Tuesday, 9 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Warm season (April to September) precipitation, especially the convective rain (CR) portion, features the most intense rainfall rate compared to the long-lasting stratiform rain (SR) portion with large coverage, which strongly corresponds to the flooding hazard. Due to the different physical and dynamical factors (such as solar heating, moisture availability, synoptic patterns), the occurrence frequency, duration, coverage, intensity, and life cycle of CR demonstrate great latitudinal variation across the entire Great Plains. In order to validate and improve the NSSL WRF’s simulated warm season CR, the Southern Great Plains (SGP) and Northern Great Plains (NGP) are selected as regions of interest for their marginal locations within mid-latitude (30o N to 60o N), and the corresponding NSSL WRF simulated CR events are evaluated using NCEP Stage IV observation. Through the 3-yr comparisons between simulations and observations, we have reached the following conclusions: 1) SGP features larger CR amount, duration and coverage than those at NGP, 2) WRF simulations agree with Stage IV better at SGP than at NGP, and 3) SGP demonstrates a strong dynamic contrast for light (frontal) and heavy (dryline) CR, while positive precipitation bias dominates NGP region. Additionally, even though the total precipitation amount and coverage are well simulated over SGP, their distribution in different CR intensity category are poorly constructed, represented as extreme over-simulation for light (cases with daily CR intensity < 25% CDF based on Stage IV observation) and moderate cases (CDF between 25 % and 75 %), as well as considerable under-simulation for heavy cases (CDF > 75 %). Through verification score analysis on 24-hour interval, the highest Probability of Detection (POD) score is found for heavy CR cases over SGP, and moderate cases over NGP. On 6-hour interval, the best performance of NSSL WRF is always found during 1800 – 2300 UTC over both regions and for all CR intensity categories, which coincides with the period when NSSL WRF can optimally mimic Stage IV in diurnal cycle. As demonstrated in this study, SGP and NGP CR amounts, duration and coverages are significantly different, should modify the current WRF CR parameterization in the middle latitudes?
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