Accuracy of a coherent doppler lidar for the urban boundary layer measurements

A large field experiment, dubbed Joint Urban 2003 (JU2003), took place in Oklahoma City in July 2003. The key component of this experiment was to study the movement of inert tracer gases through the urban environment. A large variety of atmospheric measurement systems have been deployed during the experiment - providing unprecedented opportunities to investigate questions related to the urban flows and dispersion. Two Doppler lidars were deployed with the main purpose to give a deeper insight to the coupling between the free-stream wind and urban centers. The Coherent Doppler lidar technology provides an effective way for remote measurements of the mean winds and turbulence, however, lidars are prone to the noise related measurement errors. Generally, the accuracy of the lidar measurements decreases with the increasing distance due to the weakening of the signal-to-noise ratio (SNR). The first step in the analysis was the calculation of power density spectra of the radial velocity for each range gate. During the experiment the sampling rates of 10 and 5 Hz corresponding to the Nyquist frequency of 5 and 2.5 Hz, respectively, were used. In order to alienate the effect of estimation errors due to the high frequency noise, the constant part of spectrum was examined. The estimate variance as well as standard deviation of estimation errors were determined from the integral of this constant spectrum. To give more accurate description of estimation errors for the cases far away from the lidar where the noise becomes significantly large, the data was filtered using sample averaging technique. This study emphasizes the situations when the lidar measurements should be taken with caution. The recommendations for noise reduction and filtering techniques are given.