Decomposing the Precipitation Response to Climate Change in Convection Allowing Simulations over the Conterminous United States

Explicit representation of finer-scale processes can affect the sign and magnitude of the precipitation response to climate change between convection-permitting and convection-parameterizing models. We compare precipitation across two 15-year epochs, a historical (HIST) and an end-of-21st-century (EoC85), between a set of dynamically downscaled regional climate simulations at 3.75 km grid spacing (WRF) and bias-corrected Community Earth System Model (CESM) output used to initialize and force the lateral boundaries of the downscaled simulations. In the historical climate, the downscaled simulations demonstrate less overall error than CESM when compared to observations for most portions of the conterminous United States. Both sets of simulations overestimate the incidence of environments with moderate to high precipitable water while CESM generally simulates rainfall that is too frequent but less intense. Within both sets of simulations, EoC85 rainfall amounts decrease in low-moisture environments due to reduced rainfall frequency and intensity while rainfall amounts increase in high-moisture environments as they occur more often. Overall, reductions in rainfall are stronger in WRF than in CESM, particularly during the warm season. This reduced drying in CESM is attributed to relatively higher rainfall frequency in environments with high concentrations of precipitable water and weak vertical motion. As a result, an increase in the occurrence of high moisture environments in EoC85 naturally favors more rainfall in CESM than WRF. Our results present an in-depth examination of the characteristics of changes in overall accumulated precipitation and highlight an extra dimension of uncertainty when comparing convection-permitting models against convection-parameterizing models. Model resolution determines what features can be explicitly resolved within a model and what features must be parameterized. In cases where precipitation is convective or interacts with fine-scale geographic features such as mountains, coarser model resolutions employed by most global climate models may misrepresent the end-of-21st century precipitation response to climate change. An examination of this precipitation response for the conterminous United States is examined for a coarse resolution global climate model and a dynamically downscaled regional climate model at 3.75 km resolution. Using a decomposition approach, it is shown that the downscaled model better captures precipitation and its characteristics against observations for most portions of the United States. In western portions of the United States, over the northern Rockies and the Northwest, the downscaled simulation produces the right precipitation but for the wrong reasons and overestimates the frequency of high-moisture environments. Within the end-of-21st-century response, reductions in rainfall during the summer months are generally more pronounced in the downscaled simulations than in the global climate model. This reduced drying at coarser scales is due to the higher overall frequency of precipitation, particularly in environments with weak upward motion. A discussion diagnosing potential causes for this response within the convective parameterization and future avenues of work is given. We compare the end-of-21st-century precipitation response between a global climate model and a dynamically downscaled regional climate modelCompared to observations, the downscaled model best captures the amount, frequency, and intensity of precipitationProjections show stronger warm-season drying in the downscaled simulation due to relatively lower precipitation frequencies