WRF SIMULATIONS OF THE 2009 SOUTHEASTERN UNITED STATES CONVECTIVE SEASON ONSET IN CURRENT AND FUTURE CLIMATE SCENARIOS

Abstract

Southeast US (SE US) precipitation occurs year-round from at least two regimes. In the winter, baroclinicity creates mesoscale precipitation features (MPF) and in the summer, thermodynamic instability forces smaller, isolated precipitation features (IPF). Between April and June, a sudden transition between these precipitation regimes occurs in the SE US. Climate model projections indicate increases in average global precipitation, varying in magnitude by region, with greenhouse gas emissions. Additionally, projected temperature increases may elevate atmospheric instability and potentially enhance IPF in the SE US. Furthermore, atmospheric flow changes are associated with projected warming, such as the westerly expansion of the North Atlantic Subtropical High (NASH) western ridge. These projected changes may affect the convective regime onset mechanisms and timing. Therefore, this study analyzes the effect of climate change on the SE US convective season onset timing. Numerical model simulations of current (WRF-CC) and future (WRF-RCP8.5) climates were performed using the Weather Research and Forecasting (WRF) model to estimate effects of climate change on the SE US convective season onset. Atmospheric conditions were simulated between 02 March and 19 June 2009. In WRF-RCP8.5, the 2090 decade mean temperature anomalies from selected Coupled Model Intercomparison Project Phase 5 models, assuming Representative Concentration Pathway 8.5, were added to the simulation. Daily area average Total, IPF, and MPF rain rates over land were greater in WRF-CC compared to those in the National Mosaic and Multi-Sensor Quantitative Precipitation Estimates (NMQ) dataset over the study period. In agreement with literature, the WRF-RCP8.5 total and MPF rain rates over land were greater than those in WRF-CC. No difference in daily area average IPF rain rate over land was detected. The convective season onset was then identified using criteria from literature, revealing that the onset occurred in pentad 25 in the NMQ dataset, pentad 24 in WRF-CC, and much earlier in pentad 22 in WRF-RCP8.5. Onset mechanisms were investigated using reanalysis 250 mb upper-level jet streams, 850 mb NASH western ridge position and lower-level jets, and 850 mb CAPE. The NASH western ridge was over the SE US in WRF-RCP8.5 pentad 23 and WRF-CC pentad 24 and created 850 mb southerly jets into the SE US. By pentad 24, the upper-level midlatitude jet stream shifted northward in both simulations, creating conditions favoring slower midlatitude cyclones and greater IPF rain rates. The dynamic precursors that led to the IPF season onset in WRF-CC in pentad 24 were not yet present at the onset in WRF-RPC8.5. Instead, in WRF-RCP8.5 the warming contributed to increased CAPE and to the intensification of a transient midlatitude cyclone that carried high CAPE air from the Gulf of Mexico to the SE US, causing greater IPF rain rates and the early onset of the IPF season. This study therefore isolates and highlights the importance of thermodynamics on the SE US convective season onset timing in temporal proximity to an expected onset pentad.