The most desirable aspect of the FLENUMOCEANCEN frontal analysis model is its objectivity. The fact that mean potential temperature is the only parameter used to determine frontal positions makes it very objective. Potential temperatures are obtained by calculating the 1,000- to 700-millibar thickness field and converting the thickness to the mean potential temperature of the 1,000-to 700-millibar layer. The program then computes the gradient of the mean potential temperature gradient (GG). A GG analysis accurately marks the division between two air masses having different thermal structures.

The least desirable aspect of the frontal analysis model is its overall accuracy. Large grid size, and the possible inaccuracy of upper-air temperature analyses over data-sparse areas precludes frontal positions from being more accurate than 100 miles. The program also has a few other weaknesses, as follows:

It handles occlusions poorly, because of the lack of thermal contrast across occluded fronts.

It handles fast-moving cold fronts poorly, because the major temperature contrast occurs well behind the front.

It produces false frontal indications in regions where fast-forming strong inversions develop.

For all the above reasons, CG frontal analyses should NOT be used as the final determination in positioning fronts. We recommend that you use the CG analysis as a first guess or simply as a guide in your analysis procedure. You should analyze as many frontal placement parameters as possible before settling on your final frontal positions.

Learning Objective: Recognize how FLENUMOCEANCEN models derive the heights of the tropopause and the freezing level.

TROPOPAUSE HEIGHT-ANALYSIS MODEL

The tropopause is defined by characteristic changes in the temperature lapse rate. In computing the height of the tropopause, the FLENUMOCEANCEN model combines the lapse rate between the 500- and 400-millibar levels extrapolated upward and the lapse rate between 150- and 100-millibar level extrapolated downward. The height of the tropopause is found at the point where the two lapse rates intersect. This level averages out to be 700 feet below the observed tropopause. A 5-year evaluation period of the above method also showed that, on the average, the level of maximum winds (in the jet core) is found 2,300 feet below derived tropopause heights. The model incorporates a 3,000-foot constant to account for the 700- and 2,300-foot deviations. This means a tropopause height chart actually represents the level of maximum winds. The true tropopause height is 3,000 feet higher than indicated on the chart.

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