A Severe Frontal Rainband. Part I. Stormwide Hydrodynamic Structure

Abstract

A narrow cold frontal band of intense precipitation is examined by means of triple Doppler radar and supporting observations. As the band passed through the Central Valley of California, it was accompanied by strong gusty winds, electrical activity, tornadoes and pressure jumps. Part I delineates the stormwide kinematic and thermodynamic structure. A highly two-dimensional pre-frontal updraft of 15?20 m s?1 results primarily from intense planetary boundary layer forcing of a low-level jet by the gravity-current propagation mechanism. Maximum updraft speed occurs at 2.1 km and the maximum radar echo depth is 6.6 km. Diabatic cooling, due to melting hydrometers, is proposed as a likely mechanism for control of gravity current depth and maintenance of density contrast together with synoptic-scale cold air advection. Available convective potential energy is shown to be small and kinetic energy of the environmental vertical wind shear is proposed as a likely source of energy on the updraft scale. Tornadoes develop at Helmholtz type inflectional instabilities along the surface front. Frontal zone horizontal shear, convergence and relative vorticity average 10?2 s?1. The results represent a particularly intense case in a class of storms previously studied by several other investigators. Part II will present the stormwide vorticity structure together with a detailed kinematic analysis of a small-scale vortex which spawns a tornado. Vorticity production terms will be discussed in view of the larger scale context.