Observed Evolution of Tropical Deep Convective Events and Their Environment

Steven C. Sherwood School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand

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Ralph Wahrlich School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand

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Abstract

Using a compositing technique, the temporal progression of tropical convective systems and the mean atmospheric state in their vicinity is constructed from a time series of geostationary satellite and operational rawinsonde data. The technique establishes the stage in the life cycle of convection prevailing at a given place and time, by a simple objective method using time series of satellite brightness temperature (Tb) histograms collected from a region surrounding the site. Soundings are classified according to their placement in the convective life cycle, and composites formed that represent the areal-mean state of the convecting atmosphere at each stage, for several scales of horizontal averaging.

The temporal structure found here for the mesoscale-mean atmosphere closely resembles existing observations of the horizontal structure of a tropical squall line, albeit with certain reductions in amplitude and stabilization rate. This supports the generality of previous findings that the physical mechanisms documented for squall line systems are characteristic of other forms of tropical convection, and quantifies their imprint on thermodynamic mean fields at large spatial scales. The results show an instability decay time during convection of about 3 h at the 120-km horizontal scale. This time grows with scale, as does the duration of the mature stage of convection, which is of similar magnitude. The results also show a column-integrated loss of water vapor and moist static energy following convection. These results may be of use in model validation and theoretical treatments of convective interaction with dynamics.

Corresponding author address: Dr. Steven Sherwood, NASA/Goddard Space Flight Center, Mail Code 916, Greenbelt, MD 20771.

Email: sherwood@zephy.gsfc.nasa.gov

Abstract

Using a compositing technique, the temporal progression of tropical convective systems and the mean atmospheric state in their vicinity is constructed from a time series of geostationary satellite and operational rawinsonde data. The technique establishes the stage in the life cycle of convection prevailing at a given place and time, by a simple objective method using time series of satellite brightness temperature (Tb) histograms collected from a region surrounding the site. Soundings are classified according to their placement in the convective life cycle, and composites formed that represent the areal-mean state of the convecting atmosphere at each stage, for several scales of horizontal averaging.

The temporal structure found here for the mesoscale-mean atmosphere closely resembles existing observations of the horizontal structure of a tropical squall line, albeit with certain reductions in amplitude and stabilization rate. This supports the generality of previous findings that the physical mechanisms documented for squall line systems are characteristic of other forms of tropical convection, and quantifies their imprint on thermodynamic mean fields at large spatial scales. The results show an instability decay time during convection of about 3 h at the 120-km horizontal scale. This time grows with scale, as does the duration of the mature stage of convection, which is of similar magnitude. The results also show a column-integrated loss of water vapor and moist static energy following convection. These results may be of use in model validation and theoretical treatments of convective interaction with dynamics.

Corresponding author address: Dr. Steven Sherwood, NASA/Goddard Space Flight Center, Mail Code 916, Greenbelt, MD 20771.

Email: sherwood@zephy.gsfc.nasa.gov

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