C LIMATE D ATA B ASED ON GNSS R ADIO O CCULTATION M EASUREMENTS

1. CLIMATE DATA BASED ONGNSSRADIO OCCULTATION MEASUREMENTS Hans Gleisner Danish Meteorological Institute (DMI), Denmark & GRAS SAF

2. Outline–– • What is measured with the RO technique – sensor data & geophysical data • Coverage: n:o of measurements, geographical coverage, altitude range, etc. • Applications: use of RO data (NWP, climate, re-analysis) • RO missions • Role of EUMETSAT and the GRAS Satellite Application Facility

3. Measurement principle–– t2 Measurement of phase and amplitude gives a vertical atmospheric profile at the ray’stangent point. MetOp t1 t3 GPS GPS signals: L1: ≈19 cm L2: ≈24 cm

4. Atmospheric sounding with GPS RO– what is measured – 1. From phase to bending angle 2. From bending angle to refractive index Bending angle is related to the vertical gradient of the refractive index. Under the assumption of spherical symmetry, the bending-angle integral can be inverted with an Abel transform.

5. Atmospheric sounding with GPS RO– what is measured – 3. Refractivity => Pressure, Temperature, Humidity refractive index refractivity “wet” term “dry” term When the ”wet” term is negligable, the assumption of ideal gas and hydrostatic equilibrium gives {p,T} profiles

6. Atmospheric sounding with GPS RO– what is measured – 3. Refractivity => Pressure, Temperature, Humidity Temperature-humidity ambiguity in the troposphere. A statistically optimal {p,T,q} profile is found through a 1D variational procedure, using a priori information from a model, e.g. ECMWF. Background Background error covariance Refractivity: observed and background Observational error covariance

7. Atmospheric sounding with GPS RO– atmospheric profiles – L ~ 300 kilometers Z ~ 200 meters – 1.5 kilometer

8. GRAS SAF climate data– bending angle, refractivity & 1D-Var products –

9. GRAS SAF climate data– 1DVar relative to ECMWF –

10. Sampling the atmosphere with GPS RO– distribution of profiles –

11. Sampling the atmosphere with GPS RO– distribution of profiles – Longitude vs. local time Latitude vs. local time

12. RO characteristics summary–– • microwave frequencies, active sounding: independent of clouds, surface, day/night • based on time measurements, not radiances: intrinsically stable and calibration free • limb sounding: high vertical resolution (0.1-1.0 km from troposphere to stratosphere) low horizontal resolution (300 km along the ray) • global coverage: sampling all lats & lons, troposphere and stratosphere • measures refractivity: temperature/humidity ambiguity in the troposphere Important for climate: global coverage, high vertical resolution, long-term stability. An observation made today can be directly compared to an observation made 50 years from now without intermediate inter-calibrations.

13. Applications–– • Numerical Weather Prediction:–assimilated in real-time at ECMWF, Met Office, Meteo France, NCEP, NRL Monterey, HMC (Moscow), etc. –complementary to radiance measurements => RO provides bias corrections for other type of observations, which are then used more efficiently. • Re-analysis: – assimilated into current re-analysis systems: ERA-Interim, NCEP-NCAR reanalysis, and into future re-analyses: ERA-75 (the ERA-Clim project) • Climate:– climate research applications: climate change studies, climate model testing, stratospheric dynamics, tropopause, low-latitude planetary boundary layer, … – climate monitoring: alternative to MSU/AMSU temperatures

14. RO missions– past, current, and future – SatelliteInstrumentTime spanProfiles/month CHAMP BlackJack 2001-09 – 2009-08 5,000 COSMIC (FM1-6) IGOR 2006-07 – 9,000 / satellite METOP (A-C) GRAS 2007-01 – 20,000

15. EUMETSAT & GRAS SAF–– • Metop / GRAS mission: • More than 15 years lifetime (3 satellites, Metop-A launch October 2006) • Follow-on RO mission extends operations well into the 2030s • Provision of data in NRT (3 hours) and Offline (30 days) • Re-processing into climate-quality, homogeneous data sets • EUMETSAT supports a Satellite Application Facility (SAF) dedicated to RO • EUMETSAT, centrally: • Raw processing up to Level 1a (phase observations) • Processing from Level 1a to Level 1b (bending angles) in NRT and Offline • Re-processing of data from all major RO missions for ERA CLIM (FP7 project) • Continued development of RO processing methods

16. EUMETSAT & GRAS SAF–– • GRAS SAF: • Processing from Level 1a to Level 1b, 2, and 3. • Reprocessing of RO data from all available RO missions. • Current phase (2007-2012) has been focused on NRT data to NWP applications. • Next phase (2012-2017) will have a stronger focus on climate data. • Deliver data in NRT and Offline • Deliver climate data from re-processing activities (planned for 2013 and 2016) • Deliver relevant software (forward modelling, assimilation, 1D-Var retrivals, etc.) • Helpdesk functions to users http://www.grassaf.org

17. STOP––

18. GRAS SAF climate data– sampling – MetOp is in a Sun-synchronous orbit, whereas CHAMP and COSMIC slowly drift in local time. i=99 deg Sun-sync orbit (MetOp) i=72 deg precessing orbit (COSMIC) i=87 deg precessing orbit (CHAMP)

19. Atmospheric sounding with GPS RO– measurement principles – • Limb sounding: • High vertical resolution • Horizontally integrating (and moving) over 200 – 400 km • measurement of opportunity only • GPS frequencies (19 / 24 cm, 1.575 / 1.227 GHz): • Weather independent • No information gained from absorption measurements • Measurement principle: • Measuring time differences – traceable to SI standards • No instrument degradation or drift • Calibration free • Highly accurate temperature soundings (< 1 K) up to the mid stratosphere • Complementary to radiance measurements