The Development and Motion of Typhoon "Doris," 1950

Transcription

1 326 BULLETIN AMERICAN METEOROLOGICAL SOCIETY The Development and Motion of Typhoon "Doris," 1950 GEORGE P. CRESSMAN Hq.y Air Weather Service, Washington, D. C. ABSTRACT The development and motion of typhoon "Doris," which was observed during the first two weeks of May, 1950, are studied. The development of the storm is examined with respect to previously published theories of storm formation. The original deepening occurred in the low latitude portion of an extended trough, after the fracture of the trough. This is in agreement with a model proposed by Riehl. The motion of the deepening storm relative to the high-level flow patterns differed from previously studied examples in that the deepening occurred as the low-level cyclone moved from under the west side of an upper anticyclone toward a position under an upper cyclone. The storm developed as two cyclonic vortices, which gradually merged into one, in agreement with a principle of Fujiwhara. The motion of the storm northward, as it broke through the subtropical ridge line, is shown. After examination of several possibilities, this motion is attributed to the resultant of all the Coriolis forces acting on the storm, as discussed by Rossby. The suggestion is made that this resultant force becomes prominent in determining the motion of the storm due to changes in the i radial velocity profile and the increasing geographical extent of the storm. "^HE development and movement of typhoon "Doris," which occurred in the west easterlies. This is suggested by a west wind at southern hemisphere and the northern hemisphere Pacific in the first two weeks of May 1950, 10,000 feet at Honiara (09 S, 160 E) on 1 May, offer an opportunity for the testing of several new and is confirmed by the streamline chart of 2 May. theories relating to tropical cyclones. There were By 03GCT, 2 May, the northern part of the west an unusually large number of upper-air observations available for this part of the world, and a reasonably complete life history of the cyclone can be derived. THE DEVELOPMENT STAGE It is convenient to trace the development by starting with the 10,000-ft streamline chart (FIG. 1) and the 700-rnb chart (FIG. 2) for 0300 GCT, 1 May The pattern of middle latitude long waves consists of a trough over Manchuria, a ridge east of Japan and Sakhalin, a trough at about 167 E, and a ridge at about 170 W. As a result of the short wavelength from trough to trough in this pattern, the west Pacific trough was moving rapidly eastward. It was shown in a previous investigation [1] that if the higher-latitude portion of an extended trough is moving rapidly eastward, it will be unable to retain its tropical extension. In the present example, then, the portion of the extended trough which lies in the tropics should break off from the middle latitude trough. According to Riehl and Burgner [10] this fracture produces a situation which is favorable for low-latitude cyclogenesis. An additional factor of some importance is the existence of a line of shear between cross-equatorial westerlies from the FIG ,000-ft streamlines, 0300 GCT, 1 May In this and subsequent charts winds from aircraft are included from a total period of twelve hours, centered on the map time. No correction in position is made in the plotting for aircraft wind reports computed between fixes. Pilot balloon reports six hours off map time are indicated by a dashed shaft. Apparent discrepancies occasionally seen between winds and streamlines arise primarily because the winds are off time or represent mean values between fixes.

2 VOL. 32, No. 9, NOVEMBER, ception of the one ascent on 15 GCT, 4 May, strengthening of the winds through 5 May. This interpretation of the Truk winds is supported by the cloud-distribution charts. The altostratus deck over the Marshalls on 1 May moves toward the W S W as a relatively small patch until 3 May. A large extension of the area of towering cumulus in the Carolines area can be observed on the charts for 2 and 3 May. Then from 3 to 4 May the deck of altostratus increases greatly over the Carolines and over the route from Kwajalein to Guam. At the same time, the cumulus activity over the Carolines decreases along with the increasing horizontal circulation. It is at 21 GCT on 3 May that a 45 knot wind is reported from Truk at 3000 feet. By 4 May continuous rain is FIG mb chart, 0300 GCT, 1 May In this and subsequent charts, locations where the height of the 700-mb (or other) constant pressure surface was reported are indicated by heavy dots. Pacific trough had moved 13 longitude eastward. As far south as 15 N the trough moved at least 5 longitude toward the east, as shown by F I G U R E 3. By 5 May (see FIG. 7) the northern part of the trough is just west of Wake and Midway, which it passes before 6 May. However, the southern end of the trough broke off from the eastward moving part, beginning 2 May. The gradual formation of a cyclonic center near Truk ( 0 7 N, E ) is shown by the time-section for Truk (FIG. 4) and from the cloud-distribution charts (FIG. 5). In the time section it appears that after the weak trough passage at Truk on the first, there is a gradual veering and, with the ex- FIG ,000-ft streamlines, 0300 GCT, 2 May FIG. 4. Time section and 24-hr pressure changes from 1-6 May 1950, for Truk. falling at Truk and Ponape, and a closed cyclonic circulation is drawn near Truk on F I G U R E 6. On 5 May (FIG. 7) a flight from Guam to Truk passed through a new, apparently secondary, center near 11 N, 148 E. The existence of the center is indicated by a cyclonic wind circulation and by an area of cumulonimbus and bad weather which the aircraft encountered near the center of the secondary circulation. The tendency for "troughing" in the streamlines on the Kwajalein to Guam route is particularly evident on 5 May. This suggests the beginning of a split in the anticyclone north of this area. Such a split was definitely confirmed by a flight from Tokyo to Wake on the next day. A sudden backing of the low-level winds at Truk is observed at about 00 GCT 6 May (However, the surface and 1,000-ft winds are unrepresentative due to terrain effects). This observation might be suspected to be an error. However, the graph of 24-hr pressure change indicates a maximum of falls late in the fifth, and the first thunderstorm activity at Truk occurs early in the sixth.

3 328 BULLETIN AMERICAN METEOROLOGICAL SOCIETY The streamline chart for 6 May (FIG. 8) is therefore analyzed to show the original center moving eastward past Truk as the two centers begin to rotate about each other, as described by Fujiwhara [3] [4] and Haurwitz [15]. The separate identity of these centers cannot be maintained in the analysis past the sixth, as a result of the rapidly increasing circulation and a lack of sufficiently de- FIG. 5d. Cloud distribution chart 0300 GCT, 4 May FIG. 5a. Cloud distribution chart 0300, 1 May Vertical hatching indicates areas of towering cumulus (tops above 8,000 feet). Horizontal hatching indicates areas of broken or overcast altostratus. Dots indicate observation locations. FIG. 5e. Cloud distribution chart 0300 GCT, 5 May FIG. 5b. Cloud distribution chart 0300 GCT, 2 May FIG ,000 ft streamlines, 0300 GCT, 4 May FIG. 5C. Cloud distribution chart 0300 GCT, 3 May tailed data. On the sixth the surface wind at Truk remains from S to SE, and the 24-hr pressure falls continue, although at a slower rate. From this it can be concluded that the second center is moving southward in the area west of Truk. After 6 May the only further indication of two centers comes from the Air Weather Service reconnaissance

4 VOL. 32, FIG. 7. No. 9, NOVEMBER, ,000 ft streamlines, 0300 GCT, 5 May flight Vulture Three Doris on 8 May. The report of the weather observer, C W O K. R. Walters,1 includes the following paragraph: "It is believed that there were two eyes in Doris at the time of this flight, a small one lying approximately 30 miles southwest of a larger one. While the surface wind direction indicated to the navigator and weather observer that the typhoon center was slightly to the left of the aircraft heading the radar observer found an eye 90 degrees to the right of the heading and this eye which we entered and took the fix was the smaller one." The preceding analysis then suggests that the 329 double eye might be a final remnant of the original double system of circulations which, through low-level convergence operating over a period of time, gradually merged into a single system. Other solutions are also possible, in the absence of more detailed data, but they would be more complicated than the one presented above. The conclusion however, would be the same in that a system originally containing several vortices gradually intensified into a single typhoon, with the double eye as a final indication of the original complications. The development of "Doris" to typhoon strength concurrently with the appearance and amalgamation of the two centers is a remarkable verification of views presented by Fujiwhara in 1923 [3] and again in 1937 [5]. In these two papers Fujiwhara presented a theory proposing that small adjacent vortices with the same sense of rotation will tend to amalgamate, leading to the appearance of a single large vortex. Recently Riehl [11] has pointed out the importance of the high-level flow pattern for the deepening of tropical cyclones. The data available for this situation permitted the analysis of the 300-mb chart for the period from 15 GCT, 2 May to 15 GCT, 7 May, which fortunately embraces the period during which the typhoon developed. Since there were insufficient observations to indicate departures of the wind from the contours, the contours were drawn to fit the reported heights and 1 Consolidated Report, Tropical Cyclone "Doris," May Prepared by Typhoon Post Analysis Board, 15-2 Air Weather Service Detachment, Andersen Air Force Base, Guam, M. I. FIG ,000 ft streamlines, 0300 GCT, 6 May FIG mb chart, 1500 GCT, 5 May This chart was constructed with the use of (a) differential analysis from 500 mb, based on the available data and a series of correlations made by Major L. Garvin, Air Weather Service, and (b) time sections of height and wind from all stations in the tropics and subtropics.

5 330 BULLETIN winds, and may be regarded as streamlines, even at low latitudes. From a comparison of the 10,000-ft streamline charts for 1-6 May with the 300-mb chart for 5 May (FIG. 9) it can be seen that the typhoon development began at a position midway between a 300-mb cyclone and a 300-mb anticyclone, under a flow which was from a southerly direction. Both cyclone and anticyclone moved at first toward E and then toward ESE as the low-level cyclonic circulation increased throughout the period. Thus, with respect to the high-level pattern, the tropical cyclone slowly came from under the west side of a high-level anticyclone into an area under the influence of a high-level cyclonic circulation. In this respect "Doris" seems unusual, its development being apparently of a different type than that discussed by Riehl. On 2300 GCT, 6 May, an Air Weather Service reconnaissance plane left Guam for the storm area and found surface winds of 70 knots north of the center, indicating that by this time the storm could definitely be called a typhoon. It is interesting to note the low latitude at which the storm developed, most of the development having occurred at 5 or 6 N. MOTION OF T H E STORM The detailed motion of the storm is discussed in a short but excellent paper by Horn 2 in which he points out that the detailed information on the storm position, obtained by the 514th Weather Reconnaissance Squadron, indicates that the storm 2 Horn, J. D., 1951: Movement of tropical cyclones in the Pacific. 2143d Air Weather Wing Technical Bulletin, 1, 16-21; Bull. Amer. Met. Soc., v. 32, No. 9, Nov. 1951, pp FIG mb chart, 0300 GCT, 10 May AMERICAN METEOROLOGICAL SOCIETY FIG mb chart, 0300 GCT, 12 May 1950 followed an oscillatory path, similar to the trochoid described by Yeh [10] [14]. Figure 2 in Horn's paper shows the track of the storm as determined from the observations. In pointing out the similarity to the theoretical tracks derived by Yeh, Horn concludes, "It is now believed that in the past a shadow has been unjustly cast on the validity of certain reconnaissance information, and the accuracy of many good navigators has been questioned through lack of knowledge of the true nature of typhoon movement." In addition to the oscillating motion he describes, the motion of the storm relative to the larger scale circulation patterns is of interest. The 700-mb chart for 10 May (FIG. 10) shows a clearcut ridge extending from W S W to ENE north of the typhoon. The steering, if any, appears to be toward the west. Actually the motion of the storm from May was slight, amounting to an average speed toward the N W of 7 knots. On the 11th (not shown) a weak opening in the ridge between Formosa and Okinawa is evident. On the 12th the break in the ridge in this area is still evident at 700 mb (FIG. 11). At 500 mb (map not reproduced) the ridge is weakest just north of the storm, which is moving due northward by this time. One might try to account for the northward motion of the storm by the concept of "highlevel steering." However, from the 200-mb chart for 12 May (FIG. 12), it can be seen that the motion of the typhoon is opposite to the flow at 200 mb. The steering hypothesis, low level or high level, must then be discarded in an explanation of the storm motion. The 700-mb charts for 13 (FIG. 13) and 14 May (FIG. 14) show the typhoon as it breaks completely

6 VOL. 32, No. 9, NOVEMBER, FIG mb chart, 0300 GCT, 12 May This chart was constructed with the aid of time sections and with the aid of a vertical extrapolation system developed by Mr. H. S. Appleman, Air Weather Service. through the ridge. The chart for 14 May shows the ridge re-established again after the northward passage of the typhoon. This is not an isolated example of this kind of storm motion. The writer was informed in a verbal conference with some of the meteorologists at the Central Meteorological Observatory in Tokyo that the Japanese forecasters are very familiar with this phenomenon, and do not hesitate to forecast some typhoons to break northward through the subtropical ridge line. An example familiar to American meteorologists is the notable Atlantic hurricane of October 1944, which moved northward into an anticyclone. A number of recent papers report results showing that a cyclonic vortex such as a typhoon should be driven toward the north by various dynamical effects. Davies [2] neglects the effects of the Coriolis force altogether in the basic flow of the vortex. While for certain cases this is a good approximation, the effect of variation of the Coriolis force, as indicated by Petterssen [9] and Rossby [12], is at its maximum in the low latitudes. Kuo [7] discusses the vorticity distribution in the surrounding fluid. This distribution is difficult to treat when the flow in the surroundings is as stagnant as that shown in the charts from 6-14 May. Petterssen showed that a cyclonic center in which there is convergence will tend to have a northward component of motion, while divergence would result in a southward component. In a typhoon, with low-level convergence balanced by high-level FIG mb chart, 0300 GCT, 14 May FIG mb chart, 0300 GCT, 13 May divergence, the resultant force from this effect should be small, and should be relatively invariant through most of the life history of the typhoon. This type of force should result in a tendency for the axis of the storm to tilt with height. The force on the cyclonic center according to Rossby is directed northward at all levels having cyclonic rotation. It can also vary considerably as the radial velocity profile of the storm changes with time. This type of force will be discussed in the following section, with particular reference to typhoons in the tropics. Rossby gives the northward force F acting on a unit slice of a vortex as /*R F = fipir I r*udr (1) Jo

7 332 BULLETIN AMERICAN METEOROLOGICAL SOCIETY where /? is the northward rate of change of the due to the decrease in n, the value of C changes but Coriolis parameter, p is the density, r is the radius little during the growth of the storm. Referring back to ( 5 ), it can be seen that as a of curvature of particles moving in a vortex, R is the outer boundary of the vortex, and o> is the rela- typhoon matures and grows in intensity and extive angular velocity of the air participating in the tent, the value of A will increase from near zero vortex. The force F is the resultant of the Coriolis to a significant northward acceleration. forces acting on the various parts of the vortex, and The velocity profile of a typhoon probably varies arises from the lack of balance between pressure from one quadrant to another as well as with the forces on the one hand and the sum of Coriolis and life history of the storm. Consequently, only the centrifugal forces on the other hand. most reserved type of conclusions can be drawn from As the first approximation for a typhoon we the above results. However, in order to illustrate can assume a velocity profile such that from the the results of variations of the various parameters center to a distance rlf where a maximum value of the following numerical applications are made. Exro) is found, solid rotation exists, and = con- ample ( 1 ) is a hypothetical situation representing stant = C\. From r1 to R a vrn_1 vortex exists, a young immature storm. Examples ( 2 ) and (3) are computed from data gathered by weather reand wrn Constant = C. At the distance rlf o)± = connaissance flights of the Air Weather Service to and C1 = C/rxn. Then into "Doris" on 8 May and 12 May respectively. coi = C/rin, co = C/rn. 2 l 3 Example Substitution of the above wind profile in (1) gives F = pptt / n J0 nr r'uidr + ppir I r*udr, Jri (2) which after integration yields RA~n \ r=rn)- ( 4 ) Since the mass M of the unit slice of the vortex is pirr2, the northward acceleration A is R2~n \ ( 4 ^ ) ' ri (nautical miles) R (nautical miles) Latitude As a rule, n will not be much different from two. Since it is seldom observed that r1 exceeds R/10, an error of only about two percent will result from a simplication of (3) to ( fmax (knots) S < > Various studies by Macdonald [8], U. S. Weather Bureau forecasters [13], and James [6] have suggested that n varies from values of slightly greater than two for small immature systems to values near 3/2 for large mature typhoons. Thus the fraction R 2 _ n /4 n will grow from near-zero values to about 2R1/z/5 as the typhoon grows to maturity. The constant C can be expressed in terms of the maximum wind speed, found in the storm. Since C then C = r^'hw. As a typhoon grows from a small intense storm to a large mature system, vmax and r1 both increase. However, 10 N 10 N 20 N n C 1.67 X X X103 A (knots per day) From the above table one can conclude that the northward acceleration to which a typhoon is subjected will be small when the storm is intense, but covers only a small area. This is the stage of immaturity as discussed by MacDonald [8] and James [6]. As the storm spreads out to cover a large area the northward acceleration becomes pronounced. The magnitudes of A in examples (2) and ( 3 ) are only very approximate, but at least indicate that significant accelerations can arise as a storm matures. This type of storm, which attains dimensions comparable to some of the larger high latitude cyclones, is the type which has a reputation for defying forecasters, particularly with its tendency to move toward the north and destroy warm anticyclones.3 The increase of the northward force is 8 In a lecture given at Headquarters, Air Weather Service in May 1950, Dr. H. Riehl pointed out that although small hurricanes can be expected to be steered well, large ones tend to move independently of the steering flow. This statement would be in agreement with the above results.

8 VOL. 32, No. 9, NOVEMBER, 1951 primarily due to the transformation of the radial velocity profile and to the increase of the outer extent of the vortex. CONCLUSIONS The most significant features of the development of typhoon "Doris" appear to be : (1) The existence of two centers during the development of the storm. This double system is followed by the double eye found by a later; weather reconnaissance flight, and is in agreement with a theory of storm development by Fujiwhara [3], (2) The development of the storm with respect to the high-level flow patterns. The storm developed as it moved from beneath a 300-mb anticyclone to an area southeast of a 300-mb cyclone. (3) The unusually low latitude (5 to 6 N ) at which the storm formed. The most significant features of the motion of the storm are: (1) The oscillations of the cyclone about its mean path, discussed by Horn, which might be explained by Yeh's theory [14]. (2) The motion of the storm to the north and its breakthrough of the subtropical ridge. This northward motion is attributed to the resultant of the Coriolis forces acting on the storm, in accord with the principle of Rossby. The northward acceleration of typhoons in general is shown to become prominent as a result of the changes in the radial velocity profile and the increasing extent of the storm. The problem of forecasting the motion of typhoons is then a problem of evaluating the steering [10] and related effects such as the "Fujiwhara effect" (the tendency for two cyclones to rotate counterclockwise about each other [ 4 ] ), and a problem of estimating the northward acceleration which must be added to the steering effect. Due to the lack of symmetry of most typhoons, a quantitative computation seems impractical. The quantities n and rt can occasionally be evaluated from velocity profiles obtained by individual weather reconnaissance flights, but the application of values obtained from one cross section to the whole storm is not likely to be of much value. A possibility for 333 some future investigation would be the compiling of some statistics from the excellent weather reconnaissance data now available, in which the values of n would be examined with respect to the development stage, the size, and the sector of typhoons. In practice, the northward acceleration, A, must be estimated qualitatively from the size and extent of the typhoon. Thus the forecaster will be required to forecast the future growth of the intensity and area of a typhoon in order to forecast the future track of the storm. REFERENCES [1] Cressman, G. P., 1948: Relations between high and low latitude circulations. Dep. Meteor. Univ. Chicago, Misc. Rep., No. 24: [2] Davies, T. V., 1948: Rotary flow on the surface of the earth. Philosophical Mag., 39: [3] Fujiwhara, S., 1923: On the growth and decay of vortical systems and on the mechanism of extratropical cyclones. Japanese Journal of Astronomy and Geophysics, I (5) : [4] Fujiwhara, S., 1931: Short note on the behaviour of two vortices. Proceedings of the Physical and Mathematical Society of Japan, Series 3, V. 13, No. 3. [5] Fujiwhara, S., 1937: On the quick development of cyclones by amalgamation. The Geophysical Magazine, XI (1) : [6] James, R. W., 1951: On the evolution of tropical cyclones. /. Meteor., 8 ( 1 ) : [7] Kuo, H., 1950: The motion of atmospheric vortices and the general circulation. J. Meteor., 7 (4) : [8] Macdonald, W. F., 1942: On a hypothesis concerning normal development and disintegration of tropical hurricanes. Mon. Wea. Rev., 70 (1) : 1-7., also Bull Amer. Meteor. Soc., 23 : and [9] Petterssen, S., 1950: Some aspects of the general circulation of the atmosphere. Centenary Proceedings of the Royal Meteorological Society, 1950: [10] Riehl, H., and N. M. Burgner, 1950: Further studies of the movement and formation of hurricanes and their forecasting. Bull: Amer. Meteor. Soc., 31 (7) : [11] Riehl, H., 1948: On the formation of typhoons. J. Meteor., 5 (6) : [12] Rossby, C-G., 1948: On displacements and intensity changes of atmospheric vortices. J. Marine Res., V I I : [13] U. S. Weather Bureau, 1948: Hurricane notes. Weather Bureau Training Paper No. 1, 210 pp. [14] Yeh, T. C., 1950: The motion of tropical storms under the influence of a superimposed southerly current. /. Meteor., 7 (2) : [15] Haurwitz, B., 1946: The motion of tropical cyclone pairs. Trans. Amer. Geophys. Un., 27 ( V ) : 658.