Interpretation of Cloud Classification Using the Color Image Composed by Three-channel Data

Clouds are the result of atmosphere dynamical and thermo-dynamical process. Different cloud types reflect different weather situations and cloud microphysical structure features. Satellite image offers large-area and all-day information of the cloud formation, gathering or disperse. Cloud classification is one of the most important objectives of the satellite image research.At present, only one single channel can be used to identify cloud category at one time. A quick, direct and accurate method of the interpretation of cloud classification has not yet been developed. It becomes particularly important to get the composed information of cloud optical thickness, effective particle radius and cloud top height information quickly.In order to better use the multiple spectral data of Medium Resolution Spectral Imager (MERSI) on FY-3A meteorological satellite to carefully analyze the macro and micro physical parameters of cloud system, according to the fact that the 0.65, 1.6 μm and 11.25 μm channel on MERSI is respectively sensitive to cloud optical thickness, effective particle radius and cloud top height based on the Santa Barbara DISORT Atmospheric Radiative Transfer, SBDART, which is that larger 0.65 μm reflectance means larger optical thickness, larger 1.6 μm reflectance means smaller effective particle radius, larger black body temperature of cloud top means lower cloud top height or warmer surface under clear air, the method of three color compositions is used to the explanation and interpretation of the cloud classification by FY-3A meteorological satellite data on 20 June 2009. The color is composed of red for visible reflectance, green for near infrared reflectance, and blue for the infrared brightness temperature. Redder means larger optical thickness, greener represents the smaller cloud top particles, and bluer means lower cloud tops or warmer surface under clear air.The technology makes different cloud types show in different colors, which is beneficial to directly distinguish cloud pixel from clear sky area, and the land boundary also can be recognized. In this case, cloud located at typhoon eye mainly shows orange-red, while spiral clouds band shows orange-yellow, indicating the reduced optical thickness compared with the typhoon eye. Mixed cloud system covering the Yangtze River Basin shows clear multiple-layer features, low layer cloud is yellow-green, middle layer cloud is dark-red, and high layer cloud shows orange-red. Over the sea, thin dark-yellow cirrus covers the marine stratiform cloud, which is white. Blue means clear air, but there is difference between sea and land because of the different temperature and reflectance.Besides the advantage of multiple spectrums, the 1000 m even 250 m resolution of FY-3A meteorological satellite makes the cloud detailed structure more clearly. The two advantages have greatly improved the accuracy of cloud classification.Meanwhile, the characteristic values for typical cloud, providing empirical values for the initial clustering center of fuzzy clustering method.