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
λw = λ/2sinθ where, λw = surface wavelength
λ = surface projection of the radar wavelength
• For near nadir incidence angles, σ0 ↓ as U ↑
For oblique angles, σ0 ↑ 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, UHH-UVV
(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) U10 = 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
10U
*bU
2
( / 2)1
( )R
U
C
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.