North American monsoon precipitation and its precursors: processes at the seasonal and diurnal scale
The Weather Research and Forecasting (WRF) model was run at 100~km, 25~km, and 10~km resolution for the 2000 and 2004 monsoon seasons (July-September), a dry year and a wet year. These years were chosen to represent contrasting precipitation outcomes to assure that results were robust across different monsoon conditions. Model precipitation was compared to precipitation from the Modern-Era Retrospective Reanalysis (MERRA), the North American Regional Reanalysis (NARR), and Tropical Rainfall Measuring Mission (TRMM). Then WRF, MERRA, and NARR were used to investigate the relationships between precipitation and the other moisture budget variables, the large-scale flow, and atmospheric stability on the seasonal and diurnal scales. On both the seasonal and diurnal scale, flow was key to the location and intensity of precipitation. In 2004, the subtropical high over the south-central United States was about 300~km west of its location in 2000 at 700~hPa. The shift was also evident in vertically-integrated moisture flux, which then changed the pattern and intensity of moisture flux convergence (MFC), convective available potential energy (CAPE) and convective inhibition (CIN), and precipitation over Mexico and the Gulf of California. Over Arizona and New Mexico, transient disturbances, like tropical waves, were more important than the diurnal cycle to precipitation. Despite similar spatial distributions of precipitation, WRF, NARR, MERRA, and TRMM showed very different frequencies of light and heavy rain. Such uncertainty in the character of rainfall can impact a variety of stakeholders and decision makers across the NAM region. The WRF model tended to produce heavier precipitation across the NAM region compared to MERRA, NARR, and TRMM as a result of stronger MFC and higher CAPE, especially over the Gulf of California. Beyond the resolution needed to adequately reproduce the Baja California and Gulf of California, higher model resolution tended to increase and localize the heaviest rainfall to the highest terrain, accentuating the difference between WRF and TRMM-observed rainfall.
precipitation, North American Monsoon (NAM), North American Monsoon Experiment (NAME), Desert Southwest, monsoon, climate modeling, atmospheric modeling, model resolution, Weather Research and Forecasting model (WRF), Gulf of California, North American Regional Reanalysis (NARR)
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign