Background
Hurricanes (in the Atlantic) or typhoons (in the Pacific) are Earth's strongest cyclones. A
tropical storm becomes a hurricane when sustained wind
speeds reach 64 knots (74 mph). Accurate predictions of their
tracks and intensities can save lives and minimize
property loss. While hurricane tracks can be predicted
with fairly good accuracy, the prediction of hurricane
intensity remained a challenge to operational
forecasting. This page is based on the article by Kelley and Stout (2004).
Condensational heating associated
with precipitation is a major driving force of
atmospheric circulation.
Back in the fifties, it was proposed that most of the
heating of the upper troposphere in the Tropics are
concentrated in a few towering cumulonimbus clouds
rather than large scale uplifting (Riehl and Malkus,
1958 and Malkus, 1959).
The "hot towers" which refers to the tall cumulonimbus,
has been seen as one of the mechanisms by which the
intensity of a tropical cyclone is maintained. Because of the size (1-5 km)
and short duration (30 minute to 2 hours) of these hot
towers, studies of these events have been limited to
descriptive studies from aircraft observations,
although a few have attempted to use the presence of
hot towers in a predictive capacity. Before TRMM, no data set
exists that can show globally and definitively the
presence of these hot towers in cyclone systems. Aircraft radar studies of
individual storms lack global coverage. Global microwave or Infrared
sensor observations do not provide the needed spatial
resolution. With a ground
resolution of 5 km, the TRMM Precipitation Radar
provided the needed data set for examining the
predictive value of hot towers in cyclone
intensification.
Sample Applications
After six years of operations, the
TRMM satellite observed over 100 tropical
cyclones. In most cases,
the TRMM was able to sample the cyclones twice or three
times. The best case is
hurricane Isabel that made landfall in the mid-Atlantic
regions in September 2003. TRMM was able to sample the
hurricane six times during its life cycle.
Figure 1 shows a composite
of TRMM observations of Isabel. (Courtesy TSDIS)
Figure 2 shows the accumulated rainfall
for the period September 7-18, 2003 over the hurricane
path.
Figure 3 shows the vertical structure
of Isabel using PR through both its intensification and
decay stages. These
figures do show the presence of high convective towers
in its intensifying stages. (Courtesy TSDIS)
To examine the "Hot tower" theory,
Kelley and Stout (2004) collected data of these hot
towers embedded in the hurricanes observed by
TRMM. They classified the
hurricane stages as intensifying and non-intensifying
hurricanes.
Figure 4 (after Kelley and Stout, 2004)
shows the vertical distribution of the tallest rain
pixel in the eyewalls of tropical cyclones as observed
by TRMM PR. The red and
black lines show the distribution for intensifying and
non-intensifying cyclones. There are distinct double
peaks of the tallest convective PR pixel for the
intensifying cyclone.
After careful sensitivity and statistical analysis,
they concluded that intensifying cyclones are more than
twice likely to have a convective tower in their
eye-wall than non-intensifying cyclones.
TRMM Data
The data used in their analysis are Precipitation Radar (PR) Data
The precipitation radar (PR) was
developed by CRL and NASDA in Japan. The instrument is
a 128-element active phased array system, operating at
13.8 GHz. The nadir footprint of PR is 4.3 km, with a
vertical resolution of 250m. The minimum radar
reflectivity factor is about 18 dBZ, corresponding to a
rain rate of about 0.5 mm/hour. It obtains unique
rainfall information by its 215-km cross-track scan
through nadir.
TRMM 2A25 contains vertical ranfall
rate profiles for one orbit. Also provided are:
attenuation corrected Z profiles, parameters of Z-R
relation (the relation between Z and rainfall rate),
integrated rainfall rate for each ray, range bin
numbers of rain layer boundaries, and many intermediate
parameters.
A granule of TRMM 2A25 consists of
metadata, clutter flags, and swath data. See
Readme for TRMM Product 2A25 for information on
acquiring and accessing this data product.
Some of the images in Figures 1
and 3 are produced using the TSDIS OrbitViewer.
Figure 2 was produced using the TRMM Online
Visualization and Analysis System (TOVAS)
References
Kelley, O., and J. Stout, 2004:
"Convective towers in eyewalls of tropical cyclones
observed by the TRMM precipitation radar in 1998-2001,"
20th Conference on Weather Analysis and
Forecasting, American Meteorological Society, January
11-15, 2004, Seattle, Washington.
Malkus, J. S. , 1959: Recent
development in studies of penetrative convection an dan
application to hurricane cumulonimbus towers, Cumulus
Dynamics: Proceedings of the 1st Conference
on Cumulus Convection, Pergamon Press, 65-84.
Riehl, H and J. S. Malkus,
1958: On the heat balance in the equatorial trough
zone, Geophysica, 6, 503-538.
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Atmos Composition
