Frontogenesis and Symmetric Stability in a Major New England Snowstorm

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Abstract
Synoptic and Doppler radar data are used to study the roles of large-scale frontogenetical forcing and of moist symmetric instability in the New England snowstorm of 5–6 December 1981, associated with an explosively intensifying cyclone offshore. Radar reflectivity patterns showed a tendency toward banded structure, particularly near the leading (northwestern) edge of the storm. Only a minor portion of the snowfall, however, was associated with this pronounced bandedness. From a set of constant-pressure analyses, the frontogenetical forcing was measured from the variation along the temperature gradient of the geostrophic wind component in the direction of this gradient. Over southeastern New England maximum forcing, found near 500 mb at the outset of the storm, descended to the layer between 850 and 700 mb 24 h later. Magnitudes were (3–7) × 10−10 deg m−1 s−1. Observed rates of strengthening of temperature gradient were less than half this value, implying relative adiabatic cooling in the rising warmer air. Doppler radar observations showed strong convergence just above the zone of maximum frontogenesis and at the base of a region of vigorous ascent, with magnitude of a few tens of cm s−1. Symmetric stability was evaluated, for a geostrophic base-state flow, from a series of vertical cross sections as claw as possible to the radar site. Only small areas of instability appeared in the saturated middle and upper troposphere near the outset of the storm. An evaluation based on gradient-wind balance, on the assumption that the base-state flow 1ocally represented a portion of a steady circular vortex, enlarged these regions of small or negative stability in the northwestern portions of the major cloud mass. Strong (moist or dry) symmetric stability was indicated, however, in the inner portions of the developing cyclonic circulation. The small stability initially accompanying the frontogenetical forcing was consistent with recent analytic and numerical models showing a vigorous and concentrated frontal updraft. Details of the structure shown by the Doppler data, and in particular the prominence of the bandedness at the northwestern edge of the storm, could be attributed to symmetric instability. The ascent was driven, however, by the frontogenetical forcing, but with an intensity and sharpness due to the small stability of the warmer air.

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