High frequency wind and temperature measurements, obtained in March 1995 from a 10-m tower array situated in southeast Kansas, are used to analyze and compare the structure of a shallow density current
and that of a synoptic-scale cold front in the stable nighttime surface layer.
The density current is approximately 7 m deep and exhibits a current head that is estimated to be about twice the current depth. The event lasted approximately 900 s and its origin appears to be a shallow
slope 2-5 km to the west of the site, where cold air drainage occurs. The onset of the event is marked by a 4 C temperature drop over 30 s, increased dissipation of kinetic energy, measured by a hot-wire anemometer at the 3-m level, and by a strong mean vertical temperature advection between the 3- and 10-m levels that is determined by the sonic anemometer data. Postulated frontogenesis, prior to the arrival of the apparent equilibrated front of approximately 120-m width at the site, is examined by means of a theoretical model (Kay 1992). The absence of upstream data is determined, however, to be a limitation on the analysis performed.
The passage of the synoptic-scale cold front is marked by a 150 degree change in the wind direction, from south to northwest, and a sharp jump in wind speed, from 5 to 15 m s-1. The duration of the frontal passage, defined by the time taken for the completion of the wind shift, was approximately 1500 s. The peak average turbulent kinetic energy (TKE) value of 2.1 m2 s-2 and the peak average dissipation rate value of 1.0 m2s-3 both occur approximately 850 s after the initial wind shift associated with the frontal passage. For the density current, the peak average TKE value (0.065 m2 s-2) and the peak average dissipation rate value (0.025 m2 s-3) occur approximately 40 s after the head of the current passes. The distribution of TKE and dissipation rate in the density current and in the cold front, relative to the onset of the events, is also similar, opening the possibility of scaling relationships existing between the events.
Reference:
Kay A., 1992: Frontogenesis in gravity-driven flows with nonuniform density gradients. J. Fluid Mech. 235, 529-556