Long-term Trend of Cloud Optical Thickness in East Asia and Its Impact on Regional Radiation Budget

The long-term changes of cloud optical thickness (COT) in East Asia and their influence on local radiation budget were studied quantitatively in this paper. Firstly, we analyzed the 18-year (March 2000 to February 2018) trend of COT over East Asia based on retrievals from the Moderate Resolution Imaging Spectrometer (MODIS) by different statistical methods, and found that there exist obvious long-term trend of COT over four sub-regions in East Asia, respectively. The annual mean water cloud COT_w increased trend of +0.05/a was found over northeastern China and the eastern China ocean, mainly caused by cloud microphysical process, whereas decreased trend of -0.07/a and -0.05/yr was found over southern China and southern Japan, respectively, mainly caused by atmospheric dynamical processes. An increasing trend of ice cloud COTi was found over northeastern China and northern Japan, with +0.06/yr and +0.08/yr, respectively caused by both microphysical and dynamical processes. Whereas a decreasing trend over the eastern Mongolian Plateau (-0.02/yr) and Tibetan Plateau (-0.07/yr) was found, mainly caused by the dynamical process. Then, the above observational trends are put into the radiative transfer model of BCC_RAD to calculate cloud radiative forcing (CRF) changes. Thus, the impact of COT long-term changes on local radiation budgets was obtained. The annual mean shortwave-, longwave- and total CRF at the top-of-atmosphere (TOA) caused by COT_w changes in East Asia were -72.4, 15.7 and -56.7 W m^-2, respectively. The corresponding CRF changes caused by COT_i changes were -88.5, 55.7 and -32.7 W m^-2, respectively. Both positive CRF (e.g. southern China, 0~0.4 W m^-2/yr) and negative CRF (e.g. northeastern China, -0.4~0 W m^-2/yr) occurred, indicating the effects of long-term COT changes on local radiative budget is large but the sign is opposite.