13th Symposium on Meteorological Observations and Instrumentation


Toward the estimation of the refractive index gradient from clear air wind profiler echoes

Catherine Gaffard, Met Office, Reading, United Kingdom; and L. Bianco, M. Matabuena, and V. Klaus

Wind profilers have been primarily designed to provide measurement of the wind; they also contain information about the gradient of the refractive index that might be used in combination with a passive sensor like a radiometer in order to increase the vertical resolution of the retrieved humidity profile. Stankov (Stankov 1996) used the wind profiler estimate of humidity gradient to improve the humidity profile retrieval of a two-channel microwave radiometer. She developed a Multisensor Retrieval of Atmospheric Properties (MRAP) (Stankov 1998), which combines measurements from diverse remote sensing instruments such as passive radiometers, wind profilers and other in situ instruments. Recently, she also showed encouraging comparison between humidity gradient profile retrieved from wind profiler data and computed from radiosonde measurements (Stankov 2003).

The international COST 720 Temperature, hUmidity and Cloud (TUC) profiling experiment was organised during 3 months in winter 2003/2004 at Payerne Switzerland. Various insitu and active/passive ground based remote sensing systems, including three microwave radiometers, a cloud radar, a wind profiler and radiosondes were operated in the same location. This paper describes the investigation done on the wind profiler data in order to know if they can be used to detect and quantify change in the refractive index.

A first analysis of the data showed that the wind profiler is detecting structures in the humidity and temperature associated with sharp temperature inversions and hydrolapses. However, the site suffered from a lot of contamination, and the operational software didn't allow a quantitative estimation of the gradient of the refractive index. By comparing the moments obtained on different antennas, a bias between the vertical beam and the oblique beam was found on both the signal to noise ratio and the spectral width. A statistical analysis based on the distribution of the Doppler shift showed that the direct current removal algorithm and the ground clutter removal algorithm are responsible for these biases. A simple empirical correction using the bias value obtained is therefore not recommended to get a quantitative estimation of a profile of signal to noise ratio and width. This analysis also revealed that the turbulent part of the spectral width was poorly estimated. An impact study of different parameters like the antenna beam width, the pulse length on the accuracy of the turbulent part of the width was performed. The results show the importance of knowing with a good accuracy the illumination volume of the radar. The general out-come of the study showed that a better estimation of the moment would benefit for a quantitative evaluation of the refractive index.

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Session 5, Remote Sensing II
Wednesday, 22 June 2005, 8:30 AM-9:45 AM, South Ballroom

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