Improving Operational Use of Scanning Rain Radar Estimates with Vertically Pointing Radar
Stormwater Conference
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Accurate estimation of the spatial and time variation of rainfall across urban catchments is essential for accurate sewer and stormwater modelling and operations activities.
In previous work, the authors have described the automated processing and quality control of observations from the Auckland MetService radar, required for meaningful real-time calibration of the radar precipitation estimates using the Auckland Council rain gauge network. The high quality radar derived accumulations (1 minute time step, 500m resolution rasters) are then fed into a cloud-based GIS platform and can be interacted with by council staff, for example to extract a catchment averaged accumulation or raster stack for model input.
Some form of calibration of radar precipitation estimates is required because the absolute bias of the [MetService] scanning radar is unknown, signal losses occur due to radome and path attenuation and the relationship between the radar measurement aloft (reflectivity) and rainfall rate at the surface (mm/hr) is dependent on the raindrop size distribution, which is not able to be estimated by the scanning radar. The sampling scale differences between rain gauges (10-20 cm diameter gauges) and radar (~500x500m pixels) make it very difficult to directly compare radar and gauge measurements and can confound reliable estimation of correction factors in real time.
An alternative approach to the scanning radar calibration problem is to use small Vertically Pointing Radar (VPR) to calibrate the MetService radar measurements directly while also estimating the variability in attenuation and reflectivity-rainfall (Z-R) relationship. The skill of the VPR calibrated radar product was compared with the conventional gauge correction approach for detecting significant rainfall events. The VPR and gauge products were found to have similar skill for the detection of short duration/high intensity rainfall in the retrospective tests. A possible operational advantage of the VPR calibration approach over using the gauge network to adjust the radar measurements is reduced dependency on the gauge telemetry system, which could significantly reduce the delay between the radar measurement being made and alarms being issued.