Fluctuations of refractivity as a systematic error source in radio occultations

The fact that fluctuations of refractivity may result in a systematic negative shift of the phase of waves propagating in a random medium is known for a long time. Tatarskii was the first to reveal it, and von Eshleman put this into the context of the radio occultation sounding of planetary atmospheres. In this paper, we show that this effect may also be one of the causes of the negative bias of refractivity retrieved for radio occultation observations of the Earth's atmosphere. We perform theoretical estimates of this effect based on the Rytov approximation. These estimates, however, do not consider the regular refraction, which may significantly change the magnitude of this effect. We perform numerical simulations of radio occultations, based on the Kolmogorov-von Karman isotropic spectrum of refractivity fluctuations, with the internal and external scales and magnitude tuned so as to reproduce the realistic level of the variance of retrieved refractivity and the amplitude fluctuations of the modeled signals. The model of the regular atmosphere is based on analyses of the European Centre for Medium-Range Weather Forecasts. We show that it is possible to set up a vertical profile of the structural constant of the fluctuation spectrum such that it will result in a systematic shift and variances of the retrieved refractivity consistent with those observed for COSMIC measurements.