Relationships between Stability and Monsoon Convection

John L. McBride Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia

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William M. Frank Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Abstract

Rawinsonde data from the Australian Monsoon Experiment are analyzed to determine the manner in which the atmospheric stratification of density and moisture respond to large amounts of convective latent heat release. The study focuses on time series of data from a ship located at the northern end of the Gulf of Carpentaria during active and break periods of the monsoon.

Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to midtropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occurred in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection.

Convective activity is weakly but inversely related to convective available potential energy variations. Day-to-day variations in CAPE are dominated by variations in equivalent potential temperature of the source level (boundary layer) air. The physical effect is one of changing the moist adiabat along which the air parcel rises. In temperature–log pressure space, moist adiabats diverge in the upper half of the troposphere. Since CAPE variations are dominated by changes in the moist adiabat of the rising parcel, the day-to-day CAPE changes occur almost totally in positive area variations above the 600-hPa level.

The above results are discussed in the context of other studies in the literature. It is proposed that stabilization of the atmosphere in response to deep convection occurs almost entirely through the modification of CAPE through decreasing θe of the source air in the boundary layer. This occurs over relatively small spatial scales, whereas variations in lapse rate through the deep troposphere are hypothesized to occur over the relatively large scales associated with monsoon active and break events.

Corresponding author address: John L. McBride, BMRC, GPO Box 1289 K, Melbourne, Victoria 3001, Australia.

Abstract

Rawinsonde data from the Australian Monsoon Experiment are analyzed to determine the manner in which the atmospheric stratification of density and moisture respond to large amounts of convective latent heat release. The study focuses on time series of data from a ship located at the northern end of the Gulf of Carpentaria during active and break periods of the monsoon.

Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to midtropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occurred in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection.

Convective activity is weakly but inversely related to convective available potential energy variations. Day-to-day variations in CAPE are dominated by variations in equivalent potential temperature of the source level (boundary layer) air. The physical effect is one of changing the moist adiabat along which the air parcel rises. In temperature–log pressure space, moist adiabats diverge in the upper half of the troposphere. Since CAPE variations are dominated by changes in the moist adiabat of the rising parcel, the day-to-day CAPE changes occur almost totally in positive area variations above the 600-hPa level.

The above results are discussed in the context of other studies in the literature. It is proposed that stabilization of the atmosphere in response to deep convection occurs almost entirely through the modification of CAPE through decreasing θe of the source air in the boundary layer. This occurs over relatively small spatial scales, whereas variations in lapse rate through the deep troposphere are hypothesized to occur over the relatively large scales associated with monsoon active and break events.

Corresponding author address: John L. McBride, BMRC, GPO Box 1289 K, Melbourne, Victoria 3001, Australia.

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