Sensitivity of Orographic Precipitation to Microphysics Parameter and Process Perturbations

Orographic precipitation plays a vital role in providing water resources to mountain communities. Complex interactions among the dynamics, thermodynamics, and cloud microphysics can control precipitation type, amount, and its location over a mountain. Orographic precipitation has been shown to be sensitive to the upwind sounding, mountain geometry, and microphysics, but disentangling these factors is a challenge. Through idealized Cloud Model 1 (CM1) simulations of moist neutral flow over two-dimensional topography, the microphysical controls on surface precipitation and liquid and ice water path are explored. One-parameter perturbation experiments of multiple parameters within the Morrison microphysics scheme (e.g., snow density, snow fallspeed coefficient, collision efficiencies), as well as physical processes (i.e., rain autoconversion and accretion, snow deposition) are performed. Results show mostly monotonic responses to changes in microphysical parameters, with the dominant controls on precipitation for this case being: snow fallspeed coefficient, snow particle density, ice-cloud water collection efficiency, and cloud water autoconversion. Overall, the perturbations performed result in a larger effect on precipitation location rather than amount. These perturbation experiments will help assess the quantitative sensitivity of orographic precipitation to changes in microphysical parameters, with future work involving two-parameter and multivariate perturbations to explore the full nature of possible multi-parameter interactions.