Concepts, observations, and simulation of refractive index turbulence in the lower atmosphere

Advances in computers and in computational techniques now allow the calculation of electromagnetic (EM) wave propagation through simulated refractive index turbulence in the lower atmosphere. Such applications call for instantaneous turbulence fields, not turbulence statistics, the traditional focus of the turbulence community. We clarify their important differences and review what is known about key statistics of refractive index turbulence. We discuss the calculation of EM propagation with a parabolic equation model that uses composite refractive index fields, the larger scales being calculated with a dynamical mesoscale model and the smaller scales being calculated through large-eddy simulation. The locally, instantaneously sharp top of the atmospheric boundary layer can have a profound effect on forward scatter of EM waves. This top appears to be even sharper than is revealed by conventional measurements, particularly in the convective boundary layer.