Interpreting OLYMPEX Precipitation Distributions Using Quasi-Idealized Numerical Simulations

Observations from the Olympics Mountains Experiment (OLYMPEX) indicate a sensitivity of Olympics precipitation to the phase of the frontal system in which it forms. Ahead of warm or occluded fronts (WF), a broad area of orographic precipitation enhancement extends well upstream of the Olympics, with a clear rainshadow in the lee. Larger enhancements are observed within warm sectors (WS), but these are focused over the high terrain and extended to the lee slope, with a much weaker rainshadow. Post-frontal (PF) precipitation is characterized by intermittent and brief convective showers, focused between the coastline and the lower windward slopes, that produce much lighter area-averaged precipitation than WF and WS periods. To physically interpret these differing precipitation distributions, quasi-idealized simulations are conducted with the WRF model. These simulations use realistic initial conditions and Olympics terrain, along with parameterized large-scale forcing in the form of domain-wide ascent (WF/WS) or cold advection and surface heating over the upstream ocean (PF). The simulations reasonably reproduce the observed differences in Olympics precipitation distributions between the three frontal phases. Most of these differences can be explained on the basis of three parameters: the impinging vertically integrated horizontal moisture flux (I), the upstream precipitation rate (r_up), and the nondimensional mountain height (M). The large precipitation enhancements in the WF and WS periods stem from the combination of large I and substantial r_up, which produce deep orographic clouds with large precipitation efficiencies due to an active seeder-feeder process. Relative to WS periods, the weaker and upstream-shifted precipitation enhancements in WF periods stem from the strong static stability of the frontal inversion (and hence large M), which enhances upstream blocking and the upstream tilt of mountain waves. Orographic enhancements in the PF periods are the weakest due to the combination of small I, negligible r_up, and large M, the latter associated with weaker and nominally unsaturated impinging flow.