Quantitative precipitation estimation with weather radar using a data- and information-based approach

Abstract. In this study we propose and demonstrate a data-driven approach in an information-theoretic framework to estimate precipitation quantitatively. In this context predictive relations are expressed by empirical discrete probability distributions directly derived from data instead of fitting and applying deterministic functions as it is standard operational practice. Applying a probabilistic relation has the benefit to provide joint statements about rain rate and the related estimation uncertainty. The information-theoretic framework furthermore allows integration of any kind of data considered useful and explicitly considers the uncertain nature of quantitative precipitation estimation (QPE). With this framework we investigate the information gains and losses associated with various data and practices typically applied in QPE. To this end we conduct six experiments using four years of data from six laser optical disdrometers, two micro rain radars MRR regular rain gauges, weather radar reflectivity and other operationally available meteorological data from existing stations. Each experiment addresses a typical question related to QPE: First, we measure the information about ground rainfall contained in various operationally available predictors. Here weather radar proves to be the single most important source of information, which can be further improved when distinguishing radar reflectivity – ground rainfall relationships (Z-R relations) by season and prevailing synoptic circulation pattern. Second, we investigate the effect of data sample size on QPE uncertainty using different data based predictive models. It shows that the combination of reflectivity and month of the year as a double predictor model is the best trade-off between robustness of the model and information gain. Third, we investigate the information content in spatial position by learning and applying site-specific Z-R relations. The related information gains are only moderate and especially lower than when distinguishing Z-R relations according to time of the year or synoptic circulation pattern. Fourth, we measure the information loss when fitting and using a deterministic Z-R relation, as it is standard practice in operational radar based QPE applying e.g. the standard Marshal-Palmer relation, instead of using the empirical relation derived directly from the data. It shows that while the deterministic function captures the overall shape of the empirical relation quite well, it introduces additional 60% of uncertainty when estimating rain rate. Fifth, we investigate how much information is gained along the radar observation path, starting with reflectivity measured by radar at height, continuing with the reflectivity measured by a MRR along a vertical profile in the atmosphere and ending with the reflectivity observed by a disdrometer directly at the ground. The results reveal that considerable additional information is gained by using observations from lower elevations by avoiding information losses caused by ongoing microphysical precipitation processes from cloud height to ground. This underlines both the importance of vertical corrections for accurate QPE and of the required MRR observations. In the sixth experiment we evaluate the information content of radar data only, rain gauge data only and a combination of both as a function of the distance between the target and predictor rain gauge. The results show that station-only QPE outperforms radar-only QPE up to a distance of 7 to 8 km and that radar-and-gauge QPE performs best, even compared to radar-based models applying season or circulation pattern.