2 nd GRAS-SAF USER WORKSHOP

1. Assimilation of GPS radio occultation measurements at DAO (soon GMAO) P. Poli1,2 and J. Joiner3 Data Assimilation Office (DAO) NASA Goddard Space Flight Center Greenbelt, MD 20771 USA Effective June 15, 2003: DAO becomes GMAO Acknowledgments: We would like to thank EUMETSAT and the Organizing Committee of the Workshop Some of the work presented was done in collaboration with the Radio Occultation group at JPL 1 Joint Center for Earth Systems Technology, University of Maryland Baltimore County 2 Permanent Affiliation: Meteo France Centre National de Recherches Meteorologiques, Toulouse, France 3 NASA Laboratory for Atmospheres 2nd GRAS-SAF USER WORKSHOP

2. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark OUTLINE • 1st-generation Data Assimilation System for GPS Radio Occultation: DAOGPS (1999-2002) • Review • Analysis and forecast impact studies with GPS/MET • Next-generation Data Assimilation System for GPS Radio Occultation: (2002-...) • Work in progress

3. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark 1st generation DAS for GPSRO at DAO: DAOGPS • Radiosonde-like assimilation in two steps: • 1DVAR retrievals of temperature, humidity, and surface pressure from GPS refractivity and latest available 6-hour forecast (Poli, Joiner, and Kursinski, 2002) • Assimilation of 1DVAR geopotential heights into the Finite Volume Data Assimilation System (FVDAS) (Poli and Joiner, 2003)

4. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark 1st generation DAS for GPSRO at DAO: DAOGPS Indiv. Forecast 19950630_00Z+ 19950630_06Z-19950705-00Z With assimilation of GPS/MET refractivity Anomaly Correlation CONTROL With assimilation of GPS/MET refractivity Forecast Day Anomaly Correlation CONTROL Height Anomaly Correlation at 500hPa Average over 14 forecasts 19950621-19950704 Northern Hemisphere (pre-AMSU era) Forecast Day

5. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark Next-generation DAS for GPSRO at GMAO:(1) Next-generation GPSRO data Geom. Optics Canon. Transf. Back-Prop. (Obs-Analysis) Refractivity Biases (Obs-Analysis) Refractivity Stdev.

6. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark Next-generation DAS for GPSRO at GMAO:(2) Next-generation 1DVAR retrievals Canonical Transform Refractivity data -> Reduced Bias in lower Trop.

7. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark Next-generation DAS for GPSRO at GMAO:(3) Next-generation obs. operator: FARGO Problem: slow because each step requires knowledge of the previous step and three 2D interpolations (n,dn/dr,dn/dq) Advantage: accurate solution to the forward problem • Assume: tangent point location (lat, lon) is known, occultation plane remains the same • Simulated 288 occultations with FVDAS analysis fields (1ox1.25o) and 2D ray-tracing 2D ray-tracing Integration of [dn/dr(r,q) cosq] along ray path obtained by 2D r.t. Problem: requires 2D ray-tracing to calculate ray path Advantage: once ray path is known, 2D interpolations (dn/dr only) can be done afterwards (hence parallelized) Integration of [dn/dr(r,q) cosq] along ray path obtained by 1D r.t Problem: still requires 1D ray-tracing all the way through the atmosphere (from transmitter to receiver) to get ray path Extra Advantage: ray determination is fast (requires only 1D interp) Integration of [dn/dr(r) cosq] along ray path obtained by 1D r.t Problem: does NOT account for horizontal gradients of refractivity Advantage: fast 1D inverse Abel transform Equivalent to:

8. Next-generation DAS for GPSRO at GMAO:(3) Next-generation obs. operator FARGO Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark 1200km L Integration of [(dn/dr(r,q)-dr/dn(r)) cosq] along ray path obtained by 1D r.t Integration of [dn/dr(r,q) cosq] along ray path obtained by 1D r.t + 1D inverse Abel transform • Define our Fast Atmospheric Refractivity Gradient Operator (FARGO): • Advantages of the scheme: require 1D ray-tracing along a reduced portion of the ray (many less interpolations required), and dn/dr(r,q) 2D interpolations are fast (can be parallelized) • Other advantage for data assimilation: scheme clearly separates vertical structure from the 2D correction -> possible to derive a computationally inexpensive Adjoint for vertical structure only Integration of [(dn/dr(r,q)-dr/dn(r)) cosq] along section L ray path obtained by 1D r.t + 1D inverse Abel transform

9. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark Next-generation DAS for GPSRO at GMAO:(3) Next-generation obs. operator FARGO 288 occultations simulated from DAO analyses horizontal resolution 1ox1.25o 2001-05-31 at 00GMT 1D INVERSE ABEL TRANSFORM minus 2D RAY-TRACING (errors due to horizontal gradients) FARGO minus 2D RAY-TRACING

10. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark Next-generation DAS for GPSRO at GMAO:(3) Next-generation obs. operator FARGO 288 occultations simulated from DAO analyses fields 2002-01-31 at 00GMT hor. resol. 0.5ox0.625o hor. resol. 1ox1.25o hor. resol. 2ox2.5o (retained only the rays that did not undergo critical refraction) 1D INVERSE ABEL TRANSFORM minus 2D RAY-TRACING (errors due to horizontal gradients) FARGO minus 2D RAY-TRACING

11. Poli and Joiner 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore Denmark Conclusions • Limited impact with radiosonde-like assimilation of refractivity data (biases in GPS/MET refractivities, few profiles per day, 1D approach, intermediate 1DVAR step, assimilation of temperature only) • CHAMP and SAC-C GPS RO data processed with advanced methods show better fit with DAO (GMAO) analyses • Developed a Fast model for bending angle: FARGO: (2D integral operator + inverse Abel transform) • Look forward to testing FARGO with real data in order to determine relevance of 2D approach • Future GRAS data: access to occultation geometry parameters (transformation matrix ECF 3D -> 2D plane, local center of curvature), orbital parameters