In this paper we put forward a possibility of refractive index profile retrieval using field measurements at an array of radio receivers through the variational adjoint approach. The derivation of the adjoint model begins with the electromagnetic parabolic equation for a smooth, perfectly conducting surface and horizontal polarization conditions. To deal with the ill-posed difficulties of the inversion, the regularization idea is introduced into the establishment of the cost function. Based on the variational assimilation idea, the retrieval iterative format is constructed. Numerical experiments demonstrate the feasibility of theoretic algorithm for refractivity estimation. However, using the split-step Fourier algorithm to solve the forward model and the adjoint model, the intrinsic error of the solutions will increase with the extension of the propagation range, which reduces the inversion accuracy at long distance propagation. Through adopting a good initial refractivity profile and introducing the background fields in the cost function the inversions could generally be improved.