Abstract
Following several years of experimentation with the GPS radio occultation technique, the 6-satellite FORMOSAT-3/COSMIC (F3C) mission was launched mid-2006 and has been collecting data since then. In this paper we present early findings of research performed at Meteo-France regarding the use of these data for assimilation in numerical weather prediction. Benefiting from the dense global coverage allowed by F3C refraction-induced observations, we first assess the quality of these data at four levels: bending angle, refractivity, refractivity lapse rate, and temperature. We compare them with calculations from Meteo-France numerical weather forecasts. Leaning from these various levels of data we devise quality control procedures that rely on the refractivity lapse rate. Applying a recent methodology developed in data assimilation we calculate observation bending angle error variances for our assimilation system. Using these new quality control procedures and observation error estimates we run an assimilation and forecast experiment with Meteo-France's operational global 4DVAR data assimilation system used as a reference. Our results indicate a very clear positive impact of the assimilation of F3C bending angle data in the Southern hemisphere for the prediction of geo potential heights and winds. We also observe an improvement in wind forecast skill in the Northern hemisphere, albeit such an improvement is smaller than in the Southern hemisphere.
Highlights
In 1995, The GPS radio occultation technique demonstrated its ability to collect low cost observations of the Earth’s atmosphere with a passive instrument involving neither optics nor moving parts (Ware et al 1996)
We find that the derivative of the background and observation refractivity lapse rate with respect to height (d2N/dz2) is useful to locate areas where the physical phenomena considered simulated by the background may differ from the observation conditions
The F3C mission has been providing a global dataset of GPS refraction observations since mid-2006. We have assessed these data processed by University Corporation for Atmospheric Research (UCAR) in a comparison with the global operational assimilation and forecast system of Météo-France (ARPEGE)
Summary
In 1995, The GPS radio occultation technique demonstrated its ability to collect low cost observations of the Earth’s atmosphere with a passive instrument involving neither optics nor moving parts (Ware et al 1996). The technique requires placing an advanced radio equipment (i.e., a GPS receiver) onboard a satellite whose orbit determination needs to be precise. With this setup the tracking data of the highly stable signals transmitted by the constellation of GPS satellites can be inverted to yield ionospheric and atmospheric information. That section details the observation operators used to simulate F3C data.
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