Remote sensing of multiple cloud layer heights using multi-angular measurements
Cloud top height (CTH) affects the radiative properties of clouds. Improved CTH observations will allow for improved parameterizations in large-scale models and accurate information on CTH is also important when studying variations in freezing point and cloud microphysics. NASA's airborne R...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-06-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/10/2361/2017/amt-10-2361-2017.pdf |
| Summary: | Cloud top height (CTH) affects the radiative properties of
clouds. Improved CTH observations will allow for improved parameterizations
in large-scale models and accurate information on CTH is also important when
studying variations in freezing point and cloud microphysics. NASA's airborne
Research Scanning Polarimeter (RSP) is able to measure cloud top height using
a novel multi-angular contrast approach. For the determination of CTH, a set
of consecutive nadir reflectances is selected and the cross correlations
between this set and collocated sets at other viewing angles are calculated
for a range of assumed cloud top heights, yielding a correlation profile.
Under the assumption that cloud reflectances are isotropic, local peaks in
the correlation profile indicate cloud layers. This technique can be applied
to every RSP footprint and we demonstrate that detection of multiple peaks in
the correlation profile allows retrieval of heights of multiple cloud layers
within single RSP footprints. This paper provides an in-depth description of
the architecture and performance of the RSP's CTH retrieval technique using
data obtained during the Studies of Emissions and Atmospheric Composition,
Clouds and Climate Coupling by Regional Surveys (SEAC<sup>4</sup>RS) campaign. RSP-retrieved cloud heights are evaluated using collocated data from the Cloud
Physics Lidar (CPL). The method's accuracy associated with the magnitude of
correlation, optical thickness, cloud thickness and cloud height are
explored. The technique is applied to measurements at a wavelength of 670
and 1880 nm and their combination. The 1880 nm band is virtually insensitive
to the lower troposphere due to strong water vapor absorption.
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It is found that each band is well suitable for retrieving heights of cloud
layers with optical thicknesses above about 0.1 and that RSP cloud layer
height retrievals more accurately correspond to CPL cloud middle than cloud
top. It is also found that the 1880 nm band yields the most accurate results for
clouds at middle and high altitudes (4.0 to 17 km), while the 670 nm band is
most accurate at low and middle altitudes (1.0–13.0 km). The dual band performs
best over the broadest range and is suitable for accurately retrieving
cloud layer heights between 1.0 and 16.0 km. Generally, the accuracy of the
retrieved cloud top heights increases with increasing correlation value.
Improved accuracy is achieved by using customized filtering techniques for
each band with the most significant improvements occurring in the primary
layer retrievals. RSP is able to measure a primary layer CTH with a median
error of about 0.5 km when compared to CPL. For multilayered scenes, the
second and third layer heights are determined median errors of about 1.5
and 2.0–2.5 km, respectively. |
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| ISSN: | 1867-1381 1867-8548 |