Observations of Nighttime Winds Using Pilot Balloons in Anderson Creek Valley, Geysers, California

Abstract

Nighttime drainage or downslope winds along the east-facing slope of Anderson Creek Valley located in the Geysers area of northern California are examined using pilot balloons as air parcel tracers. Observations made over four nights show a persistent pattern of winds. Before sunset strong westerly winds blow into the valley. These winds are believed due to the late afternoon sea breeze. Drainage winds become fully developed within 2 h after sunset. The drainage wind has an average speed of ∼3 m s⁻¹ regardless of the speed of the ambient wind. The drainage layer thickness grows at a rate of ∼80 m per kilometer of downwind distance. Balloons in the drainage current quickly spread throughout the shallow, sidewall canyon containing Anderson Creek. The transition from the local valley wind system to the free, ambient wind occurs at about ridge height, i.e., ∼500 m above the valley basin, for weak ambient winds, and at ∼300 m above the valley basin for strong ambient winds.