CAREER: An A Priori Test of Retrieval of Coherent Structures in the Atmospheric Boundary Layer Using an Adjoint Model

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The wind near the ground is mainly turbulent and irregular but imbedded in it are often eddies, vortices, or swirls that persist long enough to be recognizable as entities that form, grow, and eventually dissipate. These are called coherent structures and are an important class of small-scale atmospheric motions. They account for substantial transports of heat, momentum, water vapor, and pollutants, and generate small-scale turbulence. New methods of atmospheric remote sensing, in particular lidar because of its fine spatial resolution, hold the promise of observing and studying coherent structures. However, instruments like lidar that are based on the Doppler principle are limited to observing only the radial component of velocity at a given point in space, not the three-dimensional velocity vector. A challenging problem is the deduction of the three-dimensional velocity field from measurements of only the radial component. The solution is not unique: more than one wind pattern can have the same pattern of radial velocity. The approach to the problem is to constrain the number of possible solutions by requiring that the three-dimensional, time-evolving flow pattern conform to the known laws of atmospheric dynamics. This is called the adjoint method of four-dimensional data assimilation (FDDA). This project will apply FDDA to lidar data to observe and study coherent structures. Initially it will be based on simulations. A large-eddy simulation (LES) model will generate a turbulent wind field with coherent structures. Lidar measurements will be simulated by determining the radial components of velocity observable from one or more prescribed observing points in the wind field. Then FDDA will be applied to recover the complete wind field. Real observations will be simulated by degrading the resolution of the synthetic observations and adding measurement errors. These studies will indicate the accuracy and limitations of real lidar measurements and serve to define the optimum observing conditions for lidar systems of realistic capabilities. Based on the understanding gained from the simulations, experiments on observing coherent structures using real lidar data will then be undertaken. For the experimental portion of the work, the PI will collaborate with lidar specialists at his home institution and at the laboratories in Boulder, Colorado. The educational component of the CAREER grant will focus on developing instructional material for an engineering curriculum with examples from meteorology and geophysical fluid dynamics.