Accurate rain drop size distribution models for the tropical region
Lakshmi Sutha Kumar
Date of Issue2011
School of Electrical and Electronic Engineering
In this thesis, a detail study on the modeling of rain drop size distribution is undertaken. The rain drop size data measured using the Joss distrometer during the years 1994 to 1995 and 1997 to 1998 and the RADAR data during the year 1998 are used in this study. Gamma model is found to be the preferred model for drop size distribution modeling in the Singapore climatic zone. The method of moments is used to retrieve the parameters of the gamma distribution. By studying the contribution of individual bins on rain rate estimation, it was found that the contributions of lower drop diameters are small as compared to the central drop diameters. Therefore, the lower drop diameters are removed from the drop size data before the gamma model is redesigned for its moments. The effects of this removal on the specific rain attenuation (in dB) and the slant-path rain attenuation calculations using ITU-R P.838-3 model and using forward scattering coefficients for vertical polarization are analyzed at Ku-band, Ka-band and Q-band frequencies. It is concluded that the sensitivity of the Joss distrometer although affects the rain rate estimation at low rain rates, does not affect the slant path rain attenuation on microwave links. Therefore, the small drop diameters can be ignored for slant path rain attenuation calculations in the tropical region. The research work continues to find the suitable reflectivity to rain rate (Z-R) relations using a data set which consists of nine rain events selected from Singapore’s drop size distribution. The variability of the rain integral parameters R, Z, Nw, D0 and gamma model parameter μ are used for the classification of rain into convective, stratiform and transition. Z-R relations are derived for each type of rain after classification. The Z-R relations for different rain types for the Singapore climatic zone are compared and analyzed. Reflectivities are extracted from RADAR data above NTU site for rain events and compared with the reflectivities derived from the distrometer data. Rain rates retrieved from RADAR data using the proposed Singapore Z-R relations are compared with the distrometer rain rates. It was found that the Singapore Z-R relations is able to extract the rain rate from RADAR data well although they are found to be constantly lower than the distrometer derived rain rates. Finally the thesis examines the possibility of using a two parameter gamma models to retrieve the rain rates from dual polarized RADAR data. A two parameter gamma model can be found either by fixing μ or by deriving an appropriate shape-slope (μ-Λ) relation for the tropical region. In order to find an appropriate μ value, observed DSDs are fitted with different μ values to estimate the rain rates. In order to find an appropriate μ-Λ relation, different μ-Λ relations are fitted for different categories according to the rain rate and the number of drops. The derived μ-Λ relationships for the Singapore region are compared to published results from Gadanki and India. Two parameter gamma models are compared by retrieving the rain rate using the polarimetric RADAR variables found from the T-Matrix code. The use of the μ-Λ relation for rain retrieval is recommended for the tropical region.
DRNTU::Engineering::Electrical and electronic engineering::Antennas, wave guides, microwaves, radar, radio