Improved rainfall estimates in convective storms using polarisation diversity radar

Errors arise when using conventional radar reflectivity, Z, to estimate rai nfall rate, R,and these can be particularly severe during severe convective storms; the very events when accurate estimates are needed so that action can be taken to mitigate the effects of flooding. Concentration is on three problems associated with heavy rainfall: hail, attenuation and absolute ca libration of the radar, and consider how polarisation radar parameters, dif ferential reflectivity, Z(DR), and specific differential phase shift K-DP, might lead to their alleviation. It is essential to consider the fundamenta l limits to the accuracy with which these parameters can be estimated. If Z DR can be measured to an accuracy of 0.2 dB, then it provides a measure of mean raindrop shape which is sufficiently precise to improve rainrate estim ates. This can be achieved at S-band (10 cm), but seems very difficult for operational C-band (5 cm) radars; differential attenuation by the heavy rai n introduces a negative bias into Z(DR) which increases with range. However , the magnitude of this bias at C-band can then be used to correct for the total attenuation of Z. Differential phase, K-DP, has the advantage that it is a phase measurement and so is unaffected by attenuation. It only respon ds to the rainfall and is unaffected by the hail, but K-DP is a noisy param eter and is only useful for heavy rainfall above 30-60 mm hr(-1). Fortuitou sly, K-DP and Z(DR) are not independent and one use of K-DP and Z(DR) may w ell be to exploit this redundancy to identify rain areas as opposed to hail , and in rainfall to use the redundancy to provide an automatic calibration of the absolute reflectivity, Z, to 0.5 dB (12%). Finally, the noisy chara cter of both ZDR and KDP together with the low level of the co-polar correl ation coefficient provide the first reliable means of detecting and removin g anomalous propagation which is a major operational problem for all weathe r radars.