GLOBAL MODEL INVESTIGATION OF WARM SEASON PRECIPITATION FOR NORTH AMERICAN MONSOON EXPERIMENT
Link to NOAA Strategic Plan: Goal 2: Understand Climate Variability and Change to Enhance Society’s Ability to Plan and Respond
RESEARCH OBJECTIVES AND SPECIFIC PLANS TO ACHIEVE THEM
1. To improve the GCM simulation of the North American monsoon system.
2. To simulate the diurnal cycle of monsoon convection and understand its relationship with mean climate, including its impact on the surface and atmospheric energy budgets.
To accomplish the above objectives, the PI and his team make use of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM3) and a revised Zhang-McFarlane convection parameterization scheme developed by the PI. They performed multi-year ensemble simulations of the North American monsoon using the CAM3 global climate model, examining the sensitivity of the monsoon simulation to the model resolution and convection parameterization. The simulated seasonal monsoon evolution and diurnal cycle are compared extensively with in situ field observations as well as satellite estimates.
RESEARCH ACCOMPLISHMENTS
The National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3) is evaluated in terms of its warm-season response to a previous winter's positive SST anomalies in the Pacific, as associated with the El Nino / Southern Oscillation. Four independent simulations of CAM3 are executed for each of 3 historic boreal winter El Nino episodes from the past 27 years. Validation focuses on ensemble-average warm season statistically significant responses of precipitation in western North America, specifically the North American Monsoon region. Observational correlation analysis indicates that a localized response might be expected over Arizona and extreme northwestern Mexico for SST deviations with short lead times. Such a response is realized for the 1983 monsoon, in which the SST anomaly of the previous winter persists into summer. A local precipitation increase on the order of 120% is caused by an enhanced moisture flux convergence, a response not produced by the model, whose monsoon for this region is consistently biased late. An analysis of daily-mean precipitation, surface winds, and meridional moisture flux indicates that a significant portion of the enhanced precipitation is derived from persistent daily light rainfall, as might be expected from daily convection over the region. Concurrent with this persistent rainfall are southerly or southwesterly surface winds and a southerly moisture flux from the Gulf of California. The model shows no response for the monsoon this year but instead long rain-free periods during the months of July, August, and September, associated with long periods of westerly or northwesterly low-level flow. However, consistent with the observations, simulated precipitation appears to be quite sensitive to the occurrence of low-level southerly winds and a deep southerly moisture flux from the Gulf of California.
