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MODIS Infrared Imagery (IR) | 89 GHz AMSR-E |
NASA's satellite CloudSat captured an extraordinary eye overpass of a
category 4 Hurricane Bill on August 19, 2009 at 1720 UTC (1220 EDT).
Bill's maximum sustained winds are 132 mph (115 knots) with a central
pressure of 947 mb.
The 89 GHz AMSR-E onboard the Aqua spacecraft (full color image) reveals
deep convection and low brightness temperatures ( < -117 F) as shown by
the bright red and deep brown colors. The half red circle surrounding the eye
center on the AMSR-E image depicts the eye wall undergoing a replacement
cycle. Hurricane Bill has undergone several eye wall replacement cycles
(EWRC) causing the hurricane to intensify and weaken. One of the key advantages
of 89GHz imagery is to help identify areas of strong convection by looking at the
brightness temperatures, colder brightness temperatures revel deeper areas
of convection. 89GHz imagery is also used to help clearly identify storms
centers by penetrating thin cirrus shields.
The CloudSat overpass (shown by the red line on the AMSR-E image) reveals
the cirrus free eye (~50 km wide), surrounded by an intense eyewall
(orange and red reflectivities) with cloud tops on the northwestern edge
extending almost 16 km into the atmosphere. The eyewall bends backward and
then inward likely from northerly shear along the top of the storm.
The tallest thunderstorm cloud top in the northwestern eyewall edge is a
"hot tower," a cumulonimbus cloud penetrating the TTL (Tropical
troposphere layer). The hot tower extends all the way from the surface
along the inner eyewall edge to > 15 km. Hot towers form as a result of
large amounts of latent heat released as water vapor condenses into
liquid. The release of this energy provides fuel for the storm to further
intensity and strengthen as Bill was experiencing during the CloudSat
overpass.
The purple and blue colors in the cirrus regions of the CloudSat overpass
show small ice crystals. Ice crystals are found in the upper portion of
cumlonumibus cloud tops and cirrus canopies as atmospheric conditions
remain quite cold. The sensitivity of the CloudSat CPR (Cloud Profiling
Radar) helps detect these features.
The infrared imagery (IR) from the Moderate Resolution Imaging
Spectroradiometer (MODIS) onboard Aqua reveals white cloud tops indicating strong
thunderstorms and intense convection. By comparing the IR imagery with the
CloudSat overpass, multiple cloud layering is shown throughout the storm,
a feature not apparent from IR imagery alone. The CloudSat overpass reveals
convection free areas between the heavier rain bands called moats. IR
imagery indicates strong convection is found throughout the storm, on the
north and south edges but the CloudSat track shows the strongest area of
convection on the northern edge of system, verified by the AMSR-E image.
The southern edge of the storm contains drier air at the lower eye wall
levels and less convection not easily detected from either AMSR-E or IR
imagery. The cloud layering is indistinguishable from the IR imagery as all
of the cloud layer tops appear to be at the same height.
Quicklook Images can viewed at the CloudSat Data Processing Center.