4D RADAR/ACOUSTIC MEASUREMENT OF ATMOSPHERIC TURBULENCE
IN THE STABLE BOUNDARY LAYER

Stephen Frasier and Andrew Pazmany
Electrical & Computer Engineering
University of Massachusetts

John Wyngaard
Department of Meteorology
Penn State University

ABSTRACT

A four-year research project is planned to perform a detailed study of boundary layer turbulence under stably stratified conditions. To achieve this goal, two unique radar sensors developed by the University of Massachusetts will be used: the Turbulent Eddy Profiler (TEP) and a S-band FMCW Profiler. Because stably stratified conditions tend to limit the intensity and scale of turbulence, our primary objective is to determine the minimum spatial and temporal scales at which useful and reliable remote measurements of turbulence are obtainable with current boundary layer profiler technology. In collaboration with Penn State University (J. Wyngaard) we plan to apply the measurements of these sensors to obtain, among other things, validation of turbulence closures under stable conditions. Because these 4D space-time measurements of stably stratified boundary layers will be unique, we intend to make the TEP and FMCW measurements available to other interested investigators as appropriate within the VTMX program.

TEP is a volumetric 915 MHz radar wind profiler. It may be thought as a densely packed array of ninety boundary layer wind profilers sharing a common transmitter. Using software beam-forming techniques, TEP simultaneously generates over 40 contiguous beams within a 25 degree field of view. Height resolution is 30 m and the width of each beam is 3.5 degrees. As a result TEP provides a four-dimensional (a 3D volume plus time) view of atmospheric turbulence structure within a volume of the boundary layer at spatial resolutions comparable to large eddy simulations (LES). The four-dimensional fields (x,y,z,t) obtained by TEP are unique compared to other clear-air radars. Most profilers employ switched-beam antennas requiring a dwell time on each of 3 to 5 beams (typically, at least 30 seconds per beam). In contrast, TEP records all the data from each receiving antenna. This provides advantages over more conventional profilers: (1) TEP obtains signals on over 40 contiguous beams simultaneously, and (2) Dwell times are selectable in post-processing. Thus coherent integration and averaging can be tuned to conditions.

MIRSL has recently been funded to augment TEP with an acoustic source to enable RASS measurements of virtual temperature. We plan to integrate TEP with sodar with two possible modes of operation. In the first mode, the sodar will operate independently of TEP, providing acoustic backscatter and wind estimates for heights from 50 m to 500 m. This will permit measurements at the lowest altitudes, where radar measurements are problematic. In its second mode, the sodar will serve as an acoustic source for RASS measurements with TEP. In principle, TEP will be capable of imaging virtual temperature fields; however, spatial resolution will be limited in practice by the size of the focused reflection incident on the receiving array.

With the acoustic component, TEP should estimate many of the mean-field and turbulence statistics known to be important in the dynamics of stably stratified turbulence. These include the vertical gradient of mean virtual potential temperature, the mean wind shear, the kinematic Reynolds shear stresses, the vertical velocity variance, the viscous dissipation rate, and horizontal and vertical components of the temperature flux. In principle then, TEP can estimate the important terms in the TKE budget. TEP's multiple pixels provide outputs with spatial resolution of the order of that in LES. It also allows spatial averaging in the horizontal, decreasing statistical scatter in flux measurements. Finally, the 3-dimensional, time varying "picture" of the turbulence structure should prove quite useful in characterizing the nature of the flow.

TEP's range resolution is limited to 30 m. This bounds its capability to discern thin layers of refractive index gradient and turbulence at interfaces. The FMCW radar is designed to complement TEP by providing finer resolution profiles through the TEP volume. The FMCW's beamwidth is matched to the focused TEP resolution. For reflectivity, the FMCW can provide over 10 sub-pixels for each TEP pixel. Coincident measurements will allow us to examine the biases that strong turbulent layers of sub-pixel dimensions may make on TEP's measured quantities.

The TEP and FMCW sensors are participating in the CASES'99 experiment. During the Salt Lake City experiment, we propose to collocate the TEP and FMCW sensors operated from a 48' semi-trailer that will house the RF and data acquisition subsystems. We will include basic meteorological measurements at the field site. To establish surface flux conditions, however, we must deploy, at a minimum, one 3-component sonic anemometer. PSU will provide these if necessary; however, we would prefer to site ourselves near a facility or investigator making more comprehensive surface micrometeorological measurements.

The relatively low data rate from the FMCW makes it reasonable to operate this sensor continuously during field operations. TEP, operated at full capability, produces about 16 GB/hour. Therefore, it is not reasonable to operate continuously during the experiment, but rather to enable full operation during several planned Intensive Observation Periods. These we expect would be coordinated among several investigators.

CONTACT:

Stephen Frasier, tel: (413) 545-4582, e-mail: frasier@ecs.umass.edu

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