Precipitation Estimation Using C-Band Dual Polarimetric Weather Radar
Hassan, Diar M.
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Radar Quantitative Precipitation Estimation (QPE) plays an important role in weather forecasting, especially nowcasting, and hydrology. This study evaluates the current QPE algorithm implemented by the Canadian Radar Network of Environment Canada, suggests an improved algorithm, and also evaluates the use of polarimetric radars for estimation of Snow Water Equivalent (SWE), solid snowfall, and rainfall rates. Data from the dual polarimetric C-band King City radar (CWKR) near Toronto, Ontario, SWE and solid snowfall rates from Oakville, Ontario, SWE from the CAN-Now project at Pearson International Airport (CYYZ), Toronto, Ontario, and Mount Pearl, Newfoundland were used in this project. The ground observations show that the polarimetric variables could be used to infer a few of the microphysical processes during snowfall. It is suggested that the co-polar correlation coefficient (hv) could be sensitive to the size ranges of different snow habits within the radar sampled volume. Also, higher differential reflectivity (ZDR) values were measured with large aggregates due to the Mie resonance effect, lower fluttering angles, or induced field transverse. Data from the three sites were used to develop S(ZeH)-based algorithms at 1 hr interval SWE, where ZeH is the radar equivalent reflectivity factor. Similarly, two additional algorithms were developed using SWE at 10 min intervals from CYYZ and Mt. Pearl but they were found to have less skill. A modest difference was found between S(ZeH) and the polarimetric algorithm, S(ZeH, ZDR), in estimating SWE. The 1 hr interval SWE accumulation from the three sites were combined to develop an additional S(ZeH) algorithm which had statistically better results. The results show a severe underestimation of SWE and solid snowfall rates by the current Environment Canada algorithm. The similarity of the S(ZeH) algorithms for CYYZ and Mount Pearl suggests that the same algorithm could be used for many sites. A strong correlation was found between radar reflectivity factor and ground solid snowfall measurement. Accordingly, S(ZeH) and S(ZeH, ZDR) algorithms were established to directly estimate solid snowfall rates on the ground. The S(ZeH) was found to have superior results compared to the S(ZeH, ZDR). Finally, the polarimetric variables were found to be useful in estimating rainfall rates. Thus, three rainfall algorithms (R(ZeH), R(ZeH, ZDR), R(KDP)) were established and compared against the current algorithm employed by the Environment Canada and counterpart algorithms established by Bringi et al. (2010). A logic tree was devised with certain polarimetric thresholds to choose the optimal algorithm among the three established ones. It appears that for rain, unlike for snow, the polarimetric parameters are very useful for quantitative precipitation estimation.