Orographic Precipitation Enhancement in the Presence of Large-Scale Weather Systems

Abstract

Heavy precipitation in midlatitude mountain ranges is driven by the episodic passage of weather systems. It is a common paradigm that this precipitation results from the large inte- grated vapor transport (IVT) observed in the warm-sectors of these storms. Consequentially, prior idealized simulations of midlatitude orographic precipitation have simulated precipita- tion events with unidirectional, vertically sheared airstreams encountering terrain. Here we refute the hypothesis that the dominant control on orographic precipitation is the upstream IVT. We compare the IVT and resulting precipitation in published observations of real-world events with the same quantities in published studies of idealized moist airstreams impinging on a ridge. We find that numerical simulations of the upslope precipitation produced by moist horizontal unidirectional environmental winds, including 2D, 3D and real topography, tend to significantly underestimate the precipitation in comparison to actual events having the same IVT, suggesting an alternative driver of precipitation.To explore the dynamic mechanisms behind this orographic precipitation enhancement during storm events, we conducted a novel set of idealized numerical simulations. This set of simulations was designed to isolate the effects of mesoscale circulations induced by the terrain modified flow from the effect of forced uplift of a moist airstream alone. We conducted an idealized simulation of an archetypal midlatitude cyclone encountering an isolated 1-km tall, 500-km long, North-South topographic ridge. The cyclone is initialized with a PV anomaly which develops into a mature cyclone over the course of 2.5 days. Our focus is on the enhancement of the orographic precipitation as the warm sector of the storm traverses the ridge. The unique idealized setup of the simulation allows for two addi- tional, closely related experiments to be conducted. In the first additional experiment, the cyclone simulation is run with flat terrain. In the second, a unidirectional, parallel shear flow simulation is run with the flow encountering the same idealized ridge as in the original experiment. The shear flow is initialized with a vertical profile of wind, temperature, and moisture taken from the warm sector of the midlatitude cyclone, and therefore has the same cross-terrain integrated vapor transport (IVT) as the Cyc+Mt case. Hereafter, the simula- tion with both the cyclone and the ridge, the flat terrain cyclone simulation, and the parallel shear flow simulations will be referred to as the Cyc+Mt, Cyc-Flat, and Shear simulations, respectively. We found that there was significantly more precipitation on the upslope side of the terrain in the case with the cyclone and the mountain compared to the shear flow case. This precipitation enhancement is strongly correlated in space and time with ridge-scale low-level moisture flux convergence (MFC) and the enhancement of embedded convection in the nearly neutrally stratified moist airstream. The large MFC in the Cyc+Mt simulation stems from two features of the mesoscale flow pattern produced by the interaction of the large-scale disturbance with the terrain. The first arises from differences in the mountain- wave structure in comparison to the shear-flow case. The cross-mountain flow at the crest is slower than in the shear flow case, thereby increasing the East-West mass-and moisture- flux convergence over the windward slope. Second, a low-level jet on the upslope side of the terrain in the Cyc+Mt case brings in warm moist air that increases the North-South moisture flux convergence on the windward slope relative to the shear flow case. The net result of these differences in moisture-flux convergence is that there is about four times more precipitation over the mountain in the warm sector of the cyclone and mountain simulation than in the shear flow simulation, despite the two simulations having identical upstream IVT. The precipitation generated by the cyclone in the absence of the mountain is also far weaker, with nearly zero precipitation accumulation in the warm sector of the storm until the arrival of the front. Our simulations demonstrate that high values of upstream IVT are not responsible for large orographic precipitation values, and that mesoscale features of the terrain-induced flow control rainfall.