Precipitable water vapor in regional climate models over Ethiopia: model evaluation and climate projections

Precipitable Water Vapor (PWV) has strong relations with extreme rainfall and their increments in a future warming world are typically associated. It is, however, unclear how different climatic conditions and orographic effects modulate these changes in the equatorial region. We investigate PWV and heavy rainfall over Ethiopia using Regional Climate Models (RCMs) from the Coordinated Regional Climate Downscaling Experiment (CORDEX). An in-depth RCM evaluation is first provided by comparing the modeled annual cycle of PWV with those obtained from Global Positioning System observations and reanalysis, and, by investigating the changes in PWV before and after a heavy-rainfall event. Two characteristic timescales are found for the buildup and decline of PWV before and after such events: a short of about 2 days and a long timescale extending beyond ten days. Overall RCMs reproduce well the PWV annual cycle but substantial biases appear for some models in the very dry and in the tropical wet climate zones. CORDEX models simulate well the peak in PWV anomalies at the day of a heavy-rainfall event but strongly overestimate the timescales of buildup and decline. Future scenarios all point towards a PWV increase (up to 40%) for end-of-the-century RCP8.5 with limited spatial and seasonal variations. PWV changes align with near-surface temperature changes at a rate of 7.7% per degree warming. Changes in daily heavy rainfall, on the other hand, are lower especially in northwestern Ethiopia in the far future (RCP8.5), potentially caused by an overall drying.