Problems and Future Directions in Remote Sensing of the Ocean and Troposphere Dahai Jeong AMP

Outline Background Introduction Problem areas - directional ocean wave spectra - ocean surface winds - a subset of atmospheric measurements - air-sea interaction Conclusion

Background Two kinds of oceanic backscatter Reflection and scattering of a radiance that is normally and obliquely incident on specular and wave-coverd ocean surface

Bragg scatter Strong oceanic backscatter for incident angles θ as up to 70º λ w = λ/2sinθ where, λ w = surface wavelength λ = surface projection of the radar wavelength For near nadir incidence angles, σ 0 ↓ as U ↑ For oblique angles, σ 0 ↑ as U ↑ Bragg scatter generated by the interaction between and incident radiance and a specific water wavelength

Introduction Purpose - identify the gaps and limitations in our ability to remotely sense the oceans and troposphere from air and space platforms

Directional Ocean Wave Spectra Lack of remote sensing data - The spatially evolving directional wave number spectrum (~1000 km at best) - The temporally evolving directional wave number spectrum (~ a few days)

Directional Ocean Wave Spectra Recent measurements - systematic spatial and temporal variability of the spectrum : impact on understanding of the physics of wind-wave generation, operational wave forecasting, ship routing, local global wave climatology, off shore tower design and coastal erosion research. - Seasat synthetic aperture radar (SAR) data - Surface contour radar (SCR) - Radar ocean wave spectrometer (ROWS)

Existing Methods and Techniques for Remote Measurement of Ocean Wave spectra

General Research Questions

Summary and Conclusions For further progress in the practical application of remote sensing techniques - Interrelationships between the ocean and the atmosphere at all levels - Analyses of data - Physical interpretation of the results for oceanographic significance - High-quality intercomparison data sets

Ocean Surface Winds Problems with the SASS algorithm Vertically polarized (VV) and horizontally polarized (HH) pairs of observation (upper panel) - discrepancies in the low-speed and high-speed ranges - agreeing only in the range 8-14 m/s (lower panel) - plot of speed difference, U HH -U VV (Woiceshyn et al.)

Basic Considerations in the Model Function Relating Radar Scattering to Ocean Surface Conditions Correct wind measure : problem of relating radar signal to wind speed - Power law or (based on friction velocity) where, σ° = scattering coefficient (NRCS) U 10 = neutral-stability wind at 10 m α,є,b,ρ = constants for a given geometry - Donelan and Pierson : σ° have better correlation with where, U(λ/2) = average wind at half Bragg wave length above the surface C(λ) = phase speed of the Bragg resonant wave

Sea State, Currents, and Internal Waves Modulations of the ripples - winds - sea state - currents - internal waves - slopes of the larger waves - rain striking the water

Experiments Needed Continuing analysis of the wealth of data from Seasat Analysis of data to be obtained with new spaceborne scatterometers in conjunction with well-designed surface comparison experiments Properly designed aircraft measurements Careful measurements from well-instrumented towers Measurements to validate the ability to correct for the attenuation of a scatterometer signal using a coincident radiometer beam

Summary and Conclusions No theory for backscatter at high wind speeds What wind to use in specifying the relation ship between wind speed and radar return Numerous measurements

Atmospheric Measurements Winds, both surface and aloft Temperature and water vapor profiles Precipitation Surface fluxes of heat, moisture, and momentum

Measurement Methods Surface winds - scaterometer - microwave radiometer - radar altimeter Winds at altitude - airborne Doppler radar (ADR) - spaceborne Doppler lidar Water vapor - passive microwave remote sensing Precipitation - spaceborne visible or IR sensors

Air-Sea Interaction Physics behind the correlation between surface winds or stress and the microwave signal Planetary boundary layer: solar insolation is transferred to the atmosphere to drive the general circulation

Air-Sea Interaction Important factor in scales - stratification in the PBL Mesoscale phenomena - heat fluxes Wave spectrum - microwave wind detection algorithms

Conclusion Remote sensing of the atmosphere and the ocean has demonstrated some of what can be done, but it has also illuminated vast areas of ignorance.