The Next Generation Ocean Vector Wind Mission

1. The Next Generation Ocean Vector Wind Mission E. Rodríguez, B. Stiles, S. Chan, Y. Gim, S. Durden, D. Fernandez, M. Spencer Jet Propulsion Laboratory California Institute of Technology Miami, June 7, 2006

2. Next Generation OVWM Science Goals • Quantifying the variability of vector wind forcing over the oceans and land radar cross section at synoptic (few days), seasonal, annual, and decadal time scales • Understanding the interaction between winds, sea surface temperature and ocean mixing • Generating the inputs required to drive future climate models • Current Numerical Weather Prediction models have insufficient accuracy and effective resolution to serve as inputs for ocean circulation models • Obtaining ocean vector wind data at high resolution • Coastal regions have high societal impact • Coastal upwelling governs nutrient availability (key for fisheries). Coastal winds force storm surges and influence trajectories of hazardous material spills and incapacitated vessels • Coastal winds are dominated by short cross-shore scales • Small-scale wind features determine the surface location and time evolution of atmospheric fronts • Providing unique measurements for the monitoring of marine storm development and genesis • Providing 1 km resolution land and sea ice radar images at high temporal resolution to better understand the changes in sea ice, snow melt, and flooding hazards.

3. Design Options Considered • SAR’s high resolution winds • Medium Earth Orbit Scatterometers (MEOScat) • Multi-frequency active-passive Ocean Vector Wind System

4. Side-Looking SAR High Resolution Winds • Wide-Swath SAR can produce high resolution (~300m) wind speed measurements • Wind direction inferred from wind roll features • A promising technique. But… • Conventional SAR limitations set swath limits to be ~500 km (single side) • Typical temporal resolution: ~72 hours • Data rate/volume limitations make this technique (at the highest resolution) impossible to apply globally at this time (and probably in the next decade) • Many research questions remain before high accuracy wind vectors can be produced operationally with the desired accuracy Beal et al. 2004, High resolution wind monitoring with wide-swath SAR, Users Guide

5. Is Highest Resolution SAR Necessary? With apologies to Beal et al. 2004, High resolution wind monitoring with wide-swath SAR, Users Guide

6. + MEOScat: Attempting Faster Sampling MEO (1500 km) medium resolution (10km) • MEOScat (Medium Earth Orbit Scatterometer) is a design concept that attempts to improve temporal resolution by going to a higher orbit => bigger swath • Due to limitations due to the Earth curvature, the optimal orbit is a modest increase from 800km to 1500 km • Typical temporal resolution drops from about 18 hours (QuikSCAT) to about 12 hours • However the spatial resolution which can be achieved is limited: • SAR resolutions require larger apertures • Longest time to make a wind measurement is ~ 12 minutes (QuikSCAT ~5 minutes) • Expected resolution limit for MEOSCAT: ~10 km • To include C-band (for high wind speeds) would require antennas sizes ~11m • Higher altitudes require more expensive launch vehicles

7. Temporal Sampling with Constellations Inertial Motion Sampling Requirement By combining satellites, it is possible to decrease revisit periods significantly. However, achieving a 6 hour typical revisit period will require ~3 satellites. To decrease this to 1-3 hours requires a significant constellation. From Milliff et al., OceanObs99, 1999

8. A Compromise Solution: Circular Scanning Synthetic Aperture Radar

9. A Compromise Solution: Circular Scanning Synthetic Aperture Radar Due to antenna rotation, the SAR antenna size is smaller (degraded resolution) but the swath is larger (improved revisit time) and a full wind vector is obtained.

10. Next Generation Scatterometer Concept • Philosophy: incremental technology advance leads to order-of- magnitude science and operations advance • Build on the heritage of the QuikSCAT and SeaWinds pencil beam scatterometers to reduce cost and risk and preserve wide-swath and high temporal sampling capabilities • Increase antenna size (to 2.5 m) and add onboard processing capability to achieve 1km - 5km spatial resolutions while maintaining the wide swath and frequent temporal sampling of legacy systems • Add C-band channels to enable accurate measurement of ocean vector winds over the full range of wind and rain conditions, allowing the monitoring of maritime storms and tropical cyclones, and providing improved estimates of soil moisture, snow cover on land, and sea ice boundaries and conditions • Add passive radiometer channels to improve wind vector retrieval, backscatter correction, and estimation of rain

11. High-Resolution System Description • Ku-band frequency (same as QuikSCAT/SeaWinds) • 1 km - 5 km true spatial resolution • Order of magnitude improvement in resolution is enabled by SAR onboard processing • Unfocused SAR processing does not present a technological challenge • Onboard SAR processing reduces the data rate so that it can be accommodated by two ground stations • Onboard processing reduces data latency • SAR processing enables accurate measurements near land, which is difficult using real aperture (e.g., QuikSCAT) systems • SAR resolution cell is a true resolution: no deconvolution processing is required • Nadir cells cannot be processed using SAR technique, but use super-resolution processing (a la D. Long, BYU) • Resolution is improved relative to QuikSCAT due to finer range resolution and larger physical antenna

12. Example of QuikSCAT Undersampling Simulated high-res wind field QuikSCAT Sampling of Simulated Field High resolution wind field example: winds associated with a squall line generated by numerical modeling. Wind vector posting: 2-km spacing.

13. Next-Generation Sampling Example Simulated high-res wind field Next-generation Sampling of Simulated Field High resolution wind field example: winds associated with a squall line generated by numerical modeling. Wind vector posting: 2-km spacing.

14. Next-Generation Performance:Moderate to High Wind-Speeds

15. Next-Generation Performance:Moderate to High Wind-Speeds

16. Rationale for Active-Passive Combination • Multi-channel passive instrument can provide crucial information to flag scatterometer data for rain and help develop rain corrections • Passive signatures are also sensitive to wind speed and direction and can help improve ocean vector winds estimates • Passive channels provide key complementary science measurements for studying ocean and land microwave signatures

17. Impact of Passive Channels on Hurricane Wind Speed Rain Correction Tropical cyclones are responsible for a significant fraction of ocean mixing. The mixing is driven by the wind stress curl through Ekman pumping. Results using the latest processing of SeaWinds on Adeos-2 together with AMSR show the improvements which can be obtained by combining active and passive measurements of hurricanes. These results indicate the approach for fulfilling requirements for future ocean vector wind measurement systems. Hurricane Fabian Wind stress curl Courtesy of R. Milliff & J. Morzel, CoRA

18. Using AMSR for Rain Flagging and Correction AMSR Rain High Resolution Winds (white barbs are rain flagged) Hurricane Isabel (16 September, 2003) Data courtesy of JAXA and D. Long (BYU)

19. Rationale for Multi-Frequency Scatterometer • Ku-band scatterometers have limited performance under rain or high wind speeds (e.g., hurricanes) • C-band scatterometers have limited performance at lower wind speeds and problems achieving high resolution wind measurements • Combination of Ku and C-band measurements leads to a system with significantly better performance and resolution at all wind speeds and rain conditions • See presentations by S. Yueh and D. Fernandez

20. Comparison of SeaWinds/QuikSCAT with Next Generation Scatterometer Example configuration for scatterometer with a rotating 2.5m antenna. Actual configuration will depend on the selected spacecraft bus capabilities.

21. Conclusions • The technology is at hand to be near many of the NOAA accuracy and spatial resolution requirements with modest changes to current technology • The instrument cost and resource implications, relative to QuikSCAT are moderate • The proposed instrument combines the best of existing technologies: • Ku/C-bands active scatterometer • Multi-frequency passive channels • Meeting the temporal sampling requirements cannot be done with a single platform • An optimal configuration will probably involve 2-3 satellites with potentially different orbit planes and/or inclinations