Precipitation Over Complex Mountain Terrain in a Convection-Permitting Regional Climate Model

Abstract

Orographic precipitation is a critical freshwater source and major flooding hazard, but its distribution and behavior over complex terrain are often uncertain due to sparse observations. We examine precipitation and its drivers in one of the wettest regions in the world, the Southern Alps of New Zealand (NZ), using the first multi-decadal simulation by a convection-permitting regional climate model across all mainland NZ at 2.2 km grid-scale. Model skill is primarily assessed against direct measurements by more than 170 rain gauges to avoid uncertainty commonly introduced by gridded observations in remote regions. Peak intensity and duration of sub-daily rainfall over mountains appear markedly improved in the 2.2 km model relative to the 12 km driving model. The orientation of water vapor flux relative to the mountain barrier strongly affects both climatological and daily extreme precipitation. Transects illustrate the influence of steep local topography on strong landfalling atmospheric rivers to produce high vertical velocities and extremely high accumulations of rainfall over windward upper mountain flanks, which do not appear unreasonable against available gauge observations. These transects also reveal the finer spatial structure of mountain waves in the 2.2 km model, which may contribute to its more realistic windward enhancement of orographic precipitation, but with excessive leeward precipitation and an annual mean dry bias over mountains. Despite the computational burden, these results support further targeted dynamical modeling at kilometer scales to improve physical understanding of precipitation in the current climate and its potential future change in NZ and other mountainous regions of the world.