In this paper we use a novel observational approach to investigate MODIS satellite retrieval biases of τ and re (using three different MODIS bands: 1.6, 2.1 and 3.7 m, denoted as re1.6, re2.1 and re3.7, respectively) that occur at high solar zenith angles (0) and how they affect retrievals of cloud droplet concentration (Nd). Utilizing the large number of overpasses for polar regions and the diurnal variation of 0 we estimate biases in the above quantities for an open ocean region that is dominated by low level stratiform clouds. We find that the mean τ is fairly constant between 0 = 50° and ~65–70°, but then increases rapidly with an increase of over 70 % between the lowest and highest 0. The re2.1 and re3.7 decrease with 0, with effects also starting at around 0 = 65–70°. At low 0, the re values from the three different MODIS bands agree to within around 0.2 m, whereas at high 0 the spread is closer to 1 m. The percentage changes of re with 0 are considerably lower than those for τ, being around 5 % and 7% for re2.1 and re3.7. For re1.6 there was very little change with 0. Evidence is provided that these changes are unlikely to be due to any physical diurnal cycle. The increase in τ and decrease in re both contribute to an overall increase in Nd of 40–70% between low and high 0. Whilst the overall re changes are quite small, they are not insignificant for the calculation of Nd; we find that the contributions to Nd biases from the τ and re biases were roughly comparable for re3.7, although for the other re bands the τ changes were considerably more important. Also, when considering only the clouds with the more heterogeneous tops, the importance of the re biases was considerably enhanced for both re2.1 and re3.7. When using the variability of 1 km resolution τ data () as a heterogeneity parameter we obtained the expected result of increasing differences in τ between high and low 0 as heterogeneity increased, which was not the case when using the variability of 5 km resolution cloud top temperature (CTT), suggesting that is a better predictor of τ biases at high 0 than CTT. For a given 0, large decreases in re 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 0 affects the sign and magnitude of the relative differences between re1.6, re2.1 and re3.7, which has implications for attempts to retrieve vertical cloud information using the different MODIS bands. The relatively larger decrease in re3.7 and the lack of change of re1.6 with both 0 and cloud top heterogeneity suggest that re3.7 is more prone to retrieval biases due to high 0 than the other bands. We discuss some possible reasons for this. Our results have important implications for individual MODIS swaths at high 0, which may be used for case studies for example. 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 0 values. Thus, some biased data are likely to be included. It is also likely that some of the 0 effects described here would apply to τ and re 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.
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