Refractivity from Clutter by Variational Adjoint Approach

Inferring refractivity profiles from radar sea clutter is a complex nonlinear optimization problem. Previous works treat this problem as a model parameter estimation issue by using some idealized refractivity models, such as the Log linear evaporation duct model, bilinear model, and trilinear model, to describe the synoptic structure of the real atmospheric conditions. However, these idealized models can not describe the exact information of the refractivity profile. Rather than estimating a few model parameters, this paper puts forward possibilities of retrieving the refractivity values at each point over height by variational adjoint approach for RFC measurement geometry. The adjoint model is derived from the parabolic equation method for a smooth, perfectly conducting surface and horizontal polarization conditions. Evaporation duct profiles collected at East China Sea are provided as the true refractive environment. The performance of this approach is determined via simulations and is evaluated as a function of: 1) the initial guess profile; 2) the measurement noise; and 3) the spatial samples.