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Range Errors in Wind Profiling Caused by Strong Reflectivity Gradients

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  • 1 Cooperative Institute for Research in Environmental Science, University of Colorado, and NOAA/Aeronomy Laboratory, Boulder, Colorado
  • | 2 NOAA/Aeronomy Laboratory, Boulder, Colorado
  • | 3 NOAA/Aeronomy Laboratory, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado
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Abstract

Comparisons of data taken by collocated Doppler wind profilers using 100-, 500-, and 1000-m pulse lengths show that the velocity profiles obtained with the longer pulses are displaced in height from contemporaneous profiles measured with the shorter pulses. These differences are larger than can be expected from random measurement errors. In addition, there is evidence that the 500-m pulse may underestimate the wind speed when compared with the 100-m pulse.

The standard radar equation does not adequately account for the conditions under which observations are made. In particular, it assumes that atmospheric reflectivity is constant throughout the pulse volume and that observations can be assigned to the peak of the range-weighting function. However, observations from several tropical profilers show that reflectivity gradients with magnitudes greater than 10 dB km−1 are common. Here, a more general radar equation is used to simulate the radar response to the atmosphere. The simulation shows that atmospheric reflectivity gradients cause errors in the range placement. Observed reflectivity gradients can be used to calculate a correction to the range location of the observations that helps to reduce these errors.

Examples of these errors and the application of the correction to selected cases are shown. The evidence presented shows that reflectivity gradients are the main cause of the pervasive differences observed between the different radar observations.

Corresponding author address: Paul E. Johnston, NOAA/Aeronomy Laboratory, R/AL3, 325 Broadway, Boulder, CO 80305-3328. Email: Paul.E.Johnston@noaa.gov

Abstract

Comparisons of data taken by collocated Doppler wind profilers using 100-, 500-, and 1000-m pulse lengths show that the velocity profiles obtained with the longer pulses are displaced in height from contemporaneous profiles measured with the shorter pulses. These differences are larger than can be expected from random measurement errors. In addition, there is evidence that the 500-m pulse may underestimate the wind speed when compared with the 100-m pulse.

The standard radar equation does not adequately account for the conditions under which observations are made. In particular, it assumes that atmospheric reflectivity is constant throughout the pulse volume and that observations can be assigned to the peak of the range-weighting function. However, observations from several tropical profilers show that reflectivity gradients with magnitudes greater than 10 dB km−1 are common. Here, a more general radar equation is used to simulate the radar response to the atmosphere. The simulation shows that atmospheric reflectivity gradients cause errors in the range placement. Observed reflectivity gradients can be used to calculate a correction to the range location of the observations that helps to reduce these errors.

Examples of these errors and the application of the correction to selected cases are shown. The evidence presented shows that reflectivity gradients are the main cause of the pervasive differences observed between the different radar observations.

Corresponding author address: Paul E. Johnston, NOAA/Aeronomy Laboratory, R/AL3, 325 Broadway, Boulder, CO 80305-3328. Email: Paul.E.Johnston@noaa.gov

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