Re-examining the Non-Linear Moisture-Precipitation Relationship in SSM/I Version 7

Bretherton et al. (2004) used the Special Sensor Microwave Imager (SSM/I) version 5 product to derive an exponential curve that describes the relationship between precipitation and column relative humidity (CRH) over the tropical oceans. The curve, which features a precipitation pickup at a CRH between 0.7 and 0.8 and a rapid increase of precipitation with CRH after the pickup, has widely been used in the studies of the tropical atmosphere.

This study re-examines the moisture-precipitation relationship by using the version 7 SSM/I data, in which several biases in the previous version’s retrieval algorithm are corrected, and evaluates the relationship in the Coupled Model Intercomparison Project phase 5 (CMIP5) models. In the new curve, the precipitation pickup occurs at a higher CRH and precipitation increases more slowly with CRH. In most CMIP5 models, the precipitation pickup is too early due to the common model bias of overestimated (underestimated) precipitation in the dry (wet) regime.