Abstract
Global positioning satellite system (GPS) radio waves that reach the tropical lower troposphere are strongly affected by small-scale water vapor fluctuations. We examine along-the-ray simulations of the impact parameter at every ray integration step using the high-resolution European Centre for Medium-Range Weather Forecasts ERA5 reanalysis as the input model states. We find that disturbances to the impact parameter arise when ray paths go through the top of the sub-cloud layer, where there is a pronounced reduction with increasing height in the humidity, and wet refractivity has a strong local vertical gradient, creating multipath. Additionally, the horizontal gradients of refractivity cause the impact parameter to vary along the ray. The disturbances to the impact parameter are confined to an area about 250 km horizontally and 4 km vertically from the perigee point. Beyond 250 km from the perigee, the impact parameter remains constant. The vertical gradient of refractivity is largest at the top of the sub-cloud layer, usually between 1.5 and 3.0 km, and becomes negligibly small above 4 km.
Highlights
Ying ZhuSince the proof-of-concept GPS/MET experiment launched in 1995 [1], global positioning satellite system (GPS) radio occultation (RO) has proven to be a valuable atmospheric sounding technique from space, contributing to research and operational weather forecasting [2,3]
A strong3 vertical in a multipath occurrence, presence of astrong of refractivity renders the spherical companies local horizontal variation gradients in the impact parameter, variations in thesymmetry impact parame assumption invalid (Healy, 2001), so that the impact parameter and impact height of a ray along the rays are caused by the horizontal gradient of Figure shows the distri passing through the local perturbation is no longer constant
This study investigates the impacts of small-scale water vapor fluctuations in the GPS
Summary
Since the proof-of-concept GPS/MET experiment launched in 1995 [1], global positioning satellite system (GPS) radio occultation (RO) has proven to be a valuable atmospheric sounding technique from space, contributing to research and operational weather forecasting [2,3]. As the radio waves slice through the atmosphere in about 1–2 min, RO measurements (time series of excess Doppler shift) are observed [5]. From these measurements, the bending angle as a function of the impact parameter and the refractivity as a function of pressure can be derived. The atmospheric refractivity (N) is the sum of dry refractivity (Ndry ). The atmospheric refractivity (N) is the sum of dry refractivity (Ndry) and wet refractivity (Nwet) as shown in Equations (3) and (4) below. On and wet refractivity (Nwet ) as shown in Equations (3) and (4) below.
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