18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
Cold front or baroclinic trough?
Frederick Sanders, Sanders Research Enterprises, Marblehead, MA
Recent research shows that about half the cold fronts appearing in National Weather Services surface analyses are not associated with a significant contrast of surface temperature. In extreme instances the passage of a cold front may be accompanied by an abrupt temperature RISE. A significant density contrast is required for a frontal circulation, in which the warmer, less dense, air rises over the colder.

When the contrast is not present, the wind shift and change of weather may still be sufficiently marked that some designation on the analysis is desired. The wind shifts from southwesterly to northwesterly and there is a minimum of surface pressure at the time of the shift. There is warm advection ahead of the shift and cold advection to the rear, so that cooling over a period of 6 to 24 hours may occure. There is cloudiness and perhaps precipitation, often in bands, ahead of the shift and rapid clearing afterward. This pattern differs markedly from the abrupt rainfall that starts with the shift and continues afterward, as required by the classical frontal concept.

On occasion the wind shift may be very abrupt and precipitation intense, although there is little or no temperature change. In these cases, the feature may be driven by release of latent heat. In any case,it is urged that the shift be denoted a baroclinic trough unless there is a significant surface baroclinic zone associated with it. The "trof" notation in present practice, not now adequately defined, is appropriate.

Examples are shown.

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