Temperature and CCN sensitivity of orographic precipitation enhanced by a mixed-phase seeder-feeder mechanism

Abstract. Orographic precipitation is a key driver of flooding in mountainous areas. This article investigates the microphysical response of orographic rainfall to perturbations of temperature and cloud condensation nuclei (CCN) concentration. The study is motivated by the increased water vapour capacity of the atmosphere in a warming climate and the increasing frequency of extreme rainfall events. A case study for the Cumbria flood in December 2015 is performed with sensitivities using a realization of the `piggybacking' method implemented into a limited-area setup of the ICON model. A 6 % K-1 enhancement of rainfall results for the highest altitudes, caused by a `mixed-phase seeder-feeder mechanism', i.e. the interplay of melting and accretion. Total 24 h rainfall is found to increase by only 2 % K-1, significantly less than the 7 % K-1 increase in atmospheric water vapour. A rain budget analysis reveals that the negative temperature sensitivity of the condensation ratio and the increase of rain evaporation dampen the rainfall enhancement. Decreasing the CCN concentration speeds up the microphysical processing, which leads to an increase in total rainfall. At low CCN concentration the rainfall sensitivity to temperature is systematically smaller. It is shown that the CCN and temperature sensitivities are to a large extent independent (with a ±3 % relative error) and additive.