Accuracy of Various Techniques for Estimating Boundary-Layer Trajectories

Abstract

Fifty-five constant volume balloons (tetroons) were released in the fall of 1971 at Oklahoma City, Okla., and were ballasted to float at about 300 m over the city (elevation 400 m MSL) to trace local air currents. Twenty seven tetroons were recovered and, of these, the thirteen that were found between 400 and 1300 km from the city were included in this study. The trajectory end points of these thirteen tetroons were used to measure the accuracy of boundary-layer trajectory estimates begun at Oklahoma City and computed for the same time periods as the tetroon flights. Boundary-layer trajectory estimates were computed from sea level geostrophic vectors, layer-average winds (surface to 1000 m above terrain) and surface winds adjusted for “normal” shear to 300 m above terrain. Based on this study, it would appear that reasonably accurate boundary-layer trajectories can be estimated by making the following adjustments to generally available data: sea level geostrophic vectors should be backed by 20° in southerly and westerly flow and 50° in northerly flow; layer-average winds require no adjustment in northerly, 10° backing in southerly and 20° backing in westerly flow; and surface wind data need no adjustment in southerly, 10° veering in northerly and 30° veering in westerly flow. Boundary-layer trajectory estimates made from geostrophic vectors are easily constructed graphically on sea level weather charts, and trajectory forecasts can be made by using National Weather Service forecast maps distributed by facsimile. Layer-average wind and adjusted surface wind trajectories are more suited to post analysis by computer since the data are available on magnetic tapes and the wind vector data are processed objectively.