1. Status of EUMETSAT Study on Radio Occultation

2. Study Background
For years it was unclear whether there is a saturation number of radio occultation (GNSS-RO) observations, beyond which the NWP impact would be negligible
In October 2011, ESA initiated a study with ECMWF on “Estimating the optimal number of GNSS radio occultation measurements for Numerical Weather Prediction and climate reanalysis applications”:
Use “Ensemble of Data Assimilations” (EDA) analyses approach to estimate optimal number of GNSS-RO. Technically not OSSEs because it only uses simulated RO data, all other data real.
Used up to 128,000 simulated observations per day (currently about 3,000 real observations)
Conclusion: 16,000 occultations per day gives 50% improvement of 128,000 observations
Limitation: assumed randomly distributed events in time and space
Study results led to recommendations at IROWG and WMO workshops to aim for a constellations to make 10,000 to 16,000 GNSS-RO observations operationally available per day

3. EUMETSAT Study Overview
To address the random distribution limitation, EUMETSAT initiated a study on “Impact of different Radio Occultation Constellations on NWP and Climate Monitoring” in Proposal received from ECMWF was selected, study was kicked-off in March Main study aims:
Work with simulated LEO and GNSS orbits for realistic occultation distribution
Identified different scenarios to address the following questions:
Refine earlier ESA/ECMWF study with realistic future satellite orbits
Assess best observation constellation to achieve best distribution in space and time
Provide guidance on RO instrument deployments on future LEO satellites
Study duration 18 month, final results expected Q4/2015
Addresses CGMS Actions/Recommendations:
Plenary IV.4 A40.06
WGIII/2.2 A41.36
WGII A40.23
WGIII/2.1 R41.14
Main ECMWF scientists involved: Sean Healy, Andras Horanyi

4. Preliminary Study Results: Comparison Setup
Comparison of Scenario 08, 09 and 11 with no RO available:
Scenario 08:
8 LEO Satellites: EPS-SG A1 & B1, COSMIC-2 Eq; GNSS: 30 GPS, 27 Galileo
About 10,000 occultations/day
Address possible scenario when COSMIC-2 Polar is not funded and launched
Scenario 09:
14 LEO Satellites: EPS-SG A1 & B1, COSMIC-2 Eq, COSMIC-2 Po; 30 GPS, 27 Galileo
About 18,000 occultations/day
Address possible scenario when COSMIC-2 Polar is funded and launched
Scenario 11:
10 LEO Satellites: EPS-SG A1 & B1, COSMIC-2 Eq, RO-Night, RO-Early Morning; 30 GPS, 27 Galileo
About 13,000 occultations/day
Address whether RO instruments on operational orbits can replace COSMIC-2 Polar
No RO:
No RO instruments available
Notes:
COSMIC-2 will observe GLONASS, for simplicity we assumed Galileo here, since this is used for EPS-SG. Distribution is very similar between Galileo and GLONASS
RO-Night and RO-Early Morning are assumed to carry RO instruments, could be provided by e.g. FY3 or research satellites.
assumed about 1200 occultations are available per day per RO instrument observing 2 GNSS (GPS, Galileo), a bit on the optimistic side

5. Preliminary Study Results: Comparison
Global temperature improvements for different RO observing scenario against one with no RO instrument available. Scenarios shown:
sc08: EPS-SG A1,B1; C2-E
sc09: EPS-SG A1,B1; C2-E; C2-P
sc11: EPS-SG A1,B1; C2-E; RO-N; RO-EM
noro: No RO instrument assimilated

6. Preliminary Study Results: Conclusions / Outlook
a full COSMIC-2 constellation gives the highest temperature improvements
COSMIC-2 Equator plus RO sensors on all the 3 operational polar orbits would give a bit less than half of the improvement of having the full COSMIC-2 constellation
Outlook:
final study results in Q4/2015
will include impact of GLONASS, Beidou