The main objectives are to determine through the use of observations and model simulations:
We will accomplish these objectives through a series of experiments that will simulate typical conditions during the Salt Lake City field study. Our plan is to examine a range of forcing conditions representative of the climatology as analyzed by D. Whiteman and X. Bian. Experiments will be designed for both slope regions and the central basin of the Salt Lake City field site. These tests will allow us to determine how kinetic energy generated by buoyant forcing in slope flows and by large-scale wind fields is partitioned among turbulent and mean motion under differing synoptic scale conditions. We will use initial conditions and surface forcing derived from field data, so that our results can be easily compared to observations and other VTMX analyses (e.g., mesoscale model results).
The LES model will be applied to 3-4 specific cases selected from the field campaign, focusing on slope flows and central basin studies.
a. Slope Flow Experiments
In the slope flow experiments, we will examine the evolution of a packet of air as it descends from the top of the slope to the level plain. This will be accomplished by subtracting the horizontally averaged surface downslope velocity component from model velocity field, thereby translating the model domain with the downslope flow. Cooling in these experiments will be prescribed based on 1-m temperature measurements that will be taken during the field program (D. Whiteman, personal communication).
Comparison of the average fields will provide a validation of the model against the field data. For example, the mean velocity and temperature profiles will be compared with tethersonde and RASS measurements taken along the slope. Turbulence parameters, such as the velocity variance, will also be compared with sodar and lidar measurements that will provide high frequency (5 Hz) and spatial resolution (~ 5 m) that are comparable to the LES results.
b. Basin Experiments
Case studies from the basin region will examine the role of synoptic and mesoscale forcing on the erosion of the SBL. We plan to select cases from the VTMX field study that best represent situations with a diurnal cycle of SBL formation and breakup, slow erosion of the SBL through synoptic forcing, and a strong synoptic forcing leading to a complete scouring of the basin. Initial conditions for the model will be derived from tethersonde and radiosonde data collected during the field experiment, with surface forcing based on observed heating rates. Simulations of the basin problem will be somewhat limited by the scale of the observed SBL; cases with very deep stable layers cannot be modeled as effectively because of increased vertical domain size. Therefore, our experiments will focus on cases with SBL depths of order 200-300 m. For specific cases with deep SBLs and weak near surface wind, we plan to implement a version of the model with open upper and lower boundaries. With this configuration, the model can be used to focus on shear-generated mixing at the very top of the SBL and avoid simulating a deep surface layer with little or no turbulence.
Analysis for each case will utilize calculations of the mean and turbulent energy budgets to examine the importance of turbulent processes. This will allow us to more completely understand how energy is transferred from the background flow into TKE and determine how efficient the turbulence is in mixing the SBL. Our plan is to compute these fields for each experiment and use them to explain why specific instability processes are active for a given set of surface forcing conditions. We will also use the energy budgets to quantify the importance of turbulent processes in changing the potential energy or vertical temperature structure.
It is straightforward to estimate bulk parameters from the LES model that are used by turbulence parameterizations. We will compare these estimates with values obtained using, for example, the Mellor and Yamada (1982) turbulence closure under conditions of equivalent forcing and mean temperature and horizontal velocity structure. We will also examine how mixing coefficients are affected by changes in the bulk Richardson number, which is a fundamental diagnostic for parameterized mixing in stratified flow. One assumption made in many mixing parameterization is that the mixing coefficients have a fixed functional relationship to Ri. By plotting values of KM and Kr against Ri, we can provide a test of this hypothesis. Current one-dimensional parameterizations typically ignore the effects of internal waves that propagate in stratified flow. Therefore, in addition to the analysis of the total energy budget, we also plan to examine how momentum and energy are transferred by internal waves above the SBL.
We anticipate working closely with several of the funded VTMX projects. Work planned by D. Whiteman and X. Bian will provide a framework for identifying when specific turbulence processes, such as scouring of the SBL, are important. For measurements of specific processes, we will rely heavily on the observations made by D. Whiteman and S. Zhong for slope flows, and W. Shaw for basin flows. In turn, the LES results will help in the interpretation of the observations. Collaboration with also be pursued with J. Fast on mesoscale modeling issues and J. Fast and S. Zhong on improving turbulence parameterizations. Because the majority of data collected during the VTMX field program will be one-dimensional in nature, the three-dimensional picture produced by the LES model provides a much needed analysis tool for understanding turbulent mixing and transport processes.
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