Diagnosis of Convective and Mesoscale Motions During Phase IH of Gate

Richard H. Johnson Department of Atmospheric Science, Colorado State University, Fort Collins 80523

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Abstract

Observational evidence from GATE suggests that a significant fraction of precipitation in tropical disturbances over the eastern Atlantic occurs beneath anvil-type mesoscale convective systems that are frequently associated with intense cumulonimbus convection. A diagnostic model is developed that includes heat, moisture and mass transports by both the convective-scale and mesoscale components of these systems. An entraining plume model is used for individual convective-scale elements yielding a spectral distribution of cumulus sizes. Mesoscale downdrafts are modeled using a bulk cloud model. A parameterization links properties of the mesoscale systems to those of associated deep convective-scale (cumulonimbus) clouds. A method based on a subjective comparison of model results to observed cloud cover is used to determine mesoscale downdraft areas in relation to cumulonimbus areas. For this reason only qualitative results derived from application of the theory can be considered reliable.

The model is applied to GATE tropical-wave composite data (Thompson et al., 1979) for Phase III. Radiative beating rates derived from observations during this period and storage of water in clouds determined from satellite and ground-based whole-sky camera cloud coverage data are included in the study. Diagnostic model results may be interpreted as indicating the existence of deep convection and mesoscale downdraft systems well in advance of the composite wave trough, followed by suppressed low-level cloud activity with a later increase in deep cumulus convective and mesoscale downdrafts to a maximum centered at the trough axis. Multiple layers of cumulus cloud detrainment are diagnosed to occur in the vicinity of the wave trough. Convective-scale and mesoscale downdrafts are diagnosed to be important contributors to the mass, heat and water budgets of the composite wave, indicating that their effects should be included in cumulus parameterization schemes.

Abstract

Observational evidence from GATE suggests that a significant fraction of precipitation in tropical disturbances over the eastern Atlantic occurs beneath anvil-type mesoscale convective systems that are frequently associated with intense cumulonimbus convection. A diagnostic model is developed that includes heat, moisture and mass transports by both the convective-scale and mesoscale components of these systems. An entraining plume model is used for individual convective-scale elements yielding a spectral distribution of cumulus sizes. Mesoscale downdrafts are modeled using a bulk cloud model. A parameterization links properties of the mesoscale systems to those of associated deep convective-scale (cumulonimbus) clouds. A method based on a subjective comparison of model results to observed cloud cover is used to determine mesoscale downdraft areas in relation to cumulonimbus areas. For this reason only qualitative results derived from application of the theory can be considered reliable.

The model is applied to GATE tropical-wave composite data (Thompson et al., 1979) for Phase III. Radiative beating rates derived from observations during this period and storage of water in clouds determined from satellite and ground-based whole-sky camera cloud coverage data are included in the study. Diagnostic model results may be interpreted as indicating the existence of deep convection and mesoscale downdraft systems well in advance of the composite wave trough, followed by suppressed low-level cloud activity with a later increase in deep cumulus convective and mesoscale downdrafts to a maximum centered at the trough axis. Multiple layers of cumulus cloud detrainment are diagnosed to occur in the vicinity of the wave trough. Convective-scale and mesoscale downdrafts are diagnosed to be important contributors to the mass, heat and water budgets of the composite wave, indicating that their effects should be included in cumulus parameterization schemes.

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