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Water & Energy Cycles, Weather

This research is motivated by the discrepancy between the observed and simulated diurnal cycle of precipitation in the U.S. Great Plains in the GEOS-5 GCM. It consists of an observation-driven effort to simulate the correct diurnal cycle of precipitation, and to properly represent the mechanisms that produce it. Rigorous comparison of model output with cloud and radiation observations, in addition to observations of precipitation, will ensure consistency between the simulated hydrologic cycle and its associated effects on the radiative budget. Adjustments to the model’s convective parameterization combined with grid spacing that accommodates mesoscale organization of convection will enable the model to more realistically simulate the mechanisms that produce the observed distribution of clouds and precipitation. Modifications to the GEOS-5 convective parameterization will be process-based. Our goal is not to tune the model, but to allow the convection scheme to flexibly accommodate a wider variety of observed modes of convection. To this end, we will implement modifications to the GEOS-5 relaxed Arakawa-Schubert scheme that allow for elevated convective base height, as well as constraints on the distance between parcel origination level and LFC in the trigger function. In addition, approaches that have been implemented in mesoscale models will be tested in the GEOS-5 GCM. These include trigger functions that depend on the grid-scale vertical motion and the lifting depth. We will examine the interaction between the convective scheme, boundary layer parameterization, and grid-scale moist physics to ensure the model is operating as a coherent system. At each stage of model development, simulation output will be evaluated using A-Train-based cloud and radiation observations, in addition to high spatial and temporal resolution observations of precipitation. This multifaceted evaluation strategy will ensure a complete assessment of the simulated hydrologic cycle and its associated effects on the radiative balance in the model. The proposed research addresses the following NASA strategic objectives: improved predictive capability for weather and extreme weather events, bridging the study of weather and climate, development of robust techniques for comparison of model output and space-based observations, and translation of proven methodologies from fine-scale and mesoscale models to global climate models. The research described in this proposal is timely, and its potential impact is broad. An increase in predictive skill of the hydrologic cycle over the continental United States, as well as improved ability to represent extreme events (e.g., the 1993 U. S. Great Plains flood), will translate directly into improved utility of GCM output for policy and planning. In addition, the methodology outlined in this proposal is applicable to other contexts in which organized convection plays an important role in the climate system (e.g., the North American, Indian, and African monsoons, Madden-Julian oscillation, convection over the Amazon basin, among others). One key outcome of the proposed research is the development of a pathway to improving GCM simulations of the hydrologic cycle for multiple regions and seasons.

Project PI: Derek Posselt/University of Michigan

Email: dposselt@urnich.edu

Phone: (734)936-0502

http://www.engin.umich.edu/directory/uniq.do?uniq=DPOSSELT

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Started: Aug 10, 2010

Last Activity: Dec 15, 2010

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