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.