Validation of operational GRAS radio occultation data

Abstract

GRAS RO (radio occultations) are validated against co-located ECMWF and COSMIC data and by ECMWF impact trials. We focus on closed-loop data at impact heights above 8 km. Results confirm the high GRAS quality and robustness, showing lower noise than COSMIC and more occultations per day/satellite. Mean differences to ECMWF and COSMIC from 18 km to 35 km show about 0.1% smaller GRAS BAs (bending angles). Around 40 km, ECMWF shows on average about 1% smaller BAs, which may be related to microwave radiances assimilation. Recent ECMWF updates, putting more weight on RO here, reduce this bias. COSMIC co-locations reveal smaller GRAS BAs up to about 50 km, probably partly caused by COSMIC smoothing; this is currently revised. ECMWF forecast trials show similar positive GRAS, COSMIC impacts for Southern latitudes standard deviations, although GRAS provides about 60% fewer occultations. It also demonstrates that more RO instruments are beneficial, particularly for the tropics and Northern latitudes. Copyright 2009 by the American Geophysical Union.