1. 2 nd GRAS-SAF USER WORKSHOP Assimilation of GPS radio occultation measurements at DAO (soon GMAO) P. Poli 1,2 and J. Joiner 3 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

2. OUTLINE  1 st -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 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner

3. 1 st 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) 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner

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

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

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

7.  Assume: tangent point location (lat, lon) is known, occultation plane remains the same  Simulated 288 occultations with FVDAS analysis fields (1 o x1.25 o ) and 2D ray-tracing Next-generation DAS for GPSRO at GMAO: (3) Next-generation obs. operator: FARGO 2D ray-tracing Integration of [dn/dr(r,  ) cos  along ray path obtained by 2D r.t. Integration of [dn/dr(r,  ) cos  along ray path obtained by 1D r.t Integration of [dn/dr(r) cos  along ray path obtained by 1D r.t Equivalent to: 1D inverse Abel transform Problem: slow because each step requires knowledge of the previous step and three 2D interpolations (n,dn/dr,dn/d  ) Advantage: accurate solution to the forward problem 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) 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) Problem: does NOT account for horizontal gradients of refractivity Advantage: fast 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner

8. 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,  ) 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 Next-generation DAS for GPSRO at GMAO: (3) Next-generation obs. operator FARGO Integration of [(dn/dr(r,  )-dr/dn(r)) cos  along ray path obtained by 1D r.t + 1D inverse Abel transform Integration of [dn/dr(r,  ) cos  along ray path obtained by 1D r.t Integration of [(dn/dr(r,  )-dr/dn(r)) cos  along section L ray path obtained by 1D r.t + 1D inverse Abel transform 1200km L 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner

9. 1D INVERSE ABEL TRANSFORM minus 2D RAY-TRACING (errors due to horizontal gradients) FARGO minus 2D RAY-TRACING Next-generation DAS for GPSRO at GMAO: (3) Next-generation obs. operator FARGO 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner 288 occultations simulated from DAO analyses horizontal resolution 1 o x1.25 o 2001-05-31 at 00GMT

10. 1D INVERSE ABEL TRANSFORM minus 2D RAY-TRACING (errors due to horizontal gradients) FARGO minus 2D RAY-TRACING 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner hor. resol. 0.5 o x0.625 o hor. resol. 1 o x1.25 o hor. resol. 2 o x2.5 o Next-generation DAS for GPSRO at GMAO: (3) Next-generation obs. operator FARGO (retained only the rays that did not undergo critical refraction) 288 occultations simulated from DAO analyses fields 2002-01-31 at 00GMT

11. 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 2nd GRAS SAF User Workshop, 11-13 June 2003 Elsinore DenmarkPoli and Joiner