A Systematic Tropospheric Dry Bias in the Tropics in CMIP5 Models: Relationship between Water Vapor and Rainfall Characteristics

This study investigated the absolute values of column-integrated water vapor (precipitable water; PW) in the climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5), in terms of the relation ships between PW and precipitation characteristics. We identified that global mean PW values are systematically much lower in CMIP5 models than in observations. This dry bias is most profound over the tropical ocean. The dry bias is partly due to biases in sea surface temperatures in the CMIP5-coupled climate models. However, the dry bias is also present in Atmospheric Model Intercomparison Project (AMIP) experiments, which implies the existence of other factors. The relationship between PW and rainfall characteristics shows that rainfall occurs when water vapor levels are lower than in observations, particularly in models with a relatively strong dry bias. This suggests that the reproducibility of rainfall characteristics may be associated with the dry bias.