Multi-year 100-metre-scale urban precipitation study based on X-band radar observations

An operational, single-polarized X-band weather radar monitors precipitation within a 20 km scan radius around Hamburg’s city center for almost eight years. This weather radar operates at an elevation angle (~3.5°) with a high temporal (30 s), range (60 m), and sampling (1°) resolution refining observations of the German nationwide C-band radars. Studies on short time periods (several months and case studies) proofs the performance of this low-cost local area weather radar. The synergy of observations of the X-band radar, vertically pointing micro rain radars, and rain gauges yields a reliable eight-year precipitation climatology with 100 m resolution. The two guiding questions of this presentation are: Is the variability of this precipitation climatology representative and not contaminated by measurement errors? Which sub-hourly precipitation characteristics can we infer from this precipitation climatology?

Several sources of radar-based errors were adjusted gradually affecting the precipitation estimate, e.g. the radar calibration, alignment, attenuation, noise, non-meteorologial echoes. Additionally, statistical relations (k-Z and Z-R relation) increase the uncertainty of the precipitation estimate. However, the deployment of additional vertically pointing micro rain radars yields drop size distributions at relevant heights, which increases the data quality effectively and assesses the statistics of the long-term precipitation observations. The resulting climatology allows studies on the spatial and temporal scale of urban precipitation. We outline the performance of the climatology, present first results on sub-hourly precipitation characteristics and discuss open issues and limitations.

This multi-year urban precipitation analysis is groundwork for further hydrological research in an urban area within the project Sustainable Adaption Scenarios for Urban Areas – Water from Four Sides of the Cluster of Excellence Climate Climatic Change, and Society (CliCCS). Future urban precipitation studies will be improved by the extension of networked observations with a second X-band weather radar site and additional micro rain radars in Hamburg measuring since the beginning of 2021.