2006: The very stable boundary layer on nights with weak low-level jets

The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very weak turbulence and negligible turbulent mixing. The focus of this paper is on the effects of this quiescent layer just above the shallow BL, and the impacts of this quiescent layer on turbulent transport and nu-merical modeling. High-frequency time series of temperature T on a 60-m tower showed that 1) the amplitudes of the T fluctuations were dramatically suppressed at levels above 30 m in contrast to the relatively larger intermittent T fluctuations in the shallow BL below, and 2) the temperature at 40- to 60-