Simulating Precipitation Efficiency across the Deep Convective Gray Zone

Precipitation efficiency (PE) relates cloud condensation to precipitation and thus reflects how much of the total atmospheric condensate reaches the surface as precipitation. Because the PE in convective storms is directly linked to their updraft- and downdraft dynamics, it is a helpful metric to identify convective processes that influence precipitation. In this research, we investigate the PE of mid-latitude mesoscale convective systems using a set of idealized Weather Research and Forecast (WRF) model simulations at horizontal grid spacings ranging from 12 km to 250 m. We present two novel methods to derive PE from standard model output. The first method leverages the saturation adjustment technique and estimates PE from the state variables vertical velocity, temperature and pressure. The second method leverages a bulk water budget approach to estimate PE from ice water path (IWP) and precipitation. We validate both methods with the explicitly calculated PE and find that PE can be reliably estimated from state variables with an error of less than 5%, partly due to error cancellation effects. Moreover, PE can be simulated by km-scale models within ~15% accuracy compared to sub-km simulations that better represent entrainment processes. The second method is less accurate and shows a stronger grid spacing dependency of the error. However, since it is based on the observable quantities IWP and precipitation, it allows for a validation of simulated PE with satellite observations. Finally, we analyze the grid spacing dependency of the climate change signal of PE and find that future decreases in PE are consistent across grid spacings.