NRL Monterey, Marine Meteorology Division
|Satellite Detection of Tropical Upper-Level Vortices|
|The appearance and structure of tropical cyclones, as viewed from space, was the subject of intense scrutiny following the launch of TIROS-I in April 1960. One feature discovered then, but which has not received much attention in recent years, was the tendency for an upper level vortex to develop, generally to the southeast, in the wake of a major tropical cyclone. The vortex was postulated to develop in connection with the strong anticyclonically-curving upper level outflow generally found in the divergent region above a strong storm (Fett, 1964). Fig. 1 is taken from the article just referenced.|
|Fig. 1. TIROS III Nephanalysis. 10 Sep 1961, 2225Z (200mb Winds Superimposed)|
|Fig. 1 shows a strong upper level "hurricane jet" encircling Hurricane Carla in September 1961. A cold core, upper level vortex, developed east southeast of Carla, as strong northerly flow on the east side of Carla was diverted to turn cyclonically about this feature. As pointed out in the article, this diversion could be anticipated due to the requirement for conservation of potential vorticity (according to this concept relative vorticity must increase to compensate for the decreasing value of the Coriolis parameter obtained in the southward flow of the jet stream current). This point is important because it suggests that such development is not haphazard but can be routinely expected as part of normal tropical cyclone development.|
|The upper level vortex, being cold core, decreases in intensity downward,
so that there is normally little evidence of disturbance in the flow at
lower levels. Nevertheless, the effects of the cyclonic-turning from
aloft are often enough to cause a subtle, but still evident, swirling of
lower level cloud forms in the boundary layer. This provides a clue for
the savvy Navy satellite meteorologist, when seeing such evidence in
satellite visible or infrared data, to suspect the existence of a cold low
aloft. Since upper level wind data are very sparse over many ocean
regions, the satellite data may be the only clue that such a feature
exists. Knowledge of upper level wind conditions can be very important
for many tactical applications including aircraft flight planning,
aircraft refueling, missile launch considerations and many others. It has
also been shown that such upper level cold core features can on occasion
develop into warm core tropical systems (Fett, op. cit).|
An example documenting such a feature appearing in more recent times is the subject of this study.
|Fig. 2.||Fig. 3.||Fig. 4.|
|Fig. 2 shows tropical storm Humberto near 28ßN, 63ßW, due east of the
Florida Peninsula. Considering the implications in the discussion of Fig.
1, an upper level low might be anticipated somewhere to the east or
southeast of Humberto.|
Fig. 3 shows a suspicious region northeast of the Lesser Antilles, where low clouds seem to define a cyclonic swirl, sometimes referred to as a "Screaming Eagle" (Fett, 1979). Such a pattern has been successfully used to detect weak easterly waves. Fig. 4 illustrates the cyclonic turning of low clouds that forms the pattern. The "Screaming Eagle" pattern can be imagined by noting the flaring of "wings" stretching from SW to NE and the "talons" perched on latitude 20ßN, longitude 50ßW.
|Fig. 5. GOES-8 Visible Image, 22 Sep 2001, 1800Z (Low Level Winds Superimposed)||Fig. 6. GOES-8 Water Vapor Image, 22 Sep 2001, 1800Z (Upper Level Winds Superimposed)|
|Fig. 5 shows satellite derived low-level wind vectors (GOES cloud-track winds) superimposed on a visible image acquired within an hour of the infrared data, previously used to illustrate the storm pattern. The winds confirm a weak wave in the region of the "Screaming Eagle". Much more emphatically revealed by winds derived from a GOES water vapor image (GOES vapor drift winds)(Fig. 6) is an upper level vortex in the same region, overlying the "Screaming Eagle".|
|Fig. 7. GOES-8 Infrared Image. 22 Sep 2001, 1700Z (Surface Winds Superimposed)||Fig. 8. GOES-8 Infrared Image. 22 Sep 2001, 1700Z (300 mb Winds Superimposed)|
|Fig. 7 shows the NOGAPS surface wind pattern superimposed on the GOES-8
data. This product shows that a weak cyclonic turning of winds is
associated with the pattern - a turning which implies a weak easterly wave
in that region.|
Fig. 8 shows the NOGAPS 300 mb wind pattern superimposed on the GOES-8 data. These winds are in agreement with the water vapor data (Fig. 6) indicating that an upper level low center lies directly over the weak swirl of clouds defining the "Screaming Eagle".
1. An upper level low can frequently be anticipated to develop in the region to the east or southeast of a major tropical cyclone.
2. The upper level low is cold core and tends to decrease in intensity downward.
3. A weak cyclonic turning of low cloud forms can frequently be detected in the region below the upper level low. This cloud pattern has sometimes been referred to as a "Screaming Eagle".
4. Surface level winds in the region typically also show a slight cyclonic turning. This pattern is often used to define the axis of an "easterly wave".
1. Fett, R. W., 1964, Aspects of hurricane structure: New model considerations suggested by TIROS and Project Mercury observations, Mon. Wea. Rev., vol. 92, No. 2, 43-60.
2. Fett, R. W., W. A. Bohan, et al., 1979, Navy Tactical Applications Guide, Vol. 2, Environmental Phenomena and Effects, Naval Research Laboratory, Monterey, CA, 93940, pp191.
Author: Bob Fett
Last Updated: Mon Dec 9 10:50:11 2002
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