Two-dimensional idealized simulations of the impact of multiple windward ridges on orographic precipitation

Recent observations, linear analytical models, and mesoscale models have shown that gravity waves forced by narrow windward ridges can enhance surface precipitation by more than a factor of two locally. In addition, recent sensitivity simulations of the 13-14 December 2001 IMPROVE-2 IOP over the central Oregon Cascades using the Penn State-NCAR Mesoscale Model (MM5) have suggested that these narrow ridges can increase the net precipitation along windward Cascades by 10-20% as compared to a smooth slope of equal barrier height. This presentation will generalize some of these results by summarizing over one hundred two-dimensional MM5 simulations at 1-km grid spacing using a 2000 m high barrier of 50-km half width (similar in scale to the Oregon Cascades), Thompson (graupel-based) microphysical scheme, and MRF PBL parameterization (no radiation or surface fluxes). For these numerical experiments, a series of uniformly spaced sinusoidal windward ridges (n) were varied from n = 0 to 16 using ridge/valley amplitudes of 200, 400, or 800 m, moist static stabilities of Nm = 0.005 and 0.01 s-1, uniform cross barrier flow of 8, 15, or 30 m/s, and freezing levels (FLs) of 500, 750, or 1000 mb. The horizontal distribution of surface precipitation and hydrometeors aloft were compared between experiments as well the drying ratio (ratio of windward precipitation and water vapor influx), microphysical time scales, and dominate microphysical processes.

This presentation will highlight that 200-800 m windward ridges can have a significant impact on the surface precipitation and hydrometeor production locally as well as averaged over the windward slope. For example, for a relatively weak stability (Nm = 0.005 /s), FL = 750 mb, and windward ridge height of 400 m, the net precipitation and associated drying ratio over the windward slope increases by 20-30% for cross barrier wind speeds >= 15 m/s when the windward ridge number (n) is increased incrementally from 0 to 12. Much of the initial 5-10% surface precipitation increase for n = 2 to 8 is associated with a cloud water enhancement over the ridges at all wind speeds, while for n = 8 to 12 the precipitation increase is associated with net snow production aloft over the windward slope associated with gravity waves. For n > 12, this net snow increase slows as the gravity waves become more evanescent over the narrower ridges, resulting in little net precipitation increase over the windward slope for U < 30 m/s, while increased graupel further increases the net precipitation by an additional 5-10% for n > 12 and U = 30 m s-1. These ridge impacts on net precipitation are maximized for moderate moist Froude numbers, Fr = U/(HN), between 1.0 and 2.0, while there is little ridge impact in the blocked flow regime (Fr < 0.5). For U = 15 m/s, the percentage increase of precipitation over the windward slope with increasing ridge number varies little when the freezing levels is increased between 1000 and 500 mb.