Climate Models Simulate Extreme Precipitation With Sharply Contrasting Physical Mechanisms
We evaluated 33 climate models’ representations of the atmospheric processes associated with extreme precipitation and compared them to observational estimates of these processes in the real atmosphere. The vertical structures of temperature, moisture, humidity, and upward motion simulated by the models are shown to exhibit a wide range of characteristics across models, with some models better resembling the real atmosphere than others.
This study highlights shortcomings of climate models’ representations of physics relevant to extreme precipitation. Our results suggest how parameterizations may be improved in order to make more reliable future projections of extreme precipitation.
Daily and sub-daily precipitation extremes in historical Coupled-Model-Intercomparison-Project-Phase-6 (CMIP6) simulations are evaluated against satellite-based observational estimates. With increasing temporal resolution there is an increased discrepancy between models and observations: for daily extremes, the multi-model median underestimates the highest percentiles by about a third, and for 3-hourly extremes by about 75% in the tropics. To understand the model spread, we evaluate the 3-D structure of the atmosphere when extremes occur. In mid-latitudes, where extremes are simulated predominantly explicitly, the intuitive relationship exists whereby higher-resolution models produce larger extremes (r=–0.49), via greater vertical velocity. In the tropics, the convective fraction (the fraction of precipitation simulated directly from the convective scheme) is more relevant. For models below 60% convective fraction, precipitation amount decreases with a convective fraction (r=–0.63), but above 75% convective fraction, this relationship breaks down. In the lower-convective-fraction models, there is more moisture in the lower troposphere, closer to saturation. In the higher-convective-fraction models, there is deeper convection and higher cloud tops, which appear to be more physical. Thus, the low-convective models are mostly closer to the observations of extreme precipitation in the tropics but likely for the wrong reasons. These inter-model differences in the environment in which extremes are simulated hold clues into how parameterizations could be modified in general circulation models to produce more credible 21st-Century projections.