Earthshine blog

Earthshine blog

"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Is Earth Lambertian?

From flux to AlbedoPosted by Daddy-o May 06, 2013 09:17AM

But we - and BBSO - calculate albedo by comparing the earthshine measured on the Moon to the intensity of earthshine predicted by a terrestrial model based on a uniform Lambert sphere.

We test how well this works by taking a series of GERB satellite data for several weeks across a year and extract the total flux from the whole-disk images. The MSG satellite bearing the GERB instrument floats over lon,lat=0,0 so always sees the same part of Earth. Johanne has extracted images for every fifteen minutes for several weeks in a year. We plot that (top panel, below).

We use the eshine synthetic code Hans wrote to generate Lambertian images for a full cycle of sunrise over earth. As the code is based on what the Earth looks like from the Moon we pretend that one month is like one day and thereby can extract the phase law for the Lambertian uniform-albedo Earth in order to compare it to the GERB data. We also plot that (second panel, below). [Note that the difference between view from satellite and view from Moon may be important: Sun-Earth-Viewpoint angles should be the same, and if the Sun and Moon are not at similar latitudes as Sun and Satellite we could be generating artefacts in what follows: we should see if we can use the synthetic code for satellite viewpoints. For now we ought to find dates when the Moon was at latitude 0 (like the satellite) and the Sun also same latitudes - tricky to do.]

Lastly we divide Gerb fluxes by Lambertian fluxes, correct for the fact that Geostationary orbit and the Moon are at different distances and multiply that corrected ratio by the uniform albedo used in the Lambertian models. We plot that (bottom panel, below).

We see that the albedo does not come out constant. This is not surprising since the Earth has real clouds that drift around - but that is only what gives the thickness of the thick line of points in the last plot above. The 'wiggles' are due to the inadequacy of the Lambertian model. Near New Earth (Full Moon: never observed) the derived albedo rises. Near Full Earth (New Moon: attractive to observe due to strong eshine, but difficult due to Moon close to Sun) the albedo is flat. At intermediary values (20% and 80% of the cycle) the Lambertian albedo is relatively high so that the derived albedo is lowered.

How can we use these insights to understand what Johanne shows in plots of how derived albedo evolves during the nights?

Our aspiration is that the above can give insight into

1) the 'phase dependency' we see in derived albedos when we plot all data corresponding to all phases during the morning branch - i.e. Moon setting over Western Pacific/Australia, and

2) the nightly tendency to have falling albedo through the night, for that same branch.

As for 1 the reader should look in the May 3 presentation at slide 22; as for 2 the reader shoul dlook at slide 23.

We have to figure out whther the above plots explain any of these sightings. Could the almost quadratic phase dependency seen when all data are plotted be due to the 'dip' near 20 and 80% of the cycle? Could the 'nightly slopes' be due to the same?

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