed mixed layers are shallower and cooler than undisturbed ones. Light and moderate rainfall events moisten the mixed layer, but mixed layers in the environment around strong rainfall are drier. It is hypothesized that that the effect of a number of downdrafts spreading laterally produces the mixed layer cooling and drying. Observed changes of mixed layer structure with stability, measured by the ratio of mixed layer height to the Monin-Obukhov length, are presented.
Abstract
Obsemations presented show that the undisturbed subcloud layer near the ITCZ resembles that of the trades. Mixed and transition layers are also seen between cloud drafts during disturbed periods when shallow mixed layers (∼200 m) can persist for several hours. Modification of the lower atmosphere during disturbed periods is most marked in the lowest 1200 m. Mixed layers are observed during prolonged disturbed periods; disturbed mixed layers are shallower and cooler than undisturbed ones. Light and moderate rainfall events moisten the mixed layer, but mixed layers in the environment around strong rainfall are drier. It is hypothesized that that the effect of a number of downdrafts spreading laterally produces the mixed layer cooling and drying. Observed changes of mixed layer structure with stability, measured by the ratio of mixed layer height to the Monin-Obukhov length, are presented.
Abstract
Results of a one-dimensional mixed-layer growth model are compared with thermodynamic observations made during the GARP Atlantic Tropical Experiment (GATE). Observed drying at low levels accompanying warming in the wake of a storm is hypothesized to be the result of rapid mixed-layer growth as well as of subsidence in the environment outside the storm. Sensitivity of the model solutions to changes in subsidence and stability above the mixed layer is shown. Model solutions show protracted recovery periods only when the wind speed near the surface is moderate (∼3 m s−1). Recovery of the mixed layer in the wake of the 12 September 1974 GATE squall line is simulated by the model.
Abstract
Results of a one-dimensional mixed-layer growth model are compared with thermodynamic observations made during the GARP Atlantic Tropical Experiment (GATE). Observed drying at low levels accompanying warming in the wake of a storm is hypothesized to be the result of rapid mixed-layer growth as well as of subsidence in the environment outside the storm. Sensitivity of the model solutions to changes in subsidence and stability above the mixed layer is shown. Model solutions show protracted recovery periods only when the wind speed near the surface is moderate (∼3 m s−1). Recovery of the mixed layer in the wake of the 12 September 1974 GATE squall line is simulated by the model.
Abstract
The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h−1, based on the Z – R relationship, in conjunction with 4 km × 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region.
The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.
Abstract
The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h−1, based on the Z – R relationship, in conjunction with 4 km × 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region.
The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.
