scale dependence of cirrus horizontal heterogeneity effects on toa measurements – part i: modis brightness temperatures in the thermal infrared
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2017
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Abstract
This paper presents a study on the impact of cirrus cloud heterogeneities on
MODIS simulated thermal infrared (TIR) brightness temperatures (BTs) at the
top of the atmosphere (TOA) as a function of spatial resolution from 50 m to
10 km. A realistic 3-D cirrus field is generated by the 3DCLOUD model
(average optical thickness of 1.4, cloud-top and base altitudes at 10 and
12 km, respectively, consisting of aggregate column crystals of
Deff = 20 µm), and 3-D thermal infrared radiative
transfer (RT) is simulated with the 3DMCPOL code. According to previous
studies, differences between 3-D BT computed from a heterogenous pixel and
1-D RT computed from a homogeneous pixel are considered dependent at nadir
on two effects: (i) the optical thickness horizontal heterogeneity leading to
the plane-parallel homogeneous bias (PPHB) and the (ii) horizontal radiative
transport (HRT) leading to the independent pixel approximation error (IPAE).
A single but realistic cirrus case is simulated and, as expected, the PPHB
mainly impacts the low-spatial-resolution results (above ∼ 250 m) with
averaged values of up to 5–7 K, while the IPAE mainly impacts the high-spatial-resolution results (below ∼ 250 m) with average values of up to 1–2 K.
A sensitivity study has been performed in order to extend these results to
various cirrus optical thicknesses and heterogeneities by sampling the cirrus
in several ranges of parameters. For four optical thickness classes and four
optical heterogeneity classes, we have found that, for nadir observations,
the spatial resolution at which the combination of PPHB and HRT effects is the
smallest, falls between 100 and 250 m. These spatial resolutions thus appear
to be the best choice to retrieve cirrus optical properties with the smallest
cloud heterogeneity-related total bias in the thermal infrared. For off-nadir
observations, the average total effect is increased and the minimum is
shifted to coarser spatial resolutions.
| Reference Key |
fauchez2017atmosphericscale
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|---|---|
| Authors | ;T. Fauchez;T. Fauchez;S. Platnick;K. Meyer;K. Meyer;C. Cornet;F. Szczap;T. Várnai;T. Várnai |
| Journal | Journal of agricultural and food chemistry |
| Year | 2017 |
| DOI |
10.5194/acp-17-8489-2017
|
| URL | |
| Keywords |
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