ENVIRONMENTAL AND THUNDERSTORM STRUCTURES AS SHOWN BY NATIONAL SEVERE STORMS PROJECT OBSERVATIONS IN SPRING 1960 AND 1961

Abstract

Some examples of National Severe Storms Project observations are described, showing certain typical structures of the mesoscale environment and of large thunderstorms in the Oklahoma-Texas-Kansas region. The ?dry-line? at the cT/mT boundary is shown to be characterized by an extremely abrupt gradient of water-vapor content and temperature, accompanied by a wind-shift zone with appreciable horizontal convergence. On the moist air side, there were pronounced wavelike perturbations in the moisture and temperature fields, connected with systematic variations of the vector wind velocity. Special pilot-balloon observations show the low-level jet as a flat sheet of southerly winds, with maximum strength generally 300 to 500 m. above ground, and illustrate the great variations of wind speed associated with diurnal variability of stability in the surface layers. Appreciable streakiness of the velocity field is indicated by both pibal and aircraft measurements. A description is given of a series of intense low-pressure systems observed at the surface, in association with the northwest sides of radar echoes. These depressions, which form at night, are warm and very dry relative to their surroundings, and are attended by strong, gusty surface winds. High-level aircraft observations are illustrated, demonstrating particularly the significant disturbances of the wind field around cloud tops. An example of gust measurements in the upper portion of a thunderstorm (imbedded in strong vertical shear) shows that the lateral gust components, up to 30 m./sec., are of the same order as the vertica1 gusts. Movements of thunderstorms as shown by radar, and certain aspects of the reflectivities of tornado-bearing storms, are discussed briefly. Finally, the three-dimensional forms of large thunderstorms are illustrated, with emphasis on the organized asymmetrical structures.