Sensitivity of orographic precipitation in Switerland to atmospheric processes – simulations with the high-resolution numerical model COSMO

Dimensioning of flood protections is based on the estimation of the probable maximum flood (PMF). A reliable estimate of this quantity can only be made using a realistic estimate of the probable maximum precipitation (PMP) in the considered catchment. However, traditionally used procedures to estimate the PMP are not well suited for mountainous regions. These procedures typically transfer an extreme precipitation event observed in a nearby area to the catchment of interest with some adaptation for the differing topography. But the complex terrain does strongly affect the precipitation distribution and impose strong nonlinearities for the precipitation resulting from small variations in the atmospheric flow conditions. Therefore an in-depth knowledge of the precipitation characteristics of a catchment is needed to obtain realistic estimates of the PMP and eventually the PMF.

We use the high-resolution numerical weather prediction model COSMO to study small-scale processes induced by topography-flow interactions. A sensitivity analysis is performed to determine the influence of subtle variations in atmospheric parameters such as specific humidity, wind direction, and temperature on the precipitation distribution. For this purpose, various approaches are used to modify either the initial and boundary conditions of humidity and temperature, or the wind field via a synthetic PV modification and PV inversion. Simulations are performed for different flood events in Switzerland, including different type of synoptic forcing, such as blocked and unblocked cases, characterized by atmospheric rivers or quasi-stationary cyclone.

The results show that, for instance an increase of the specific humidity of the incident flow does not necessarily produce an increase of precipitation in the target catchment. Indeed, with increased ambient moisture, smaller mountains upstream of the catchment can be more efficient in triggering precipitation and therefore reduce the moisture available downstream. This novel approach with a set of synthetic sensitivity experiments allows estimating, for a particular catchment, the physical limits of the PMP value.