ABSTRACT

• The first goal of VTMX that we will address is to improve our ability to measure quantities required for a better understanding of vertical transport and mixing. Specifically, we will apply our extension of the ability of profiler technology to measure vertical transport and mixing to the goals of VTMX. In order to make a contribution toward this goal, we have begun to analyze data from a six-month field program completed last fall and winter along the Front Range of Colorado. In-situ measurements were made on a 300-m tower and compared to observations made by a multiple antenna wind profiler radar (MAPR). MAPR points continuously in the vertical direction in contrast to typical Doppler-based systems allowing a continuous measure of the vertical motion. We in the process of documenting vertical transports associated with fronts and gravity waves. We have used tower data to evaluate MAPR estimates of mean winds. With MAPR spaced antenna techniques are used to provide continuous vertical profiles of horizontal and vertical winds and radar reflectivity with a temporal resolution of order of a few minutes or less. This performance well exceeds that of conventional wind profilers allowing us to observe mixing events resolved by the profiler sample volumes. We have begun to evaluate several MAPR-based techniques to estimate turbulent mixing within the volume sampled by the profiler, including the variance in the time series of vertical velocity, changes in C_n², and spectral width methods of measuring turbulence intensity. The knowledge gained from these inter-comparisons, together with work at NCAR on estimating mixing with conventional profilers, will be useful to the other VTMX investigators who will use wind profilers in their research. Intercomparisons will also be made against Doppler lidar estimates made by NOAA/ETL either at the 300-m tower. Conventional profiler estimates of turbulence will be explored through deployment of our profiler systems for CASES-99. Based on our studies thus far, we plan to upgrade the system for participation in the Salt Lake City experiments through increasing the vertical resolution using multiple frequency techniques. We will add a SODAR, possibly a tether sonde system, and a backscatter lidar for the Salt Lake City experiment. We have extensive experience with integrated sounding systems. These integrated measurements will allow us to better investigate the lowest 100-150 m, which are difficult to measure with profilers. In addition, if our deployment could be near an X-band radar than we recently proposed to use multiple frequency techniques to estimate eddy dissipation. Such an approach could allow us to detect weak mixing events difficult to observed with the previously described profiler techniques.

• The second goal to which we will contribute is the development of improved treatment of vertical mixing and transport for use in conceptual and numerical models. We will begin with an evaluation of what improvements are needed in numerical simulations by testing the ability of the National Center for Atmospheric Research (NCAR)/ Pennsylvania State University Mesoscale Model Version 5 (MM5) to replicate the observed thermodynamic fields, winds and mixing. Comparisons with observations will be undertaken in the Front-Range field program and in the subsequent Salt Lake City field programs. The model simulations for last years Front-Range field program have already been completed and the data saved for the experiment. For the VTMX field programs at Salt Lake City, we will compare MM5 predictions against RAMS and ETA model simulations that will be made by other funded VTMX investigators. The model versus observational intercomparisons will better identify the strengths and limitations of the modeling approach. New schemes for treating stable boundary layers developed under VTMX will be implemented and tested in MM5.

• Finally, this study will also contribute to the VTMX goal of providing insight into the fundamental processes that govern vertical transport and mixing. The measurements will be used to determine the local vertical transport and mixing, while the model will be used to determine the role of non-local processes (terrain-induced circulations, fronts, changes in the conditions above the stable layer) in determining the location and timing of mixing. The Front Range field program will allow us to address transport and mixing events common to the lee of the mountains. These processes include the erosion of stable layers by mechanical mixing from high winds aloft and by transient weather systems, such as fronts. The relative role of terrain-induced circulations and the interactions of mesoscale frontal structures with terrain-induced circulations will be studied during the Salt Lake City field programs. We will also address transition periods in both the Front Range and Salt Lake City field programs to determine how the diurnal heating cycle, sloping terrain and transient circulations interplay to pattern mixing and vertical transport during these conditions. For the Salt Lake City experiment, it is our underlying assumption that vertical transport is strongly patterned by both orographically-induced circulations (thermally-induced slope flows, terrain channeling, gravity waves) and transient disturbances such as fronts. These circulations should be generally well resolved by the profiler. We hypothesize that these types of transport events will prove to be relatively more important for human activities through impacting air quality and surface temperature forecasts than the "classical" small-scale stable mixing events. Our integrated system deployed at VTMX should be able to well resolve the former types of mixing events.

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