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Richard J. Reed
and
Adrian J. Simmons

Abstract

The effect of surface sensible and latent heat fluxes on explosive cyclogenesis has been found to be quite variable. Sensitivity tests reveal little or no influence in some cases and dramatic effects in others. However, in nearly all cases where little impact was found, the predicted deepening rate was significantly less than the observed rate, opening the possibility that the unexplained deepening might be a consequence of a shortcoming of the flux formulation.

We present here the results of a test of the impact of the fluxes in a case of extreme deepening that was accurately predicted by the ECMWF T106 operational model. In the test the fluxes were switched off during the 24-h period of most rapid deepening. The result was to change the predicted deepening rate of 48 mb by only 1 mb. This confirms that in certain circumstances the concurrent fluxes can have negligible effect on the deepening despite their great importance in other situations.

It is shown that downward heat and moisture fluxes (cooling and drying) occurred in the warm sector close to the low center, whereas upward heat and moisture fluxes (warming and moistening) occurred in the cold air well to the rear of the low. It is known from theory that this pattern is not conducive to cyclonic development. The cause of the vast difference in the effect of the fluxes on cyclone development is discussed in terms of the stage of development, the geographical location, and the degree of atmospheric preconditioning.

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Richard J. Reed
and
Mark D. Albright

Abstract

Low-level (300 m) aircraft observations, taken within an intense extratropical storm during Intensive Observation Period 2 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic, are used to document the near-surface frontal structure in the interior of the storm at 1800 UTC 14 December 1988, when the storm was at its maximum depth (959 mb). The flight data revealed that a well-defined occluded front spiraled into the low, making one-and-one-half turns about the center. The front followed the inner boundary of a spiral cloud and moisture band seen in satellite visible, infrared, and water vapor imagery. The results provide support for the idea that sharp occluded, or occluded-like, fronts can wrap around the core of deep ocean storms and that satellite imagery can be helpful in locating such fronts.

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Steven Businger
and
Richard J. Reed

Abstract

The small-scale and rapid development of polar lows over relatively data-sparse areas results in a special forecast challenge for the operational forecasting community. This paper constitutes a review of recent advances in our understanding of cyclogenesis in polar air masses. The review is primarily comprised of a survey of the observed features of polar lows as documented in a number of case studies presented in the recent literature. The review is organized on the basis of a combination of observational and physical considerations and is aimed at diagnosing common types of developments. Theoretical ideas concerning the origins of polar lows and results of numerical modeling experiments aimed at simulating their development are also summarized. Finally, a discussion of approaches to the operational problem of forecasting polar lows is given.

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Richard J. Reed
,
Mark D. Albright
,
Adrian J. Sammons
, and
Per Undén

Abstract

Operational forecasts from the European Centre for Medium Range Weather Forecasts of three cases of explosive cyclogenesis of large magnitude that occurred in the North Atlantic during a 1-week period in January 1986 are presented, and results of numerical experiments performed on the three cases are described. Two of the cases were well predicted, and the third was not. The experiments were aimed at 1) determining the contribution of latent heat release to the explosive deepenings in the two cases that were well predicted and 2) diagnosing the cause of the poorer forecast performance in the third case.

It was found that condensation heating accounted for 40%–50% of the deepening in the well-predicted cases and that most of the heating derived from stable, frontal type precipitation rather than from convective precipitation. The results of the attempt to determine the cause of the relative failure of the third forecast were inconclusive but pointed toward problems in the initial analysis. In particular, there was evidence that the initial analysis failed to capture fully the high moisture content and low static stability of the warm sector air that was ingested into the heart of the storm during the rapidly deepening stage.

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