Errors in Mean Vertical Velocities Measured by Boundary Layer Wind Profilers

Wayne M. Angevine CIRES, University of Colorado and NOAA Aeronomy Laboratory, Boulder, Colorado

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Abstract

The accuracy of vertical velocities measured by UHF wind-profiling radars has been a matter of discussion for some time. This paper shows that there are significant errors in mean vertical velocities measured by the vertical beam of 915-MHz wind profilers. The erroneous velocities are 0.1–0.3 m s−1 downward in daytime convective boundary layers over two sites, flat farmland in Illinois and rolling forest in Wisconsin. Velocities at night are not affected, and different days have different erroneous velocities. The directly measured velocities are compared to vertical velocities calculated from the divergence of the horizontal wind to show that they are indeed in error. The erroneous velocities are not caused by detectable rain, by an error in the beam pointing direction, or by the skewness of the vertical velocity distribution. They are probably due to small targets (particulate scatterers) that have a small fall velocity and are detected by the radar. An online algorithm for removing intermittent contamination reduces the error, but does not eliminate it. The fluctuating component of the velocity is not affected by these errors since it is much larger in magnitude.

Corresponding author address: Dr. Wayne M. Angevine, NOAA Aeronomy Laboratory, R/E/AL3, 325 Broadway, Boulder, CO 80303.

Email: wayne@al.noaa.gov

Abstract

The accuracy of vertical velocities measured by UHF wind-profiling radars has been a matter of discussion for some time. This paper shows that there are significant errors in mean vertical velocities measured by the vertical beam of 915-MHz wind profilers. The erroneous velocities are 0.1–0.3 m s−1 downward in daytime convective boundary layers over two sites, flat farmland in Illinois and rolling forest in Wisconsin. Velocities at night are not affected, and different days have different erroneous velocities. The directly measured velocities are compared to vertical velocities calculated from the divergence of the horizontal wind to show that they are indeed in error. The erroneous velocities are not caused by detectable rain, by an error in the beam pointing direction, or by the skewness of the vertical velocity distribution. They are probably due to small targets (particulate scatterers) that have a small fall velocity and are detected by the radar. An online algorithm for removing intermittent contamination reduces the error, but does not eliminate it. The fluctuating component of the velocity is not affected by these errors since it is much larger in magnitude.

Corresponding author address: Dr. Wayne M. Angevine, NOAA Aeronomy Laboratory, R/E/AL3, 325 Broadway, Boulder, CO 80303.

Email: wayne@al.noaa.gov

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