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

Stable climatic patterns are essential for maintaining hydrologic, ecosystem, and resource stability. Understanding future climate pattern changes and effects are consequently vital for human survival. Climate model projections indicate that average global precipitation will increase with projected rates of greenhouse gas emissions. However, these projected precipitation increases will not be uniform everywhere on earth. In general, regions that currently experience high rain rates are expected to become wetter while those with low rain rates to become drier. For the Southeast United States (SE US), current Intergovernmental Panel on Climate Change projections are for the region to warm between 3 and 5°C while there is strong agreement between climate models that rain rate will increase between 10 and 20 percent. However, the SE US has seasonal precipitation patterns that may be vulnerable to change in a warming climate and therefore needs investigation. \nPrecipitation in the SE US occurs throughout the year by means of at least two distinct precipitation regimes which are forced by baroclinicity and diurnal thermodynamic instability and occur in the winter and summer seasons, respectively. Precipitation can be categorized by size into mesoscale precipitation features (MPF), which are large rain features associated with midlatitude cyclones in the winter, and smaller, isolated precipitation features (IPF), generally associated with ordinary cell convection in the summer. Between April and June, the SE US experiences a sudden transition between precipitation regimes, from an MPF-dominant regime to one in which IPF is more influential. This study analyzes the effect of climate change on the timing of the onset of the IPF season in the SE US.\nFuture climate projections indicate that temperature increases may lead to localized rises in atmospheric instability which may create environments with enhanced support for convective storms and seasonal IPF in the SE US. Additionally, a warming climate has been associated with other potentially influential effects, such as tropical expansion and westward expansion of the North Atlantic Subtropical High (NASH) western ridge. These projected increases in temperature and instability and synoptic scale feature changes, in turn, may affect the mechanisms and timing of the convective regime onset identified using IPF rain rate criteria. \nTo estimate the effect of climate change on the convective season onset in the SE US, numerical model simulations of current (WRF-CC) and future (WRF-RCP8.5) climates were performed using the Weather Research and Forecasting (WRF) model. In the WRF model, 6-hour Global Forecast System (GFS) reanalysis and daily low-resolution Real-Time Global sea surface temperature (RTG SST) data were used as boundary conditions to simulate atmospheric conditions at 3 km resolution during the 110-day period between 02 March – 19 June 2009. The future climate simulation, WRF-RCP8.5, was forced using GFS and RTG SST data modified to include mean temperature anomalies for the 2090 decade from a selection of models within the Coupled Model Intercomparison Project Phase 5, assuming Representative Concentration Pathway 8.5. \nDaily area average total, IPF, and MPF rain rates over land are greater in WRF-CC compared to those observed in the National Mosaic and Multi-Sensor Quantitative Precipitation Estimates (NMQ) data over the 110-day period, by 1.36 (39%), 0.63 (89%), and 0.73 (26%) mm/day, respectively. In agreement with previous studies, the WRF-RCP8.5 daily area average total rain rates over land were statistically significantly greater than in WRF-CC, with differences of 1.26 (26%) mm/day. The daily area averaged MPF rain rates over land in WRF-RCP8.5 were also statistically significantly greater than those in WRF-CC, with differences of 1.43 (41%) mm/day. However, no statistical difference in daily area average IPF rain rate over land was detected. Daily area average rain rate over ocean statistically significantly increased in total, IPF, and MPF categories by 1.37 (44%), 0.37 (34%), and 0.99 (49%) mm/day. IPF fraction over land decreased from 28% in WRF-CC to 19% in WRF-RCP8.5 but was not changed over the Atlantic Ocean. \nThe transition from an MPF-dominant regime, in which the spatial pentad-averaged IPF rain rate distribution over land is scattered and weak, to a regime in which pentad-averaged IPF rain rate has comparatively greater coverage and intensity was analyzed and convective season onset was defined using the land domain averaged pentad IPF rain rate according to previously published criteria. The convective season onset was found to occur in pentad 24 in WRF-CC, and in pentad 25 in the NMQ dataset. The WRF-RCP8.5 convective season onset occurred in pentad 22, two pentads before the onset in WRF-CC in pentad 24. In each dataset, a sharp increase in pentad-averaged IPF daily area average land rain rate occurred on the onset pentads compared to preceding pentads, consistent with the NMQ observations.\nTo investigate the mechanisms for convective season onset in the current and future climate, the 250 mb upper-level jet streams, 850 mb NASH western ridge positioning and lower-level jets, and 850 mb CAPE are analyzed. The 850 mb NASH western ridge intrudes into the SE US in pentad 23 in WRF-RCP8.5, and in pentad 24 in WRF-CC. This creates 850 mb southerly jets from the Gulf of Mexico into the SE US, thereby connecting tropical flow and increasing 850 mb CAPE in the SE US. By pentad 24, in both WRF-CC and WRF-RCP8.5, the seasonally split 250 mb upper-level midlatitude jet stream suddenly unifies and shifts northward. In the SE US, this provides upper-level synoptic conditions favoring slower midlatitude cyclone propagation and generating a favorable environment for widespread, strong IPF rain rates. \nThe sequence of dynamic events leading to the convective season onset in WRF-CC in pentad 24 is consistent with literature. In comparison, these dynamics are not yet present at the convective season onset in WRF-RPC8.5 pentad 22. The warming that was introduced in WRF-RCP8.5 intensifies midlatitude cyclones and increases CAPE values from those in WRF-CC. In WRF-RCP8.5, this is influential in the convective season onset as a midlatitude cyclone produces more favorable lower-level dynamics that transports CAPE ahead of the cold front compared to WRF-CC. These high CAPE values ahead and along the cold front trigger IPF in the warm sector over pentad 22 in the SE US, without the upper-level support or connection to the larger 850 mb NASH flow. \nThe increased temperature inherent in WRF-RCP8.5, due to the use of the pseudo-global warming (PGW) technique in this project, may alone be sufficient to produce an earlier onset in WRF-RCP8.5 by increasing CAPE and strengthening midlatitude cyclones. 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. \nFuture work in this research area could include investigating historical observations to determine if dynamics in the WRF-RCP8.5 convective season onset are found in the real world, to simulate multiple seasons and years other than 2009, run model ensembles using various PGW method configurations, and account for anticipated future climate circulation patterns in model configurations