Effect of superrefraction on inversions of radio occultation signals in the lower troposphere

Abstract

[1] Radio occultation remote sensing of the Earth's atmosphere by use of GPS encounters problems in the moist lower troposphere (planetary boundary layer). The negative errors in retrieved refractivity (bias) may not be explained by the horizontal inhomogeneity in refractivity. In part, these errors can be attributed to the use of signal tracking algorithms inappropriate for the complicated structure of radio occultation signals propagated through the moist troposphere. However, another fraction of the negative bias in retrieved refractivity can be related to the superrefraction. In this study we introduce the problem and give an estimate of the negative refractivity errors in the moist planetary boundary layer, which in some cases can be as large as $10%. We show that the magnitude of these errors significantly varies over oceanic areas. We validate the canonical transform method by use of the radio occultation signals simulated for complicated refractivity structures, including multiple superrefraction layers and small-scale irregularities. We find that this method does not introduce errors additional to those existing in geometric optics. Also, we discuss and estimate an additional error source when inverting occultation signals by radioholographic methods: insufficient extension of the acquired signal, which can contribute to about 1% error of the retrieved refractivity.INDEX TERMS: 6904 Radio Science: Atmospheric propagation; 6964 Radio Science: Radio wave propagation; 6969 Radio Science: Remote sensing; 6994 Radio Science: Instruments and techniques; KEYWORDS: radio occultations, radioholography, superrefraction Citation: Sokolovskiy, S., Effect of superrefraction on inversions of radio occultation signals in the lower troposphere,

Validation of Refractivity Profiles Retrieved from FORMOSAT-3/COSMIC Radio Occultation Soundings: Preliminary Results of Statistical Comparisons Utilizing Balloon-Borne Observations

“…1 and 2. This can be at trib uted to the negative re trieval er ror in refractivity in the trop i cal lower troposphere (e.g., Rocken et al 1997;Ao et al 2003;Sokolovskiy 2003). A sharp ver ti cal gra di ent of refractivity, or superrefraction, might of ten oc cur, which leads to the neg a tive bias in the at mo spheric bound ary layer in trop ics.…”

Assessment of global navigation satellite system (GNSS) radio occultation refractivity under heavy precipitation

“…This bias is not assessed here, since it has already been discussed previously in other studies (e.g., Ao et al, 2003;Sokolovskiy, 2003;Xie et al, 2006Xie et al, , 2012Wang et al, 2017). Similarly, other potential sources of bias have been checked, for example, the angle of incidence of the occultation ray to the receiver, with respect to the transmitter position.…”

“…This implies that the theoretical spherically symmetric atmosphere could depart from a realistic approximation and induce errors in the retrievals (Foelsche et al, 2011). Also, large incident angles correspond to low SNRs, which could be introducing positive biases (Sokolovskiy et al, 2010). Therefore, the positive bias has been checked grouping the occultation events according to its azimuth angle, in addition to rain variables.…”

A perspective on the fundamental quality of GPS radio occultation data

“…A number of studies analyzed the error sources and propagation of the errors through the retrieval process (e.g., Kursinski et al, 1997;Feng and Herman, 1999;Syndergaard, 1999;Rieder and Kirchengast, 2001;Hajj et al, 2002;Kuo et al, 2004;. Some of the error sources are particularly hard to tackle including ionospheric residuals (e.g, Syndergaard, 2000), the influence of a priori error on the statistical optimization of bending angle (e.g., Wee and Kuo, 2014), horizontal inhomogeneity in the atmosphere (e.g., Poli and Joiner, 2004), abnormal propagations of radio waves (e.g., Sokolovskiy, 2003), strong multipath effects in the lower troposphere (e.g., Gorbunov et al, 2006), and the ambiguity in separating moisture and dry air density from the bending angle or the refractivity (e.g., Kursinski et al, 1997). Different approaches that aim to reduce the errors are proposed in the literature; however, they yield slightly different results.…”