Comparison of the seasonal cycle of tropical and subtropical precipitation over East Asian monsoon area

Abstract

The East Asian monsoon includes the tropical monsoon and subtropical monsoon. They are different systems, and the formation mechanisms and major weather systems are not similar, and thus, the structures and characteristics of the associated tropical and subtropical monsoon rainfall and precipitation differ. The properties of precipitation can indicate thermodynamic and dynamic structures and cloud microphysics, and can reflect many characteristics associated with the onset and variation of monsoon, so is the focus of this paper. Four regions (one for tropical monsoon, three for subtropical monsoon) are selected. The components of precipitation (i.e., convective and stratiform rain) are studied by using 10-year (1998-2007) Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) rain products (2A25). The results show that the convective rain contribution in the tropical monsoon region (10°-20°N) is much higher than that in the subtropical monsoon regions (20°-35°N). Time series of rainfall, rain probability and rain ratios over each region are analyzed to study the seasonal variability of tropical and subtropical monsoon. Results show that after East Asian summer monsoon onset, rapid increases in convective, stratiform and total rainfall occur in all the four regions. The percentage of convective rain increases significantly in the three subtropical monsoon regions after monsoon onset, but decreases in the tropical monsoon region. TRMM PR observations show that there is no striking difference in the temporal variability in either the rain amount or probability between the East Asian tropical and subtropical monsoon; however, the associated convective rain contribution is significantly different, and East Asian tropical and subtropical monsoon precipitations can thus be separated according to the ratios of convective and stratiform precipitation. Intergovernmental Panel on Climate Change (IPCC) Coupled Model Inter-comparison Project phase 3 (CMIP3) general circulation model (GCM) rainfall simulation results (1980-1999) are evaluated against 10- year TRMM observations. The results show that the GCMs have a better simulation of the subtropical monsoon regions (20°N-35°N) than the tropical region, with a consistent variability of convective rain relative to the TRMM observations, during the convective rainfall surge after the monsoon onset, with about 10% deviation. But the GCM simulated convective rain fraction is generally overestimated in East Asia. The largest precipitation biases are found in the tropical monsoon region (0°-20°N), where there is more than 30% deviation for convective rain relative to TRMM observations. The TRMM rain measurements indicate that the contribution of convection precipitation in the tropical monsoon region falls considerably after onset of the East Asian monsoon. However, the CMIP3 GCMs show the opposite. Results from TRMM PR observations demonstrate that convective precipitation and stratiform precipitation have similar spatial- temporal distribution patterns and are closely correlated in both subtropical and tropical regions, but the GCMs simulate a small coefficient and even a negative correlation over tropical region. Current GCMs significantly underestimate stratiform rainfall over South China Sea, resulting in poor simulations of total rainfall and latent heat profiles; suggesting that some weather and climate phenomenon are not accurately simulated, and thus there is low confidence in GCM projections of East Asian monsoon. The results suggest that a better GCM parameterization scheme for convective and stratiform rain is needed for improving model simulation of the monsoons.