Homework 8
Solutions
Brightness temperature simulations
Due Thursday, April 1

Radiation objective:

To understand the spectral relationship between cirrus emissivity and brightness temperature.

MATLAB objective:

To use a computer program to explore relationships between two or more parameters.

Assignment

Compute and plot the 11 and 12 micron brightness temperature as a function of cloud emissivity for nadir radiance leaving a cirrus cloud. How might you use such observations to detect a cirrus cloud from a satellite? You can assume: 

• the mean temperature of the cloud is 210K
• the incident radiance at the base of the cloud as a brightness temperature of 282K at 12 microns and 287K at 11 microns.

Solutions

Computations and Plotting

Here is Shaima's Matlab code to generate plot: homework8.m
And here is the plot: BT plot

Discussion

We were asked to discuss how one might use the above observations to detect a cirrus cloud from a satellite. Detection of thin cirrus clouds has always been a difficult issue due to the fact that for low emissivity (high transmissivity), the contribution from the cloud to the total radiance is small. Most students mentioned this fact in their discussions. What our plots show us is that a multispectral approach involving the difference between the 11 and 12 micron brightness temperatures is useful in some cases. The brightness temperature difference (BTD) versus emissivity plot demonstrates this clearly. As emissivity increases, the BTD decreases. A satellite observed BTD near zero would imply the presence of a thicker cirrus cloud. Thinner clouds could possible be detected using the BTD approach with some foreknowledge of the surface temperature and a guess temperature profile. A moderatue BTD difference combined with BT's noticeable less than surface temperature would imply the presence of a thinner cirrus cloud. For very thin cirrus clouds, other spectral methods would probably be necessary.