Timescales of Orographic Precipitation in Idealized Simulations from a Lagrangian Viewpoint

One classical, fundamental concept to understand orographic precipitation amounts and patterns is the upslope model. In recent years this model has seen several extensions, which basically rely on the introduction of a timescale describing the precipitation processes. However, this "microphysical" timescale is quite poorly constrained by observations and theoretical considerations.

By tracing individual air parcels in numerical model simulations of orographic flow one can directly quantify the relevant timescales for individual parcels and therefore improve the constraints for the timescales required in the upslope model. We performed idealized two- and three-dimensional simulations with the non-hydrostatic limited-area model COSMO with a horizontal resolution of one kilometer and a two-moment warm-rain microphysics scheme. The Lagrangian analysis of the model data revealed an interesting pattern of timescale-variability within single clouds, which is influenced by the formation of gravity waves and the microphysical evolution of the air parcels. It is particularly interesting, that besides the usually considered influence of the advective timescale on the microphysics there is a distinct influence of precipitation formation on the advective timescale. This impact, which is due to the removal of precipitable water, varies in strength with the flow regime depending on the downdraft pattern over the downwind slope. Generally, the dependency on the gravity wave dynamics gives rise to a strong variation of the timescales with the flow regime.

Besides improving the estimates of the timescales that are essential for orographic precipitation formation, the Lagrangian perspective allows analyzing in-depth the physical mechanisms that determine these timescales. The relevant processes include the influence of the dynamics and microphysics on the cloud shape and the precipitation efficiency and the two-way interaction of dynamical and microphysical processes. Therefore the use of a Lagrangian perspective is promising to provide novel insight to the orographic precipitation problem.