Weather Forecasting Group

A Sensitivity Analysis of C-Band Polarimetric Variables in Rain

T.D. KEENAN¹
Bureau of Meteorology Research Centre, GPO Box 1289, Melbourne ,Australia

L. CAREY
Colorado State University, Fort Collins, Colorado

D. ZRNIC
NOAA, Environmental Research Laboratories, National Severe Storms Laboratory, Oklahoma

P. MAY
Bureau of Meteorology Research Centre, GPO Box 1289, Melbourne ,Australia

AND S. RUTLEDGE
Colorado State University, Fort Collins, Colorado

Draft for Submission: J. Appl. Meteor.
Date: 24 Feb 1998

¹Corresponding author address: Dr. T.Keenan, Bureau of Meteorology Research Centre, GPO Box 1289, Melbourne, Australia, 3001. t.keenan@bom.gov.au

ABSTRACT

Examination of disdrometer data and in-situ cloud microphysical measurements in a high rainfall tropical regime during the Maritime Continent Thunderstorm Experiment (MCTEX) indicates the presence of large drops up to 8 mm in diameter. The sensitivity, of polarimetric variables at a 5 cm wavelength, to raindrop size and axial ratio are examined using T-matrix modelling of the scattering process. Monodisperse raindrop distribution calculations show there are significant temperature dependent Mie resonance effects at C-band for drops greater than 4-5 mm in diameter. Using Marshall-Palmer (1948) and gamma distributions parameterised as a function of rain rate from MCTEX data, all polarimetric variables are found to be sensitive to varying degrees to dropsize distribution, the maximum imposed dropsize and the assumed axial ratio.

The sensitivity of polarimetric variables to maximum dropsize and raindrop axial ratio is then further studied using the Mueller-matrix approach of Vivekanandan et al. (1991) for gamma raindrop size distributions. These simulations demonstrate that whilst specific differential phase (KDP) based estimates of rainfall, attenuation and differential attenuation are less affected by dropsize distribution variations, they are sensitive to the relation between the equivolume diameter and axial ratio and can produce biased estimates. Attenuation is sensitive to the occurrence of large drops which can produce anomalously high attenuation associated with Mie scattering effects. The results also demonstrate that polarimetric techniques employed to classify the phase of hydrometeors are sensitive to the assumed raindrop axial ratio.