Evaluation of Microwave Scatterometers and Radiometers as Satellite Anemometers Frank J. Wentz, Thomas Meissner, and Deborah Smith Presented at: NOAA/NASA Workshop Satellite Measurements of Ocean Vector Winds Present Capabilities and Future Trends February 8-10, 2005

Wind Direction Evaluation WindSat EDR Produced July 2004 using NOAA-NESDIS/NRL Algorithm (Jelenak) Rain is Excluded from the Analysis Wind Speed Evaluation Capability of Satellite Microwave Radiometers is Well Demonstrated Midori-2 Flew both Scatterometer and Radiometer  Moored Buoys: TAO. PIRATA, NDBC, MEDS  QuikScat: RSS Version  NCEP FNL Analysis (GDAS) Two Type of Evaluations

Active Versus Passive Wind Speed Retrievals: The Physics  Scatterometer measures backscatter from capillary waves (L = /(2sin  )  1 cm)  Radiometer measures polarization mixing and sea foam emission

Active Versus Passive Wind Speed Retrievals: Regional Biases Eliminated Old Sea-Surface Emissivity Model New Sea-Surface Emissivity Model

SeaWinds and AMSR Comparison with Moored-Buoy Data AMSR – Buoy Wind Speeds Size: Standard Deviation SeaWinds - Buoy: mean speed bias m/s std deviation 1.1 m/s mean direction bias 1.23 deg std deviation 16.7 deg AMSR – Buoy: mean speed bias m/s std deviation 0.97 m/s

Conclusions On Wind Speed  The active and passive winds speed are in very close agreement. For individual observations at 25-km resolution rms difference between SeaWinds and AMSR is 0.78 m/s.  Passive wind speeds agree well with buoys (0.97 m/s standard deviation)  The passive winds are more sensitive to rain than active retrievals  The passive winds are more sensitive to nearby land  The passive winds are degraded when RFI is present (only 7 and 11 GHz so far)

The Active and Passive Directional Signal at Low Winds

Buoy Comparisons Collocation Window = 30 minutes and 12.5 km Triplets = WindSat + NCEP + Buoy Quadruplets = WindSat + QuikScat + NCEP + Buoy NDBC TAO/TRITON PIRATA MEDS

Table 1: Wind direction statistics for wind speed range from 3 to 5 m/s for buoy comparisons. Table 2: Wind direction statistics for wind speed range from 5 to 25 m/s for buoy comparisons. WindSat Compared to Buoys: Tables

Wind direction retrievals for WindSat (left) and NCEP (right) versus buoy wind direction. These results are for a wind speed range from 5-25 m/s and show the triplet collocations. A total of 1.56% of the WindSat observations and 0.37% of the NCEP values have wind directions that differ by more than 90  from the buoy direction WindSat and NCEP Compared to Buoys WindSat NCEP

WindSat, NCEP, and QuikScat Compared to Buoys WindSat NCEP QuikScat

5-6 m/s, WindSat minus buoy rms = 28 o Standard deviation of the difference between the WindSat and buoy direction (black curve) and the QuikScat and buoy direction (red curve). These results show the quadruplet collocations for which both WindSat and QuikScat see the same buoy at nearly the same time. The faint dashed line shows the wind direction requirement of a 25  -accuracy for wind speeds from 3-5 m/s and a 20  -accuracy for wind speeds above 5 m/s. If either the WindSat direction or the QuikScat direction differed by more than 90  from the buoy direction, the collocation was excluded. Head-to-Head Comparison

Table 1: Wind direction statistics for wind speed range from 3 to 5 m/s for global comparisons. Table 2: Wind direction statistics for wind speed range from 5 to 25 m/s for global comparisons. WindSat Compared to QuikScat: Tables Collocation Window = 60 minutes and 12.5 km

Bias (dashed curves) and standard deviation (solid curves) for wind direction differences between WindSat – NCEP (blue), WindSat – QuikScat (red) and QuikScat – NCEP (green). Outliers having directional differences greater than 90  are removed from these statistics. WindSat Compared to QuikScat: Plots

Individual Buoy Histograms Time Series Cross Talk Report Available containing Additional Details Assessment of the Initial Release of WindSat Wind Retrievals

Wind direction bias (dotted line) and standard deviation (solid line) for the WindSat first-ranked wind vector versus NCEP. The NOAA-NESDIS algorithm is shown in red, and the RSS algorithm is shown in blue. New and Improved Algorithms RSS CMIS Algorithm Versus WindSat First Rank Ambiguity NOAA\NESDIS

Radio Frequency Interference European TV Satellites Broadcasting at 10.7 GHz 80 K Excess Brightness Temperature Error in Retrieved Wind Speed

CMIS Compared to WindSat Channel Sets

Radiometer Bandwidths Band [GHz]WindSat BW [MHz]CMIS BW [MHz] Swath Width WindSat swath =1000 km CMIS swath = 1500 km CMIS Compared to WindSat

Future Technology Advancements Passive Technique Internal calibration/dedicated spacecraft allowing full 360 o view 2-look, fully polarimetric Marginal improvement Still limited by low winds, rain, RFI, spatial resolution Little or no signal below 5 m/s. Active Technique Dual frequency (Ku and C-band) Fully Polarimetric Enhanced spatial resolution Significant improvement particularly in storms and hurricanes

 Wind-direction retrieval accuracy strongly depends on wind speed  For winds less than 6 m/s, the WindSat directions error exceeds 20 o  For winds above 8 m/s, WindSat accuracy close to QuikScat  New and improved algorithms may results in better performance  But, below 5 m/s there is little if any signal  RFI will be a problem for the passive technique in some areas  Rain and land effects need closer study  WindSat performance needs to be mapped to CMIS Conclusions on Wind Direction from Passive Microwave Radiometry