A Case Study of Cold-Season Emergent Orographic Convection and Its Impact on Precipitation. Part I: Mesoscale Analysis

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Abstract It is not uncommon for layers within the warm conveyor belt in a frontal system to become potentially unstable, releasing elevated convection. The present study examines this destabilization process over complex terrain, and resulting precipitation, with a focus on the surface coupling, orographic ascent, and the initiation and evolution of convective cells. This study uses detailed observations combined with numerical modeling of a baroclinic system passing over the Central Idaho Mountains in the United States on 7 February 2017. The data were collected as part of the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE). Specifically, observations from a ground-based scanning X-band radar and an airborne profiling Doppler W-band radar along ∼100-km-long flight tracks aligned with the wind describe the development and evolution of convective cells above shallow stratiform orographic clouds. Convection-permitting numerical simulations of this event, with an inner domain grid resolution of 0.9 km, capture the emergence and vertical structure of the convective cells. Therefore, they are used to describe the advection of warm, moist air over a retreating warm front, cold-air pooling within the Snake River basin and adjacent valleys, destabilization in a moist layer above this shallow stable layer, and instability release in orographic gravity wave updrafts. In this case, the convective cells topped out near 6 km MSL, and the resulting precipitation fell mostly leeward of the ridge where convection was triggered, on account of strong cross-barrier flow. Sequential convection initiation over terrain ridges and rapid downwind transport led to banded precipitation structures. Significance Statement In winter storms, most precipitation falls on the upwind side of the main mountain crest, especially when there is deep subsident flow in the lee side. This is evident simply from the vegetation on opposite sides of the Cascade Mountains in the United States, for instance. This paper examines a case with significant leeward precipitation. Radar observations from aboard an aircraft and from a scanning radar located on a mountaintop, combined with numerical simulations, show that the cumulus clouds grew over terrain ridges and enhanced precipitation over and downwind of the mountains, notwithstanding the tiny amount of potential energy available for convection. Key ingredients are upstream stable near-surface conditions, an elevated layer of potential instability, and strong cross-barrier flow.

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