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Title: Observed and simulated characteristics of down-valley flow within stratiform precipitation over the Olympic Peninsula
Abstract Mid-latitude cyclones approaching coastal mountain ranges experience flow modifications on a variety of scales including orographic lift, blocking, mountain waves, and valley flows. During the 2015-16 Olympic Mountain Experiment (OLYMPEX), a pair of scanning ground radars observed precipitating clouds as they were modified by these orographically-induced flows. The DOW radar, positioned to scan up the windward Quinault Valley, conducted RHI scans during 285 hours of precipitation, 80% of which contained reversed, down-valley flow at lower levels. The existence of down-valley flow in the Quinault Valley was found to be well-correlated with upstream flow blocking and the large-scale sea-level pressure gradient orientated down the valley. Deep down-valley flow occurred in environments with high moist static stability and southerly winds, conditions which are common in pre-frontal sectors of midlatitude cyclones in the coastal Pacific Northwest. Finally, a case study of prolonged down-valley flow in a pre-frontal storm sector was simulated to investigate whether latent heat absorption (cooling) contributed to the event. Three experiments were conducted: a Control simulation and two simulations where the temperature tendencies from melting and evaporation were separately turned off. Results indicated that evaporative cooling had a stronger impact on the event’s down-valley flow than melting, likely because evaporation occurred within the low-level down-valley flow layer. Through these experiments, we show that evaporation helped prolong down-valley flow for several hours past the time of the event’s warm frontal passage.  more » « less
Award ID(s):
2042105 2113207
PAR ID:
10411823
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Monthly Weather Review
ISSN:
0027-0644
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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