Analysis of the biases in the cloud radiative feature simulations over eastern China as done by the BCC_AGCM2.1

The cloud radiative features over eastern China simulated by the general circulation model (BCC_AGCM2.1), which is developed at National Climate Center, China Meteorological Administration (CMA), are evaluated based on the observational and reanalysis data. The reasons for biases are analyzed in this paper. As for the climatological mean pattern, the results indicate that BCC_AGCM2.1 can basically reproduce the maxima center of stratiform cloud. The features of maximum cloud fractions in winter and minimum cloud fractions in summer are all reasonably reproduced, and the simulated cloud radiative forcing also has the corresponding features. On the other hand, the model can simulate the similar interaction processes between the cloud radiative forcing and the surface temperature to observations, namely the stratiform cloud increasing and negative net cloud forcing strengthening during the cooling period and the opposite evolution during the warming period. However, model systematically underestimates the continental stratiform clouds (especially in cold seasons), weakening the short wave cloud forcing. The reasons causing the stratiform cloud bias lie mainly in the evidently weakening of the southerly flow at the lower troposphere over southwest China and the shortage of land atmosphere latent heat. The weakening of southerly flow drives the low level divergence and vertical motions to go against the formation of mid level cloud. The weakening of the southerly flow is also unfavorable to the water vapor transportation from the ocean to the southwest area. Along with the discrepancy of surface upward latent heat in the model, these two aspects all result in the water vapor shortage and lower relative humidity at the lower troposphere over southwest area, which obstruct the formation and development of stratiform cloud. The shortage of water vapor further leads to the weakening of the cloud radiative forcing surface temperature feedback in the case of cold anomaly downstream of the Tibetan Plateau, namely the lower water vapor content weakening the feedback process at the temperature decreasing period.