Modeling of Passive Microwave Measurements of Surface Wind Speed in Hurricanes

1. Modeling of Passive Microwave MeasurementsofSurface Wind Speed in Hurricanes Salem El-Nimri CFRSL Dec. 10th , 2005

2. Presentation Outline • Overview • Approach • RadTb and SFMR Comparisons • Development of nadir wind speed model. • Development of Incidence angle dependence. • Summary

3. Overview • Hurricane Imaging microwave Radiometer (HiRad) is a proposed airborne/satellite remote sensing instrument for use in hurricane surveillance. • HiRad is an extension of Stepped Frequency Microwave Radiometer (SFMR) remote sensing that has successfully measured surface wind speed and rain rate in hurricanes. • Unlike SFMR, which measures only at nadir, HiRad provides wide swath measurements of ±45 degrees incidence cross-track. • The objective of this work is to develop an improved wind speed model for the CFRSL radiative transfer model, RadTb, based on SFMR measurements, to support HIRad analyses. • Improved at high, hurricane force wind speeds. • Improved incidence angle dependence.

4. Approach • Tune RadTb at nadir using SFMR hurricane measurements at high wind speeds • Use experimental foam measurements from Melville (2005). • Develop a physically realistic emissivity model in terms of foam fraction and emissivity of foam. • Determine foam emissivity dependence on frequency and incidence angle. • Model off-nadir Tb’s from aircraft banks in hurricanes Fabian 2003 and Katrina 2005 to derive incidence angle dependence for V and H pol., respectively.

5. SFMR Emissivity(Uhlhorn & Black)( new model)

6. SFMR and RadTb Comparison RadTb and SFMR Comparison at 4GHz wso<=33.2 eSFMR= (a1*(ws-wso)+a2*(ws-wso)2)*(1+.15*f) wso>33.2 eSFMR= a1*(ws-wso))*(1+.15*f) eRadTb= FOM*Efoam+(1-FOM)*Erough FOM = Foam Fraction Efoam = Emissivity of Foam Erough = Rough Surface Emissivity

7. RadTb Deficiencies Foam Fraction Characteristic Emissivity Characteristic at 4 GHz • Foam Fraction exceeds 100 % at 38 m/sec. • Emissivity exceeds 1.0 at 45 m/sec.

8. Foam Emissivity • Need a formula for the emissivity of foam that is of the form: Efoam=Efoam_freq*Efoam_IE Just as currently in RadTb, from Stogryn. • The Efoam_freq term was modeled using foam data from Melville, et.al. and the nadir looking SFMR wind speed model.

9. Monahan and others (Wu, 1988)

10. Foam Emissivity Frequency Dependence • Using , Etotal=FOM*Efoam+(1-FOM)*Erough Where, Foam fraction = 100% @ 85 m/sec. and, Efoam_IE=1 Efoam_freq=a0+a1*f, f in GHz.

11. Foam Emissivity Frequency Dependence

12. Foam Fraction Formula • Foam fraction is a function of WS only. • Used the SFMR wind speed forward model and, Etotal=FOM*Efoam+(1-FOM)*Erough • Apply MLE testing principle we get, FOM= a3*ws3 + a2*ws2 +a1*ws + a0

13. Difference between SFMR and New CFRSL

14. Typical Turn Data

15. Sample Flight Path

16. Measured and Modeled Emissivity

17. Measured and Modeled Curve Fits

18. Measured and Modeled Bias Removed

19. Summary of resultsFabian Sept. 2, 2003 Data set Results 11 turns. 21 banks. WS=7-41 m/sec. EIA=0-35 deg. Accuracy of curve fit, Measure= 0.005-0.01 Modeled= 0.0005-0.005 Measure vs modeled bias, 0.002-0.02 Max difference Measured vs Modeled, 0.001-0.006

20. Summary • Developed a wind speed model for RadTb that is useful at C-band up to 70 m/sec. at nadir. • Conducting analyses to define incidence angle dependence for H and V polarizations up to 35 deg. • Future Work: • Composite incidence angle dependence for Efoam_ang . • Study measured vs modeled bias • Do the antenna pattern correction • Find the H pol dependence using Katrina