The Low-Level Structure and Evolution of a Dry Arctic Front over the Central United States. Part I: Mesoscale Observations

Abstract

Observations taken over the period 8?10 March 1992 during the Storm-scale Operational and Research Meteorology Fronts Experiment Systems Test in the central United States are used to document the detailed low-level structure and evolution of a shallow, dry arctic front. The front was characterized by cloudy skies to its north side and clear skies to its south side. It was essentially two-dimensional in the zone of intense observations. There was a significant diurnal cycle in the magnitude of the potential temperature gradient across both the subsynoptic and mesoscale frontal zones, but imposed upon an underlying, more gradual, increase over the three days. On the warm (cloudless) side., the temperature increased and decreased in response to the diurnal heating cycle, while on the cold (cloudy) side the shape of the temperature decrease from its warm-side value (first dropping rapidly and then slowly in an exponential-like manner) remained fairly steady. The authors attribute the strong diurnal variation in potential temperature gradient mostly to the effects of differential diabatic heating across the front due to differential cloud cover. The front is described in terms of three scales: 1) a broad, subsynoptic frontal zone (?250?300 km wide) of modest temperature and wind gradients; 2) a narrower mesoscale zone (?15?20 km wide) with much larger gradients; and 3) a microscale zone of near-zero-order discontinuity (≤1?2 km wide). There was some narrowing (?50 km) of the subsynoptic frontal zone, but the authors found no evidence for any significant contraction of this zone down to much smaller mesoscale sizes. In response to the differential diabatic heating, the strongest evolution occurred in the micro-mesoscale zone, where dual-Doppler radar and aircraft measurements revealed the development of a density-current-like structure in and behind the leading edge of cold air. Here the steepest gradients developed shortly after sunrise and then increased by an order of magnitude during the day, with leading-edge vorticity, divergence, and temperature gradients reaching maximum values of 10?2 s?1 and 8 K km?1. A narrow updraft, marked by cumulus clouds, grew in intensity above the leading edge through the day to a maximum of 5?8 m s?1. Stratus clouds lay in the cold air, their leading edge receding by noon to 10?20 km behind the cumulus line.