Orographic Flow Response to Variations in Upstream Humidity

Abstract

The effects of upstream relative humidity (RH) on low-level wind and precipitation patterns for low-speed, statically stable flows over a mountain are investigated using idealized two- and three-dimensional numerical-simulation experiments in which RH is increased from 0% to 100%. For RH less than some critical threshold, the flow upstream becomes less decelerated as RH is increased; for RH greater than this threshold, the flow upstream becomes more decelerated as RH is increased. This increasing deceleration with RH is due to locally enhanced static stability resulting from enhanced condensation near the freezing level. Analyses from the simulations indicate that the lifted condensation level and the height of the freezing level are significant control parameters for the upstream-flow deceleration in the steady-state solutions. Dimensional analysis using these control parameters (as well as others) brings forth new nondimensional parameters that are shown to enter into analytic formulas for the orographic upstream-flow deceleration in a moist atmosphere.

A scaling relation for warm‐phase orographic precipitation: a Lagrangian analysis for 2D mountains

“…This behaviour is consistent with previous studies on the influence of moisture on flow blocking (e.g. Jiang, ; Reeves and Rotunno, ). The different flow patterns for different U 0 impact the cloud geometry, with a much shallower and less symmetric cloud for simulations with a small Froude number.…”

Orographic effects on precipitating clouds

“… Rotunno and Ferretti [2001] and Reeves et al [2008] show how different patterns of humidity in the airflow ahead of fronts affect the degree to which the air is decelerated and turns, or does not, as it approaches a barrier. Reeves and Rotunno [2008] show that even if the air is saturated, cooling by melting of precipitation particles can increase stability and decelerate flow toward a mountain range.…”

Sensitivities of Orographic Precipitation to Terrain Geometry and Upstream Conditions in Idealized Simulations

“…While this study examines idealized terrain shapes, the upstream environment is based on an observed prefrontal Australian event that is moist but initially unsaturated. In one way this is more realistic, but it complicates the scenario because of the mixture of both dry and moist dynamics [see, e.g., Reeves and Rotunno (2008) for discussion on the influence of upstream humidity variations on the orographic flow response].…”