Application of the 85 GHz ice scattering signature to a global study of mesoscale convective systems

Abstract

It has long been observed that tropical convection tends to cluster, organizing into multicellular mesoscale convective systems (MCS), In convective towers, updrafts on the order of 10 m s-I favor the formation of large, precipitation-sized ice, which strongly scatters upwelling 85 GHz radiation. The 85 GHz brightness temperature is related to the amount of ice in an MCS and, indirectly, to the strength of the updrafts producing the ice. This study used an objective technique to identify MCSs from the Special Sensor Microwave/Imager's 85 GHz channel to describe the size, intensity, and geographic distribution of MCSs between 35'N and 35'S for January, April, July, and October 1993. An MCS was defined as an area at least 2000 km 2 of 85 GHz polarization corrected brightness temperatures < 250 K, with a minimum brightness temperature < 225 K. The distribution of the MCS population defined by the objective technique was consistent with IR-based cloud climatologies and large scale seasonal dynamics. Tropical South America, tropical Afiica, and the oceanic wann pool had the greatest number of MCSS. These regions also had the smallest median areas. The sub-tropical oceans had the largest median areas, 10 3 kM2 greater than other regions. North America had the largest number of very large (> 1 0 5 @) MCSS. Over the oceans, median brightness temperature depression was 20-30 K below 225 K, while over land, it was 35-45 K below 225 K. Tropical Africa had the coldest median minimum brightness temperature. The sub-tropical oceans had the warmest median minimum brightness temperatures. MCSs were 35% more frequent over the oceans at sunrise than at sunset, and 60% more frequent over tropical continents at sunset than at sunrise. Both continental and oceanic MCSs tended to be larger at sunrise than at sunset. Continental MCSs tended to be colder at sunset than at sunrise, but oceanic MCSs tended to be marginally colder at sunrise than at sunset. In general, continental MCSs appeared to be smaller and more intense than oceanic MCSS, and the largest and the most intense MCSs occurred more frequently in the sub-tropics.