Winds in the Polar Regions from MODIS: Atmospheric Considerations

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Winds in the Polar Regions from
MODISAtmospheric Considerations
Jeff Key1, Dave Santek2 , Chris Velden2, and Paul
Menzel1 1Office of Research and Applications,
NOAA/NESDIS, Madison, Wisconsin2Cooperative
Institute for Meteorological Satellite Studies,
University of Wisconsin
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Characteristics of the Polar Regions
Condensate over a lead
Greenland ice sheet
SHEBA ship
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Temperature
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Water Vapor
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If its not dark, its very bright
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Tropospheric Winds
Raob data are from NCDC/FSL
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Clouds Properties
New data set The AVHRR Polar Pathfinder has been
extended to include surface, cloud, and radiative
properties for 1982-1999 at 25 km.
Arctic Cloud Amount
Near real-time AVHRR retrievals are available at
http//stratus.ssec.wisc.edu/products/rtcaspr
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Joint Frequency of Cloud Pressure and Optical
Depth Arctic Winter and Summer
Frequency of clouds that are thin (OD lt 5) and
low (P gt 600 hPa) January 22, June 34
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Joint Frequency of Cloud Pressure and Optical
Depth
Frequency of clouds that are thin (OD lt 5) and
low (P gt 600 hPa) January 22, June 34
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Clouds from AVHRR Validation
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Surface Radiation at Neumayer Station, Antarctica
Arctic Cloud Amount
Note APP results are for 1991 only others are
for 1986-1991.
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Height Assignment
CO2-Slicing Problems occur when the
clear-cloudy radiance difference is small.
Cloud pressures greater than 700 hPa (lower in
altitude) are generally not retrievable with this
method.
Note difference in horizontal scales.
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Cloud-Surface Temperature Differences from AVHRR
Small cloud-surface temperature differences are
common and are not restricted to low clouds.
Results are similar for Arctic. Note frequency
of warm clouds (warmer than surface), especially
in winter.
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MODIS CO2-Slicing Failure Rate in the Polar
Regions
No CO2 retrieval attemptedbelow 700 hPa
No CO2 retrieval found
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IR Window Currently, this approach assumes the
cloud is opaque so that the IR brightness
temperature is also the cloud temperature. Find
the temperature in the profile to get the
height. An adjustment for surface emission
should be used with thin clouds, which means
optical depth must be calculated. The ISCCP and
CASPR methods adjust cloud temperature if the IR
optical depth is less than 4.6 (gt 1
transmission), which is a larger visible optical
depth for water clouds but somewhat smaller for
ice clouds.
Joint Frequency of Cloud Pressure and Optical
Depth
Frequency of clouds that are thin (OD lt 5) and
low (P gt 600 hPa) January 22, June 34
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Note slope differences for low clouds
H2O-Intercept Problem 6.7 ?m band is
insensitive to low clouds. In theory the 7.2 ?m
band, which peaks in the lower troposphere, would
be better. In practice the method is generally
not useful for cloud pressures greater than 600
mb for 6.7 ?m and 750 hPa for 7.2 ?m.
6.7 ?m
7.2 ?m
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H2O-Intercept Two Channel Solution? The idea
The intersection of the lines connecting points
at 6.7 ?m and 7.2 ?m give a brightness
temperature (actually, 3 Tbs) that can be used to
estimate the cloud height with a model profile,
eliminating the need to compute the opaque cloud
temperature (not really the Planck
function). However, the two opaque cloud curves
diverge for mid- and low-level clouds.
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Can the 6.7 ?m band see the surface?
This figure shows the modeled change in the 6.7
?m brightness temperature as a function of total
precipitable water (TPW) when the surface
temperature is varied by 15 degrees. For a given
Arctic/ Antarctic profile, TPW is held constant
while the surface temperature is varied. The
figures below show that variations in the surface
temperature do affect the 6.7 ?m Tb when the
atmosphere is very dry. So theoretically, this
band can see the surface. The problem is more
significant for the 7.2 ?m band.
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Can the 6.7 ?m band see the surface? (cont.)
This is a MODIS image covering part of the Arctic
(SE Greenland) on 19 March 2001. Surface
features are clearly seen in the IR window band
(left), but are also apparent in the water vapor
band (right).
6.7 ?m
11 ?m
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IR Window Currently, this approach assumes the
cloud is opaque so that the IR brightness
temperature is also the cloud temperature. Find
the temperature in the profile to get the
height. An adjustment for surface emission
should be used with thin clouds, which means
optical depth must be calculated. The ISCCP and
CASPR methods adjust cloud temperature if the IR
optical depth is less than 4.6 (gt 1
transmission), which is a larger visible optical
depth for water clouds but somewhat smaller for
ice clouds.
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Converting the cloud temperature to a cloud
pressure (lookup in the profile), the adjustment
in summer will generally increase the cloud
altitude. In winter the direction of change may
be mixed due to inversions.
The point-by-point retrievals, with and without
the adjustment for optical depth, are shown above
for one summer image. Only clouds with visible
optical depths less than 5 are shown. The
relative frequency of the pressure differences is
shown at left.
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Case Study Infrared Winds
Low Level Mid Level High Level
05 March 2001 Daily composite of 11 micron MODIS
data over half of the Arctic region. Winds were
derived over a period of 12 hours. There are
about 4,500 vectors in the image. Vector colors
indicate pressure level - yellow below 700 hPa,
cyan 400-700 hPa, purple above 400 hPa.
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