THE LOW-LEVEL STRUCTURE AND EVOLUTION OF A DRY ARCTIC FRONT OVER THE CENTRAL UNITED-STATES .1. MESOSCALE OBSERVATIONS

Observations taken over the period 8-10 March 1992 during the Storm-sc ale Operational and Research Meteorology Fronts Experiment Systems Tes t in the central United States are used to document the detailed low-l evel structure and evolution of a shallow, dry arctic front. The front was characterized by cloudy skies to it north side and clear skies to its south side. It was essentially two-dimensional in the zone of int ense observations. There was a significant diurnal cycle in the magnit ude of the potential temperature gradient across both the subsynoptic and mesoscale frontal zones, but imposed upon an underlying, more grad ual, increase over the three days. On the warm (cloudless) side, the t emperature increased and decreased in response to the diurnal heating cycle, while on the cold (cloudy) side the shape of the temperature de crease from its warm-side value (first dropping rapidly and then slowl y in an exponential-like manner) remained fairly steady. The authors a ttribute the strong diurnal variation in potential temperature gradien t mostly to the effects of differential diabatic heating across the fr ont due to differential cloud cover. The front is described in terms o f three scales: 1) a broad, subsynoptic frontal zone (similar to 250-3 00 km wide) of modest temperature and wind gradients; 2) a narrower me soscale zone (similar to 15-20 km wide) with much larger gradients; an d 3) a microscale zone of near-zero-order discontinuity (less than or equal to 1-2 km wide). There was some narrowing (less than or similar to 50 km) of the subsynoptic frontal zone, but the authors found no ev idence for any significant contraction of this zone down to much small er mesoscale sizes. In response to the differential diabatic heating, the strongest evolution occurred in the micro-mesoscale zone, where du al-Doppler radar and aircraft measurements revealed the development of a density-current-like structure in and behind the leading edge of co ld air. Here the steepest gradients developed shortly after sunrise an d 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 cu mulus 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, thei r leading edge receding by noon to 10-20 km behind the cumulus line.