Distrometric Drop Size Distribution in South Brazil: Derived Z-R Relationships and Comparisons with Radar Measurements

Drop size distribution plays an important role in radar meteorology, among many other areas such as atmospheric physics, telecommunications, remote sensing, hydrological modeling and soil erosion. Radar rainfall estimation, in particular, can benefit much from accurate measurements of DSDs, as is also the case with many meteorological applications like cloud model initialization and verification, and cloud radiative transfer. Ground-based distrometers have been used extensively to validate radar based rainfall retrieval, in the process of quantitative precipitation estimation (QPE), and can contribute to keep the quality of dual-polarization radar parameters at a high level. One of the distrometers most widely used in validation and comparisons of weather radar is the OTT Parsivel. Two of those distrometers were installed in the state of Paraná in southern Brazil in association with a single-polarization radar at Teixeira Soares (from now on TXS, -50.3613, 25.5053) and a dual polarization radar at Cascavel (from now on CAS, -53.5293, -24.8700). The distrometer operating with CAS is situated at about 3 km from the radar and the distrometer associated with TXS is at a distance around 114 km. Both radars operate at S-band and cover regions of high socio-economic importance with outstanding agro-industrial activities and energy production; the latter is responsible – in the context of South Brazil – for more than 35% of the total hydropower generation in the country. This paper deals with the derivation of Z-R relationships based on data from the distrometers and comparisons of reflectivity as measured with the radars and reflectivity computed from the distrometer data. A series of approximately 3 years of data from the distrometers and the corresponding radar measurements were used in the study. At first a comparison of rainfall derived from CAS distrometer with a collocated raingage was effected with distrometer data being integrated to match the 15 min time of resolution of the raingage data. Results indicate a better agreement at the lower portion of rainfall rate range with departures from the 1:1 curve increasing with R; the fitted curve features a slope of 0.87 with an intercept of 0.01. Monthly average raindrop spectra were then elaborated covering one of the distrometer droplet size ranges, e.g., 0.3 mm to 5.5 mm The set of monthly curves are, in general, confined between the curves for July (dry season) and December (wet season); the July (uppermost) and December (lowermost) curves are approximately parallel in the range of diameters from about 1 mm to about 5 mm, i.e, the ratio of the concentration numbers of droplets is approximately constant along that diameter range. The peak concentration of droplets, occurring in the lower end of diameter range is more pronounced in the months of May to June, featuring a “kink” around D~0.4mm; for the rest of the year the maximum concentration is “smooth” and centered around D~0.5 mm. The accentuated decrease in the concentration number of drops was not observed, up to the lowest diameter registered of 0.3 mm. Reflectivity was derived from the distrometer data and compared to corresponding radar reflectivity, for both CAS and TXS. For CAS, PPI at an elevation of 1.5° was used and the distrometer time resolution was 60 sec; no corrections were made regarding the antenna near field. For TXS the 0.5° elevation PPI was used and distrometric data time resolution was also 60 sec. For both radars, reflectivities were derived for arrays of 3 x 3 and 11 x 11 range gates centered at the gate containing the distrometer. The dispersion curves show slopes varying from 0.93 to 0.99 and biases from -4.7 to -7.2 dBZ. Z-R relationships were derived for CAS and TXS using distrometer data at 30 sec resolution. Relations were stratified as following : a) General relationship for the whole set of data, b) “Seasonal” relationships, one for Summer (December-to-February) and the other for the period from March-to-November, mostly in the dry period, c) Relationships for each month of the year, and d) Relationships for each of the following daily intervals: 10-14 h, 14-17 h, 17-22 and 22-10 h. This last stratification has proved quite relevant, for instance, in flow simulation over basins in the State of São Paulo. For CAS, for the monthly relationships the multiplicative coefficient, A, showed a substantial variation from a minimum of 130 in February to a maximum of 496 in April, while the exponent, b, varied between 1.5 and 1.7, except for April (1.3) and November (1.4) while for the daily interval relationships the respective variations were 153 to 242 and 1.5 to 1.7. For TXS the monthly relationships showed a variation of A from 119 to 489 with b varying from 1.3 to 1.8 while for the daily interval relationships A went from 152 to 423 and b from 1.3 to 1.5. Particularly in reference to the distrometer - raingage comparison and the comparisons of reflectivity from both distrometer and radar results suggests compatibility with results from works reported in the literature. Regarding the Z-R relationships results indicate, in general, compatibility with prevailing convective/ stratiform conditions. Ongoing work includes verifications of the calibration of the radar and fitting standard distribution functions to the distrometer data.