Climate Processes in CMIP5: Reasons for the Large Differences in Simulated Low-Level Jet among the CMIP5 Models

Climate Processes in CMIP5: Reasons for the Large Differences in Simulated Low-Level Jet among the CMIP5 Models

Qi Hu School of Natural Resources and Department of Earth and Atmospheric Sciences University of Nebraska-Lincoln, Lincoln, NE 68583-0987

There are large differences among the simulated southerly low-level jet (LLJ) from the Gulf of Mexico to the U.S. Great Plains in April-September. They are not directly resulting from the difference in spatial resolution of the models; some finer resolution models produced less organized and weaker LLJ. Because the LLJ transports abundant moisture into the Great Plains and is an important part of the hydrologic cycle in the region during April-September, it is essential for the models to simulate it and its seasonal variation accurately. In this analysis the CMIP5 models that have difficulties to simulate the LLJ are examined and compared to the ones that can describe the LLJ in ways comparable to the observed. The foci are placed on how well the models can describe the seasonal transition in the mass field from the high pressure in the cold season to low pressure in the warm season over the Rocky Mountains (and supporting circulation), and how the summer low pressure is maintained in the models. Meanwhile, the simulations of the western portion of the North Atlantic Subtropical High pressure system and its interaction with the low pressure over the Rockies are examined and compared. In addition, the boundary layer processes and their interaction with the middle troposphere, which have been shown to play important roles in the LLJ development, are also examined. Preliminary results of the analyses suggest that model ability in describing the circulation and mess field and their seasonal variation over the Rockies is important to improve the simulation of the LLJ in the Great Plains.