The effective beam of ARM scanning radars and the shear contribution to radar-measured spectrum width

 

Authors

Ming Fang — University of Miami
Bruce A. Albrecht — University of Miami

Category

Dynamics/Vertical Motion

Description

Different from the vertically pointing cloud radar that fixes its radar beam in the vertical direction, ARM W-, X-, and Ka-band scanning cloud radars scan their beams in vertical planes with predetermined azimuth angles and provide the capability to obtain cloud volume data within a few minutes. The radar-measured spectrum width is often used to retrieve turbulence in clouds. However, other than turbulence, radar antenna motion and shear can also contribute to the measured spectrum width, and they need to be removed before the retrial of turbulence. In fact, an analytical expression that can be used to calculate spectrum width due to antenna motion alone, like that for azimuthally scanning radar, does not exist. Furthermore, using stationary beam width can cause the shear contribution to be seriously underestimated due to two reasons. One is that the vertical shear of horizontal wind is much larger than its shear in the horizontal plane; another is that radar antenna motion combined with signal processing creates an effective beam width that is significantly broader than its stationary counterpart. But the analytical expression of an effective beam pattern for a beam scanning in a vertical plane does not exist either. Starting from the correlation function of radar signals, this study derives the analytical expressions of the effective beam pattern that can be approximated by a Gaussian function under certain circumferences. The general relation between normalized effective beam width and antenna rotation rate and dwell time is plotted. This curve is even applicable to a beam scanning in the azimuth direction with a zero degree elevation angle. This study also derives analytical spectrum width equations due to beam scanning in a vertical plane and the vertical shear of horizontal wind. All expressions are given in the spherical coordinate system that is a better choice for processing radar observations. It has been shown that for the ARM Ka-band scanning cloud radar, the effective beam width is about 1.4 times broader than the stationary beam.

The results obtained in this study will help to classify some confusion or mistakes in the literature and will be applied to the data collected by ARM scanning cloud radars.