Towards assimilation of atmospheric surface layer using numerical weather prediction and radar clutter observations

Radio wave propagation on low-altitude paths over the ocean above 2 GHz is significantly affected by negative refractivity gradients in the atmospheric surface layer (ASL) which form what is often referred to as an evaporation duct (ED). Refractivity from clutter (RFC) is an inversion approach for estimation of the refractivity profile from radar clutter, and RFC-ED refers to its implementation for the case of evaporation ducts. An approach for fusing RFC-ED output with evaporation duct characterization based on ensemble forecasts from a numerical weather prediction (NWP) model is examined here. Three conditions of air sea temperature difference (ASTD) are examined. Synthetic radar clutter observations are generated using the advanced propagation model (APM). The impact of ASTD on the evaporation duct refractivity profile, atmospheric parameter inversion, and propagation factor distributions are studied. Relative humidity at a reference height and ASTD are identified as state variables. Probability densities from NWP ensembles, RFC-ED, and joint inversions are compared. It is demonstrated that characterization of the near surface atmosphere by combining RFC-ED and NWP reduces the estimation uncertainty of ASTD and relative humidity in an evaporation duct, with respect to using either method alone.