Among the specific goals of VTMX, two are directly addressed by this work; 1) understanding the mechanisms responsible for vertical transport and mixing and 2) developing improved treatments of vertical transport and mixing for use in conceptual and numerical models. The specific VTMX science issues partly addressed are: What is the nature of the interaction of terrain-induced flows (e.g., drainage winds at night, daytime upslope winds, and waves) with cold air pools in basins, and how do such flows affect the formation and erosion of those pools and the dispersion of pollutants within them? What are the fundamental processes that control vertical transport for stable and transition boundary layers? How can momentum, heat and moisture fluxes be modeled and predicted in a stratified atmosphere with multiple layers? What formulations are most appropriate for the description of vertical diffusion in stable air?
We have established collaborations for the numerical modeling, observational and parameterization improvement portions of our work with Dr. James Bossert of Los Alamos National Laboratory (LANL), Dr. Dave Whiteman of Pacific Northwest National Laboratory (PNNL) and Dr. Jerome Fast, also of PNNL, respectively.
During the first of 4 years, we would analyze relevant data from the CASES-99 program, participate in site optimization for the first VTMX field experiment and evaluate the representation of wave/instability processes in mesoscale model parameterizations. The second year would consist primarily of participation in the VTMX field project, and subsequent data analysis and modeling of the IOPs (or specific mixing events) toward quantifying the impact of wave and shear instability phenomena on basin cold pool vertical mixing and transport. The third year would focus on intensified numerical modeling to identify the dynamical causes of intermittency and the refinement of a quantitative representation of wave/ instability processes on stable cold pool evolution. The 4th year would consist of participation in the second VTMX field program and the application of research in years 1-3 toward the improvement and subsequent testing of subgrid-scale parameterizations.
Our participation in the field program would use the VTMX instrumentation for over one month of extensive measurements during Fall 2000, and again 2 years later. VTMX measurements will focus on the meso- and microscale environmental structure and evolution as well as the surface and SBL (up to and above the inversion) fluxes of heat, moisture, and momentum. Due to the generally westerly flow across the area during the cold season, gravity wave and shear instability processes should be frequent at this location, suiting our research extremely well.
Initially we will undertake the analysis of CASES-99 data relevant to both our research and related to our collaborations with DOE Laboratories. This analysis will focus on shear instability and propagating gravity wave events during IOPs within the heavily instrumented CASES-99 domain. Information on CASES-99 can be found at, www.colorado-research.com/cases/CASES-99.html. First, we will evaluate the measured fluxes and turbulence from specific gravity wave breaking and shear instability events over the CASES-99 domain. Second, by correlating the VTMX measurements from relevant platforms (FM-CW radar, aircraft and lidar) and using those instruments that may actually measure fluxes (the airborne instruments, lidar, towers), we will quantify the impact of wave and shear instability phenomena on basin cold pool vertical mixing and transport. Depending on the intensity, depth, and horizontal extent of a particular turbulent burst, the task of identifying and quantifying the source of turbulence can be either reasonably easy or extremely difficult. The characterization of burst sources will be divided into three main parts, 1) identification of the source type, 2) the vertical propagation of the turbulence and evolution of the vertical fluxes, and 3) the horizontal extent of said event.
Numerical simulations anticipated as a part of this research would use the Regional Atmospheric Modeling System (RAMS) model to describe the coupled urban-land-atmosphere system, boundary layer fluxes, and large-scale wave and shear structure of the SBL through assimilation of measurements of mesoscale atmospheric structure. More focused simulations using a highly-optimized spectral DNS code would be performed to examine specifically observed stable shear flow instability within and above the SBL, the occurrence, intensity, and intermittency of turbulence and its penetration into the SBL, and the structure of, and the turbulence fluxes accompanying, such penetration events. RAMS would be the primary modeling resource for 1) evaluating gravity wave and shear instability contributions to parameterization errors in stably stratified conditions, 2) determining propagating gravity wave evolution across the cold pool and its influence on SBL flows, and 3) reconstructing VTMX field program IOPs. It is important to recognize the irony here: we will be using a type of model subject to parameterization errors when used in stable atmospheric conditions to simulate actual stable atmospheric cases!
The results from DNS simulations will be used for 1) interpreting field measurements and RAMS out put, and 2) developing reliable subgrid-scale schemes to be employed by mesoscale models such as RAMS. Events chosen for DNS would be identified from measurements. Comparisons we shall conduct between RAMS output and DNS calculations of shear flows taken from a subset of RAMS' domain will allow refined evaluation and useful modification of the subgrid-scale strategies employed by RAMS. Evaluation of the specific instability events, using VTMX observations or mesoscale model data for initialization, will reveal the details of mixing and entrainment of scalar quantities, and provide evidence of the importance of such events to the basin-wide turbulence budget.
We will evaluate mesoscale model parameterizations that typically fail in stably-stratified conditions. Our approach will be to evaluate the stable case errors in RAMS by intercomparison with VTMX field measurements and to dissect the conceptual model upon which its parameterizations are based. RAMS model results for selected basin SBL cases will be evaluated for the representativeness of evolving atmospheric characteristics to target problem areas. Comparison will be made with the measured evolution of turbulent scalar and momentum fluxes at various heights.
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