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SATELLITE
INTERPRETATION
DISCUSSION
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NOAA
NESDIS
Regional
and Mesoscale
Meteorology
Team
Cooperative Institute for Research in the
Atmosphere (CIRA)
Colorado State University
Fort Collins, Colorado
November 9, 1998
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Size versus Intensity of
Tropical Storms
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In today's satellite discussion, the size and intensity
of a hurricane in the East Pacific is compared to a typhoon in the North
West Pacific, occurring at the same time at about the same latitude (16N).
The climatology of tropical cyclone sizes confirms that tropical cyclones
of the north west Pacific are characteristically larger than their Atlantic
and East Pacific counterparts. In the following examples, a GMS IR
image of Typhoon Zeb in the West Pacific has been remapped to the same
projection and resolution as Kay in the East Pacific, off the coast of
Baja. Kay was still tropical storm intensity at the time of the image
but was upgraded to hurricane intensity at 21 UTC on Oct 13. Click
on images below for full display.
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Figure 1
GMS IR Hurricane Zeb
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Figure 2
GOES-10 IR Hurricane Kay
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Minimum sea level pressure (MSLP) and maximum sustained
lower-tropospheric winds are usually used as measures of intensity.
Intensification refers to a decrease in MSLP or an increase in maximum
winds. The areal extent of a cyclone circulation is measured by forecasters
as the extent of winds above a certain speed, usually gale force (17 m/s
). The radius of 35 kt (17 m/s) or greater winds for Typhoon Zeb
covers an area of 300 Nautical Miles (NM) while this same measurement for
Tropical Storm Kay is only 60 NM. Coincidentally, on October
13, 1998, both of these storms were located at latitude 16 N., on opposite
sides of the world. Zeb was classified as a Super-Typhoon with
maximum surface winds of 155 kt. (Super-Typhoon is a classification
used in the NW Pacific for tropical cyclones greater than 125 kt intensity).
The visible image below further illustrates that size
and intensity are not always related. On August 25, 1998, while
Hurricane Bonnie was making landfall on the east coast, it's average radius
of 35 knot or greater winds extended 140 nautical miles, while Danielle's
extended 70 nautical miles. For a given intensity, the size of individual
Tropical Cyclones, according to the radial extent of gale force winds may
be very different.
Figure 3
GOES-8 Visible
Reference: Merrill, R.T., 1984: A comparison
of large and small tropical cyclones. Mon. Wea Rev., 112,
1408-1417.
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