Effects of Miso- and Mesoscale Obstructions on PAM Winds Obtained during Project NIMROD

Abstract

27 PAM (Portable Automated Mesonet) stations were operated between 19 May and 1 July 1978 during the operational phase of Project NIMROD (Northern Illinois Meteorological Research On Downburst), collecting ?1 000 000 records of wind data. Analysis revealed that the PAM-measured winds are influenced by the mesoscale obstruction of the Chicago metropolitan area as a whole, as well as the misoscale obstructions of individual trees and buildings identifiable in panoramic pictures taken at each PAM site where data were being collected. Mesoscale obstruction increased from near zero in the open field to 50% around the Chicago city limit, while the misoscale obstruction factor turned out to be as large as 58% in the wake of obstructing trees. The wind speed deficit extended 50-80 times the height of obstructing trees and buildings. The analysis of obstacle effects upon PAM-measured winds is empirical and not necessarily based upon sound hydrodynamical principles. Misoscale and mesoscale corrections, thus derived, were applied to various situations. An example of a gust-front analysis presented in this paper shows significant differences between corrected and uncorrected wind fields, leading to the conclusion that PAM-measured winds need to be corrected for depicting the airflow free from obstacles.