Hourly infrared (IR) brightness temperature (BT) derived from China's first operational geostationary meteorological satellite Feng-Yun (FY)-2C is analyzed over the southern contiguous China (20°N–33°N, 100°E–122°E) during 2005–2008. The main focus is to investigate the diurnal variation of clouds and compare the different features between the southwestern and southeastern China. The clouds are first divided into three categories by the cloud top temperature (CTT, derived from IR BT) according to their diurnal features in summer. The cold cloud (CC, defined as CTT lower than −30°C) occurs most frequently in the late afternoon in most regions over southern China. The frequency of the middle cloud (MC, defined as CTT between −30°C and 0°C) reaches its diurnal maxima in the early evening to the midnight. The frequency of warm cloud (WC, defined as CTT warmer than 0°C) peaks around noon. There are also distinct regional differences of the diurnal variation of summer CC and MC frequencies. The diurnal variation of CC, MC and WC frequencies also show evident seasonal changes. The late afternoon peak of CC frequency appears in almost all months, and in the cold seasons, there is also a midnight to late evening secondary peak. The MC frequency reaches its diurnal peak in the early evening in summer and in the late evening in other seasons. The diurnal amplitude of WC frequency is smaller in warmer seasons and larger in cold seasons, whereas its diurnal phase shows no evident seasonal changes.
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