We will participate in the expected VTMX field program by making measurements using the NOAA/ARL Long-EZ aircraft, an instrumented tower, and an array of sensitive electronic microbarographs. The Long-EZ measurements will be made from the top of the boundary layer down to about 50 m AGL. In addition, the aircraft will be used to measure the mean wind, and thermal structure as well as the turbulent fluxes of heat, momentum, and water vapor. To account for advection effects and gravity waves it is important to fly two widely separated headings. We thus propose to fly along two straight, nearly perpendicular legs of roughly 10 km length each. The legs will intersect near their centers, forming a cross.
Six ATDD high-precision microbarographs will be used to monitor the passages of gravity waves and other coherent pressure disturbances through the research area. Data will be taken at 10 Hz using Campbell Scientific data loggers, and 10-second average values will be formed. Data collection will be around the clock. The instrument spacing and the averaging time determines the range of gravity wave phase speeds that can be resolved. If the phase velocity of a wave is less than 100 ms-1 or if its wave length is greater than 200 m, then the wave will be detected by the proposed grid configuration. Internal gravity waves in the PBL have phase velocities on the order of 10 ms-1 and wave lengths greater than 10 km, and Kelvin-Helmholtz waves have wave lengths of a few kilometers. Thus, we should detect those waves that are important on the PBL scale.
The energy exchanges between the atmosphere and the ground surface play a significant role in the development and maintenance of the stable PBL and the resulting turbulence regimes which drive the vertical mixing processes. As a complement to the proposed airborne measurements a fixed tower location with five levels of fast response turbulence measurements is planned. The levels would be logarithmically spaced to capture the shear zones near the effective urban canopy surface. A suggested spacing might be 6, 12, 24, 48, and 96 meters. Sonic anemometers from ATDD (Solent Gill R3) would be used to measure the stream-wise, transverse, and vertical wind velocities and sonically derived air temperature (u,v,w, Tv) from which the components of TKE, momentum and heat flux will be derived. Fast response water vapor, carbon dioxide and ozone will also be measured at these same levels coincident with the sonic measurements. Measurements of air temperature gradients will be made at these same levels with shielded and aspirated chromel-constantan thermocouples.
In the second year, these data will be used to (1) monitor gravity wave activity, (2) separate gravity waves from turbulence signals, (3) evaluate the terms of the TKE equation through the depth of the PBL, (4) analyze periods of intermittent turbulence, and (5) measure turbulent fluxes through the lower 100 m of stable PBL. Of special interest are the effects on the TKE equation of horizontal advection, intermittent and sporadic turbulence, fossil turbulence, and the interactions of gravity waves with turbulence. A possible field program during the third year, perhaps at location different than the first experiment, will be used to gather data for testing new parameterizations of turbulence and dispersion. During the fourth year, data from the second field program will be analyzed, and the new parameterizations tested.
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