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Abstract Midlatitude 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 h 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 that are common in prefrontal sectors of midlatitude cyclones in the coastal Pacific Northwest. Finally, a case study of prolonged down-valley flow in a prefrontal 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. Significance StatementThis paper analyzes the characteristics of down-valley flow over the windward Quinault Valley on the Olympic Peninsula of Washington State using data from OLYMPEX, with an emphasis on regional pressure differences and blocking metrics. Results demonstrate that the location of precipitation over the Olympic Peninsula is shifted upstream during events with deep down-valley flow, consistent with blocked upstream airflow. A case study of down-valley flow highlights the role of evaporative cooling to prolong the flow reversal.
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This content will become publicly available on February 28, 2026
Orography‐Induced Atmospheric Small‐Scale Waves During Bora Using Lidar Observations and Numerical Simulations
Abstract Atmospheric flow of cold air over mountain barriers in the Alpine region often gives a rise to strong and gusty downslope wind, Bora. Such flows are often accompanied by atmospheric waves, generated by the flow passing an elevated barrier. Such phenomenon can only rarely be observed visually and can generally not be reliably reproduced by simplified numerical models. Orography‐induced atmospheric small‐scale waves were experimentally observed on 25 January 2019 during a Bora outbreak in the Vipava valley, Slovenia. A vertical scanning lidar, positioned at the lee side of the Trnovski gozd mountain and a fixed direction lidar, 5 km apart in the Vipava valley, were used to characterize the density field. The flow exhibited a stationary jump after the mountain ridge and, superimposed, an oscillatory flow pattern. High‐resolution numerical simulations complemented the observations and supported experimental results on the flow periodicity but also on the wave structures and propagation characteristics.
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- Award ID(s):
- 2327914
- PAR ID:
- 10650471
- Publisher / Repository:
- Geophysical Research Letters
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 4
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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