An accurate knowledge of radio refractivity is essential in many radar and propagation applications. Especially at low altitudes, radio refractivity can vary considerably with both height and range, heavily affecting the propagation characteristics. This paper addresses the problem of high variability in tracking surface-based ducts in marine and coastal environments. The method tracks the evolution of the range and height-dependent index of refraction using the radar sea clutter. A split-step fast Fourier transform (FFT) based parabolic equation (PE) approximation to the wave equation is used to compute the clutter return in complex environments with varying index of refraction.