Figure 1. Wind maxima of 50 knots (at one-km height) located 200 km west of Kwajalein (letter K) at 1532 UTC 6 August 1997 (DWSR-93S PPI image, 0.5( degree elevation angle).
1. INTRODUCTION
The 1997 western Pacific Tropical Cyclone (TC) season was very active in the vicinity of the Marshall Islands. This paper will look at some reasons for this activity. It will also attempt to assess the role that wind maxima and wind bursts play in the development and intensification of some tropical cyclones. The increased tropical cyclone activity in the area was consistent with an ENSO event as warmer waters shifted eastward into the central and eastern Pacific. Earlier studies have found that the genesis region for western Pacific tropical cyclones shifts eastward during ENSO events (Chan, 1990). In addition to this increase in tropical cyclone activity in the eastern portion of the Western Pacific(WPAC) area, there is also a tendency for cross-equatorial tropical cyclone pairs to be more frequent during the November and December period.
accommodation inn BourgasDuring 1997, the WPAC area experienced 33 tropical cyclones (which is two above the long-term average). Eleven of these storms reached Super Typhoon (STY) intensity. This was well above the average of four STY per year. Twelve of these tropical cyclones developed in, or moved through, the Marshall Islands area with eight becoming Super Typhoons.
Convection associated with many of these systems was monitored by the Kwajalein Missile Range DWSR-93S Doppler Radar. This included nearly 48 hours of radar coverage for the eye of TyphoonPaka. Paka passed just south of the Kwajalein area on 11 December. During the TC season several significant radar features were observed including an eyewall replacement sequence (with TyphoonPaka) and the development of significant convection associated with the initial stages of STY Keith and STY Winnie.
2. FAVORABLE CONDITIONS FOR GENESIS OF TROPICAL CYCLONES
Several authors have developed a list of conditions required for tropical cyclone development. This list includes: low-level convergence, and deep convection occurring in a pre-existing tropical disturbance. The tropical disturbance should have a favorable environment for development including good low-levelvorticity, low-level convergence, favorable sea surface temperatures, little vertical wind shear, and favorable outflow at the upper-levels (Gray, 1968; McBride andZehr, 1981; and Zehr, 1991).
The second stage of the genesis process requires some mechanism which will enhance low-level convergence resulting in increased vertical motion and increasing convection. The disturbance should also have a favorable upper-level outflow pattern to support mass convergence at low-levels along with mass removal (divergence) at the upper-levels. Some factors suspected of contributing to initiating and impacting development of tropical cyclones include westerly wind bursts (Phoebus, 1993) and wind surges in the low-levels of the atmosphere(Zehr, 1991).
Most tropical forecasters have found that the multitude of numerical models have difficulty in forecasting the initiation of convection. Usually, the subtle changes in wind flow may produce the necessary low-level convergence needed to trigger convection. In the authors' opinion, these subtle changes in wind speed (wind surges) may be the primary factor in initiating the deep convection. These wind speed maxima combine with other favorable environmental factors to provide sufficient low-level forcing to trigger this process. The inward transport of momentum toward the center of the disturbance (associated with the windmaxima) then combines with vertical motion and favorable outflow to intensify the cyclone.
3. SUPER TYPHOONS: WINNIE AND KEITH
Two excellent examples covering the genesis stage of rapidly developing tropical cyclones were observed by the Kwajalein DWSR-93S Doppler Radar in WPAC in 1997. The first example was a wind speed maxima which became the triggering mechanism for STY Winnie. This wind maxima is seen in Figure 1. At 1532 UTC on 6 August 1997, the initial stages of Winnie were captured by the DWSR-93S radar on Kwajalein Atoll. The 50-knot wind maxima (at 1 km height) served as the primary trigger for a rapidly developing area of convection which became Typhoon Winnie at 1000 UTC on 10 August. Typhoon Winnie would become a Super Typhoon at 0000 UTC on 12 August.
Figure 1. Wind maxima of 50 knots (at one-km height) located 200 km west of Kwajalein (letter K) at 1532 UTC 6 August 1997 (DWSR-93S PPI image, 0.5( degree elevation angle).
A second example is found in Figure 2. This picture captured the initial stages of STY Keith. A wind speed maxima of 55 knots was detected in the radar data at 2152 UTC on 27 October 1997. This wind speed maxima was the initial triggering mechanism for a rapidly increasing area of convection which became Typhoon Keith at 0600 UTC 30 October. Typhoon Keith developed into a super typhoon at 1800 UTC on 31 October. Note the LTG symbol in Figure 2 indicates lightning potential in the individual convective cells. The Kwajalein criteria used for lightning potential is reflectivity values of 30 dBZ extending above 30 000 feet. Both Winnie and Keith developed in a favorable environment with a pre-existing deep cyclonic circulation (persisting in the area for several days) prior to the rapid development. Other minor vortices were detected in both radar and satellite imagery during this period, but most of these features were short-lived and failed to develop beyond the minor convective complex stage. The primary difference between the vortices which developed and the ones that did not was the occurrence of the wind bursts (which preceded the development and intensification of the tropical cyclones).
Figure 2. Wind maxima of 55 knots (at one-km height) located 120 km south of Kwajalein (letter K) at 2152 UTC on 27 October 1997 (DWSR-93S PPI image, 0.5( degree elevation angle).
The occurrence of the wind bursts and subsequent intensification of the system to typhoon status is consistent with earlier observations by Zehr (1982). Other environmental factors which contributed to TC development included sea surface temperatures of 29-30(C in the area (and a favorable outflow pattern at the 250-mb level) with a strengthening sub-tropical ridge of high pressure to the north of the system. The building of the ridge to the north provided an increasingly favorable outflow pattern during the intensification process.
hoteles en Edimburgo4. SUPER TYPHOON PAKA
The third example of the intensification of tropical cyclones by wind bursts and wind speed maxima was Super Typhoon Paka. Earlier studies have suggested that the occurrence of westerly winds and wind bursts near the equator have contributed to subsequent development and intensification of tropical cyclones during ENSO years (Keen, 1982; Phoebus, 1993; Harrison and Geise, 1991). Typhoon Paka developed in an environment similar to most cross-equatorial cyclone pairs. Westerly winds were observed along the equator several days before Paka reached typhoon intensity. As Paka tracked westward just south of Kwajalein Atoll during the period 10 December to 12 December 1997, a series of volume scans from the Kwajalein radar were used to extract Doppler wind velocities. The one-km Doppler winds were overlaid on the reflectivity data to obtain a better representation of storm structure. During this two-day period, several wind speed maxima were observed in animated satellite imagery and surface data. Wind data derived from the radar was compared to Kwajalein radiosonde data for the one-km level. This radar-derived wind data was found to be consistent with the observed wind data.
The DWSR-93S radar presentation of the eyewall structure of Paka showed considerable variability on 11 December 1997. Much of this variability appeared to be associated with cyclical changes in the convective activity and the associated wind speed. The most intense convective activity would usually be found near the wind bursts or speed maxima. One example is found in Figure 3. This one-km CAPPI (Constant Altitude Plan Position Indicator) image at 1302 UTC on 11 December depicts a wind speed maxima of 115 knots just north of the eye. As this wind speed maxima approached the center of the storm, the eyewall convective activity increased (eyewall also became better defined). This was near the peak intensity of the storm during this particular cycle.
Figure 3. Wind maxima of 115 knots (at one-km height) near the eye of Typhoon Paka at 1302 UTC on 11 December 1997 (DWSR-93S CAPPI image, one-km height). Letter K denotes the radar site on Kwajalein. Maximum reflectivity values in inner eyewall exceeding 50 dBZ.
As the wind speed maxima translated into the center of the storm, the eyewall began to weaken with a 2nd eyewall forming some distance from the center. The satellite picture in Figure 4 (2035 UTC on 11 December 1997) captured the changing eyewall structure as the inner eye became more diffuse during the eyewall replacement cycle (second eyewall forming). A review of the radar reflectivity for this same time period indicated the most intense convection had shifted to the outer eyewall and wind speeds (one-km height) had decreased to 100 knots.
Figure 4. GMS-5 High Resolution Visible image of Typhoon Paka at 2035 UTC 11 December 1997. Note the appearance of the double-eye structure on both the radar and satellite imagery. Letter K denotes Kwajalein Atoll.
5. CONCLUSIONS
Low-level wind bursts and wind speed maxima play a very important role in the genesis process of tropical cyclones. The increase in low-level mass convergence associated with these wind speed maxima provides a triggering mechanism for increased convection in an existing disturbance which has a favorable environment. As these maxima move into the storm center, they bring increased kinetic energy through the transfer of momentum to the system. When these speed maxima reach the center of the storm, there is a decrease in energy (upstream away from the storm center) and therefore the amount of kinetic energy would decrease. The storm would then have a tendency to weaken after the wind maximum reaches the storm center. This would be like the ice skater that spins faster when the arms are pulled in and then slows when they are extended. This process may have some impact on the development of the second eyewall structure as there is a period of relaxation following the period of maximum inflow to the storm. In conclusion, there is a need for further study and expanded use of some of the more detailed wind data sets to properly forecast the genesis of tropical cyclones.
6. REFERENCES
Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96, 669-700.
Harrison, D. and B. Geise, 1991: Episodes of surface westerly winds as observed from islands in the western tropical Pacific. J. Geo. Research, 96, 3221-3237.
Keen, R. A., 1982: The role of cross-equatorial cyclone pairs in the Southern Oscillation. Mon. Wea. Rev., 110, 1405-1416.
McBride, J. L. and R. Zehr, 1981: Observational analysis of tropical cyclone formation, Part II: Comparison of non-developing versus developing systems. J. Atmos. Sci., 38, 1132- 1151.
Phoebus, P. A., 1993: Westerly wind anomalies associated with developing tropical cyclones in the western Pacific. Preprints, Copenhagen luxury hotels20th Conference on Hurricanes and Tropical Meteorology, San Antonio, Amer. Meteor. Soc., 127-130.
Zehr, R. M., 1991: Observational investigation of tropical cyclogenesis in the western north Pacific. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, Amer. Meteor. Soc., 235-240.
Corresponding author address:
Glenn H. Trapp
c/o Aeromet, Inc.
P.O. Box 701767
Tulsa, OK 74170
e-mail: kmr_web@aeromet.com.
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