CARROTS: een klimatologische correctie voor radarneerslag in een operationele context: CARROTS: A climatological correction product for radar rainfall in an operational setting

Real-time radar quantitative precipitation estimations (QPEs) generally show significant biases from the true rainfall amount. Despite the abundant number of adjustment methods, the absence of a timely reporting high-density rain gauge network limits the use of these methods. This especially holds for more advanced geostatistical and Bayesian methods that can also correct the radar QPE in space. As an alternative, we present CARROTS (Climatology-based Adjustments for Radar Rainfall in an OperaTional Setting), a gridded climatological QPE correction product, which corrects the radar QPE both in space and time. The publicly available CARROTS factors are based on a historical set of 10 years of 5-min radar and reference rainfall data from KNMI, which makes CARROTS independent of real-time rain gauge availability. We tested the CARROTS factors on the resulting corrected radar QPE and subsequent discharge simulations for 12 Dutch catchments. We regarded the mean field bias (MFB) adjustment method as benchmark in this study. This real-time adjustment method determines spatially uniform adjustment factors based on the 32 automatic rain gauges of KNMI and is operationally used in the Netherlands. The CARROTS factors show clear spatial and temporal patterns. From December through March, the factors are higher than in other seasons, which is likely a result of sampling above the melting layer during these months. Compared to the unadjusted radar QPE, both adjustment methods significantly improve the estimated rainfall sums, but annual rainfall sums from CARROTS outperform the MFB-adjusted QPE for catchments in the south and east of the Netherlands. In these regions, the MFB-adjusted QPE still underestimates the rainfall amounts. Differences in the rainfall estimations are amplified in the discharge simulations, where CARROTS outperforms the simulations with the MFB-adjusted product for all but one basin. Concluding, CARROTS can be a benchmark for QPE adjustment method development, and it has shown to be a better operational option than the MFB method, provided that the radar data it is used on, was processed in a similar way as the radar data the factors are based on.