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Explicit and Parameterized Realizations of Convective Cloud Systems in TOGA COARE

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Convection and cloud processes are examined in a hierarchy of two-dimensional numerical realizations of cloud systems observed during the 19–26 December 1992 period of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The hierarchy consists of cloud-resolving simulations at a 2-km resolution, and two sets of 15-km resolution simulations; one attempts to treat convection explicitly and the other parameterizes convection using the Kain–Fritsch scheme.

The Kain–Fritsch parameterization shows reasonable results but shortcomings are found in comparison with the cloud-resolving model. (i) The entraining plumes in the parameterization excessively overshoot the tropopause, which produces a cold bias mostly through adiabatic cooling. The attendant moisture detrainment overproduces cirrus cloud. (ii) Because parameterized downdrafts detrain at the lowest level they generate a surface cold bias. (iii) The scheme fails to represent the trimodal convection (cumulonimbus reaching the tropopause, cumulus congestus around the melting level, and shallow convection regimes) realized by the cloud-resolving simulation and also seen in observations. The lack of shallow convection and cumulus congestus leads to an overprediction of the low-level moisture. (iv) The simulations are sensitive to the magnitude of moisture feedback from the convective parameterization to the grid scale but less sensitive to whether the moisture is in vapor or condensed phase.

These deficiencies are mostly a consequence of the single-plume model that represents updrafts and downdrafts in the parameterization scheme, along with the lack of a shallow convection scheme. A more realistic model of entrainment and detrainment that reduces overshoot and represents the cumulus congestus is required. Realistic downdraft detrainment and relative humidity are needed to improve the downdraft parameterization and alleviate the surface temperature bias.

Corresponding author address: Dr. Changhai Liu, National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307-3000.Email: chliu@ucar.edu

Abstract

Convection and cloud processes are examined in a hierarchy of two-dimensional numerical realizations of cloud systems observed during the 19–26 December 1992 period of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The hierarchy consists of cloud-resolving simulations at a 2-km resolution, and two sets of 15-km resolution simulations; one attempts to treat convection explicitly and the other parameterizes convection using the Kain–Fritsch scheme.

The Kain–Fritsch parameterization shows reasonable results but shortcomings are found in comparison with the cloud-resolving model. (i) The entraining plumes in the parameterization excessively overshoot the tropopause, which produces a cold bias mostly through adiabatic cooling. The attendant moisture detrainment overproduces cirrus cloud. (ii) Because parameterized downdrafts detrain at the lowest level they generate a surface cold bias. (iii) The scheme fails to represent the trimodal convection (cumulonimbus reaching the tropopause, cumulus congestus around the melting level, and shallow convection regimes) realized by the cloud-resolving simulation and also seen in observations. The lack of shallow convection and cumulus congestus leads to an overprediction of the low-level moisture. (iv) The simulations are sensitive to the magnitude of moisture feedback from the convective parameterization to the grid scale but less sensitive to whether the moisture is in vapor or condensed phase.

These deficiencies are mostly a consequence of the single-plume model that represents updrafts and downdrafts in the parameterization scheme, along with the lack of a shallow convection scheme. A more realistic model of entrainment and detrainment that reduces overshoot and represents the cumulus congestus is required. Realistic downdraft detrainment and relative humidity are needed to improve the downdraft parameterization and alleviate the surface temperature bias.

Corresponding author address: Dr. Changhai Liu, National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307-3000.Email: chliu@ucar.edu

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