Inversion, error analysis, and validation of GPS/MET occultation data

摘要

The global positioning system meteorology(GPS/MET) experiment was the first practical demonstration of global navigationsatellite system (GNSS)-based active limb sounding employing the radiooccultation technique. This method measures, as principal observable and withmillimetric accuracy, the excess phase path (relative to propagation in vacuum)of GNSS-transmitted radio waves caused by refraction during passage through theEarth's neutral atmosphere and ionosphere in limb geometry. It shows greatpotential utility for weather and climate system studies in providing an uniquecombination of global coverage, high vertical resolution and accuracy, long-termstability, and all-weather capability. We first describe our GPS/MET dataprocessing scheme from excess phases via bending angles to the neutralatmospheric parameters refractivity, density, pressure and temperature. Specialemphasis is given to ionospheric correction methodology and the inversion ofbending angles to refractivities, where we introduce a matrix inversiontechnique (instead of the usual integral inversion). The matrix technique isshown to lead to identical results as integral inversion but is more directlyextendable to inversion by optimal estimation. The quality of GPS/MET-derivedprofiles is analyzed with an error estimation analysis employing a Monte Carlotechnique. We consider statistical errors together with systematic errors due toupper-boundary initialization of the retrieval by a priori bending angles.Perfect initialization and properly smoothed statistical errors allow for betterthan 1 K temperature retrieval accuracy up to the stratopause. No initializationand statistical errors yield better than 1 K accuracy up to 30 km but less than3 K accuracy above 40 km. Given imperfect initialization, biases >2 Kpropagate down to below 30 km height in unfavorable realistic cases.Furthermore, results of a statistical validation of GPS/MET profiles throughcomparison with atmospheric analyses of the European Centre for Medium-rangeWeather Forecasts (ECMWF) are presented. The comparisons indicate the highutility of the occultation data in that very good agreement of uppertroposphere/lower stratosphere temperature (better than 1.5 K rms, <0.5 Kbias) is found for a region (Europe+USA) where the ECMWF analyses are known tobe good, but poorer agreement for a region (Southern Pacific) where the analysesare known to be degraded.Key words. Atmospheric composition and structure(pressure; density and temperature), Meteorology and atmospheric dynamics(instruments and techniques), Radio science (remote sensing)