The Role of Cloud Radiative Forcing in the Asian-Pacific Summer Monsoon


Convective-cloud clusters with strong precipitation occur frequently in most of the Asian-Pacific summer monsoon (APSM) regions such as the Bay of Bengal (BOB), South China Sea (SCS), and Tropical Western North Pacific (TWNP). Cloud radiative forcing (CRF) is important in these regions. The net CRF at the top of the atmosphere (TOA) has shown large cooling over these APSM regions. This is on account of the presence of large amounts of high clouds with large optical depth. Through data analysis, the summer convective precipitation in TWNP is as strong as that in the BOB. However, the average net CRF at the TOA in the BOB (~ -36 W m-2) is twice as big as in the TWNP (~ -17 W m-2). The spectral analysis of cloud optical depth shows that in the BOB, the highest power is in the intra-seasonal timescale, while in the TWNP, the leading spectral peaks are less than 10 days. The radiative cooling from net CRF at the TOA could be associated with lowfrequency oscillation. The difference between the APSM regions is related to their sub-stages separating from CRF in time evolution.

In a convective system, convective clouds can detrain to form other high clouds. In the APSM regions, large areas of high-thin and high-thick clouds cause different CRF at the TOA. These two types of CRF relate to precipitation, atmospheric vertical motion, and cloud life cycles etc. and should be separated from the APSM time evolution. We divided the APSM precipitation into two categories. As in the heavy-precipitation stage, clouds with large optical depth shield solar radiation and cause local and instantaneous surface cooling. The outgoing longwave radiation (OLR) is generally lower than 210 (W m-2). The net CRF at the TOA is large negatively.

Besides, large high-thin clouds can be found in a stage of relatively small or no precipitation. The OLR in this stage has a broad range and the net CRF is small and could be either positive or negative. The major difference between the APSM regions occurs in this stage. In this stage at the BOB, significant high-thick clouds cause negative net CRF, while more than half of the SCS and TWNP at this stage is dominated by large amounts of cirrus clouds. The optically thin cirrus clouds with large spatial size and long lasting time are important modulators for modifying the net CRF at the TOA. The poor simulation of the APSM climate in general circulation models (GCMs) maybe associated with the inability for accurately simulating the role of cirrus clouds in this stage.

Based on the cloud classification of the International Satellite Cloud Climatology Project (ISCCP), we found a useful cloud-amount index from cloud amounts of cirrus minus the sum of deep convection and cirrostratus. The index can effectively separate different characteristics of CRF from the APSM time evolution. The cloud-amount index should be more appropriate for APSM studies and model simulations instead of considering only one cloud type in convective systems.

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