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Frederick Sanders

Abstract

Quasi-geostrophic diagnosis of the fields of vertical motion and streamfunction tendency, based on wind analysis, was undertaken for the monsoon depression of 5–8 July 1979, a uniquely well-observed case during Summer MONEX. Wind analysis, in terms of streamfunction and velocity potential, was performed for data averaged in three layers of equal pressure depth front 1000 to 100 mb. The thermal stratification near the depression above 850 mb was approximately moist-adiabatic. The wind analyses showed that the basic zonal currents, westerly in the lower troposphere and easterly in the middle levels, weakened as the depression formed. Central vorticity increased rapidly on 5–6 July to a magnitude three times the Coriolis parameter. The center tilted toward the southwest with elevation.

The quasi-geostrophic omega-equation was solved at the interfaces between layers, for a range of stabilities from the full dry-adiabatic value to 1% of it. Ascent west of the center for the near-neutral stability was sufficient to account for the storm-scale average observed precipitation. Divergences for each layer were combined with vorticity advections to calculate the quasi-geostrophic streamfunction tendencies. The observed slow westward motion of the system was reasonably well accounted for, but the temporally increasing meridional slope of the system was somewhat exaggerated. The strong increase in central vorticity during development could not be accounted for by divergence arising from advections of temperature and vorticity.

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Frederick Sanders

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Frederick Sanders

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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Frederick Sanders

Abstract

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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Frederick Sanders

Estimates of skill in prediction of daily temperature and precipitation are obtained from a six-year record of real-time forecasts for Boston made in the Department of Meteorology at Massachusetts Institute of Technology. No secular increase of skill is found during the period 1966–1972, despite continuous improvement of predicted synoptic-scale flow patterns at the surface and at 500 mb, which were used for guidance. Skill, defined as incremental accuracy of the forecasts over forecasts of the climatological mean, is near 50% for the first day. The decrease of skill with increasing range is more rapid for precipitation than for temperature, the 10% level being reached in two and one-half days and four days, respectively. A pronounced seasonal variation of skill, with a summer minimum, is attributed to a similar variation in the importance of synoptic-scale, as opposed to mesoscale, sources of weather variability. The latter sources are seen as primarily responsible for the present limit of first-day skill. Bias in the probability forecasts of precipitation is relatively small. For the first day, unbiased forecasts representing near certainty are often made, while for the fourth day probability statements representing departures of more than 10 or 15% from the climatological probability are not reliable. Comparison of our results with trends in skill of temperature and precipitation forecasts made at the National Meteorological Center confirms that an increase in the skill of synoptic circulation prognoses is no guarantee of increased skill in predicting the weather.

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Frederick Sanders

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Frederick Sanders

Abstract

For assessment of the sufficiency of the approach of an upper-level vorticity maximum as a predictor of explosive surface cyclogenesis in the western Atlantic, a study was made of all 500-mb maxima which crossed the east coast of North America between 5 October 1985 and 4 April 1986. Of 96 such events, 38 produced bombs. This overall likelihood of 40% was greatest (50%) during the period from 19 December through 1 February, when crossings were most frequent.

In the 26 cases when the strength of the vorticity maximum was at least 22 × 10−5 s−1, bomb likelihood was also 50%. When the crossing occurred at or equatorward of latitude 41°N, bombs occurred in 17 of 33 instances (52%). The likelihood rose to 47% when the speed of the upper maximum exceeded 30 kts (15.5 m s−1). The best discrimination was found when this speed was multiplied by a measure of the strength of the vorticity gradients upstream and downstream from the center. On this basis, 16% of the 25 smallest values of this product produced bombs whereas 68% of the 25 largest values did so. Thus it appears that the intensity of the baroclinic forcing influences the probability of an explosive response. The effect of tropospheric stability, static or symmetric, was not examined.

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Frederick Sanders

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No abstract available.

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