A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region. Part I: A Synopsis of Observed Features of Tropical Cyclone Kathy (1984)

Abstract

Objective streamline analyses and digitized high-resolution IR satellite cloud data have been used to examine in detail the changes in the environmental circulation and in the cloud structure that took place in and around Tropical Cyclone Kathy (1984) when it started to intensify, and during its intensification and dissipation stages. The change of low-level circulation around Tropical Cyclone Kathy was measured by the change in the angle of inflow (a 4 ) at a radius of 48 latitude from the cyclone center. When Kathy started to intensify, a 4 increased suddenly from 208 to 42.58 in the northerly airstream to the north and northeast of the depression, and decreased to 08 to the south of the depression. At that stage the low-level circulation around the depression appeared as a giant swirl that started some 600 km to the north and northeast of the depression and spiraled inward toward its center, while trade air, which is usually cool, dry, and stable, did not enter the cyclonic circulation. The angle a 4 remained the same during intensification. During the dissipation stage, a 4 returned to 208 and trade air started to participate in the cyclonic circulation.Satellite cloud data were used to determine the origin, evolution, and importance of the feeder bands in the intensification of the cyclone, to follow the moist near-equatorial air that flowed through them and to estimate the maximum height of cumulonimbi that developed in them, to observe the changes in the convective activity in the central dense overcast (CDO) area, as well as in the area around the CDO. Most of the observed changes in Kathy have also been observed in other tropical cyclones during intensification and dissipation.Using the sequence of observed changes of the circulation and of convective activity in and around the CDO of Kathy, a mathematical model has been developed to forecast the intensity of a tropical cyclone. The model and its application to three tropical cyclones in the Australian region are described in Part II of this paper.

A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region. Part II: An Analytic Method for Determining the Time Variation of the Intensity of a Tropical Cyclone*

“…3 and 13 of Part I, respectively. Similar or larger sudden falls have been observed for other storms (Lajoie 2007). Some time between 0900 and 1200 UTC 20 March the intensification rate suddenly increased to 25 hPa day 21 .…”

“…Sometimes r m can be estimated (Kossin et al 2007;Lajoie and Walsh 2008), but r e cannot be determined either because the eye has not yet developed or the eye is not visible in satellite cloud images because of the cirrus shield over it. (21), p c , the central surface pressure, can be calculated if r e and r m are known.…”

“…Tables of detailed computations for the four feeder bands can be inspected in Lajoie (2007). Table S6 (see the supplement) lists the computed surface pressure, sustained maximum mean surface winds (0.91 times the mean gradient-level wind, see Table S1 in the supplement) at different times for the four feeder bands.…”

“…Naval Research Laboratory were used to estimate the time the cyclone started to develop, the size of r e and r m following the technique of Lajoie and Walsh (2008). It started to intensify at about 0900 UTC 17 March 2006 at about 1300 km east of Cairns.…”

“…Parameters r m and r e are determined from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI; Simpson et al 1988;Lonfat et al 2004), TRMM Precipitation Radar (PR) data (Iguchi et al 2000), or digitized high-resolution IR satellite cloud data (see Lajoie 2007;Lajoie and Walsh 2008). Finally, by using a reduction factor to convert gradient-level mean wind to mean surface winds, and the mean asymmetric wind distribution around a tropical cyclone, the horizontal distribution of the sustained mean surface wind as well as the maximum gust around the cyclone can be estimated.…”