Temperature and cloud condensation nuclei (CCN) sensitivity of orographic precipitation enhanced by a mixed-phase seeder-feeder mechanism: a case study for the 2015 Cumbria flood

The formation of orographic precipitation in mixed-phase clouds depends on a complex interplay of processes. This article investigates the microphysical response of orographic precipitation to perturbations of temperature and cloud condensation nuclei (CCN) concentration. A case study for the 2015 Cumbria flood in northern England is performed with sensitivities using a realization of the "piggybacking" method implemented into a limited-area setup of the Icosahedral Nonhydrostatic (ICON) model. A 6 % K-1 enhancement of precipitation results for the highest altitudes, caused by a "mixed-phase seeder-feeder mechanism", i.e. the interplay of melting and accretion. Total 24 h precipitation is found to increase by only 2 % K--1,K- 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 in rain evaporation dampen the precipitation enhancement. Decreasing the CCN concentration speeds up the microphysical processing, which leads to an increase in total precipitation. At low CCN concentration the precipitation 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.