The effect of solar zenith angle on MODIS cloud optical and microphysical retrievals
Authors: Grosvenor D. P. , Wood R. 
Published: January 2014

Abstract. 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 μm, denoted as re1.6, re2.1 and re3.7, respectively) that occur at high solar zenith angles (θ0) and how they affect retrievals of cloud droplet concentration (Nd). Utilizing the large number of overpasses for polar regions and the diurnal variation of θ0 we estimate biases in the above quantities for the open ocean region north of Scandinavia that is dominated by low level stratiform clouds. We find that the mean τ is fairly constant between θ0 = 50° and ~65°, but then increases rapidly with an increase of over 70% between the lowest and highest θ0. re2.1 and re3.7 decrease with θ0, with effects also starting at around θ0 =65°. At low θ0, the re values from the three different MODIS bands agree to within around 0.2 μm, whereas at high θ0 the spread is closer to 1 μm. The percentage changes of re with θ0 are somewhat 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 θ0. The increase in τ and decrease in re both contribute to an overall increase in Nd of 40–70% between low and high θ0. We argue that such a change is highly unlikely to be due to any physical diurnal cycle, which is supported by the finding that the retrieved Nd is constant at local times at either side of noon for which θ0 < 65°. 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 (roughly twice the contribution for re2.1 and six times for re1.6). However, 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; τ and re bias contributions were comparable for re2.1 and for re3.7re bias contributions were ~50% greater. For a given θ0, large decreases in re were observed as the cloud top heterogeneity changed from low to high values: decreases of 25–30% for re3.7, ~20% for re2.1 and 10% for re1.6, 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 θ0 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 θ0 and cloud top heterogeneity suggest that re3.7 is more prone to retrieval biases due to high θ0 than the other bands, which is interesting since re3.7 has generally been shown to be less prone to other retrieval biases (e.g. due to sub-pixel heterogeneity) at low θ0. We discuss some possible reasons for this. Our results have important implications for individual MODIS swaths at high θ0, which may be used for case studies for example. θ0 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 consists of the averages of several overpasses for the high latitudes, which will occur at a range of θ0 values. Thus, some biased data is likely to be included.