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STEREO DERIVED CLOUD TOP HEIGHT CLIMATOLOGY OVER GREENLAND
FROM 20 YEARS OF THE ALONG TRACK SCANNING RADIOMETER (ATSR)
INSTRUMENTS
D. Fisher * * and J-P. Muller?
* Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, U.K., RH5 6NT
KEY WORDS: Climate, Environment, Atmosphere, Imagery, Matching, Pattern, Stereoscopic
ABSTRACT:
Current algorithms for the determination of cloud top height and cloud fraction in Polar Regions tend to provide unreliable results,
particularly in the presence of isothermal conditions within the atmosphere. Alternative methods to determine cloud top heights in
such regions effectively, from space borne sensors, are currently limited to stereo-photogrammetry and active sensing methods, such
as LiDAR. Here we apply the modified census transform to one month of AATSR stereo data from June 2008. AATSR is unique in
that it is the only space borne stereo capable instrument providing data continuously in both the visible, near infrared and thermal
channels. This allows for year round imaging of the poles and therefore year round cloud top height and cloud fraction estimation.
We attempt a preliminary validation of the stereo retrieved cloud top height measurements from AATSR against collocated cloud
height measurements from the CALIOP LiDAR instrument. CALIOP provides an excellent validation tool due to its excellent
height resolution of between 30-60 meters. In this validation, a pair of collocated swaths is assessed with a total of 154 inter-
comparisons; the results show that AATSR correlates well with CALIOP cloud base layers with an R? score of 0.71. However, in all
cases AATSR appears to be underestimating the cloud top height compared to CALIOP, the causes for this are currently not fully
understood and more extensive inter-comparisons are required. Once validation is completed a processing chain is in place to
process the entire ATSR time-series data generating a 20 year cloud top height dataset for Greenland.
1. INTRODUCTION
The current predominant algorithms for the determination of
cloud top height from space-borne sensors rely upon absolute
radiance measurements and knowledge of well-defined thermal
profiles. Such techniques include the use of the infrared
window technique (Rossow et al,. 1989) and CO, slicing (Frey
et al, 1999). It is well known that these methods perform
poorly where thermal gradients do not exist (Schweiger and
Key, 1992). Such isothermal conditions often occur in the lower
troposphere during polar summer (Karlsson and Dybbroe,
2010) or when thermal inversions are present and the land
surface is colder than the overlying clouds. Therefore in Polar
Regions, when using the aforementioned algorithms, cloud top
height (CTH) assignment and cloud fraction (CF) estimation is
unreliable.
The relationship between CTH and CF and the net radiation
balance in Polar Regions is complex. Over fresh snow with a
high albedo, increasing cloud cover leads an increase in the net
radiation balance (Ambach, 1974). Whilst over melting snow
or clean ice, with a lower albedo, increased cloud fraction leads
to a decrease in the net radiation balance, with a marked
dependence on the cloud type (Cawkwell and Bamber, 2002).
These effects are significant as they may lead to forcing of the
ice-albedo feedback mechanism, which is the attributed cause of
increased warming at higher latitudes in comparison to the
global mean in most climate models. Indeed, it has been shown
that a +5% change in cloud amount over Greenland is
equivalent to a 1°K change in temperature (loc. cit.). Improved
knowledge of both CTH and CF is vital to generate better
understanding of the responses of Polar Regions to climate
change,
* Corresponding author: dnf@mssl.ucl.ac.uk
Currently, effective and accurate CTH and CF estimation in
polar regions at suitable spatial and temporal scales is still to be
achieved. Datasets which can effectively resolve CTH and CF
are available; however, their spatial and temporal sampling is a
limiting factor. Such data sets include the Multi-angle Imaging
SpectroRadiometer (MISR) instrument which uses the
principles of stereo-photogrammetry to determine CTH to an
accuracy of ~500m. However, MISR is limited to the visible
spectrum so cannot provide year round coverage. Also able to
effectively resolve CTH over Polar Regions to an accuracy of
30-60m (depending upon the clouds altitude) is the LiDAR type
Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP)
instrument. CALIOP, due to the LiDAR technology involved
can operate year-round, however it only provides very limited
spatial sampling. Perhaps of more promise is the ATSR
instrument series with its long time series of ~20 years, thermal
channels to provide year round coverage and stereo capability
to eliminate the reliance upon ancillary LUTs for height
assignment. ATSR-2 data has already been successfully applied
for the determination of CTH using stereo-photogrammetric
methods over Greenland (Cawkwell ef al., 2001. Here, we hope
to demonstrate the potential of extending this dataset to provide
coverage over the entirety of Greenland for the extraction of
both CTH and CF for initially one month of AATSR data, with
the ultimate aim extending this to the entire ATSR time series.
2. CLOUD TOP HEIGHT DETERMINATION
2.1 AATSR Instrument
The Along Track Scanning Radiometer (ATSR, Mutlow, 1998)
instrument series began with ATSR launched on ERS-1 with
the mission of measuring sea surface temperature. The ATSR
instrument used four infrared channels (1.6, 3.7, 11 and 12um),
blackbody calibration to ensure consistent on board calibration
