Extended Abstract (24K)14.2
Observations of the diurnal variation of energy dissipation in the urban boundary layer using Doppler lidar
Karen E. Bozier, University of Salford, Salford, United Kingdom; and C. G. Collier
Observations of airflow over the urban conurbation of Greater Manchester, northwest England, UK have been undertaken using a ground based, pulsed infrared Doppler lidar. Values for the kinetic energy dissipation rate have been estimated from the fluctuations in the components of the wind. Estimates for the surface heat flux are possible from the turbulent kinetic energy dissipation and the vertical gradient of the third moment of the vertical velocity. Comparisons of the measurements obtained with the lidar system are made with previously published work.
Turbulent exchange in the lowest part of the boundary layer controls the exchange of heat, mass and momentum at the surface, thereby influencing the structure of the whole boundary layer. Within a city the nature and distribution of urban structures govern airflow, and the dominant environmental controls of urban turbulence are the high roughness of the surface and the urban heat island. This aerodynamically rough and inhomogeneous surface interacts with the airflow, modifying the turbulent transfer and structure. The turbulent kinetic energy dissipation rate is one of the key parameters in atmospheric turbulence theory. It represents the rate of transfer of energy to smaller eddies in the inertial subrange of inhomogeneities, and the rate of conversion of kinetic energy of turbulence into heat in the viscous subrange, characterising the flow of energy in the atmosphere.
The Remote Sensing and Hydrometeorological research group at the University of Salford operate a mobile, ground based scanning Doppler lidar system. An important consideration in the design of the system was the need to operate the lidar within an urban environment to provide wind and intensity measurements in the atmospheric boundary layer. The infrared, pulsed lidar operates with an eye safe wavelength of 10.6 µm and detects backscatter of infrared laser light from naturally occurring aerosols that are advected with the wind field. The system provides radial (line of sight) velocities and backscattered intensity measurements with a range resolution of 112 m. The system has a theoretical maximum range of 4.6 km and a wind velocity accuracy of 0.5 ms-1.
The measurement site is within the University campus, with the surrounding area consisting of low / medium height terraced housing to the north, 3 - 4 storey buildings and high rise tower blocks to the east, south and west. Data was collected during the period 10.00 - 16.00 hours on 27th August 1998, and 12.40 - 15.30 hours on 3rd April 2001. On both days the atmospheric stability was near neutral, although on 3rd April the boundary layer became increasing unstable during the latter part of the observation period. Estimates of turbulent kinetic energy dissipation rates obtained from observations made with the Salford lidar system are presented. The variation of these measurements in the context of previously published data on their diurnal variation is considered. Values of the potential temperature-vertical velocity covariance have been obtained from the balance of the vertical velocity fluctuations for horizontally homogenous turbulence, leading to estimates of surface heat flux.
Session 14, Urban boundary layer: observations
Friday, 24 May 2002, 8:00 AM-10:45 AM
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