Finescale Vertical Structure of a Cold Front as Revealed by an Airborne Doppler Radar

Abstract

In the afternoon of 24 May 2002, a well-defined and frontogenetic cold front moved through the Texas panhandle. Detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary. This paper primarily uses reflectivity and Doppler velocity data from an airborne 95-GHz radar, as well as flight-level thermodynamic data, to describe the vertical structure of the cold front as it intersected with the dryline. The prefrontal convective boundary layer was weakly capped, weakly sheared, and about 2.5 times deeper than the cold-frontal density current. The radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail (∼40 m). The echo structure and dual-Doppler-inferred airflow in the vertical plane reveal typical features such as a nose, a head, a rear-inflow current, and a broad current of rising prefrontal air that feeds the accelerating front-to-rear current over the head. The 2D cross-frontal structure, including the frontal slope, is highly variable in time or alongfront distance. Along this slope horizontal vorticity, averaging ∼0.05 s−1, is generated baroclinically, and the associated strong cross-front shear triggers Kelvin–Helmholtz (KH) billows at the density interface. Some KH billows occupy much of the depth of the density current, possibly even temporarily cutting off the head from its trailing body.

Evolution and structure of a cold front in an Alpine valley as revealed by a Doppler lidar

“…The inclination of the frontal surface is about z/ x = 1/3 as derived from lidar data and about 1/7.5 as estimated from a temperature slope profile. These values reveal a much steeper front than in an earlier study conducted in the Inn Valley (Freytag, 1990) but are consistent with other observations of atmospheric density currents (Geerts, et al, 2006). However, lidar observations also show that the structure of the head of the density current and therefore also the steepness of the frontal surface is variable in time.…”

“…The latter is supported by a subcritical Richardson number (Ri < 0.25) at the interface between the cold-air flow and the foehn. Such density current characteristics have been found for other cold fronts too (Garratt, 1988;Nielsen and Neilley, 1990;Koch and Clark, 1999;Geerts, et al, 2006). The inclination of the frontal surface is about z/ x = 1/3 as derived from lidar data and about 1/7.5 as estimated from a temperature slope profile.…”

“…The spatial resolution of the lidar dataset is not sufficient to resolve turbulent eddies in a great detail. Hence, the flow field is much smoother compared with the higher-resolution Doppler radar observations of Geerts, et al (2006). Our lidar observations indicate a relatively large eddy with a diameter of about 500-1000 m at the head.…”

“…Schematic representation of an atmospheric density or gravity current. Adapted from several sources including Charba (1974), Goff (1976), Koch (1984), Smith and Reeder (1988), Simpson (1997) and Geerts, et al (2006). According to Simpson (1997), two types of instability may occur at the head, which lead to the formation of billows or to lobes and clefts, respectively.…”

“…Cold fronts have often been described as atmospheric density (or gravity) currents (Carbone, 1982;Bond and Fleagle, 1985;Seitter and Muench, 1985;Shapiro, et al, 1985;Hakim, 1991;Darby, et al, 1999;Koch and Clark, 1999;Neiman, et al, 2001;Geerts, et al, 2006). A conceptual model of such a density current is shown in Figure 2.…”