1. Surface Pressure Measurements from the NASA Orbiting Carbon Observatory-2 (OCO-2) Presented to CGMS-43 Working Group II, agenda item WGII/10 David Crisp Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA Kenneth W. Jucks NASA Headquarters, Washington D.C.

2. The Orbiting Carbon Observatory-2 (OCO-2) Mission
The primary objective of the OCO-2 mission is to make the space based measurements needed to estimate the carbon dioxide (CO2) column averaged dry air mole fraction, XCO2, with the precision, accuracy, resolution, and coverage needed to quantify CO2 sources and sinks on regional scales.
OCO-2 was successfully launched on 2 July 2014, and joined the 705 km Afternoon Constellation (A-Train) a month later. The instrument was then cooled to its operating temperatures and began collecting about 1 million soundings over the sunlit hemisphere each day.
While optically-thick clouds and aerosols preclude full-column measurements for many soundings, between 10 and 25% (100,000 to 250,000/day) are sufficiently cloud free to yield accurate estimates of XCO2.
While XCO2 is the primary product returned by OCO-2, it returns other meteorological variables as well, including surface pressure, ps, surface winds over the ocean, and column water vapor, [H2O].

3. XCO2 = 0.20935  NCO2 dz /  NO2 dz = 0.20935 [CO2] / [O2]
Retrieving XCO2 and ps from Remote Sensing Observations
XCO2 is defined as the ratio column-integrated CO2 number densities NCO2 and the column density of dry air, which can be estimated from the column-integrated molecular oxygen (O2) number densities, NO2:
XCO2 =  NCO2 dz /  NO2 dz = [CO2] / [O2]
Estimates of [CO2] can be retrieved from precise, high resolution spectra of reflected sunlight in the shortwave infrared CO2 bands at 1610 and 2060 nm. Similarly, estimates of [O2] can be retrieved from high resolution spectra of reflected sunlight in the O2 A-band at 765 nm.
O2 A-band measurements are combined with [H2O] from the 2060 nm to yield estimates of surface pressure.
765 nm
1061 nm
2060 nm

4. Coverage Provided by Nadir and Glint Observations
OCO-2 currently alternates between nadir and glint observations on successive16-day ground track repeat cycles.
Nadir observations provide better coverage of land
Glint observations provide better coverage of ocean
A different nadir/glint strategy that alternates between glint and nadir on successive orbits is currently under development

5. Preliminary Surface Pressure Soundings
Difference between the surface pressure estimates retrieved from OCO-2 measurements and the ECMWF prior are shown for the ~15 orbit tracks observed on 10 April The width of the individual tracks has been expanded to show results for the 8 cross-track footprints. The actual width of these tracks is less than 10 km.
Histogram of difference between the surface pressure estimates retrieved from OCO-2 measurements and the ECMWF prior are shown for the ~15 orbit tracks observed on 10 April 2015.

6. PRELIMINARY CONCLUSIONS AND RECOMMENDATIONS TO CGMS
While these early OCO-2 surface pressure results do not yet meet the requirements of NWP applications, they are comparable to the very best results obtained from earlier satellite measurements.
As the instrument calibration is refined and retrieval algorithm improves, we expect to reduce the overall bias to close to zero, and the standard deviation to 1-2 hPa.
These data should be of most value for complementing the existing measurements in data poor regions (polar regions, tropical continents, ocean basins)
The next step will be to adapt a NWP reanalysis model to assimilate these data into assess their impact on weather forecasts.