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Abstract
Tropical cyclones (TC) are one of the main producers of clouds in the tropics and subtropics. Hence, most of the clouds in TCs are dense, with large water and ice content, and provide conditions conducive to investigate clouds’ impact on Radio Occultation (RO) measurements. Although the RO technique is considered insensitive to clouds, recent studies show a refractivity positive bias in cloudy conditions. In this study, we analyzed the RO bending angle sensitivity to cloud content during tropical cyclone seasons between 2007 and 2010. Thermodynamic parameters were obtained from the ERA-Interim reanalysis, whereas the water and ice cloud contents were retrieved from the CloudSat profiles. Our experiments confirm the positive mean RO refractivity bias in cloudy conditions that reach up to more than 0.5% at the geometric height of around 7 km. A similar bias but larger and shifted up is visible in bending angle anomaly (1.6%). Our results reveal that the influence of clouds is significant and can exceed the RO bending angle standard deviation for 21 out of 50 (42%) investigated profiles. Mean clouds’ impact is detectable between 9.0 and 10.5 km, while, in the case of single events, clouds in most of the observations are significant between 8 and 14 km. Almost 15% of the detectable clouds reach 16 km height, while the influence of the clouds below 5 km is insignificant. For more than half of the significant cases, the detection range is less than 3 km but for one observation this range spreads to 7–8 km.
Highlights
GPS Radio Occultation (RO) is an active limb viewing technique that takes advantage of phase and amplitude of two L-band electromagnetic signals transmitted from GPS and received on low earth orbit (LEO) satellites [1]
We identified all of the CloudSat ice water content (IWC)/liquid water content (LWC) profiles that occurred within a 200 km radius from
We found 50 collocated RO, Tropical cyclones (TC) and CloudSat observations, where 21 (42%) do not fail the sensitivity test (Equation (8))
Summary
GPS Radio Occultation (RO) is an active limb viewing technique that takes advantage of phase and amplitude of two L-band electromagnetic signals transmitted from GPS and received on low earth orbit (LEO) satellites [1]. A GPS signal passing through Earth’s atmosphere is refracted and attenuated, which induces delay and bending. A measure of the atmospheric density in terms of the bending angle parameter is derived from Doppler shifts based on precise clocks, satellite orbits and velocity measurements. Since the GPS signal is considered to be insensitive to clouds, it can be used in any weather condition [2] and enables observations with high accuracy, precision and vertical resolution [1]. Scherllin-Pirscher et al [4] demonstrated that the global mean observation errors are as low as 0.3% for bending angle, 0.1% for refractivity and 0.2 K for a dry temperature at altitudes between 4 and 35 km
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