HUMIDITY FIELD IN THE VICINITY OF FRONTAL SYSTEMS

Studies of the humidity field throughout frontal zones indicate there is a tongue of dry air extending downward in the vicinity of the front, and sloping in the same direction as the front. One study found that such a dry tongue was more or less well developed for all frontal zones investigated. This dry tongue was best developed near warm fronts; it extended, on the average, down to 700-hPa in cold fronts and to 800-hPa in warm fronts. In about half the fronts, the driest air was found within the frontal zone itself on occasion it was found on both the cold and warm sides of the zone. About half the flights through this area showed a sharp transition from moist to dry air, and the change in frost point on these flights averaged about 20C in 35 miles. Some flights gave changes of more than 20C in 20 miles.

As a frontal cloud deck is approached, the dewpoint or frost point depression starts diminishing rapidly. At distances beyond 10 to 15 nm, this variation is much less. You should keep this fact in mind when attempting to locate the edge of a cloud deck from rawinsonde data. Linear extrapolation or interpolation of dewpoint depressions cannot be expected to yield good results. For instance, when one station shows a dewpoint depression of 10C and the neighboring station shows saturation, the frontal cloud may be anywhere between them, except within about 10 nm of the driest station.

Since the frontal cloud masses at midtropospheric levels is usually surrounded by relatively dry air, it is possible to locate the edge of the cloud mass from humidity data on constant pressure charts. This is so because the typical change in dewpoint depression in going from the cloud edge into cloudfree air is considerably greater than the average error in the reported dewpoint depression.

500-hPa ANALYSIS OF DEWPOINT DEPRESSION

Figure 4-20 shows an analysis of the 500-hPa dewpoint depression field superimposed upon an analysis of areas of continuous precipitation, and of areas of overcast middle clouds. The 500-hPa dewpoint depression isopleths were drawn independently of the surface data. The analysis shows the following:

1. The regions of high humidity at the 500-hPa level coincide well with the areas of middle clouds and the areas of precipitation.

2. The regions of high humidity at the 500-hPa level are separated from the extensive dry regions by strong humidity gradients. These gradients are, in all probability, much stronger than those shown on this analysis.

3. A dewpoint depression of 4C or less is characteristic of the larger areas of continuous

Figure 4-20.-Surface fronts, areas of continuous precipitation, areas covered by 8/8 middle clouds, and isolines of 500-hPa dewpoint depression at 0300 UTC, 7 September 1995.

precipitation, and also of the larger areas of overcast middle clouds. Since the 500-hPa level dewpoint depression analysis agrees well with the surface analysis of middle cloud and precipitation, the possibility exists of replacing or supplementing one of these analyses with the other. The characteristics of the 500-hPa level dewpoint depression analysis, outlined above, make it a valuable adjunct to the surface analysis. These analyses can be compared and, by cross-checking, each can be completed with greater accuracy than if they were done independently.

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