VHF Doppler Radar Determination of the Momentum Flux in the Upper Troposphere and Lower Stratosphere: Comparison between the Three- and Four-Beam Methods

Abstract

The vertical flux of horizontal momentum in the upper troposphere and lower stratosphere can be measured by VHF Doppler radars using the mesosphere-stratosphere-troposphere (MST) radar technique. Two methods have been used: one using three beams, one vertical and two oblique, and the other using four beams, two pairs of oblique beams symmetrically offset from the vertical. According to theory the four-beam method should be more accurate, but bemuse some radars do not have the capability of using the four-beam method, it is important to assess the accuracy of the three-beam method. In this study the rapid steerability of the Japanese MU radar was used to make three- and four-beam measurements simultaneously. It is found that the three-beam flux agrees with the four-beam flux only for long-period fluctuations. For shorter periods a systematic error is caused by wind fluctuations with wavelengths that are comparable with the separation between the beams (2?7 km in this study). In this study, performed during summer at 35°N latitude, it is found that the momentum flux due to long-period fluctuations is caused primarily by synoptic-scale or mesoscale disturbances, while short-period flux may primarily be related with intense vertical air motion. Thus, during these observations, the contribution of gravity waves seemed to be unimportant.