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Frederick Sanders

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FREDERICK SANDERS

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FREDERICK SANDERS

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Frederick Sanders

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Frederick Sanders

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Frederick Sanders

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Frederick Sanders

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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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Frederick Sanders

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The skill of the Nested-Grid Model (NGM) and the global spectral model (GLBL) at the National Meterological Center in the prediction of explosive cyclogenesis was evaluated for the period 1 September 1986–30 April 1987. Manual analyses covering the eastern North Pacific, North America and the North Atlantic eastward to 20°W were used as ground truth. The criterion for a bomb event in the analyses of the forcasts was a deepening of the center of at least 24 mb at 60°N, normalized geostrophically at other latitudes, in a period of 24 h, beginning at 0000 or 1200 UTC.

Both models displayed skill out to 48 h for the NGM and 60 h for the GLBL. The NGM performed notably better in the innermost fine-grid area than in the surrounding area of overlap with a more coarse grid. For the GLBL in the Atlantic and North America, similar skill was seen through 36 h; skill was very small in the Pacific region. 12-h deepening beginning 12 h after initialization was compared with analyzed deepening for both models. Correlations ranged from 0.72 for the NGM in the inner grid over the Atlantic and North America to 0.03 in the Pacific. The GLBL values were intermediate, again better in the Atlantic than in the Pacific. All samples showed an average shortfall of predicted deepening from 12–24 h after initialization, ranging from 1 mb for the inner NGM grid to 7 mb for the overlap area, with the GLBL intermediate; again, it was much better in the Atlantic than in the Pacific.

Growth of skill over the past few years is attributable to improved analyses, better model resolution and better treatment of bounndary-layer fluxes. Initial data limitations are now the most important factor, both in models and in verifying analyses. These results alter the nature of the problem of research on explosive cyclogenesis from one of discovering a missing ingredient to one of improving the performance and extending the range of predictability.

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Frederick Sanders

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Frederick Sanders

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A study was made of the performance of the Limited-Area Fine-Mesh (LFM) operational forecasts for cases of explosive cyclogenesis in the west-central North Atlantic Ocean during 1981–84. For 51 instances in which the observed 12-h deepening was at least 10 mb, the LFM forecasts for 12–24 h range indicated 58% of this deepening on the average, but accounted for only 30% of the variance in individual cases. In a comparison of central pressures from manual analyses with those in LFM initializations and predictions, the latter were insufficiently deep once rapid intensification began, the discrepancy increasing from about 4 mb in the initializations to about 10 mb in the forecasts from 36 to 48 h. In a number of instances the LFM did not detect the initial appearance of the cyclone. Mean position errors increased from about 75 n mi (140 km) initially to about 185 n mi (340 km) at 48-h range. Mean vector errors were shortly southeast of the analyzed center initially and about 65 n mi (120 km) east-northeast finally, indicating a forecast track slightly too fast and slightly too far to the right. Two individual case studies showed that even when there are large quantitative discrepancies between events in the LFM and real atmospheres, the model is qualitatively correct. These results indicate the essentially baroclinic nature of the cyclogenesis, but the intensity of response to the baroclinic forcing remains intractable.

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