Mesoscale Studies of Convection, Tornadogenesis and Fronts

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

The Principal Investigator will perform research in the areas of: 1) tornadoes and tornadogenesis, 2) structure of bow echoes, 3) mesoscale documentation of warm fronts, and 4) participation in the field phase of the international water vapor project (IHOP). The research on the first three areas is based upon a thorough analysis of existing data collected from past field experiments. Two case studies based on data collected during VORTEX (Verification on the Origins of Rotation in Tornadoes Experiment) are being examined. A well-documented tornado associated with a supercell that developed near Sweetwater, Texas will be studied in order to investigate the generality of prior research results. The Sweetwater storm also spawned several low-level mesocyclones that did not produce tornadoes. Accordingly, there is a unique opportunity to compare tornadic and non-tornadic low-level mesocyclones associated with the same supercell using data of comparable spatial and temporal resolution. A second case study is based on airborne Doppler data collected on a bow echo that developed within a squall line. The bow echo is a common radar signature that is often associated with downbursts and derechos. The present case provides an opportunity to examine the evolution of this echo from a linear structure into a bow-shape. During the winter of 1997, an international field experiment called FASTEX (Fronts and Atlantic Storm Track Experiment) was organized to study oceanic cyclogenesis. The primary data platforms were several research aircraft which released dropwindsondes, recorded in situ measurements at flight level, and collected high-resolution radar reflectivity and Doppler velocity information using on-board radars. Observational and numerical studies on cold fronts have dominated the literature largely because of its well-defined structure, often manifested as abrupt wind shifts and temperature discontinuities, and its frequent association with severe convective weather. In comparison, there have been relatively few observational studies of surface warm fronts. This is due, in part, to the fact that warm fronts are often not associated with strong discontinuities especially over the continents. A unique data set was collected on a warm front during FASTEX. Detailed kinematic and thermodynamic data were collected on the frontal zone using Doppler wind syntheses and dropwindsonde information. Specific goals of this research include examining the degree to which the flow was in geostrophic balance and the terms of the frontogenesis equation. The primary goal of IHOP (International H20 Project) is to improve measurements of the four-dimensional distribution of water vapor and better understand the impact of water vapor on precipitation forecasting. The field phase of the experiment is planned for Spring 2002. In this project the PI will address a number of research objectives that deal with the convection initiation component of IHOP. These objectives include: 1) a thorough documentation of the impact of horizontal convective rolls on moisture variations within the convective boundary layer; 2) determining the generating mechanism(s) of inflections/circulations that develop along convergence boundaries and their impact on the evolving moisture fields; and 3) a quantitative assessment of the importance of the balance between the horizontal vorticity generated by the cold pool from thunderstorm outflows with the environmental vorticity produced by the low-level shear vector in determining the potential for convection initiation.