THE TRANSITION ZONE AND SECONDARY MAXIMUM OF RADAR REFLECTIVITY BEHIND A MIDLATITUDE SQUALL LINE - RESULTS RETRIEVED FROM DOPPLER RADAR DATA

Thermodynamic and microphysical retrieval techniques are applied to du al-Doppler synthesized air motion fields for a midlatitude squall line , which passed through the Oklahoma-Kansas Preliminary Regional Experi ment for the Stormscale Operational and Research Meterology Program (P RE-STORM) observational array in Kansas and Oklahoma on 10-11 June 198 5. The retrieved pressure and potential temperature fields are consist ent with surface network and sounding data, while the retrieved microp hysical fields show the characteristic secondary maximum of radar refl ectivity in the stratiform region and the band of low reflectivity, or transition zone, lying between the leading convective line and the se condary maximum. The retrieved fields indicate the processes producing the secondary maximum and transition zone minimum of radar reflectivi ty more quantitatively than has been possible in previous studies. The primary processes accounting for these features of the radar reflecti vity pattern were 1) the substantial increase in precipitation mass co ncentrations by vapor deposition within the region of mesoscale ascent in the stratiform region and the increase in particle size resulting from the strong aggregation of ice particles above the bright band in the region of the secondary band, 2) the suppression of growth in the middle to upper level descent just behind the convective region, which enhanced the minimum of radar reflectivity in that zone, and 3) the t rajectories of ice particles detrained from the convective line, which qualitatively accounted for the general location of the secondary ban d. Additional insights into these processes are discussed.