An Assessment Of Land Radiative Energy Balance Over East Asia And Study Of Its Potential Factors

The genesis and evolution of Earth’s climate is largely determined by the global energy balance and its spatial and temporal variations.Current estimates of Earth’s energy balance are not only limited to the global scales,the regional budgets show larger uncertainties due to their unique geographic and climatic characteristics.This paper focuses on East Asia with high emissions of aerosols and greenhouse gases,and nearly one fifth of its land area is covered by the known world’s“Third Pole”of the Tibet Plateau（TP）,significantly affecting the climate system through its orographic and thermal effects.Thus,knowledge on the energy budget over this region has been widely concerned.This study first attempts to examine the performances of the radiation budgets from multiple lines of evidence over East Asian land by combining surface and satellite observations,as well as the Coupled Model Intercomparison Project phase 6（CMIP6）simulations and atmospheric reanalysis.Then,we estimate the energy budget over this region and make deeper insights into the energy budget differences over East Asian and global land to explore the important roles of aerosols,clouds,and TP in between them.The cloud radiative effects（CREs）at the TOA,within the atmosphere,and at the surface is further quantified over this region.The main conclusions are summarized as follows:（1）Compared to the radiation budgets at the top of the atmosphere（TOA）derived from CERES product,the representations of aerosols and clouds over TP,southern China,especially over East TP,should be improved in the CMIP6 models and ERA5 reanalysis.The CERES-derived surface solar radiation（SSR）among all the three datasets shows the highest consistency with the surface observation under both all-sky and clear-sky conditions.Additionally,among all the aforementioned SSR estimates,the East Asian urban sites are in general more significantly overestimated than the rural sites on average compared to the surface observations,indicating that rural stations might show higher accuracy than the urban ones due to less effects from human activities.（2）Compared to the global land budget,a substantially larger fraction of atmospheric shortwave radiation of 5.2%is reflected,highly associated with the higher aerosol loadings and more clouds over East Asian land.While a slightly smaller fraction of atmospheric shortwave absorption of 0.6%is unexpectedly estimated,possibly related to the lower water vapor content effects due to the thinner air over the TP to overcompensate for the aerosol and cloud effects over East Asian land.The weaker greenhouse effect and fewer low clouds due to the TP are very likely the causes for the smaller fraction of East Asian-land surface downward longwave radiation.Hence,aerosols,clouds,and the TP over East Asia play vital roles in the shortwave budgets,while the TP is responsible for the longwave budgets during this regional energy budget assessment.（3）The importance of clouds on land energy balance over East Asia.Comparisons between the all-sky and clear-sky energy budgets indicate that the overall effects of clouds greatly reduce the surface solar absorption by about 15.3%and enhance that within the atmosphere by 1.5%.The further obtained CREs suggest that the presence of clouds results in a net CRE at the TOA of-22 W m^-2,a more cooling effect on the regional climate system than that over globe(-19W m^-2).Moreover,the area-weighted averages of total and low cloud cover from CERES-derived product indicate a slightly more total cloud cover of 1.1%with more reflecting solar radiation within the atmoaphere and fewer low cloud cover of 3.7%over East Asian land,possibly contributing to the relative lower percentage of surface downward longwave radiation compared to global land.As a major component of Earth’s energy balance,the solar radiation reaching the Earth’s surface also shows the largest uncertanties among all the substantially differing all-sky surface radiation components.To improve the accuracy of the surface energy balance assessment,deeper insights into the major factors influencing the SSR variations are of particular significance.Considering that the majority of surface sites and the largest land area over East Asia are located in China,the annual mean SSR trends under all-sky,clear-sky,all-sky-no-aerosol,and clear-sky-no-aerosol conditions as well as their possible causes（aerosols,clouds,water vapor,and ozone）are analyzed during 2005-2018 over China based on different satellite-retrieved datasets to determine the major drivers of the trends.Then,the yearly annual and seasonal means of these factors during this period are used as inputs into a radiative transfer model BCC＿RAD to examine the relative contributions of cloud cover from different cloud types to all-types-of-cloud-cover-induced（ACC-induced）SSR.The results confirm clouds and aerosols as the major contributors to such all-sky SSR trends over China compared to those of water vapor and ozone but playing different roles over sub-regions.Aerosol variations during this period result in a widespread brightening,while cloud effects show opposite trends from south to north.Moreover,aerosols contribute more to the increasing all-sky SSR trends over northern and northeastern China,while clouds dominate the SSR declines over southern China.Furthermore,simulations point out that the relative contributions of cloud cover from different cloud types to the ACC-induced SSR trends over China in recent years largely depend on sub-regions and seasons,and the changes in HCC（High cloud cover）usually contribute more to the ACC-induced SSR trends in summer over most regions in China,which is possibly associated with the deep convection in this season.However,the contributions from LCC（Low cloud cover）or mid-LCC（Medium low cloud cover）are responsible for the ACC-induced SSR trends over most regions all year round.