Computer Modeling Laboratory 6

Written report due: 16 October

Passive remote sensing of atmospheric aerosols

RELATED MATERIALS:
Lectures 7 and 8

TASK 1

The atmosphere of a planet X consists of gases and non-spherical dust particles. A passive radiometer with a visible channel observes the planet X at scattering angles from 1000 to 1700. Assume that the planet surface is dark.

  1. For aerosol-free conditions, find a scattering angle at which the radiometer will measure the largest radiance. Assume that the optical depth due to molecular scattering is small.
  2. Now consider the case of a dust layer and find a scattering angle corresponding to the largest observed radiance. Figure shows a representative scattering phase function for non-spherical dust particles. Ignore molecular scattering.
  3. The above figure also shows the scattering phase function of surface-area equivalent spherical particles. At what scattering angle there will be a largest error in the retrieved aerosol optical depth if the retrieval algorithm treats dust particles as spheres? Ignore molecular scattering.
  4. If the radiometer measures the same radiance over the dust layer and in the aerosol-free case, would you expect the optical depth of the dust layer to be higher or lower than that of molecular scattering?

 

TASK 2

Giovanni is a web-based application that provides a straight forward way to visualize, analyze, and download selected NASA remote sensing products. In this task you will be analyzing aerosol products retrieved from MODIS on the Terra and Aqua satellites and comparing them to AERONET aerosol measurements.

  1. Go to MODIS Terra and Aqua, Daily products. On a map, draw a box over the Northern Africa to cover the land and ocean off the West Coast of Africa, extending from about 14N to 30N. Select Aerosol Optical Depth retrieved from MODIS/Terra for 6 May, 2008. Use option "Lat-Lon map, Time-averaged" to visualize AOD retrieved over the selected domain. Download the image and data. Then select Aerosol Small Mode Optical Depth, generate and download the image and data. Note that AOD are not retrieved over the land. Briefly discuss possible reasons.
    Now compare MODIS retrievals to AERONET data from the station located in Dakar. Do you find a good agreement between AERONET and MODIS measurements of AOD and fine mode AOD? Briefly discuss the factors that may be responsible for observed differences.
  2. Select Aerosol Optical Depth retrieved from MODIS/Terra and MODIS/Aqua during May of 2008. Use option "Scatter Plot" to generate a graph. Do you find a good correlation between these two products? Why or why not?
  3. Would you expect to find MODIS AOD retrieved over the Greenland? Why or why not? Test your predictions by positioning a box over that region to see whether there any AOD data.

 

TASK 3

The accuracy of aerosol retrievals from satellite observations strongly depends on sensitivity of the TOA (top-of-the-atmosphere) radiances to aerosol and underlying surface characteristics. In this task you will investigate how type and loading (aerosol optical depth) of aerosols and land surfaces affect reflectances observed by satellite sensors. To perform your modeling experiments, you will be using a SBDART radiative transfer code. Consider urban aerosol (strong light absorption) and oceanic aerosol (negligible absorption). Consider the MODIS channel #4.

To compute the TOA radiances click on RUN SBDART
Notations in SBDART output:
solar-TOA-dw denotes the downward solar radiation at the top of the atmosphere multiplied by the cosine of solar zenith angle;
solar_dir-sur-dw denotes the direct solar radiation reaching the surface.

NOTE: SBDART is freely available at http://www.icess.ucsb.edu/esrg/pauls_dir/ For a general description and review of the program refer to Ricchiazzi et al 1998, Bulletin of the American Meteorological Society.