Forecasting 24-hour vorticity change at the 500-MB level.
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Authors
Allison, John K.
Subjects
Advisors
Date of Issue
1957
Date
Publisher
Monterey, CA; Naval Postgraduate School
Language
en_US
Abstract
Fjortoft's graphical method of prognosis of the 500-mb surface assumed relative vorticity change as being due to advection only. This study considers the total 24-hour relative vorticity change as consisting of two components, that due to 100$ advection and that due to non-advective changes. Objective methods are developed for the prognosis of the latter. These changes are then added to the advective changes to give the total 24-hour relative vorticity change from which the 24-hour height change can be recovered. The 500-mb velocity field is subjectively categorized into five types. For each type the non-advective relative vorticity change is correlated with a parameter determined from the spreading of the contours. Quantitative objective forecast rules are developed from the regression line's relating the non-advective relative vorticity change and the parameters for each of the five cases. Using the developed technique the results of five 24-hour 500-mb test forecasts for the United States are presented. These tests show an Improvement of four feet in forecast height value for the entire forecast area as compared to the Fjortoft method. However, in considering only those points at which the computation was made an average improvement of 34 feet is noted. Finally the persistence of non-advective relative vorticity changes is considered for its value as a height-change forecast tool. The results of seven forecasts prepared using this assumption indicate an improvement of 32 feet over the Fjortoft method for the entire forecast area.
Type
Thesis
Description
Series/Report No
Department
Aerology