Defining Mesoscale Convective Systems by Their 85-GHz Ice-Scattering Signatures

Karen I. Mohr Department of Meteorology, Texas A&M University, College Station, Texas

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Edward J. Zipser Department of Meteorology, Texas A&M University, College Station, Texas

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Mesoscale convective systems are composed of numerous deep convective cells with varying amounts of large, convectively produced ice particles aloft. The magnitude of the 85-GHz brightness temperature depression resulting from scattering by large ice is believed to be related to the convective intensity and to the magnitude of the convective fluxes through a deep layer. The 85-GHz ice-scattering signature can be used to map the distribution of organized mesoscale regions of convectively produced large ice particles. The purpose of this article is to demonstrate the usefulness of the 85-GHz ice-scattering signature for describing the frequency, convective intensity, and geographic distribution of mesoscale convective systems.

Objective criteria were developed to identify mesoscale convective systems from raw data from January, April, July, and October 1993. To minimize the effects of background contamination and to ensure that bounded areas contained convective elements, a “mesoscale convective system” was defined as an area bounded by 250 K of at least 2000 km2 of 85 GHz, with a minimum brightness temperature ≤ 225 K. Mesoscale convective systems extracted from the raw data were sorted and plotted by their areas and by their minimum brightness temperatures. Four area and brightness temperature classes were used to account for a spectrum of organized convection ranging from small to very large and from less organized to highly organized. The populations of mesoscale convective systems by this study's definition were consistent with infrared-based climatologies and large-scale seasonal dynamics. Land/water differences were highlighted by the plots of minimum brightness temperature. Most of the intense mesoscale convective systems were located on or near land and seemed to occur most frequently in particular areas in North America, South America, Africa, and India.

Corresponding author address: Dr. Edward Zipser, Department of Meteorology, Texas A&M University, College Station, TX 77843-3150. E-mail: zipser@ariel.tamu.edu

Mesoscale convective systems are composed of numerous deep convective cells with varying amounts of large, convectively produced ice particles aloft. The magnitude of the 85-GHz brightness temperature depression resulting from scattering by large ice is believed to be related to the convective intensity and to the magnitude of the convective fluxes through a deep layer. The 85-GHz ice-scattering signature can be used to map the distribution of organized mesoscale regions of convectively produced large ice particles. The purpose of this article is to demonstrate the usefulness of the 85-GHz ice-scattering signature for describing the frequency, convective intensity, and geographic distribution of mesoscale convective systems.

Objective criteria were developed to identify mesoscale convective systems from raw data from January, April, July, and October 1993. To minimize the effects of background contamination and to ensure that bounded areas contained convective elements, a “mesoscale convective system” was defined as an area bounded by 250 K of at least 2000 km2 of 85 GHz, with a minimum brightness temperature ≤ 225 K. Mesoscale convective systems extracted from the raw data were sorted and plotted by their areas and by their minimum brightness temperatures. Four area and brightness temperature classes were used to account for a spectrum of organized convection ranging from small to very large and from less organized to highly organized. The populations of mesoscale convective systems by this study's definition were consistent with infrared-based climatologies and large-scale seasonal dynamics. Land/water differences were highlighted by the plots of minimum brightness temperature. Most of the intense mesoscale convective systems were located on or near land and seemed to occur most frequently in particular areas in North America, South America, Africa, and India.

Corresponding author address: Dr. Edward Zipser, Department of Meteorology, Texas A&M University, College Station, TX 77843-3150. E-mail: zipser@ariel.tamu.edu
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