Microphysical Studies Using the CSU-CHILL radar during HEAT

 

Abstract

This proposal is focused on polarimetric and dual-wavelength radar analyses using the CSU-CHILL radar, which are being requested for the Houston Environmental Aerosol Thunderstorm (HEAT) project to be held in the summer of 2005. The Scientific Overview Document (available at www.met-tamu.edu/ciams/heat) gives the rationale, scientific background, overall objectives and experimental plan for HEAT. Here the interest is in the microphysical objective related to observations of storms that develop within the Houston urban area (and downwind), and the non-urban areas, and in understanding via well-documented case studies the possible reasons for the rainfall and lightning anomalies that occur within the urban area and downwind of it.

The specific objectives of this proposal are:

i)   To determine if there are systematic differences in the average raindrop size distribution in the rain layer between storms
     developing over the urban heat island (and down wind), and over the non-urban regions.

ii)  To examine the evolution of warm rain processes, the development of the precipitation "balance" level, the lofting of
     supercooled rain above the freezing level and the development of frozen drops and wet ice aloft for events over both
     the urban and non-urban regions.

iii) To coordinate with other HEAT investigators performing kinematic retrievals, airborne measurements and
     cloud-resolving numerical models to enhance and validate our analyses and to contribute to realization of the
     overall HEAT objectives.

One platform of interest is the CSU CHILL polarimetric radar, which is expected to be upgraded with a new dual-offset Gregorian antenna in time for HEAT. The improvement in polarimetric radar data quality is expected to be significant especially in regions of strong reflectivity gradients that frequently occur in vigorously growing storm cells.  We propose to use Zh, Zdr, and Kdp data from both radars to estimate the two important parameters of the drop size distribution in the rain layer (mass-weighted mean diameter or Dm and the "normalized" intercept Nw), and to evaluate the average dsd trajectories in the log(Nw) versus Dm plane that are known to exhibit systematic differences between continental and maritime precipitation events and the transition between them if and as they occur.  The vertical structure of Zdr, Kdp, and LDR will be used to examine the evolution of the mixed phase region, in particular, the evolution of the positive Zdr and Kdp columns expected in the vigorous growth phase of storm cells and the appearance of the enhanced LDR 'cap' signature aloft.  The broader impact of this research is related to the upgrade of the WSR-88D radars for dual-polarization by the National Weather Service which is of national importance.  The techniques and algorithms developed herein in the moist sub-tropical Houston environment should greatly benefit future algorithm development for the WSR-88D system especially for hydrology and severe storms. The research results obtained herein will also be integrated into the advanced weather radar class taught regularly at Colorado State University which should benefit graduate education.