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Verification of ESAS-Light demonstration version - Task 4

During the ESASLight study two completely new algorithms are developed: one is for the calculation of Raman scattering, the other is for the computation polarization using the Monte Carlo method. These new algorithms are carefully validated.

We have collected published results that can serve as benchmark for validation. Furthermore we have defined several test cases for solver intercomparisons as well as for physical consistency tests. We also compare the simulations of polarized radiances to measurements and take part in a model intercomparison study for models including polarization.

Details are given in the verification plan.

Polarization

Comparison against benchmark results by Coulson, 1960

Seven test cases have been defined for the verification of the implementation of polarization in MYSTIC. The following figure shows the result of one case as an example:

Test case 6

In general there is a very good agreement between the results by Coulson et al. (1960) and MYSTIC. The small differences are due to Monte Carlo noise and could further be reduced by using more photons. The error bars correspond to two standard deviations (2σ). The noise is larger where the polarized radiance is very small (close to 0). In all test cases there is a systematic difference of about 1% between Coulson et al. (1960) and polradtran.

Test reciprocity principle for realistic 1D atmospheres with molecules, aerosols, and clouds

Realistic simulations in spherical atmospheres including molecules, aerosols, and clouds have been simultated. The computations have been performed in the solar principal plane for a solar zenith angle of 60° at 350 nm wavelength. All simulations were done in forward and backward tracing mode. This test shows that the reciprocity principle (von Helmholtz, 1867) is fullfilled in MYSTIC.

Intercomparison between libRadtran radiative transfer solvers

In order to test the consistency of the various solvers that are available in libRadtran, we performed polarized calculations using the Monte Carlo solver MYSTIC and the doubling-and-adding solver polradtran. The intensity results have also been compared with the well tested discrete-ordinate solver DISORT2.

Three aerosol types of the OPAC database have be used:

  • water soluble (consists of various kinds of sulfates, nitrates, and other, also organic, water-soluble substances
  • soot (absorbing black carbon)
  • sea salt accumulated mode

For each aerosol type the radiation field for two aerosol optical thicknesses has been calculated: 0.05 (very low aerosol content) and 0.4 (very high aerosol content).

The following figure shows the result for the high aerosol content.

aerosol tau=0.4 For soot (small particles), MYSTIC and polradtran agree well for I and Q. I in DISORT is different because polarization can not be considered. For larger particles (water soluble and sea salt) with enhanced forward scattering polradtran intensities are not correct in the forward scattering region. Here MYSTIC and DISORT agree well. The agreement for Q between MYSTIC and polradtran is quite good, even for sea salt aerosol. The difference due to neglecting the forward scattering peak (by delta-scaling) is 5% at maximum.
 
task4.1245071377.txt.gz · Last modified: 2009/06/15 14:09 by esaslight     Back to top