Forecast impact experiment with GPS radio occultation measurements

Abstract

[1] Refractivity profiles from CHAMP GPS radio occultation measurements have been assimilated into the Met Office numerical weather prediction (NWP) system. A forecast impact experiment was run using 16 days of CHAMP data from May/June 2001, in addition to conventional and satellite observations which are used in operational forecasts. Although typically only 160 CHAMP profiles are assimilated daily, it is demonstrated that they reduce NWP temperature analysis and forecast mean and root mean square (RMS) differences against radiosondes in the upper troposphere and lower stratosphere. A significant impact is found in the Southern Hemisphere where 24 to 96 hour forecast 250 hPa temperature RMS differences are reduced by $0.1 K. No improvement in humidity forecasts is found because refractivity values below 4 km are not used in the experiment. These results are encouraging and would support the case for using CHAMP measurements in NWP if they were available in near real time. Citation: Healy, S. B., A. M. Jupp, and C. Marquardt (2005), Forecast impact experiment with GPS radio occultation measurements, Geophys. Res. Lett., 32, L03804,

Operational NWP assimilation of GPS radio occultation data

“…The model has 50 vertical levels extending to approximately 65 km, with a horizontal resolution of ∼40 km in mid-latitudes (N320 L50), and employs a 4D-VAR data assimilation system (Lorenc et al, 2000). This is in contrast to the results described in Healy et al (2005), where the model had a resolution of only 60 km in mid-latitudes with 38 vertical levels extending to approximately 40 km and a 3D-VAR data assimilation system. Two main forecast cycles out to T + 120 are run per day, from analyses valid at 00Z and 12Z, with update cycles at 06Z and 18Z, each cycle having an assimilation time window of 6 h centred on the analysis time.…”

“…The refractivity forward model employed for the operational assimilation is conceptually the same as that described in Healy et al (2005). We have assumed an observation error similar to that suggested by Steiner et al (1999) for the CHAMP satellite but with a correlation length fixed to 1.5 km for the whole profile.…”

“…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.…”

“…Following previous 3D-VAR data assimilation experiments at the Met Office (e.g. Healy et al, 2005), further forecast impact experiments for two different seasons were performed as a final test of GPSRO's impact and to ensure the robustness of the 4D-VAR assimilation system before the operational implementation. On 26 September 2006 the Met Office began assimilating CHAMP refractivity profiles in its operational global NWP model.…”

Assessment of global navigation satellite system (GNSS) radio occultation refractivity under heavy precipitation

“…Usually, these a priori values are obtained from global meteorological analyses or reanalyses. On the other hand, bending angle and refractivity profiles are directly assimilated into NWP (e.g., Healy et al, 2005;Cucurull et al, 2007), with a high positive impact in the weather forecasts (Cardinali and Healy, 2014). …”