Finescale Structure and Dynamics of an Atmospheric Temperature Interface

Abstract

Near-ground observations of an atmospheric temperature interface in the stable nocturnal boundary layer are reported. The thermal effect of the interface passage was a 5-K decrease in temperature during a 5-min period in which changes in wind speed and wind direction were also observed. The interface is of unknown origin and horizontal and vertical scale. A 32-Hz time series of temperature measured with a sonic anemometer?thermometer at 1.5 m above ground level revealed a sharp, nonintermittent decrease in temperature (?1 K s?1) nested within a more gradual, intermittent decrease (?1 K min?1). A first attempt to extricate this subtle feature with regard to the relevant physics of generalized density currents is presented. It is shown that a very similar feature has been observed in the literature, but not interpreted for its physical significance. The contribution of this paper is its unique interpretation of the high-resolution temperature time series as it corresponds to the finescale physical character of an interface of density or temperature, and the motion of such an interface along the ground. Parameters of interest, including a spatial temperature gradient normal to the interface (?1 K m?1), an interfacial thickness (2.8 m), and a thermal boundary layer for the interface (288 m), are calculated. Additionally, theoretical and empirical arguments are presented for a viscous suppression of turbulent mixing in the immediate vicinity of an interface of density or temperature.