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| Titel |
The effect of solar zenith angle on MODIS cloud optical and microphysical retrievals within marine liquid water clouds |
| VerfasserIn |
D. P. Grosvenor, R. Wood |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 14 ; Nr. 14, no. 14 (2014-07-17), S.7291-7321 |
| Datensatznummer |
250118893
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| Publikation (Nr.) |
 copernicus.org/acp-14-7291-2014.pdf |
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| Zusammenfassung |
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. |
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