The rapid morning boundary-layer transition.

Abstract

A case study from the Haswell, Colorado, experiment of 1975 shows in detail, through a variety of observations, the sequence of events that occurs during this rapid morning transition. As surface heating begins, the valley air, which is about 4 K colder than the air over the upstream slope and plateau, becomes less stably stratified and increasingly turbulent. Eventually, the shear stress at the top of the boundary layer becomes large enough to pull the cold air out of the valley. The valley air is then replaced by warmer upstream air that is already well mixed. The criteria necessary for this transition to occur are evaluated and generalized for application to other situations. These criteria are then applied to several previous observational studies of the dissipation of cold air pools formed in valleys through nighttime radiational cooling.- from Authors