Rainfall‐aerosol relationships explained by wet scavenging and humidity

Abstract

AbstractRelationships between precipitation rate and aerosol optical depth, the extinction of light by aerosol in an atmospheric column, have been observed in satellite‐retrieved data. What are the reasons for these precipitation‐aerosol relationships? We investigate relationships between convective precipitation rate (Rconv) and aerosol optical depth (τtot) using the ECHAM5‐HAM aerosol‐climate model. We show that negative Rconv‐τtot relationships arise due to wet scavenging of aerosol. The apparent lack of negative Rconv‐τtot relationships in satellite‐retrieved data is likely because the satellite data do not sample wet scavenging events. When convective wet scavenging is excluded in the model, we find positive Rconv‐τtot relationships in regions where convective precipitation is the dominant form of model precipitation. The spatial distribution of these relationships is in good agreement with satellite‐based results. We further demonstrate that a substantial component of these positive relationships arises due to covariation with large‐scale relative humidity. Although the interpretation of precipitation‐aerosol relationships remains a challenging question, we suggest that progress can be made through a synergy between observations and models.