Momentum Flux Measurements over Mountains: Problems Associated with the Symmetrical Two-Beam Radar Technique

Abstract

An experiment to measure the vertical transport of horizontal momentum was carried out using the Poker Flat, Alaska, MST radar operating in a symmetrical two-beam mode. We show that the difficulties inherent in this measurement are magnified by problems associated with the effects of orographic flow over the local mountainous terrain. These difficulties show up most clearly when comparing the average vertical velocities measured using antenna beam positions in two orthogonal vertical planes. The measured fluxes are dominated by large values of the long-period (>6 h) fluctuations. Although this observation is consistent with previous measurements at other locations, we show that the accuracy of the symmetric two-beam method under these geographic conditions is questionable.