Biases Due to Gravity Waves in Wind Profiler Measurements of Winds

G. D. Nastrom St. Cloud State University, St. Cloud, Minnesota

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T. E. VanZandt Aeronomy Laboratory, NOAA, Boulder, Colorado

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Abstract

Radar reflectivity for Bragg scattering is proportional to the atmospheric static stability. In vertically propagating gravity waves the perturbations to the static stability and to the winds occur either in phase or out of phase, so that the radar reflectivity and the wind perturbations are correlated. This correlation leads to a bias in winds observed by radars and any other remote sensors that rely on Bragg scattering. The magnitude of the biases of the vertical and horizontal wind components due to a monochromatic wave propagating in the radar beam are found to be proportional to the amplitude squared of the gravity wave, and for a spectrum of waves they are proportional to the spectral energy. For radar systems with two coplanar beams, the bias to the observations of horizontal wind speed is about 0,2 m s−1 for gravity wave amplitudes typically encountered over flat terrain at midlatitudes and increases to 1 m s−1 or more for wave amplitudes seen over mountainous terrain and in the vicinity of fronts, etc. For radars with only one oblique beam, the magnitude of the bias in the horizontal wind speed due to waves with typical amplitudes ranges from near zero to several meters per second, depending on wave amplitudes and on the zenith angle of the beam. The bias to the mean vertical velocity is a few centimeters per second for similar wave conditions. Variances of velocities along oblique beams also have a bias due to vertically propagating gravity waves; ranging from near zero to about 0.5 m2 s−2 depending on the zenith angle of the beam and on the ratios of the radar vertical range-gate size and temporal averaging period relative to the wave vertical wavelength and the wave period. The observed vertical momentum flux is about 20% smaller than the true momentum flux due to this bias effect.

The theoretical predictions of biases due to gravity waves are compared with observations from the Flatland 50-MHz radar, located in the very flat terrain of central Illinois. It is found that the magnitude and the sign of the observed differences between eastward- and westward-directed beams are about the same size as expected for gravity waves with amplitudes typically observed at Flatland. The mean momentum flux for all cases combined is also consistent with the predictions of this theory for wave energy propagation upward toward the east, whereas the momentum flux for those cases with large variances in the midtroposphere at Flatland is about −0. 1 5 m2 s−2 and is consistent with wave energy propagation downward toward the cast (or upward toward the west).

Abstract

Radar reflectivity for Bragg scattering is proportional to the atmospheric static stability. In vertically propagating gravity waves the perturbations to the static stability and to the winds occur either in phase or out of phase, so that the radar reflectivity and the wind perturbations are correlated. This correlation leads to a bias in winds observed by radars and any other remote sensors that rely on Bragg scattering. The magnitude of the biases of the vertical and horizontal wind components due to a monochromatic wave propagating in the radar beam are found to be proportional to the amplitude squared of the gravity wave, and for a spectrum of waves they are proportional to the spectral energy. For radar systems with two coplanar beams, the bias to the observations of horizontal wind speed is about 0,2 m s−1 for gravity wave amplitudes typically encountered over flat terrain at midlatitudes and increases to 1 m s−1 or more for wave amplitudes seen over mountainous terrain and in the vicinity of fronts, etc. For radars with only one oblique beam, the magnitude of the bias in the horizontal wind speed due to waves with typical amplitudes ranges from near zero to several meters per second, depending on wave amplitudes and on the zenith angle of the beam. The bias to the mean vertical velocity is a few centimeters per second for similar wave conditions. Variances of velocities along oblique beams also have a bias due to vertically propagating gravity waves; ranging from near zero to about 0.5 m2 s−2 depending on the zenith angle of the beam and on the ratios of the radar vertical range-gate size and temporal averaging period relative to the wave vertical wavelength and the wave period. The observed vertical momentum flux is about 20% smaller than the true momentum flux due to this bias effect.

The theoretical predictions of biases due to gravity waves are compared with observations from the Flatland 50-MHz radar, located in the very flat terrain of central Illinois. It is found that the magnitude and the sign of the observed differences between eastward- and westward-directed beams are about the same size as expected for gravity waves with amplitudes typically observed at Flatland. The mean momentum flux for all cases combined is also consistent with the predictions of this theory for wave energy propagation upward toward the east, whereas the momentum flux for those cases with large variances in the midtroposphere at Flatland is about −0. 1 5 m2 s−2 and is consistent with wave energy propagation downward toward the cast (or upward toward the west).

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