Springtime Onset of Isolated Convection in the Southeastern United States: Initial Results from a 10-Year Radar Analysis

In the southeast United States (SE US), precipitation is present year-round. While precipitation occurs during each part of the year, the driving mechanisms behind the precipitation vary seasonally. Over 70% of total winter precipitation is associated with extratropical (ET) cyclones, with most of this precipitation organized as mesoscale precipitation features (MPF), as determined over a four-year period (Nieto-Ferreira et al. 2013). Summer has a more varied set of precipitation regimes, with rain associated with thermodynamic instability, sea-breeze circulations, and tropical cyclones - in addition to ET cyclones. During summer, convection organized as isolated precipitation features (IPF), accounts for nearly 40% of the total rain (Rickenbach et al. 2015). Recent studies, limited to a four-year period, have shown that the onset of the IPF-based summer precipitation regime is abrupt, occurring over just a few weeks in May (Rickenbach et al. 2015, Rickenbach et al. 2019). The present study represents the initial steps to extend this analysis from four to ten years, to study year-to-year differences in the timing and geographic pattern of the spring to summer transition of isolated convection in the SE US.

This work builds on previous studies of the springtime onset of the IPF season in the SE US by extending the analysis to a ten-year period (2002-2011) with an archive of the high-resolution Multi-Radar/Multi-Sensor (MRMS) NEXRAD-based precipitation dataset (Zhang et al. 2011) produced at NOAA NCEI. The data consist of hourly instantaneous rain rates on a 0.01^o x 0.01^o grid covering the contiguous US. Preliminary analysis focuses on the period March-August for several of the years. The Grid Analysis and Display System (GrADS) is used to create hourly radar imagery covering the southeast United States using the MRMS data. From this imagery, movies of hourly radar imagery for the ten-year period have been created to visualize qualitatively the evolving structure and organization of precipitation during the springtime onset season. Using Interactive Data Language (IDL) scripts, precipitation features in each hourly image will be identified and objectively separated into IPF and MPF categories (Rickenbach et al. 2015) using a feature maximum length threshold of 100 km. Hourly rainfall from each category will be summed to a daily total (mm day^-1), and stored in binary netCDF files. From these daily values, time series of IPF and MPF precipitation will be constructed for the March-August period covering the springtime onset transition. Preliminary analysis of the timing and geographic pattern of IPF onset for several seasons will be presented at the conference.

References:

Nieto-Ferreira, R, Hall, L, Rickenbach, T. 2013. A climatology of the structure, evolution, and propagation of midlatitude cyclones in the southeast United States. Journal of Climate, 26 8406-8421.

Rickenbach, T. M., Nieto-Ferreira, R., Zarzar, C., Nelson, B. 2015. A seasonal and diurnal climatology of precipitation organization in the southeastern United States. Quarterly Journal of the Royal Meteorological Society, 141 (690), 1938-1956,https://doi.org/10.1002/qj.2500

Rickenbach, T. M., Nieto-Ferreira, R., Wells, H. 2019. Springtime onset of isolated convection precipitation across the southeastern United States: Framework and regional evolution. Conditionally accepted, Monthly Weather Review.

Zhang, J, Howard, K, Langston, C, Vasiloff, S, Kaney, B, Arthur, A, Van Cooten, S, Kelleher, K,Kitzmiller, D, Ding, F, Seo, D-J, Wells, E, Dempsey, C. 2011. National Mosaic and multi-sensor QPE (NMQ) system: Description, results, and future plans. Bulletin of the American Meteorological Society. 92 1321– 1338, doi: 10.1175/2011BAMSD-1100047.1