MODIS Aerosol Project

"Remote sensing of Spectral aerosol properties over the ocean

A Classroom Experience"

**Objectives/General Tasks:**

This project introduces the process of retrieving aerosol properties
from satellite (MODIS) over ocean (Tanré et al., 1997), and
understanding the quality of retrieval by comparison with ground based
measurements (e.g. AERONET). The objectives are to:

-   Understand how aerosols interact with reflected solar radiation

-   Obtain working knowledge of the MODIS inversion algorithm over ocean

-   Use the MODIS algorithm to retrieve aerosol properties from
    observations

-   Understand that aerosol properties differ with geography and season

-   Evaluate satellite retrievals in the context of ground based aerosol
    measurements

To meet the objectives stated above, the project directs performance of
the following tasks.

-   Examine spectral MODIS data for specific cloud free aerosol
    conditions, for two satellite/sun angle geometries

-   Determine the effects of different aerosol types on the spectral
    reflectance observed by MODIS.

-   Use a simplified look-up table to derive aerosol properties ("by
    hand") from an example of MODIS observed spectral reflectance.

-   Compare aerosol properties retrieved by MODIS with those from
    AERONET.

**Specific instructions**:

The following instructions will aid in completing the listed general
instructions. For ease of reading, different types of information are
noted with different fonts.

General information is in regular font.

***Tasks/Questions to answer are in italics/bold.***

File/directory names and computer commands are Courier Font)

All input data (Tables 1, 3 and 4) are also contained in
"comma-deliminated" ('.csv') files and can be found in the Project_Data/
directory. The main algorithm code, lookup tables and input files are in
Code/. The "by-hand" retrieval lookup tables are in Mini_LookupTables/.
Individual 'readme' files are available for help. Good luck! Rob Levy
(<levy@atmos.umd.edu>).

1.  Table 1 (also: MODIS_angle_refl.csv) shows Terra-MODIS spectral
    reflectance ('Level 2') at 7 wavelengths, for the oceanic area
    surrounding or nearby the named AERONET site. There are two groups,
    each group having similar sun/satellite geometry. DOY is the Julian
    day.

-   For each group, plot spectral reflectance observations against
    wavelength.

-   Why are there significant spectral differences even within each
    group?

2.  ***For each entry in Table 1, compute the following angles:***

    a.  ***The relative azimuth angle, φ *** (the difference between the
        solar and satellite azimuth angles -- defined as 180 degrees
        when the sun and satellite are at the same azimuth)***.*** That
        is: *φ* = \|180.0 - \|*φ~sun~-φ~satelliet~*\|\|. Note that your
        final *φ *must be less than 180°, and the symbols \| \|
        represents absolute values.

    b.  ***Scattering angle***

    c.  ***Glint angle*** (the angle difference between direct
        "specular" reflection off the surface and actual reflection
        toward the satellite.) Hint: The formula for glint angle looks a
        lot like scattering angle except for a sign change.

Questions: Think about how sunlight interacts with the ocean surface:

-   What does wind do to the water surface?

```{=html}
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-   Why might the operational MODIS algorithm restrict retrievals to
    glint angles greater than 40 degrees?

3.  The operational MODIS lookup table is created by performing
    radiative transfer calculations to model radiance (reflectance) at
    the satellite level. This includes radiation contributions from both
    the atmosphere and the surface. The atmospheric contribution
    includes multiple scattering by gas and aerosol, as well as
    reflection from the sea surface. Contributions from the ocean
    surface include the direct radiance from sun glint (specular)
    reflection, and diffuse radiance arising from reflection from waves
    and underwater material. As for the aerosol, their properties are
    calculated using the values for aerosol optical properties shown in
    Table 2 (see Tanré et al, 1997 for physical description, table
    details in by Levy et al, 2003). Note there are four choices of
    'fine' mode and five choices of 'coarse' mode. Total satellite
    reflectance is then calculated for many combinations of angles (9
    solar zenith angles, 16 satellite zenith angles, and 16 relative
    azimuth angles), and for six aerosol "loadings" (optical depths
    between 0.0 and 3.0 at 550 nm).

MODIS assumes that natural aerosol shows bi-modal (each lognormal) size
distributions and may be described by combining a single small mode from
Table 2a and a single large mode from Table 2b (Tanré et al, 1997).
MODIS is free to choose any combination of fine ('*f*') and coarse
('*c*') mode, to match the observed satellite reflectance. The retrieved
parameters are the spectral total optical depth (at each wavelength),
*τ*~λ~, the model choices *f* and *c,* and the relative contribution of
*f* to the total optical depth, known as 'fine mode weighting' or *η.*

For this exercise, you will use a simplified lookup table containing
only modes F1 and C5, to compute the parameters *τ*~λ~ and *η*. This
lookup table is contained in two files, C5.lut.dat and F1.lut.dat and
can be found in the Mini_LookupTables directory. The files are ASCII
format, and clues on how to read them are given in lut.README.

> ***Given the MODIS angle and reflectance data in Table 3, (***also
> angle_refl_forMini.csv***) use the lookup tables to retrieve
> information about the optical thickness at each wavelength and the
> relative contribution of the small mode to the total optical depth at
> 550 nm, η.***
>
> Follow these steps for doing the retrieval:

a)  ***For each wavelength and optical depth index (*Tau_Index*),
    interpolate the lookup table radiance (*Total_Rad*) to the MODIS
    geometry****.* Your new radiances, L~λ,τ~, should be a function of
    wavelength and optical depth only.

b)  ***Convert the resulting radiances to reflectance ρ~λ,τ~ assuming
    isotropy*.**

> (i.e. ρ~λ,τ~ = (L~λ,τ~ \* 3.14159) / cos(θ~0~) ). The reflectance
> should be a function of wavelength and optical depth only.
>
> Steps c) through g) minimize the difference between the computed
> reflectance and observed reflectance. This is the heart of the
> inversion algorithm.

c)  ***Assume that the observed optical depth at 865 nm is only fine
    mode aerosol (i.e. η = 100%.). Use the F1 lookup table to find the
    optical depth at 865 nm that is "responsible" for the reflectance
    matching the observed reflectance at 865 nm.*** When interpolating
    linearly between reflectance, interpolate logarithmically between
    optical depths (think Beer\'s Law).

d)  ***Using the weightings computed between optical depth entries (at
    865 nm), find the associated spectral reflectance at the other
    wavelengths. Again use Beer\'s law to compute reflectance
    (***linearly interpolate optical depth at new wavelengths -\>
    exponentially interpolate reflectance***)***

e)  ***For each wavelength (not including 466 nm), square the difference
    between the observed reflectance and the calculated reflectance. Now
    sum up this "error" over the six wavelengths to estimate the total
    error of the fit.***

f)  ***Repeat steps c) through e) for:***

-   ***Only the C5 lookup table (i.e. η = 0%.)***

-   ***An equal mix (i.e. η = 50%.).***

-   ***What is the optical depth contribution from each mode?***

-   ***Which η gave the best fit?***

-   ***Can you see where we are going?***

g)  ***Compare the total errors for η = 50% with η = 100% and with η =
    0%. Which direction should you go towards smaller errors, to 25% or
    75%?***

> Operationally, MODIS keeps "halving" the interval and iterates until
> the total error is less than 3% or stops at 10 iterations. Then, the
> algorithm does the same thing for every combination of modes in the
> lookup table. Finally, it reports the optical depths "responsible" for
> the matching reflectance.

4.  In this exercise, we will use the operational MODIS algorithm to
    compute some aerosol optical properties. You will need a Fortran 77
    compiler. The code, Process_Ocean_V2.f, and helpful 'readme' files
    can be found in the Code directory.

-   ***Use the MODIS angle and reflectance data from Table 1 as
    input.***

-   ***Compute the aerosol optical properties for each case.***

-   ***Plot the spectral dependence of the aerosol retrieval.***

-   ***How is the ratio η related to the spectral dependence of the
    optical depth?***

-   ***Use the MODIS angle and reflectance data from Table 3 as
    input.***

-   ***How does the MODIS retrieval compare with your "by hand"
    retrieval?***

5.  The corresponding AERONET observations of aerosol optical depth for
    each case in Table 1 are found in Table 4 (also aeronet.aod.csv).

-   Plot the spectral dependence of each AERONET observation.

-   Compare each AERONET observation with the spectral dependence of the
    corresponding MODIS retrieval.

-   Is there any relationship between the total MODIS error (ERR) and
    the ability of MODIS to fit the AERONET spectral dependence?

6.  ***Provide a short write-up interpreting results found in this
    project. One or more of the references may be useful.***

**Some References:**

[http://modis-atmos.gsfc.nasa.gov](http://modis-atmos.gsfc.nasa.gov/)

[http://modis.gsfc.nasa.gov](http://modis.gsfc.nasa.gov/)

[http://climate.gsfc.nasa.gov](http://climate.gsfc.nasa.gov/)

Carlson, T. and R. Caverly, 1977: Radiative characteristics of Saharan
dust at solar wavelengths. Transactions-American Geophysical Union, 58
(6): 555-555.

Eck, T. F., B. N Holben, J. S. Reid, O. Dubovik, A. Smirnov, N. T.
O'Neill, I. Slutsker and S. Kinne, 1999: Wavelength dependence of the
optical depth of biomass burning, urban and desert dust aerosols, J.
Geophys. Res.-Atmos., 104, D8, 31,333-31,349.

Ichoku, C., D. A. Chu, S. Mattoo, Y. J. Kaufman, L. A. Remer, D. Tanre,
I. Slutsker, and B. N. Holben, 2002: A spatio-temporal approach for
global validation and analysis of MODIS aerosol products. Geophys. Res.
Lett., 29, doi: 10.1029/2001GL013206.

Intergovernmental Panel on Climate Change, *Climate Change 2001: the
scientific basis*, Cambridge Univ. Press, New York, 2001.

Holben, B. N., and Coauthors, 2001: An emerging ground-based aerosol
climatology: Aerosol Optical Depth from AERONET. J. Geophys. Res., Vol.
106, No. D11, 12,067-12,097.

Kaufman, Y. J., D. Tanré , and O. Boucher, 2002: A satellite view of
aerosols in the climate system. Nature, Vol. 419, 215-223.

Levy, R. C., L. A. Remer, D. Tanré, Y. J. Kaufman, C. Ichoku, B. N.
Holben, J. M. Livingston, P. B. Russell and H. Maring, 2003: Evaluation
of the MODIS retrievals of dust aerosol over the ocean during PRIDE. J.
Geophys. Res., 108 (D14), 10.1029/2002JD002460

Pinker, R. T., G. Pandithurai, B. N. Holben, O. Dubovik, and T. O. Aro,
2001: A dust outbreak episode in sub-Sahel West Africa, J.Geophys.Res.,
106, D19, 22 923 - 22 930.

Remer, L. A., and Coauthors, 2002: Validation of MODIS aerosol retrieval
over ocean. Geophys. Res. Lett., 29, 10.1029/2001GL013204.

Tanré, D., M. Herman, and Y. J. Kaufman, 1996: Information on aerosol
size distribution contained in solar reflected spectral radiances. J.
Geophys. Res., 101 (D14), 19 043--19 060.

Tanré, D., Y. J. Kaufman, M. Herman, and S. Mattoo, 1997: Remote sensing
of aerosol properties over oceans using the MODIS/EOS spectral
radiances. J. Geophys. Res., 102 (D14), 16 971--16 988.

Table 1: MODIS reflectance at 7 wavelengths over the ocean, for the
named AERONET stations. Reflectance has been averaged for a 50 km x 50
km box.

  ---------- --------- --------- ------------------ ---------- --------- --------- --------- ---------- ---------- ----------- ----------- ----------- ----------- ----------- ----------- -----------
  **Date**   **DOY**   **Time    **Location**       **Long**   **Lat**   **Solar   **Solar   **Sensor   **Sensor   **reflect   **reflect   **reflect   **reflect   **reflect   **reflect   **reflect
                       (UTC)**                                           Zen**     Az**      Zen**      Az**       470nm**     550nm**     660nm**     870nm**     1240nm**    1640nm**    2130nm**

  4/11/01    102       15:45     Arica              -70.31     -18.47    30.29     28.15     58.87      -83.84     0.147       0.095       0.061       0.033       0.016       0.011       0.007

  8/15/02    227       11:55     Ascension_Island   -14.42     -7.98     27.09     35.67     58.83      -82.54     0.139       0.087       0.057       0.035       0.026       0.022       0.018

  8/2/01     215       11:10     Venise             12.51      45.31     27.73     177.84    59.62      -69.12     0.176       0.12        0.078       0.043       0.02        0.011       0.006

  8/19/02    231       11:15     Villefranche       7.33       43.68     31.02     173.02    57.4       -70.65     0.166       0.109       0.072       0.041       0.023       0.015       0.01

                                                                                                                                                                                           

  7/11/02    192       9:30      Venise             12.51      45.31     31.01     130.14    52.44      95.18      0.161       0.1         0.058       0.028       0.015       0.009       0.006

  3/23/01    83        11:45     Capo_Verde         -22.94     16.73     31.13     117.26    56.88      96.3       0.172       0.104       0.065       0.037       0.023       0.02        0.014

  9/13/02    256       11:15     Dahkla             -15.95     23.72     31.37     125.65    51.88      96.47      0.151       0.1         0.061       0.032       0.019       0.014       0.011

  8/17/01    230       15:25     COVE               -75.71     36.9      32.18     130.91    52.67      95.18      0.169       0.105       0.063       0.031       0.016       0.009       0.005
  ---------- --------- --------- ------------------ ---------- --------- --------- --------- ---------- ---------- ----------- ----------- ----------- ----------- ----------- ----------- -----------

Table 2a: Refractive Indices, Median, Standard Deviation and Effective
Radius for small mode particles used in the MODIS Lookup Table
(small_v2c1.dat.v3)

  --- ----------------------- -------------- ------------- ------------- -------- ------- ---------- --------------
  S   **λ=0.47\--\>0.86μm**   **λ =1.24μm**  **λ =1.64μm** **λ =2.13μm** **rg**   **σ**   **reff**   **comments**

  1   1.45-0.0035i            1.45-0.0035i   1.43-0.01i    1.40-0.005i   0.07     0.40    0.10       Wet Water
                                                                                                     Soluble type

  2   1.45-0.0035i            1.45-0.0035i   1.43-0.01i    1.40-0.005i   0.06     0.60    0.15       Wet Water
                                                                                                     Soluble type

  3   1.40-0.0020i            1.40-0.0020i   1.39-0.005i   1.36-0.003i   0.08     0.60    0.20       Water Soluble
                                                                                                     with humidity

  4   1.40-0.0020i            1.40-0.0020i   1.39-0.005i   1.36-0.003i   0.10     0.60    0.25       Water Soluble
                                                                                                     with humidity
  --- ----------------------- -------------- ------------- ------------- -------- ------- ---------- --------------

Table 2b: Refractive Indices, Median, Standard Deviation and Effective
Radius for large mode particles used in the MODIS Lookup Table
(big_v2c1.dat.v3).

<table style="width:100%;">
<colgroup>
<col style="width: 2%" />
<col style="width: 0%" />
<col style="width: 18%" />
<col style="width: 12%" />
<col style="width: 14%" />
<col style="width: 14%" />
<col style="width: 7%" />
<col style="width: 6%" />
<col style="width: 7%" />
<col style="width: 17%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>B</strong></td>
<td colspan="2"><strong>λ =0.47--&gt;0.86μm</strong></td>
<td><strong>λ =1.24μm</strong></td>
<td><strong>λ =1.64μm</strong></td>
<td><strong>lλ =2.13μm</strong></td>
<td><strong>rg</strong></td>
<td><strong>σ</strong></td>
<td><strong>reff</strong></td>
<td><strong>comments</strong></td>
</tr>
<tr class="even">
<td>5</td>
<td colspan="2">1.45-0.0035i</td>
<td>1.45-0.0035i</td>
<td>1.43-0.0035i</td>
<td>1.43-0.0035i</td>
<td>0.40</td>
<td>0.60</td>
<td>0.98</td>
<td>Wet Sea salt type</td>
</tr>
<tr class="odd">
<td>6</td>
<td colspan="2">1.45-0.0035i</td>
<td>1.45-0.0035i</td>
<td>1.43-0.0035i</td>
<td>1.43-0.0035i</td>
<td>0.60</td>
<td>0.60</td>
<td>1.48</td>
<td>Wet Sea salt type</td>
</tr>
<tr class="even">
<td>7</td>
<td colspan="2">1.45-0.0035i</td>
<td>1.45-0.0035i</td>
<td>1.43-0.0035i</td>
<td>1.43-0.0035i</td>
<td>0.80</td>
<td>0.60</td>
<td>1.98</td>
<td>Wet Sea salt type</td>
</tr>
<tr class="odd">
<td colspan="2">8</td>
<td><p>1.53-0.003i (0.47)</p>
<p>1.53-0.001i (0.55)</p>
<p>1.53-0.000i (0.66)</p>
<p>1.53-0.000i (0.86)</p></td>
<td>1.46-0.000i</td>
<td>1.46-0.001i</td>
<td>1.46-0.000i</td>
<td>0.60</td>
<td>0.60</td>
<td>1.48</td>
<td>Dust-like type</td>
</tr>
<tr class="even">
<td>9</td>
<td colspan="2"><p>1.53-0.003i (0.47)</p>
<p>1.53-0.001i (0.55)</p>
<p>1.53-0.000i (0.66)</p>
<p>1.53-0.000i (0.86)</p></td>
<td>1.46-0.000i</td>
<td>1.46-0.001i</td>
<td>1.46-0.000i</td>
<td>0.50</td>
<td>0.80</td>
<td>2.50</td>
<td>Dust-like type</td>
</tr>
</tbody>
</table>

Table 3: MODIS reflectance at 7 wavelengths over the ocean, for the
named AERONET stations. Reflectance has been averaged for a 50 km x 50
km box.

  -------- ----- ---------- ---------- ------- -------- ------- --------- -------- -------- --------- --------- --------- --------- --------- --------- ---------
  Date     DOY   Time (UTC) Location   Long    Lat      Solar   Solar Az  Sensor   Sensor   reflect   reflect   reflect   reflect   reflect   reflect   reflect
                                                        Zen               Zen      Az       470nm     550nm     660nm     870nm     1240nm    1640nm    2130nm

  1/1/02   190   11:20:00   Gotland    18.95   57.917   36.0    -167.61   60.0     -58.65   0.165     0.119     0.075     0.038     0.018     0.01      0.006
  -------- ----- ---------- ---------- ------- -------- ------- --------- -------- -------- --------- --------- --------- --------- --------- --------- ---------

Table 4: AERONET optical depths corresponding to MODIS data in Table 1.
Optical depths have been averaged over a one hour period (usually 4 or 5
retrievals).

  ---------- --------- ---------- ------------------ ---------- --------- --------- ------------- ------------ ------------ ------------ ------------ ------------ ------------
  **Date**   **DOY**   **Time**   **Location**       **Long**   **Lat**   **Solar   **AOT1020**   **AOT870**   **AOT670**   **AOT500**   **AOT440**   **AOT380**   **AOT340**
                                                                          Zen**                                                                                    

  4/11/01    102       15:45      Arica              -70.313    -18.472   30.29     0.126         0.153        0.24         0.382        0.458        0.622        0.653

  8/15/02    227       11:55      Ascension_Island   -14.415    -7.976    27.09     0.218         0.221        0.239        0.258        0.281        0.3          0.409

  8/2/01     215       11:10      Venise             12.508     45.314    27.73     0.101         0.138        0.203        0.364        0.441        0.568        0.814

  8/19/02    231       11:15      Villefranche       7.329      43.684    31.02     0.118         0.14         0.225                     0.444                     

                                                                                                                                                                   

  7/11/02    192       9:30       Venise             12.508     45.314    31.01     0.102         0.138        0.229        0.387        0.468        0.595        0.997

  3/23/01    83        11:45      Capo_Verde         -22.935    16.733    31.13     0.164         0.157        0.2                       0.261                     

  9/13/02    256       11:15      Dahkla             -15.95     23.717    31.37     0.197         0.192        0.214        0.256        0.26         0.302        0.317

  8/17/01    230       15:25      COVE               -75.71     36.9      32.18     0.088         0.122        0.209        0.351        0.423        0.522        0.59
  ---------- --------- ---------- ------------------ ---------- --------- --------- ------------- ------------ ------------ ------------ ------------ ------------ ------------