Abstract
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 an open ocean region that is dominated by low level stratiform clouds. We find that the mean τ is fairly constant between θ0 = 50° and ~65–70°, but then increases rapidly with an increase of over 70 % between the lowest and highest θ0. The re2.1 and re3.7 decrease with θ0, with effects also starting at around θ0 = 65–70°. 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 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 θ0. 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 θ0. 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 (γτ) as a heterogeneity parameter we obtained the expected result of increasing differences in τ between high and low θ0 as heterogeneity increased, which was not the case when using the variability of 5 km resolution cloud top temperature (σCTT), suggesting that γτ is a better predictor of τ biases at high θ0 than σCTT. For a given θ0, 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 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. 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 consist of the averages of several overpasses for the high latitudes, which will occur at a range of θ0 values. Thus, some biased data are likely to be included. It is also likely that some of the θ0 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.
Highlights
The MODIS (Moderate Resolution Imaging Spectroradiometer) instruments onboard the Aqua and Terra polar orbiting satellites are capable of retrieving cloud optical depth (τ ) and cloud top effective radius information from liquid clouds based upon the combination of one non-absorbing optical wavelength (0.86 μm is used by MODIS for retrievals over the ocean) and one absorbing near-infrared band (Foot, 1988; Nakajima and King, 1990; King et al, 1997; Platnick et al, 2003); this can be either 1.6, 2.1 or 3.7 μm
It seems that restricting analysis to regions where the MODIS cloud fraction is high is one way to increase the probability of homogeneity, since it was shown by Wood and Hartmann (2006) that, over the scale of ∼ 200 km, cloud fraction is strongly correlated with a measure of homogeneity based on the MODIS liquid water path
There are a lot of clouds that likely reside within the boundary layer and which would be well suited to the application of the Nd estimate using MODIS, as described in Sect. 3.1 and Appendix A
Summary
The MODIS (Moderate Resolution Imaging Spectroradiometer) instruments onboard the Aqua and Terra polar orbiting satellites are capable of retrieving cloud optical depth (τ ) and cloud top effective radius (re) information from liquid clouds based upon the combination of one non-absorbing optical wavelength (0.86 μm is used by MODIS for retrievals over the ocean) and one absorbing near-infrared band (Foot, 1988; Nakajima and King, 1990; King et al, 1997; Platnick et al, 2003); this can be either 1.6, 2.1 or 3.7 μm. The re retrieved using these different bands will hereafter be referred to, respectively, as re1.6, re2.1 and re3.7, with re2.1 being the value provided as standard from MODIS (e.g. in Level products) This information is invaluable for a range of cloud microphysical studies, especially given the global coverage and the long time period of the data set available from these instruments (Terra MODIS since mid-2000 and Aqua MODIS since 2002). This information can be used to estimate cloud droplet number concentrations (Nd) within liquid clouds (Boers et al, 2006; Bennartz, 2007, hereafter B07). Precipitation can be an important sink process for Nd and, insight into such processes can be gained through knowledge of Nd (e.g. Wood et al, 2012)
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