Scale Effects and Heterogeneity in Land-atmosphere Interactions: Large Eddy Simulation Studies, Parameterizations and Field Validations

Johns Hopkins University, Baltimore MD

Investigators

Abstract

1Center for Environmental and Applied Fluid Mechanics (CEAFM), 2Dept. of Mechanical Engineering, The Johns Hopkins University, Baltimore MD 21218, 3Ecole Polytechnique Federale Lausanne, Switzerland, and Dept. of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore MD 21218. Summary: Intellectual merit: In hydrology, the main practical approach to obtain regional scale evaporation continues to be based on classical similarity theory of the atmospheric boundary layer (ABL). The theory assumes a uniform land surface yet has also often been found acceptable in flows over heterogeneous natural land surfaces due to the turbulent flow in the ABL, which efficiently blends the various sources and inhomogeneities across the landscape. It is thus essential to understand and predict how land heterogeneity and atmospheric stratification affect the relevant length-scales associated with this blending process. Specifically, the height of blending layers, and effective 'average') roughness lengths for surface fluxes of temperature and humidity are of needed to formulate simplified models. In the proposed research, Large Eddy Simulations (LES) of turbulent flow and transport in the ABL will be conducted to identify the blending properties of turbulent mixing under various conditions of stratification and spatially heterogeneous roughness properties, including fractal distributions. Numerical simulations will employ the newly developed and tested Lagrangian scale-dependent dynamic model, which has been shown to be particularly well suited to capture unresolved small-scale turbulence physics in complex environments in which turbulence deviates from the classical assumptions of isotropic, inertial-range behavior. The dynamic procedure eliminates the need to specify tunable model coefficients. Extensions of the dynamic model to account for scalar transport will be implemented. Through a parametric series of high-resolution simulations (parametric LES) where surface roughness, geometric arrangements, and surface temperature and/or heat flux are varied systematically, those features which drive the dynamics of land-atmosphere exchange over heterogeneous terrain will be quantified, and their relevant lengthscales established. This information will be used in new strategies for obtaining regional scale fluxes such as heat flux and evaporation over realistic terrain. Field validations of the LES and parameterizations will be conducted using data collected during the planned Swiss Mosaic Experiment. Broader impacts: Large-scale (regional or global-scale) earth-system simulations, used to simulate weather and the Earth's climate and project temperature changes in the coming decades, rely heavily on parameterizations to represent the atmospheric boundary layer and the land surface. LES with accurately tested subgrid models allows us to undertake assessments and developments of new, carefully and individually tested, components of the larger scale models. Increasing the trustworthiness of individual components of large-scale models is a necessary step for predictions to be taken more seriously by policy-making bodies, and ultimately also by the broader public. Broader impact of the proposed research activity is also achieved through our unique graduate studenttraining program. The students are trained rigorously in science through course-work in various departments, possibly leading to a M.S. degree in Mechanical Engineering and a PhD in the Department of Geography and Environmental Engineering through the CEAFM's Dual Degree Program. And, through the program in Geography and Environmental Engineering they will have a broader knowledge at the interface between society and environment. This program is distinctive in the scope of its focus on the interaction between human behavior and nature over time and in its structure, which combines political and human aspects with the science of the environment. The Hopkins program has garnered considerable recognition for its success in influencing public policy through solid science.