Hourly Precipitation Biases and Clausius-Clapeyron Scaling in Convection-Resolving and Convection-Parameterizing Regional Climate Models

Regional climate models (RCMs) are crucial tools for understanding and predicting climate change and its impacts, such as precipitation extremes. We investigate the characteristics of hourly precipitation and the associated extremes in RCM ensembles with two resolutions: km-scale (the CORDEX-FPS Convection ensemble with ~3 km grid spacing, where deep convection is represented explicitly), and coarser-scale (~12 km grid spacing, with parameterized convection). The km-scale ensemble is downscaled from the coarser one, and both cover three time periods: evaluation, historical, and end-of-the-century period under the RCP8.5 warming scenario (2000-2009, 1996-2005, and 2090-2099, respectively). Evaluating the model ensembles against data from 179 weather stations in Austria, we study how the intensity, duration, and the time of onset of precipitation depend on mean daily temperature. We then examine how these characteristics change under warming conditions.

It is well established that over the Alps the coarser RCMs produce too much light and persistent precipitation which is triggered too early in the day. We find that these shortcomings in models with parameterized convection become more pronounced with rising temperatures. We show that the km-scale ensemble closely matches observations and greatly outperforms the coarser ensemble in capturing the investigated hourly precipitation characteristics, especially at higher temperatures and on days with heavy rainfall. As high temperatures are expected to become more common in future climates, our results imply that coarser RCMs suffer from more severe biases in hourly precipitation in the future than under present climate conditions, especially for short-duration extremes. 

In this light, we also assess the ability of both km-scale and coarser RCM ensembles to capture the Clausius-Clapeyron scaling of extreme precipitation with temperature, and discuss how model deficiencies in the coarser ensemble affect this relationship.

In summary, our findings highlight the importance of km-scale RCMs for accurate simulations of hourly precipitation and its extremes, particularly in the warming climate.