Evaluation of Severe-Weather Forecasting Parameters Derived from Quasi-Real-Time Mesoscale Analyses using VORTEX2 soundings

An important part of short-term forecasting of severe weather is the diagnosis of the pre-convective and near-storm environment from observations. For the past several years, the Storm Prediction Center (SPC) has generated a comprehensive surface objective analysis scheme (SFCOA), which uses a blend of current surface observations and the latest Rapid Update Cycle (RUC) forecast/analysis of upper-air data on a 40-km grid, to produce a 3-dimensional current analysis of the mesoscale environment. A sounding analysis routine is then used to calculate diagnostic fields related to severe storms forecasting. These fields are generated hourly, and are generally available to the forecasters, and the public in the form of a web display, about 20 minutes after the top of the hour that the analysis is valid. These fields are used extensively at the SPC, and at National Weather Service forecast offices nationwide, in the short-term forecasting of severe weather.

Recently, the National Severe Storms Laboratory (NSSL) has been a part of a collaborative effort to develop an ensemble-based data assimilation system using the Weather Research and Forecasting (WRF) model to explore the impact of conventional observations on mesoscale analyses and forecasts of severe weather events. Routinely available observations from surface stations and rawinsondes are assimilated into a multi-member ensemble that is constructed from initial and boundary conditions provided by the North American Mesoscale Model (NAM) forecast cycle. The square-root ensemble Kalman filter (EnKF) approach encoded in the Data Assimilation Research Testbed (DART) framework is used in the assimilation process. Although this system is primarily designed to be an integrative analysis and short-term forecast system, the capability exists to produce quasi-realtime analyses every hour from this EnKF-based system, similar to the SFCOA system used operationally at the SPC.

Performance of these two mesoscale analysis systems from a portion of the Spring 2009 convective season (and the Spring 2010 season, if time permits) is assessed through comparison to pre- and near-storm soundings that were obtained as part of the second field phase of the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). This comparison is unique, in that the VORTEX2 soundings were not used in the RUC assimilation procedure, so that they form an independent source of 3-dimensional observations that can be used to evaluate the analyses. Environmental characteristics of the analyses related to severe-storms forecasting (e.g. CAPE and shear), as well as other severe weather parameters related to the analyzed structure of the boundary layer (boundary-layer depth, moisture depth, lifting condensation level, etc.) will be discussed and implications for the future use of mesoscale analysis and forecast systems will be examined.