The Rapid Morning Boundary-Layer Transition

Abstract

Even slight terrain inhomogeneities can cause large horizontal variations in the clear, stably stratified, nocturnal boundary layer largely through cold air drainage. By early morning the valleys and depressions can be several degrees cooler than the adjacent slopes and plateaus. As surface heating begins in the morning, these horizontal variations can lead to abrupt changes in temperature and wind speed at valley observation sites, as the boundary layer warms and becomes unstably stratified. Temperature and wind speed changes of 12 K and 6 m s?1 respectively, within a 30 min period are observed even in valleys as shallow as 50 m with slopes of only 0.007. These changes are too large to be accounted for by vertical convergence of turbulent beat flux. Rather, it appears that a well-mixed boundary layer is advected into the valley from the upstream slopes or plateaus. Data from the National Hail Research Experiment (NHRE) 1976 surface mesonet are used to show that, statistically, this abrupt change is a frequent occurrence, throughout the summer, even in broad shallow valleys, but almost never occurs on plateau observation sites. 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. The observed transition in temperature typically precedes the velocity transition by 20?40 min. This lag appears to be due to both the adverse pressure gradient developed during the temperature transition, and the difference in the shear and temperature gradient production terms in the equations for shear stress and heat flux.