A Numerical Study of Moisture Forecast in the Planetary Boundary Layer By

A dynamical forecast model is applied to study thlJ moisture variation in the planetary boundary layer under the chosen synoptic condition. The observed l2-hour moisture patterns are compa.red with the forecast patterns. Four numerical experiments, using the same initial data on 0600 CST, March 25, 1967, are made. However, in one case only the effects of evaporation from the Gulf of Mexico, and the latent heat release are considered in the model, whereas in the rest of the cases;. the topographical effects are also included. The results indicate that: (a) The moisture tongue shows a good correspondence in location between the forecast and ob:;erved values, at 200 meter-level above the ground. (b) The ridge line of the forecast moisture patterns display a tendency of tilting westward when compared wi th the initial observed values. The degree of tilting depends, very closely, on the speed of the cold front. (c) The forec:ast moisture in the central U. S. proves to be acceptable, but in the eastE;~rn U. S. -1 along the Appalachian Mountains the error is about 2 grn kg . (d) For the l2-hour forecast in the central U. S. region, the :Latent heat release seems to be more important than the evaporation effect"