Sub-Daily Scale Extreme Precipitation in Future Climate-Change Scenarios: A Pilot Study

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Sub-daily extreme rainfall events and their associated convective precipitating systems will be explicitly simulated over North America by employing a hierarchical, scale-spanning modeling approach that culminates in 3D cloud resolving model simulations. The experimental design consists of global climate model (GCM), nested regional climate model (RCM), and sub-nested cloud-resolving model (CRM) integrations. The PIs will employ two basic approaches: GCM-CRM nesting and GCM-RCM-CRM nesting. This is the first attempt to simulate historical severe convective weather events over the contiguous United States using these two approaches. The case study events will include the tornado super-outbreak of 3 April 1974, the flash flood and hailstorm event of 5 May 1995, and others as deemed necessary. The majority of the work "to include the analysis and verification of the events, as well as their use in the development of the statistical tools" will be devoted to these retrospective experiments. However, once the optimum method for has been established, the PIs will perform an initial experiment forced by an IPCC SRES emissions scenario to examine the ability of that method to simulate the response of sub-daily extreme rainfall events to enhanced greenhouse forcing. The primary objectives are: 1. To identify the optimum methodology for one-way coupling of a GCM, a nested RCM and a sub-nested CRM by simulating historical events using a range of configurations and comparing the various simulations against instrumental data. 2. To yield an initial projection of the characteristics, frequency, and geographical distribution of sub-daily extreme rainfall events for a future emissions scenario, by comparing the simulated future events to similar historical events. 3. To yield an initial projection of where and when the sub-daily extreme rainfall will be accompanied by other hazardous weather phenomena such as tornadoes and hail, by associating the future events with specific types of precipitating weather systems and hence with the most likely attendant hazardous phenomena. The broader impacts of the project include the education and training of two graduate students. If this pilot project is successful, it will demonstrate the potential for projecting statistics of severe weather events in a climate forced by human activities. Information from such projections could be of significant benefit to policy makers.