The effect of upstream stability on the formation and movement of precipitation systems over idealized terrain

It is well established that for dry flow impinging on a mountain, that if the Froude number (F=U/Nh) is low, the flow will be in a blocked or flow-around regime while for high Froude number cases, the flow will be in an unblocked or flow-over regime. The critical Froude number for which the transition from blocked to unblocked flow occurs is about 1. There have been numerous investigations in the literature on the dependence on F for moist flow over a mountain. While the results of most of these papers indicate that whether moist flow is in a blocked or unblocked regime is dictated by F, this possible dependence has not been systematically tested, such is the goal of this research.

The problem of understanding flow regimes for moist flow is complicated by the variety of candidate control parameters including not just F, but the CAPE, the vertical distribution of moisture, potential instability, etc. We start our investigation with two-dimensional, idealized simulations of conditionally unstable flow impinging on an idealized mountain. Three sets of simulations were performed , one with a 1 km high mountain, one with a 2 km high mountain, and one with a 3 km high mountain. In all sets of experiments, the basic state wind speed, U, was varied from 1 to 20 m/s. These simulations reveal that regardless of the mountain height, the U wind speed for which the transition from blocked to unblocked flow occurred was 10 m/s, meaning that the critical F was 0.42, 0.69, and 0.73 for the 1 km, 2 km, and 3 km simulations, respectively. These findings suggest F is not a reliable control parameter for use in determining precipitation distribution and movement of precipitation systems relative to orography for conditionally unstable flow. Additional sensitivity experiments reveal that the critical F is different for different upstream temperature and moisture profiles, even if the CAPE is held constant.

A second series of experiments was devised where the upstream thermodynamic profile had very low or no CAPE. In this series of simulations, N was the held constant, the U wind speed and mountain height were varied as above, and the moisture content was also varied. These simulations show that when there exists sufficient moisture in the upstream profile, the release of latent heat via condensation acts to increase air parcel buoyancy on the upstream side of the mountain, which, in turn, propels the flow into a flow-over type regime. Again, this phenomena is independent of F. When the moisture content was sufficiently reduced, this effect of latent heating was negligible and two distinct flow regimes were observed that, ultimately, appeared to be determined by F.