Extreme Daily Precipitation in North American Climate Simulations: Scales and Processes

Abstract:

We analyze the ability of global and regional climate models to simulate extreme daily precipitation and supporting processes for midlatitude and Arctic regions of North America. Regional model output comes from the NARCCAP archive and simulations by an Arctic version of WRF; global model output comes from the CMIP5 archive. The NARCCAP results also include output from a time-slice, high-resolution global simulation. All regional model output is at half degree resolution, whereas the CMIP5 resolutions vary but are coarser than the regional model resolutions. The combined analysis allows us to assess added value of finer resolution in simulating extreme precipitation. Analysis focuses on selected regions of North America for winter (DJF) and summer (JJA), building on several previous analyses focused on this region. In addition to comparing results from the different models, we also compare simulated precipitation and supporting processes with those obtained from observed precipitation and reanalysis atmospheric states.

In the central U.S., the models generally reproduce well the precipitation-vs.-intensity spectrum seen in observations, with a tendency for coarse-resolution global models to produce somewhat less intense precipitation. In contrast, all models are deficient in high intensity precipitation in Alaska. Further analysis focuses on precipitation events exceeding the 99.5 percentile that occur simultaneously at several points in the region, yielding so-called “widespread events”. Analysis of 500 hPa heights, near-surface circulation and fields such as temperature and humidity reveal the processes leading to extreme events in the models and observations. The finer resolution models generally reproduce the physical behavior of these extreme events, with the coarser models showing a smoother rendition. In the central U.S., for winter, these events are produced by slowly moving low-pressure systems that all models simulate fairly well. In Alaska, these events tend to occur through orographic uplift in both observations and simulations. This difference in physical behavior between the two regions suggests reasons for the deficiency in Alaska’s simulated, high intensity precipitation.