LIDAR based Measurements of Turbulent Dissipation above Urban Areas

A large field-experiment focusing on the dispersion phenomena of urban and suburban flows, took place in Oklahoma City during the summer of 2003. A variety of instruments were deployed, including two Doppler-LIDARs. The LIDARSs were situated in a way to provide insight into dynamical flow structures caused by the urban core. This communication presents progress in using the structure function in conjunction with single radial velocity field measurements, from coherent Doppler-LIDAR, to measure turbulent dissipation. Turbulent dissipation is conventionally measured using hotwires or sonic anemometers utilizing the Kolmogorov. -5/3 law for the inertial subrange. These are both point measurements limited to a small stationary control volume. A common way of measuring characteristics of turbulence, high above the ground, is through expensive airplane base measurements. On the contrary, LIDAR gives a three dimensional field of the radial velocity component within a range of several kilometers, is ground deployed, and is relatively cheap. In addition to the large range, LIDAR has a very high sampling frequency. In the presented case, the sampling frequency is 500 Hz and velocity data is subsequently averaged over 50 and 100 pulses giving the actual sampling rate of 10 and 5 Hz, respectively. This sampling frequency is fast enough for resolving the turbulence timescale. Here, we advance the idea of using LIDAR as a remote sensing instrument for the dissipation measurements above urban areas. Using the second-order structure function, a dissipation field is extracted from the radial velocity field. The redial velocity field is filtered to remove all data with the low signal to noise ratio (SNR). The filtering is applied individually to each range-gate over the entire time interval of measurement. Results show a very strong and substantiated relationship between the calculated structure function and the theoretical prediction allowing the calculation of dissipation. The dissipation results corroborate with previous work. Measured dissipation varies around 0.0035 [m^2/s^-3] depending on mean wind conditions and location relative to the urban core. Undergoing efforts are directed towards the better understanding of the dissipation dependence on flow conditions and urban vicinity and geometry. Results presented here are invaluable to the urban dispersion models in which a more reliable turbulent dissipation model is desperately needed.