Impact of 837 GPS/MET bending angle profiles on assimilation and forecasts for the period June 20–30, 1995

Abstract

Abstract. With a ray-tracing procedure and variational data assimilation techniques, it is now possible to make direct use of radio occultation bending angles, rather than their derived temperature and moisture retrievals, in atmospheric data analysis and assimilation. This paper describes results obtained from including more than 800 GPS/MET bending angle profiles, available over an l 1-day period, 20-30 June 1995, into the National Centers for Environmental Prediction spectral statistical interpolation analyses. The methodology for assimilating the bending angles (including an impact parameter offset correction) is briefly summarized. Verified with 56 collocated radiosonde profiles, the assimilation of only GPS/MET bending angles improves the temperature and specific humidity analysis above 850 mbar. Even though the number of GPS/MET soundings is still far less than conventional data and operational satellite soundings, our results from two continuous l 1-day data assimilation cycles demonstrate a closer fit of both GPS/MET and conventional observations to the analyses between 850 and 200 hPa when the bending angles are incorporated. Including the bending angles also results in a small but consistent improvement in the short-range (6 hours) and medium-range (1-5 days) forecast skills, especially in the Southern Hemisphere. IntroductionAs shown by the Global Positioning System Meteorology (GPS/MET) experiment [Kursinski et al., 1996; Ware et al., 1996], the availability of Global Positioning System (GPS) satellites and the development of GPS receivers carried on a Low Earth Orbiting (LEO) satellite create an opportunity for active remote sounding of the Earth's atmosphere by radio occultation technology. Phase delay measurements of GPS radio signals as they are occulted by Earth's atmosphere depend on the atmospheric refractivity, which is determined by the atmospheric temperature and humidity distributions. Atmospheric profiles of bending angle and refractivity can be provided globally in all weather conditions. At temperatures below 250 K (above 5-7 km), where the effects of water vapor are negligible, vertical profiles of temperature (dry-temperature retrieval), density, and geopotential height may be obtained using the GPS limb-sounding technique. At temperatures above 250 K in the troposphere (below 5-7 km) the contribution of water vapor to the refractivity is significant, and the accuracy of the dry temperature retrieval (which assumes water vapor is negligible) is degraded [Ware et al., 1996; Kursinski et al., 1996, 1997]. It is generally impossible to compute both temperature and moisture variables from an occultation sounding using a traditional retrieval method, an underdetermined problem.

Prospects of the EPS GRAS Mission For Operational Atmospheric Applications

“…However, when this is not feasible, the assimilation of refractivity profiles is superior to the assimilation of temperature and humidity profiles derived from the RO measurements. Positive results from assimilation experiments with bending angle profiles from the GPS/MET and CHAMP missions have been presented by Liu et al (2001) and Zou et al (2004), but these studies did not assimilate all the other satellite measurements that were available at that time. Healy et al (2005) has demonstrated a positive impact on stratospheric temperatures in a 16-day trial with the assimilation of CHAMP refractivity profiles into the operational Met Office 3DVAR system.…”

Operational NWP assimilation of GPS radio occultation data

“…Liu et al, 2001;Healy et al, 2005), GPSRO data available in near real time has not previously been operationally assimilated into NWP models. Following previous 3D-VAR data assimilation experiments at the Met Office (e.g.…”

Assimilation experiments with CHAMP GPS radio occultation measurements

“…All of these approaches have been investigated in recent forecast impact experiments. To briefly summarize, Liu et al (2001) presented results from an 11-day trial assimilating GPS/MET bending-angle profiles using a ray-tracing forward model in a threedimensional variational (3D-Var) system. Although they claimed a small but consistent improvement in the forecast scores, the results were limited because other satellite data available at that time were not assimilated.…”