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The Coupling of Vertical Velocity and Signal Power Observed with the SOUSY VHF Radar

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  • 1 Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany
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Abstract

The perturbations to the static stability (and hence to the radar reflectivity) and to the velocity in a vertically propagating gravity wave are correlated, and the sign of the correlation depends on whether the wave is propagating upward or downward. The wave-induced correlation between radar reflectivity and vertical velocity is the basis of a hypothesis to explain the downward bias in long-term averages of the vertical velocity seen at extratropical sites by wind profiler radars, and for predictions of biases in the horizontal wind speeds and in the vertical momentum flux seen by profiler radars. In this study, the hypothesis that mean vertical velocity is related to the correlation between perturbations to vertical velocity and signal power is tested. Observations with very high time and vertical resolution from the SOUSY VHF radar are used. It is found that the mean vertical velocity in the midtroposphere (2.4–6.3 km) is downward (upward) when the perturbations to vertical velocity and to backscattered power over this height range are negatively (positively) correlated. Similar results are found during summer and winter periods. Results are also similar when the radar was upwind of the nearby Harz Mountains compared to when the radar was downwind of the mountains.

* Current affiliation: Department of Earth Sciences, St. Cloud State University, St. Cloud, Minnesota.

Corresponding author address: Dr. Gregory D. Nastrom, Department of Earth Sciences, St. Cloud State University, 720 Fourth Avenue South, St. Cloud, MN 56301-4498.

nastrom@stcloudstate.edu

Abstract

The perturbations to the static stability (and hence to the radar reflectivity) and to the velocity in a vertically propagating gravity wave are correlated, and the sign of the correlation depends on whether the wave is propagating upward or downward. The wave-induced correlation between radar reflectivity and vertical velocity is the basis of a hypothesis to explain the downward bias in long-term averages of the vertical velocity seen at extratropical sites by wind profiler radars, and for predictions of biases in the horizontal wind speeds and in the vertical momentum flux seen by profiler radars. In this study, the hypothesis that mean vertical velocity is related to the correlation between perturbations to vertical velocity and signal power is tested. Observations with very high time and vertical resolution from the SOUSY VHF radar are used. It is found that the mean vertical velocity in the midtroposphere (2.4–6.3 km) is downward (upward) when the perturbations to vertical velocity and to backscattered power over this height range are negatively (positively) correlated. Similar results are found during summer and winter periods. Results are also similar when the radar was upwind of the nearby Harz Mountains compared to when the radar was downwind of the mountains.

* Current affiliation: Department of Earth Sciences, St. Cloud State University, St. Cloud, Minnesota.

Corresponding author address: Dr. Gregory D. Nastrom, Department of Earth Sciences, St. Cloud State University, 720 Fourth Avenue South, St. Cloud, MN 56301-4498.

nastrom@stcloudstate.edu

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