Classification of clouds over the western equatorial Pacific Ocean using combined infrared and microwave satellite data

A new cloud classification scheme is presented that combines infrared and microwave satellite data. Because microwave radiation can penetrate deep into the cloud layer, this scheme is able to determine characteristics for both thin and deep clouds. Additionally, the new scheme can provide information on precipitation, which traditional infrared-visible cloud classification schemes have been unable to. The proposed cloud classification scheme utilizes the cloud top temperature obtained from infrared measurements and a microwave index that includes both emission and scattering signals. The following eight cloud classes are defined: warm nonprecipitating cloud, warm precipitating cloud, midtop nonprecipitating cloud, midtop precipitating cloud, thin high-top nonprecipitating cloud, deep high-top nonprecipitating cloud, anvil with stratiform precipitating cloud, and deep convective precipitating cloud. The classification scheme is validated by aircraft radar data obtained from Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The new cloud classification scheme is used to investigate the clouds in the western equatorial Pacific Ocean warm pool region for the period from November 1992 to February 1993, allowing us to obtain for the first time the spectra of cloud coverage and precipitation distribution as a function of cloud types. The analysis shows that most of the cloudiness in this region is associated with warm nonprecipitating clouds. Precipitating pixels comprise only about 15% of total cloud pixels. Of the precipitating pixels, about 50% have cloud top temperatures warmer than −40°C. Using rainfall rates obtained from microwave satellite data, the relative contribution by each precipitating cloud type is investigated. It is found that the precipitating clouds with cloud top temperature warmer than −40°C contribute at least 23% of the total rainfall amount, while the remaining 77% of total precipitation is divided almost equally by anvil with stratiform precipitating cloud and the deep convective precipitating cloud.

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