Publications

Grosvenor, D. P.; Wood, R. (2014). The effect of solar zenith angle on MODIS cloud optical and microphysical retrievals within marine liquid water clouds. ATMOSPHERIC CHEMISTRY AND PHYSICS, 14(14), 7291-7321.

Abstract
In this paper we use a novel observational approach to investigate MODIS satellite retrieval biases of tau and r(e) (using three different MODIS bands: 1.6, 2.1 and 3.7 mu m, denoted as r(e1.6), r(e2.1) and r(e3.7), respectively) that occur at high solar zenith angles (theta(0)) and how they affect retrievals of cloud droplet concentration (N-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 strati-form clouds.We find that the mean tau is fairly constant between theta(0) = 50 degrees and similar to 65-70 degrees, but then increases rapidly with an increase of over 70% between the lowest and highest theta(0). The r(e2.1) and r(e3.7) decrease with theta(0), with effects also starting at around theta(0) = 65-70 degrees. At low theta(0), the re values from the three different MODIS bands agree to within around 0.2 m, whereas at high theta(0) the spread is closer to 1 mu m. The percentage changes of r(e) with theta(0) are considerably lower than those for tau, being around 5% and 7% for r(e2.1) and r(e3.7). For r(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.The increase in tau and decrease in r(e) both contribute to an overall increase in N-d of 40- 70% between low and high theta(0). Whilst the overall r(e) changes are quite small, they are not insignificant for the calculation of N-d; we find that the contributions to N-d biases from the tau and r(e) biases were roughly comparable for r(e3.7), although for the other r(e) bands the tau changes were considerably more important. Also, when considering only the clouds with the more heterogeneous tops, the importance of the r(e) biases was considerably enhanced for both r(e2.1) and r(e3.7).When using the variability of 1 km resolution tau data (gamma(tau)) as a heterogeneity parameter we obtained the expected result of increasing differences in tau 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 tau biases at high theta(0) than sigma(CTT). For a given theta(0), large decreases in r(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 r(e1.6,) r(e2.1) and r(e3.7), which has implications for attempts to retrieve vertical cloud information using the different MODIS bands. The relatively larger decrease in r(e3.7) and the lack of change of r(e1.6) with both theta(0) and cloud top heterogeneity suggest that r(e3.7) is more prone to retrieval biases due to high theta(0) than the other bands. We discuss some possible reasons for this.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 degrees can occur at latitudes as low as 28 degrees 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 tau and r(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.

DOI:
10.5194/acp-14-7291-2014

ISSN:
1680-7316; 1680-7324