The role of cloud height and warming in the decadal weakening of atmospheric heat source over the Tibetan Plateau

The warming over the Tibetan Plateau (TP) is very significant during last 30 years, but the thermal forcing has been weakened. The thermal weakening is attributed mainly to the enhancement of the TOA (top of atmosphere) outgoing radiation. This enhancement is opposite to the greenhouse-gas-induced weakening of the global mean TOA outgoing radiation and is also unable to be explained by the observed decrease of total cloud cover. This study presents the importance of cloud height change and the warming over the TP in modulating the TOA radiation budget and thus the thermal forcing during spring and summer. On the basis of surface observations and satellite radiation data, we found that both the TOA outgoing shortwave radiation and longwave radiation were enhanced during this period. The former enhancement is due mainly to the increase of low-level cloud cover, which has a strong reflection to shortwave radiation, especially in summer. The latter enhancement is caused mainly by the planetary warming, and it is further enhanced by the decrease of total cloud cover in spring, as clouds extinguish outgoing longwave radiation emitted from the land surface. Therefore, the radiative cooling enhancement and thus the thermal weakening over the TP is a response of the earth-atmosphere system to the unique change of cloud cover configuration and the rapid warming of the land surface. However, these trends in cloud cover and TOA outgoing radiation are not well represented in four reanalyses.