The Suppression of Deep Moist Convection near the Southern Great Plains Dryline

Abstract

Deep moist convection failed to initiate over the Texas Panhandle on 6 May 1995 despite expectations to the contrary by the forecasters for the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). The National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) reanalysis is used to identify a midlevel migratory short-wave ridge propagating across the area during the time of maximum diurnal heating. Subsidence values of 1.0 ? 10?3 hPa s?1 for a period exceeding several hours were associated with the ridge and led to substantial (?3 K) warming in the midlevels and to the formation of a strong inversion around 600 hPa. A number of independent observations are examined to corroborate the hypothesis that the passage of the midlevel short-wave ridge at the time of maximum diurnal heating is ultimately responsible for the absence of deep moist convection. Twelve-hour forecasts of the 1995 NCEP operational model suite and the Pennsylvania State University?NCAR fifth-generation Mesoscale Model (MM5) show only minor indications of the ridge passage. Sensitivity tests using MM5 showed that variations in the horizontal and vertical resolution, the convective parameterization schemes, the boundary layer schemes, nudging, and others did not lead to any substantial improvement in the forecast of the observed midlayer warming and lid formation. However, when MM5 was initialized with the NCEP?NCAR reanalysis (containing the initial short-wave ridge) the forecast improved significantly. A likely cause of the operational model deficiencies is the movement of the short-wave ridge into the North American radiosonde network after 1200 UTC on 6 May 1995, the time of the operational model initialization.