Incorporation of Cloud-Scale and Mesoscale Downdrafts into a Cumulus Parameterization: Results of One- and Three-Dimensional Integrations

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  • 1 Department of Atmospheric Science, State University of New York at Albany, Albany, NY 12222
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Abstract

Cumulus and mesoscale downdrafts are incorporated into the cumulus parameterization of Kuo. Convection is driven by grid-scale moisture supply, and distributed vertically by temperature and specific humidity differences between the environment and an idealized cloud. The moisture supply is defined to minimize the problem of lag between instantaneous moisture accession and rainfall. Downdrafts are added to the idealized cloud profile by determining a weighted mean of the equivalent potential temperatures (θe) for cumulus updrafts, saturated cumulus downdrafts, and unsaturated mesoscale downdrafts, and by extracting the cloud temperature and specific humidity iteratively from the mean θe. The θe values are weighted by the mean vertical eddy flux convergence of moist static energy by each component.

The addition of downdrafts sharply increases the rate of stabilization of the grid scale by the Kuo approach. Stabilization characteristics are also shown to depend upon precipitation efficiency, strength of grid-scale forcing, downdraft relative humidity, downdraft weighting, and intensity of surface fluxes.

The approach was tested in a real-data, three-dimensional primitive equation prediction of a mesoscale convective complex (MCC) on a 1° latitude/longitude mesh. Prediction of total rain volume was most accurate when downdrafts were included. Without downdrafts, a feedback instability occurred at the MCC center and rainfall was greatly overestimated. When convective heating was omitted, so that rainfall could be produced only after grid-scale saturation, predicted rainfall was less than 10% of that observed and the MCC decayed. Difference vectors between the full and no convection integrations showed strong outflow developing in the upper troposphere, evolving to a large anticyclonic eddy following the MCC by hour 12 of the forecast. Corresponding inflow and a weak cyclonic eddy developed at low levels. Influence of the MCC spread rapidly over several hundred kilometers through this divergent flow. The results indicate, not surprisingly, that maintenance of the MCC depends critically on the presence of cumulus convection. The failure of the explicit (nonparameterized) approach suggests that cumulus parameterization is necessary for realistic prediction of convective systems in meso-α scale models.

In the integration with downdrafts incorporated, a life cycle behavior occurred in the heavy rainfall region. The level of maximum upward motion shifted from middle to upper levels over several hours, and downward motion developed at the lowest levels. The, apparent heat source was initially positive at all levels, then became negative in the lower troposphere and more strongly positive aloft. Stratiform precipitation fell from saturated upper levels for a brief period after convection ceased. This life cycle behavior, which contains several aspects of that observed, took place only when cumulus and mesoscale downdrafts were incorporated.

Abstract

Cumulus and mesoscale downdrafts are incorporated into the cumulus parameterization of Kuo. Convection is driven by grid-scale moisture supply, and distributed vertically by temperature and specific humidity differences between the environment and an idealized cloud. The moisture supply is defined to minimize the problem of lag between instantaneous moisture accession and rainfall. Downdrafts are added to the idealized cloud profile by determining a weighted mean of the equivalent potential temperatures (θe) for cumulus updrafts, saturated cumulus downdrafts, and unsaturated mesoscale downdrafts, and by extracting the cloud temperature and specific humidity iteratively from the mean θe. The θe values are weighted by the mean vertical eddy flux convergence of moist static energy by each component.

The addition of downdrafts sharply increases the rate of stabilization of the grid scale by the Kuo approach. Stabilization characteristics are also shown to depend upon precipitation efficiency, strength of grid-scale forcing, downdraft relative humidity, downdraft weighting, and intensity of surface fluxes.

The approach was tested in a real-data, three-dimensional primitive equation prediction of a mesoscale convective complex (MCC) on a 1° latitude/longitude mesh. Prediction of total rain volume was most accurate when downdrafts were included. Without downdrafts, a feedback instability occurred at the MCC center and rainfall was greatly overestimated. When convective heating was omitted, so that rainfall could be produced only after grid-scale saturation, predicted rainfall was less than 10% of that observed and the MCC decayed. Difference vectors between the full and no convection integrations showed strong outflow developing in the upper troposphere, evolving to a large anticyclonic eddy following the MCC by hour 12 of the forecast. Corresponding inflow and a weak cyclonic eddy developed at low levels. Influence of the MCC spread rapidly over several hundred kilometers through this divergent flow. The results indicate, not surprisingly, that maintenance of the MCC depends critically on the presence of cumulus convection. The failure of the explicit (nonparameterized) approach suggests that cumulus parameterization is necessary for realistic prediction of convective systems in meso-α scale models.

In the integration with downdrafts incorporated, a life cycle behavior occurred in the heavy rainfall region. The level of maximum upward motion shifted from middle to upper levels over several hours, and downward motion developed at the lowest levels. The, apparent heat source was initially positive at all levels, then became negative in the lower troposphere and more strongly positive aloft. Stratiform precipitation fell from saturated upper levels for a brief period after convection ceased. This life cycle behavior, which contains several aspects of that observed, took place only when cumulus and mesoscale downdrafts were incorporated.

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