Fitting of NWM ray-traced slant factors to closed-form tropospheric mapping functions

Abstract

In this paper we investigate the fitting of ray-tracing results to closed-form expressions. We focus on the variation of the delay with elevation angle and azimuth. For the elevation angle-dependence we compare the continued fraction form of Yan and Ping (Astron J 110(2):934-993, 1995) with that of Marini (Radio 11 Sci 7(2):223-231, 1972) (normalized to yield unity at zenith and found negligible differences between the two functional formulations for the hydrostatic case, while for the non-hydrostatic case, the Yan and Ping model performed marginally better. Since the ray-tracing results do not necessarily assume azimuthal symmetry, we have to account for the azimuth-dependence. For that we compare the linear gradient model of Davis et al. (Radio Sci 28(6):1003-1018, 1993) with the inclusion of second order terms (Seko et al., J Meteorol Soc Jpn 82(1B):339-350, 2004) and arbitrary spherical harmonics. These functional forms performed very well for the hydrostatic case, although for the non-hydrostatic case there were some large biases, particularly in the spherical harmonics of order 1, degree 1 and the 2nd order polynomial case. © Springer-Verlag Berlin Heidelberg 2012.