Diurnal Characteristics Of Cloudiness And Precipitation Over The Asia Monsoon Region

Up to now, climate models have difficulties in simulating the weather and climate in the East Asia, especially in simulating cloud and precipitation. One important reason is lacking of comprehension of cloud and precipitation variations. Nowadays, the diurnal and seasonal cycles of cloud and precipitation have been paid more and more attention to and have become one of the most important aspects for evaluating model's simulation ability. In this paper, we analysed multi-temporal and multi-spatial distribution of cloudiness and focused on the diurnal characteristics of cloud and precipitation over the Asia monsoon region for the period from July 2005 to June 2007. We also explored the possible physical mechanism of nocturnal precipitation over the Sichuan basin. Many datasets about cloud and precipitation have been used, including the cloud detection dataset derived from Fengyun-2C (FY-2C), the International Satellite Cloud Climatology Project (ISCCP), the Moderate Resolution Imaging Spectroradiometer (MODIS) on board Terra and Aqua satellites, the conventional ground observation cloudiness dataset (GO), NOAA CPC Morphing Technique (CMORPH) precipitation dataset, and NCEP reanalysis 2 dataset. The qualities of FY-2C cloudiness and CMORPH precipitation have also been evaluated. The main conclusions are as follows:1. FY-2C is the first operational geostationary meteorological satellite in China. It is shown that the FY-2C cloud mask performs consistently with other cloud mask products over the Asia monsoon region. In the boreal winter, the whole domain is dry with little cloudiness. More extensive cloudiness can be observed over the Sichuan Basin, in the East China Sea and the South China Sea, along the northwestern border of China and around the ITCZ in the Southern Hemisphere. In the boreal summer, the summer monsoon is the dominant system for the studied domain, which is generally humid with extensive cloudiness, corresponding to zones of strong convective activities. Diurnal cycles of FY-2C cloudiness for the four seasons of a year are analyzed. The diurnal range of cloudiness is generally larger over land than over ocean. Remarkable diurnal variation is found over the Tibetan Plateau, the northern part of the Indian Peninsula, and central Asia where there is generally less precipitation. The peaks of diurnal cycle of cloudiness appear around local noon over the subtropical land, in the morning over the Indian Peninsula, and in the afternoon near the equator.2. Results also reveal considerable discrepancies among different cloud products over extended areas of North China and Mongolia. The Sichuan Basin is another region of large discrepancies among the four cloud products. It may be related with the different ways between satellite and surface observations, the retrieval methods among different satellite datasets, the characteristics of underlying surface, and the continuing cloud cover over some regions.3. Comparing to the ground observation precipitation dataset, the CMORPH product well captures each precipitation events, but overestimates rainfall frequency and underestimates its intensity. If we use 0.01 mm/hour instead of 0 mm/hour to estimate whether it rains or not, the excessive frequency shown by the CMORPH product could be well modulated and seemed more resonably.4. Large difference in the diurnal cycle of rainfall between land and sea is evident in terms of intensity and frequency in the season of spring, summer and autumn. The diurnal cycle of annual mean precipitation over land tends to reach the maximum in either late-morning or afternoon, while precipitation over sea has a late-evening or early-morning peak. In contrast with the weak diurnal variation of precipitation over sea, rainfall at land shows a robust diurnal variation which depends on different regions. The characteristics of diurnal cycle over maritime continent resemble to that over land. The diurnal cycle of cloudiness is coincident to that of precipitation, and also depends on regions.5. Precipitation over Sichuan basin generally occurs in the period from mid-night to early morning. The possible physical mechanism responsible for that may be as follows: there is large temperature difference between day and night over the Tibetan Plateau which is exactly west to the Sichuan basin. The coldest atmospheric temperature over the Tibetan Plateau just occurs from mid-night to early morning so that there forms the strongest thermal gradient between the Tibetan Plateau and its surrounding area. It causes the strongest southwesterly wind in the lower troposphere over the basin which brings rich water vapor transported from the Bay of Bengal.