Validation of hourly precipitation extremes in the Regional Atmospheric Climate Model (RACMO) for the Netherlands and surrounding regions

Hourly precipitation extremes in the Regional Atmospheric Climate Model (RACMO) are validated for the Netherlands and surrounding regions, primarily with the goal of assessing whether hourly extremes are sufficiently well represented in RACMO for its use in hydrological applications. This is done by a comparison to HARMONIE (in climate mode) at convection-permitting resolutions, and to observations from measurement stations within the  Netherlands. Firstly, the spatial precipitation distribution in RACMO is assessed by comparing individual events of the largest hourly precipitation extremes in RACMO to the same events in HARMONIE. This comparison shows that precipitation in RACMO is substantially more clustered, is frequently spatially shifted and/or occurs over different regions compared to HARMONIE. Thereby, RACMO seems to misrepresent spatial precipitation patterns, although a comparison to observations is required to definitively conclude this. Furthermore, data pooling is used to validate return periods of hourly precipitation in RACMO by a comparison to HARMONIE and observations, from which we conclude that hourly precipitation extremes in RACMO up to return periods of ∼1000 years are too low. Conversely, for return periods of ≳1000 years it seems that the hourly extremes in RACMO are too high, although more data from observations and HARMONIE is required to conclusively determine this. Finally, we validate the dew point temperature (Tdew) in RACMO due to its essential role in governing precipitation extremes, and compare the effect of Tdew on hourly precipitation extremes between RACMO, HARMONIE and the observations. Based on this evaluation, we conclude that the Tdew mean and distribution in RACMO compare well to the observations, while RACMO overestimates the T dew extremes. Finally, the effect of Tdew on hourly precipitation extremes differs considerably between RACMO and both HARMONIE and the observations. Based on the results presented in this report, we believe that RACMO generally does not perform well in reproducing hourly precipitation extremes, and that the misrepresentation of spatial precipitation patterns during hourly extreme events is especially problematic when RACMO is adopted for hydrological applications.