Sensitivity to Aerosol Thickness

 

In this section, the impact of aerosol vertical thickness is assessed. This is done by simply scaling the standard profile of Figure 5.1 by factors of 0.6, 0.8, 1.2 and 1.4 for a maximum variation of $\pm40\%$ in number density and aerosol optical depth.

Results are presented in Figure 5.4 at 750 nm. Decreasing the aerosol acted to decrease the radiance and increase polarization. This is simply the result of fewer scatterers and hence less scattering into the line of sight by the depolarizing aerosols. Below $-4^{\circ}$ there was little effect due to the large contribution of surface reflected radiation. Comparing radiance curves representing scaling factors of 0.6 and 0.8, there is approximately a 20% difference for uplooking angles. This difference drops to 6% at $-3^{\circ}$ and is nearly zero below $-5^{\circ}$. Also, the larger the scaling factor, the smaller the difference between neighbouring curves; differences of about half those quoted above were found between factors of 1.2 and 1.4. Between neighbouring curves polarization was observed to change by roughly 0.02 between scaling factors of 0.6 and 0.8 and less than 0.01 between scaling factors of 1.2 and 1.4. The decreasing impact with increasing aerosol amount is a saturation-type effect as the slant optical depths get increasing larger as does the fraction of Mie scattering. A similar analysis was carried out previously in which a sharply peaked aerosol profile was used (McLinden et al., 1998). Results similar to those above were found.