Remote sensing of inhomogeneous clouds

Remote sensing makes use of observed radiances. Radiances are clearly affected by cloud geometry. A very simple example is shown in the following: An array of cubes was assumed, with an integrated optical thickness of 20. The diurnal course of the sun is simulated by varying the solar zenith angle from 90 degrees (horizon, left) to 0 degree (zenith) down to 90 degree (horizon, right).

Clouds A regular array of cloud cubes was assumed. The cubes are homogeneous and the integrated optical thickness of each cube (in all three dimensions) was 20. A Henyey-Greenstein phase function was assumed, with an asymmetry parameter of 0.85, and the single scattering albedo was set to 1 (pure scattering clouds). Background atmosphere was neglected and the surface albedo was set to 0. The image shows the view from the top. The blue line shows the course of the sun - shining parallel to the cube sides. Calculations were done for two viewing directions:
  • Nadir (viewing zenith angle: 0 degree)
  • Inclined (viewing zenith angle: 30 degree; azimuth: green line)


Nadir viewing direction

In the Nadir viewing direction, the domain-averaged radiance is symmetric about noon (solar zenith angle: 0 degree). The stars show the exact MYSTIC results while the solid line is the independent pixel approximation. There are two notable features: First, the exact radiation is considerably smaller than the independent pixel approximation for overhead sun. This is due to the fact that in the 3D simulation many photons "drop" out of the cube sides and don't contribute to the signal which is not considered by the independent pixel approximation. Second, the maximum radiance does not occur for overhead sun but for about 45 degree. At this angle, all incoming solar radiation is intercepted by the cloud, either by the top or the side of the cube (which again is not the case in the independent pixel approximation). At about 60 degrees the independent pixel approximation and the 3D calcultion coincide. At this angle the loss of radiation through the cube sides is obviously compensated by the gain due to interception of direct solar radiation at the cloud sides.

View an animation of the radiance distribution, as the sun wanders from the left (SZA -90 degree) through the zenith (SZA 0 degree) to the right (SZA 90 degree) of the szene

 Clouds


Inclined viewing direction

The results are similar to the nadir findings, except for an asymmetry between the backward and forward viewing directions.

View an animation of the radiance distribution, as the sun wanders from the left (SZA -90 degree) through the zenith (SZA 0 degree) to the right (SZA 90 degree) of the szene

 Clouds