The effect of solar zenith angle on MODIS cloud optical and microphysical retrievals within marine liquid water clouds

• Main Authors: D. P. Grosvenor, R. Wood
• Format: Article
• Language: English
• Published: Copernicus Publications 2014-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/14/7291/2014/acp-14-7291-2014.pdf

In this paper we use a novel observational approach to investigate MODIS satellite retrieval biases of τ and <i>r</i>_e (using three different MODIS bands: 1.6, 2.1 and 3.7 &mu;m, denoted as <i>r</i>_e1.6, <i>r</i>_e2.1 and <i>r</i>_e3.7, respectively) that occur at high solar zenith angles (&theta;₀) and how they affect retrievals of cloud droplet concentration (<i>N</i>_d). Utilizing the large number of overpasses for polar regions and the diurnal variation of &theta;₀ we estimate biases in the above quantities for an open ocean region that is dominated by low level stratiform clouds. <br><br> We find that the mean τ is fairly constant between &theta;₀ = 50° and ~65–70°, but then increases rapidly with an increase of over 70 % between the lowest and highest &theta;₀. The <i>r</i>_e2.1 and <i>r</i>_e3.7 decrease with &theta;₀, with effects also starting at around &theta;₀ = 65–70°. At low &theta;₀, the <i>r</i>_e values from the three different MODIS bands agree to within around 0.2 &mu;m, whereas at high &theta;₀ the spread is closer to 1 &mu;m. The percentage changes of <i>r</i>_e with &theta;₀ are considerably lower than those for τ, being around 5 % and 7% for <i>r</i>_e2.1 and <i>r</i>_e3.7. For <i>r</i>_e1.6 there was very little change with &theta;₀. Evidence is provided that these changes are unlikely to be due to any physical diurnal cycle. <br><br> The increase in τ and decrease in <i>r</i>_e both contribute to an overall increase in <i>N</i>_d of 40–70% between low and high &theta;₀. Whilst the overall <i>r</i>_e changes are quite small, they are not insignificant for the calculation of <i>N</i>_d; we find that the contributions to <i>N</i>_d biases from the τ and <i>r</i>_e biases were roughly comparable for <i>r</i>_e3.7, although for the other <i>r</i>_e bands the τ changes were considerably more important. Also, when considering only the clouds with the more heterogeneous tops, the importance of the <i>r</i>_e biases was considerably enhanced for both <i>r</i>_e2.1 and <i>r</i>_e3.7. <br><br> When using the variability of 1 km resolution τ data (&gamma;_&tau;) as a heterogeneity parameter we obtained the expected result of increasing differences in τ between high and low &theta;₀ as heterogeneity increased, which was not the case when using the variability of 5 km resolution cloud top temperature (&sigma;_CTT), suggesting that &gamma;_&tau; is a better predictor of τ biases at high &theta;₀ than &sigma;_CTT. For a given &theta;₀, large decreases in <i>r</i>_e were observed as the cloud top heterogeneity changed from low to high values, although it is possible that physical changes to the clouds associated with cloud heterogeneity variation may account for some of this. However, for a given cloud top heterogeneity we find that the value of &theta;₀ affects the sign and magnitude of the relative differences between <i>r</i>_e1.6, <i>r</i>_e2.1 and <i>r</i>_e3.7, which has implications for attempts to retrieve vertical cloud information using the different MODIS bands. The relatively larger decrease in <i>r</i>_e3.7 and the lack of change of <i>r</i>_e1.6 with both &theta;₀ and cloud top heterogeneity suggest that <i>r</i>_e3.7 is more prone to retrieval biases due to high &theta;₀ than the other bands. We discuss some possible reasons for this. <br><br> Our results have important implications for individual MODIS swaths at high &theta;₀, which may be used for case studies for example. &theta;₀ values > 65° can occur at latitudes as low as 28° in mid-winter and for higher latitudes the problem will be more acute. Also, Level-3 daily averaged MODIS cloud property data consist of the averages of several overpasses for the high latitudes, which will occur at a range of &theta;₀ values. Thus, some biased data are likely to be included. It is also likely that some of the &theta;₀ effects described here would apply to τ and <i>r</i>_e retrievals from satellite instruments that use visible light at similar wavelengths along with forward retrieval models that assume plane parallel clouds, such as the GOES imagers, SEVIRI, etc.