New sensors and models for complex environmental conditions

John N. PorterUniversity of Hawaii

If one cannot accurately measure the in situ optical propertiesof the environment, then developing a satellite remote sensing algorithm is like trying to hit a target with poor vision

1) Hard to see the target (you will use inadequate/incorrect algorithms),

2) Hard to see if you hit the target (difficult to determine if your results are correct)

While new satellite algorithms are moving forward, more effort is needed for environmental optical characterization.

What is still missing in our environmental optical toolbox ?

1) Aerosol Polar Nephelometer

2) Automatic Sun-Sky Photometer for use on ships and aircraft

3) Monte Carlo radiative transfer programs which can deal with land, ocean and atmosphere inhomogeneity

4) All-sky camera network to map out spatial distribution of clouds

We are now working on these problems.

Ground-based aerosol polar nephelometerModified to make polarization measurements

Field measurements planned for summer.

Sea salt phase function (unpolarized light)

UAE dust/pollution phase function(vertical and depolarization)

Mini custom polar nephelometer - Tried various approaches with low cost components - System still under testing with more expensive components

Porter, J.N. A. Clarke, J. Reid, G. Shaw, H. Maring, E. Reid, D. Kress, Handheld

Sun Photometer Measurements From Light Aircraft, J. Atmos. Ocean. Tech., 24, 1588-1597, 2007.

Aircraft handheld sun photometer measurements

Many bad values must be removed!

Agreement with ground Cimel

AircraftHeight

WebCam Sun-Sky Photometer (WCSSP) (for ships and aircraft) Basic design already tested, now incorporating faster components

Calibration of new automatic sun-sky photometer (at MLO, Hawaii)

Beers Law

I = Io exp(- τ /cos(θ)

(define AirMass = 1/cos(θ) )

V = Vo exp(-τ AirMass)

ln(V) = - τ AirMass + ln(Vo)

determine Vo by extrapolating to zero air mass

New sun-sky photometer system tracked sun automatically on several days with excellent results. The concept of using webcams for sun alignment works well. The System upgrades for faster performance are near completion.

A new Monte Carlo radiative transfer model (AO3D). To study light fields in complex

cases with inhomogeneous conditions

Example of complex coastal site (UAE 2, MAARCO site)

AO3D tracks photons through the atmosphere-ocean and uses Monte Carlo techniques to solve radiation problems.

AO3D accounts for:ocean surface roughness and whitecapsmultiple aerosol layersrefraction (index of refraction layer changes)earth curvatureuser specified optical properties

AO3D compares well with Modtran4COARTKattawar and Adams

Bates, D. and J. Porter, AO3D: A Monte Carlo Code for Modeling of Environmental Light Propagation, accepted in Journal of Quantitative Spectroscopy and Radiative Transfer, January 2008.

AO3D example of photons entering the ocean surface (laser beam)

(double click to start movie)

AO3D model of laser beam entering ocean.

Points show where photon was absorbed

0 100 200 300 400 500 600 700 800 900 1000-14

-13

-12

-11

-10

-9

-8

-7

Time (ns)

Base-1

0 L

ogarith

m o

f Lid

ar

Dete

cto

r F

luore

scent

Sig

nal P

ow

er

Standard Lidar Eq. (green)Monte Carlo simulation (blue)

Monte Carlo time-resolved calculations.AO3D compared with lidar equation.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0 20 40 60 80 100

Viewing Nadir Angle

Rad

ian

ce (

W/m

^2/s

r/u

m)

Adams & Kattawar

AO3D

Calculations of total-scatter TOA radiance for spherical-shell molecular atmosphere with no surface reflection.

AO3D top of the atmosphere radiance compared with Kattawar and Adams (1978)

Kattawar and Adams (circles)AO3D (triangles)

Calculation of height of aerosol layer over Mauna Loa Observatory

-2.00

-1.80

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0 5 10 15

Airmass

ln(V

)

Measured Langley

ln(V) all aerosol 15-30 km

ln(V) 0.0253 OD layer 4-5 km

Sun photometer measurements made at Mauna Loa Observatory using new automated sun tracking sun photometer out to air mass20 (small dots). Larger dotsshow expected values calculated with AO3D MonteCarlo radiation model for aerosol layer placed at differentheights.

Best agreement isfound when aerosol layer is placed slightly above the observatory. Only coarse aerosolpositioning was attempted. Furtherstudies could provide a better fit. Bates and Porter, 2007

AO3D Monte Carlo code showing that an inhomogeneous surface has a significant impact on sky radiance.

Tropical Atmosphere

Sulfate Aerosols

SZA = 60deg

Figure above shows sky radiance for a coastal sitewith part of the sky over land and the other over ocean. Three different aerosol optical depths are shown. Surface inhomogeneity turns out to significantly affect the sky radiance for all aerosolloadings ! (unpublished results)

Land | Ocean

Example of Azimuth Angle Scan

Cloud Mapping Stereo Camera System

Cloud cover, cloud shape, cloud microphysical properties, and location all affect surface and satellite radiation measurements. In order to model these light fields it is therefore important to quantify cloud properties as much as possible.

In addition to radiation problems, there is also a need for new wind measurements aloft where little data exists. Cloud tracking can be used to derive winds aloft.

For these reasons we have begun testing a new approach to map out cloud fields and to derive wind fields at different heights using ground based stereo cameras.

Example of clouds passing over Honolulu. (double click image)

Calculation of relative wind speed based on feature tracking with spatial correlation.

Wind vectors calculated from two images and overlain on one of the images.

Cloud simulation model to test how far apart the stereo cameras need tobe and what the accuracy is needed for camera azimuth and zenith angles.(work in progress)

In order to derive cloud position accurately from stereo cameras, we need to know the camera internal and external calibrations (azimuth and zenith angles for each pixel). Internal calibration was carried out with a reference pattern and an example is shown on the right.

External calibration is achieved by external reference points. To be discussed in detail in the future publication.

In order to test the camera internal calibration we carried out a set of independent measurements using a pan-tilt system. A single bright light source was placed ~80 m away and the camera was panned and tilted under computer control. Preliminary results are shown below. The error seen in the azimuth angle figure is likely due to cases with near zero zenith angle (azimuth angle poorly defined). (work in progress)

Camera on pan-tiltSystem.

Two different camera angular calibration approaches plotted versus each other.

Conclusion

Good progress is occurring in each of these 4 areas and new papers will be appearing soon.

• Aerosol Polar Nephelometer

2) Automatic Sun-Sky Photometer for use on ships and aircraft

3) Monte Carlo radiative transfer programs which can deal with land, ocean and atmosphere inhomogeneity

4) All-sky camera network to map out spatial distribution of clouds and wind speeds.