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| Titel |
Evaluating MODIS cloud retrievals with in situ observations from VOCALS-REx |
| VerfasserIn |
N. J. King, K. N. Bower, J. Crosier, I. Crawford |
| 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 ; 13, no. 1 ; Nr. 13, no. 1 (2013-01-08), S.191-209 |
| Datensatznummer |
250017546
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| Publikation (Nr.) |
 copernicus.org/acp-13-191-2013.pdf |
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| Zusammenfassung |
| Microphysical measurements collected during eleven profiles, by the UK
BAe-146 aircraft, through marine stratocumulus as part of the Variability of
the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study
Regional Experiment (VOCALS-REx) are compared to collocated overpasses of the
Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua and
Terra satellite platforms. The full depth of the cloud is sampled in each
case using a Cloud Droplet Probe (CDP) and a Two-Dimensional Stereo Probe
(2DS) together sizing cloud and precipitation droplets in the diameter range
2–1260 μm. This allows the total optical depth (τc)
of the cloud and effective radius (re) of the droplet size
distribution to be compared to MODIS cloud retrievals of the same quantities
along with the secondarily derived total liquid water path. When compared to
the effective radius at cloud top, the MODIS retrieved re using
the 2.1 μm wavelength channel overestimates the in situ
measurements on average by 13% with the largest overestimations coinciding
with the detection by the 2DS of drizzle sized droplets. We show through
consideration of the full vertical profile and penetration depths of the
wavelengths used in the retrieval that the expected retrieved values are less
than those at cloud top thus increasing the apparent bias in re
retrievals particularly when using the 1.6 and 2.1 μm channels,
with the 3.7 μm channel retrievals displaying the best agreement
with in situ values. Retrievals of τc also tend to overestimate
in situ values which, coupled with a high bias in re retrievals,
lead to an overestimation of liquid water path. There is little apparent
correlation between the variation of the three near-infrared re
retrievals and the vertical structure of the cloud observed in situ.
Retrievals are performed using measured profiles of water vapour and
temperature along with an accurate knowledge of the width of the droplet size
distribution which improve agreement between in situ and retrieved values but
cannot completely explain the observed biases. Additionally we show that
cloud heterogeneity and three-dimensional radiative effects may high skew the
mean when averaging over comparison domains but cannot explain all of the
apparent high bias. An intercomparison between in situ measurements from the
BAe-146 and C-130 platforms is also presented, highlighting the uncertainties
associated with in situ observations. |
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