Abstract from a Dynamical Point of View, Three Dimensional Analyses Of

551.515.2

An Aerological Investigation of the Structure of the Typhoon

- Analyses of the Typhoons Jane and Kezia-

by Y. Masuda and M. Takeuchi

RescarchMerteorological Institute (Receive (I July 25, 1951)

Abstract

From a dynamical point of view, three dimensional analyses of the typhoons Jane and Kezia, that is, the analysis of the pressure field, the temperature field, the tropopause and the stream lines, etc. are performed, and a typhoon model deduced from thee analyses is proposed. It is concluded that the typhoon is not a simple convective system, but a system superposed by a few dynamical secondary convective systems, and each meteorological element corresponds to the distribution of then e systems.

1. Introduction The typhoon is a large cyclonic eddy accompanied with strong winds and heavy rains, bringing about great damage in its passage. Therefore, since former times, studise of forecasting of several phenomena associated with it have been made, and many laws (mainly empirical laws) have been obtained. However, these laws are not reliable yet. This may be partly due to the fact that the three-dimentional structure of the typhoon has not been held exactly. Therefore, it is most necessary to make a detailed three-dimensional analysis of typhoon. Two-dimensional analyses of the typhoon have been performed in some detail using abundant data of surface observations. Especially, Dr. SY0N0 S. [11, [2] showed theoretically the existence of negative vorticity area in the typhoon and he and his collaborators [3] verified this theory after analysing the typhoon from the dynamical point of view. Furthermore, T. Fujita's study of the fine structure of the typhoon Dela [4] is also remarkable. Recently, especially during and after World War II, in company with the development of aerological observation nets, a branch of meteorology, known as tropical synoptic meteorology, has developed and a tendency to study the formation and the structure of the typhoon in its three-dimentional aspect has been growing. However, in this tropical synoptic meteorology there are many difficulties in method of analysis addition to the scarcity of data. That is, thermal methods of analysis familiar in the middle latitudes ---frontal, lapse-rate, and air mass analysis, etc.- -are not useful. The system with which tropical synopic meteorology is concerned has 1951An Aerological Investigation of the Structure of the Typhoon235 a dynamical character and then, in its analysis, dynamical and kinematical methods must be mainly used. Some studies of the three-dimensional structure of the typhoon from these points of view- have been performed. And especially, as some detailed data are gained in the case of the typhoon Kitty, many studies, for example the studies of YAMA_DA H. & MATUHASHI S. [5], of Dr. ARAKAwA H. [6], [7], of HASHIMOTO M. & SAMPE1 J. [8] and of IVIAsuDA Y. [9], . etc. are reported. Conc- erning the hurricane, studies of SimPSON R. H. [10], [11] and PALMEN E. [12] are also remarkable. However, these analyses are all fragmentary and a synthetic analysis has not been performed yet. In this report, from a dynamical point of .view, three-dimensional analyses of the typhoons Jane and Kezi .a are performed by using aerological data of Japan, and a typhoon model deduced from these analyses is proposed. As typhoons, coming near Japan, change their intensities through the mature to the dissipative stage, so the model proposed in this, report belongs to these .stages. In future, it is necessary to settle the typhoon model after accumulations of sufficient data, to study the mechanism of typhoon phenomena theoretically, and further to study applications of aerological data to typhoon forecast in relation to surface data.

2. General aspects

The typhoon Jane was born in the area south of Iwozima on about 30 Aug. 1950, moved through the Kinki district of Ja- pan on 3 Sep. , and went away south of Kuril. Islands on 5 Sep.. The scale of this typhoon was smaller than that of the typhoon Mu- roto, but in several places it was stronger than the later, and wrought great damage. The typhoon Kezia came into existence in the 236Y. Masuda and M. Takeuchi Vlo. 11 Nos.3--4 area north-west of Gum Island on about 7 Sep. 1950, landed in the south of Kylisyet, on 13 Sep entered into the Japan Sea after crossing Ky6syit, and moved away to the Okhotsk Sea on 15 Sep.. Fig. 1 shows the tracks of the typhoons Jane and Kezia, and positions of the aerological observatories using in this report. In this figure, denominators of fractional numbers of side of tracks represent dates, numerators times (in L. M. T. ), and the abbreviations Wak. , Sap. , Mi. , Aki. , Sen. , Tat., Ha. , Waz. , Ko. , Shi. , Yo. , Ita. , Kag. , and Tare represent Wakkanai, Sapporo, Misawa, Akita, Sendai, Tateno, Haneda, Wazima, Komaki, Shionomisaki, Yonago, Itazuke, Kagoshima and the Ocean Station at 135°E, 29°N respectively. These abbreviations will be used throughout this report. Figs. 2 and 3 show relative positions of the main aerological observatories to Jane and Kezia. In these figures, concentric circles represent distances 2°, 4°, 6°, 8° and 10° in latitude from the surface centers. Fig. 4 and 5 indicate the weather charts at those times when Jane and Kezia were located at the Shikoku and the Kyrisyri district respectively. At these times, isobars of the each typhoon system were almost circular and no front was found yet. It is thought that Jane changed into an extratropical cyclone of the temperate latitudes near Hokkaido, and Kezia in the middle of the Japan Sea. Figs. 6 and 7 represent time isopleths for Tare and Wazima in Jane, Fig. 8 space cross-section taking from NE to SE direction through the center of Kezia at 1951 Fig. 6 The temperature and potential temperature isopleth for Tare in Jane. In this figure, full lines represent isotherms for each 5°C, dotted lines isentropic lines for each 5°K, and dashed and dotted line tropopause.

0000 L. M. T. 14 Sep. and Fig. 9 time isopleth for Itazuke in Kezia respectively. In these figures solid lines indicate isotherms for each 5°C, dotted lines isentropic lines for each 5°K, and dashed and dotted lines tropopauses respectively. The definition of tropopause will be seen in paragraph 5. Fig. 6 is the most characteristic among these figures, and from this figure, a warm core which is extremely warm up to. Fig. 7 The temperature and potential temperature isopleth for Wazima in Jane. the upper levels exists at 13.00, 2 Sep. nearest to the typhoon center, and around it fairly cold areas exist. From this fact, it is suggested that areas of strong descending currents exist in the vicinity of the typhoon center and areas of strong ascending currents surround it. As, however, in a humid system such as typhoon, Fig. 8 The temperature and potential temperature cross-section taking from NE to SE direction through the cent(r of Kezia at 0000 L.M. T. 14 Sep. 1950. it occasionally happens that temperature rises when the humid air near the earth surface ascends in wet adiabatic change, so the above description probably does not hold in the lower layers. Other figures are essentially the same as in Fig. 6. However, in these figures time intervals of observations are too long to describe fine structure any more. Fig. 9 The temperature and potential temperature isopleth for itazuke in Kezia.

3. Analysis of the pressure field (a) Vertical pressure distributions In order to investigate vertical pressure distributions in the vicinity of the typhoon, time isopleths for each place and space cross-sections of pressure are 242 , Y. Masuda and M. TakeuchiVol. II Nos. 3-4 drawn. On that occasion, in order to eliminate diurnal variations of pressure and temperature in outline, time isopleths of pressure and temperature anomalies are drawn. The anomalies are computed with respect to the monthly mean values at 0 h, 6 h, 12 h and 18 h in September 1950 at the each aerological station. In this report, " pressure " means heights of standard pressure levels. Figs. 10 (a and b) show distributions of pressure and temperature anomalies for Wazima and Yonago in Jane, and ' Figs. 11 (a arid h) those for Itazuke and Kagoshima in Kezia. In these figures, solid lines represent pressure isanomals for each 40 dyn. m. and dotted lines temperature isanomals for each 2°C. Abscissas of these figures are times of observations and vertical axes are logarithms of pressure. However, in the figure for Itazuke in Kezia, the distances from the typhoon center are taken an abscissa. As, in a system such as the typhoon , the relation between time intervals of observations and distances from the center is not linear , and as the typhoon rapidly fills after landing, so that utility of time isopleths is limited within some degrees. However, from the common characters of time isopleths, characteristic vertical pressure distributions in the typhoon system are understood in some details. Space cross-section of pressure and temperature

Figs. 10 (a and b) Time isopleths of pressure and temperature anomalies for Nazi ma (a), Yonago (b). In these figures, solid lines represent pressure anomalies or each 40 dyn. m. , dotted lines temperature anomalies for each 2°C, II and L relatively high and low pressure areas, and IV and C relatively warmer and colder areas. Figs. ii (a and b) Time isopleths of pressure and temperature anotnalies for Itazuke (a), Kagoshiina(b). The explanation of represent- ation are same as Figs. 10. distributions taken NE to SE through the center of Kezia near Itazuke .iE shown in Fig. 12. In this figure, solid lines. represent distributions of heights of the standard pressure levels, dott. ed lines that of temperature on those levels, i.e. forms of isentropic surface, Therefore, it is warm where dotted lines hang, and it is cold where they ascend As the system moves changing its stage in its passage, time isopleths do not represent space

distributions by themselves. And space cross- sections have also the defect that they do not show fine structures because of roughness of aerological station nets. Therefore, after combining time isopleths and space cross-sections, the following characteristic features of vertical pressure distribution are obtained. i) In the upper layers (above about the 500 mb surface in Jane, and above about the 400 mb surface in Kezia), the secondary pressure field in which relatively high and low pressure areas distribute at about 200 km. intervals reciprocally, superposes on the primary pressure field in which pressure uniformly decreases towards the typhoon center. Therefore, it is suggested that secondary circulations superposes on the prima/ y circulation system of the typhoon. ii) In the lower layers (below about the 700 mb surface), pressure decreases steeply towards the typhoon center as the surface pressure field. iii) The middle layer is the transitional layer between these two. iv) Above the surface center, pressure is relatively higher, but the maximum pressure area shifts slightly forward. v) The axis of the minimum pressure is almost vertical up to about the 500 mb surface, and above this level steeply tilts towards the rear. (b) Horizontal pressure distributions Because of scarcity of data, it is almost impossible, except for the surface, to draw a horizontal pressure distribution in the typhoon. Therefore, in this report, the method of superposition of relative positions is taken up. This method is based on the assumption that the system moves without change in stage during a a certain period, but, in reality, absolute values, such as pressure of the typhoon center, change considerably. So, in this report we assume that though the absolute values of pressue, temperature and etc. change, some characteristic features, such as areas of relatively high or low pressure, are almost steady. On this assumption, the distributions of the characteristic features the typhoon are examined. Figs. 13 and 14 represent horizontal distributions of relatively high and low pressure areas at the 300 mb surface in Jane and Kezia respectively. Relatively high or low pressure areas are determined by pressure anomalies of each station. In these figures, dotted concentric circles represent

Fig. 13 The horizontal distribution of relatively high and low pressure areas in Jane at 300mb.. In this figure, dotted concentric circles represent distances from the surface center 2°, 4°, 6°, 8° and 10° in latitude, and shaded areas relatively low pressure areas.

Fig. 14 The horizontal distribution of relatively high and low pressure areas in Kezia at 300 mb. In this figure, dotted concentric circles represent distances from the surface center 2°, 4°, 6°, 8° and 10° in latitudes, shaded areas relatively low - pressure areas and symbols and 0 the typhoon center at surface and 300mb respectively. 246Y. Masucla and M. TakeuchiVol. fi Nos.3--4 distances from the surface center in 2°. 4°. 6°. 8° and 10° in latitude, and shaded areas relatively low pressure areas. The mark 0 in Fig. 14 shows the typhoon center on the 300 mb surface determined by pressure anomaly and vertical variation of wind directions at 0000 L. M. T. 14 Sep. at Itazuke. Concentric circles having the center on this mark approximately surround areas of relatively high and low pressure. Therefore, it is suggested that the pressure distribution of the upper levels is not symmetrical with the typhoon center of• the earth surface, but with that of the upper levels. This fact is different from the opinion that the up per pressw e distribution is symmet?'bird with the high pressure part above the surface center. From the above descriptions we may conclude for the horizontal distribution of the upper pressure field as follows. 1) In the upper levels relatively high and low pressure areas distribute reciprocally. ii) The upper pressure center is located in the south-west sector of the surface typhoon and is about 200 km distant from the surface center. iii) The upper pressure distribution is not symmetrical with the surface center but with the low pressure of the upper levels. (c) Vertical variations of the pressure gradient In order to investigate vertical variations of the pressure gradient near the typhoon center, 6 hour pressure tendencies before and after 0000 L.M.T. 14 Sep. at Itazuke located about 40 km in the rear of the typhoon center are calculated and these 1951An Aerological Investigation of the Structure of the Typhoon247 tendencies are converted into pressure gradients and then vertical distributions of these gradients are obtained. In Table I the vertical distributions of pressure gradients and their percentages to the surface pressure gradient are represented, and for the sake of comparison, those calculated by RTEHL H. [13] in a hurricane are shown in the last column. In general, pressure gradients decrease with height, but not so systematically as RIEHL's example, situations of variations in front of and in .rear of the typhoon are somewhat different from each other. This seems to be caused by the fact that the axis of the minimum pressure tilts backward. In the rear, decreases of pressure gradients with height is steeper than RIEHL's example and signs of there gradients change at 7 km already. And further, decreases of pressure gradients are systema- tical in the rear, but not so in front.. This probably due to the fact that the observation values at 0000 L. M. T. 14 are not exactly the values of the typhoon center. However, we may conclude seemingly that pressure gradients decrease with the heights systematically. As the vertical variation of pressure gradients is necessary for the mechanism of outflow of the upper air in the typhoon, we must examine it in more detail after further accumulation of data. (d) Tilt of the axis of the minimum pressure The axis of the minimum pressure in the typhoon is almost vertical up to about the 500 mb level, and tilts towards the rear steeply above this level. This fact is shown, in the case of Jane, from the pressure anomaly for Misawa and in the case of Kezia, from the pressure anomaly for Itazuke and from the sudden change of wind direction at 5 km for Itazuke. Though we can not speak exactly because of scarcity of data, we can deduce that the gradient of the axis is about 1/28 for Jane, and about 1/25 for Kezia. According to KRASNER W. and LONDON J. [14], a typhoon axis tilts towards the rear in the case of development of a young typhoon and tilts forwards in the case of dissipation. Therefore, we may predict that most axes of typhoons near Japan tilt towards the rear.

4. Analysis of the temperature field Using the same data as in the analysis of the pressure field, the analysis of the temperature field is performed. As the results, We may conclude as follows. 1) Near the surface center, the two warmest areas are located at about the 700 mb and about the 300 mb levels (See Fig. 11, a). It is probable that the lower maximum is produced by wet adiabatic ascents of the humid air near the surface and the upper one is produced by descending currents. ii) As the characteristic feature of time isopleths, relatively warm and cold areas distribute themselves reciprocally in the upper layers. In comparison with pressure fields, the warmest areas are located under the relatively high pressure •areas and the coldest areas are located above the relatively low pressure areas. This fact suggests that the temperature field in the typhoon is formed by the secondary circulation of the tS7Ohoon. The fact that the temperature field in the typhoon is produced by the dynamical mechanism may be one of the most remark- 248Y. Masuda and N. TakeuchiVol. II Nos. 3'-.-4 able character of tropical cyclones, as against extra-tropical cyclones having thermal structures. iii) The hights of the warmest areas become lower with the distance from the centers This fact suggests that the height of the typhoon system decreases with the horizontal breadth and therefore the heights of •the secondary circulations decrease with the distance from the center. iv) The phases of pressure and temperature fields are inverse up to about the 600 mb level, and above this level, they coincide with each other. This fact is not different from the extra-tropical cyclone essentially.

5. Analysis of the tropopause

Analysis of the tropopause in the case of the typhoon have been also performed recently. It was natural, however, that different results were obtained according to different definitions of the troporia .use. Therefore, in this report, the following definition of the tropopause considered as the most proper is used .' The tropopause is lower surface of the transitional layer between the troposphere and the stratosphere. This transitional layer must have a considerable thickness (more than 1 km) and in this layer, ascent curves indicate one of the following characters, that is, i) inversion, ii) isothermal, iii) lapse-rate less than 0.3°C/10)m. According to " Constant pressure analysis " of Chief of Naval Oneration of U. S. A. [15], the 3rd condition is less than 0 2°C/100m. Therefore, it is more suitable to adopt the latter condition. However, in this paper, in the 'first place the above definition is used. In the same manner, as in the analysis of the pressure and the temperature field, entries of absolute values of tropopauses in the chart of , relative positions are avoided, and distribution charts of relatively high and low tropopause areas are constructed. Fig. 15 is constructed in this manner, and shows the hori ontal distribution of the tropopause in Kezia. In this 1951 An Aerological Investigation of the Structure of the Typhoon249 figure, dashed areas represent relatively low tropopause areas. This figure resembles Fig. 14 in shape, especially in the neighbourhood of the typhoon center. That is to say, the tropopause is relatively low where pressure is relatively low, and the tropopause is relatively high where pressure is relatively high. Furthermore, from the space cross-section (see Fig. 8) and time isopleths (see Figs. 6 and 7) through the typhoon center, it is deduced that the tropopause becomes the lowest above the axis of the minimum pressure. Therefore, we may deduce that the tropopause in the case of the typhoon is the lowest above the axis of the minimum pressure, becomes higher around this axis and again becomes lower outside. This shape of the tropopause is the same as the "tropopause funnel" (Tropopausentiichter) and the " tropopause ridge " (TropopausenhOcker) i,n the case of the extra-tropical cyclone. Therefore, wemay conclude that the tropopause in the typhoon coincides in shape with that of the extra-tropical cyclone. However, the relation between the tropopause height and the tropopause temperature in the typhoon is widely different from that of the middle latitudes. That is, in most cases of the typhoon, a low and warm ora high and cold tropopause as found in the middle latitudes does not exist, but a low and cold or a high and warm tropopause exists. Figs. 16 (a, b and c) show the relation between iso- thermals and isentropic lines near the tropopau,9.e. In these figures, solid lines represent isotherm3 and dotted lines isentropic lines. From these figures it is shown that the phases .of isentropic lines are the same up to the, stratosphere, and the tropopause not always ascends on the position where isentropic lines ascend. Therefore, contrary to the case in the middle latitudes, the low and cold or the high and warm tropopause is observed. This fact is inconsistent with PALMEN's theory on the formation of the tropopause [16], [17]. Therefore, another reasonable cause for the formation of the tropopause must be considered. P_A_LAthNassumed that the change and the formation of the tropopause depend on the vertical motion. This assumption seems to be useful also in our case. However, contrary to the systems of the middle latitudes treated by PALMEN, the heights of the systems near Japan in summer, such as the anticyclone and the typhoon, are considerable. Therefore, it seems to be possible that the motion in the troposphere extends up to the stratosphere, and then, in the stratosphere, the vertical motion of the same sign as in the troposphere also occurs. Actually, the fact that, as in Figs. 16, the phases of isentropic lines in the troposphere are the same up to the stratosphere is likely to support the above theory. The tropopause is defined by the position where the lapse-rate becomes discontinuous, that is, isentropic lines are crowded. If we assume adiabatic change, for the concentration of isentropic lines, two cases are considered. One is the case when the signs of the vertical motions in the troposphere and in the stratosphere are inverse to each other, the other is the case when these signs are, the same but the difference of the magnitudes of vertical motions is large. The former is PALINIkN's 1951An Aerological investigation of tile Structure of the Typhoon251 case and the latter ours. That is to Fay, when ascending currents near the tropopause decrease steeply with the heights, intervals of isentropic lines become smaller with the heights, isentropic lines concentrate just below the original level and then the new tropopause is formed at a lower level than the original. Inversely, when descending currents near the tropopause decrease steeply with the height, intervals of isentropic lines becomes large just above the original level, and then the new tropopause forms at a higher level than the original. And furthermore, as temperature becomes colder where ascending currents exist, becomes warmer where descending currents exist, so the low and cold or the high and warm tropopause forms. The fact that, near the typhoon center, pressure is low where the tropopause is low and pressure is high where the tropopause is high, is also likely to support the above descriptions. Fig. 17 represents schematically the above descriptions of the formation of the tropopau,.e. The Palmen type predominates in the middle latitudes, and the typhoon type predomintes in the strong convective system such as the typhoon. Fromthis point of view on the formation of the tropopause in the typhoon,HAsHim OTO M. and SAmPEI J. [8] classified tropopauses in the typhoon Kitty, and from the distribution chart of the tropopause, deduced very interesting results.4'

G. Analysis of the wind field (a) Vertical distributions of the three-components of wind In order to study vertical distributions of wind in the typoon, vertical distributions of the three-components of wind at Itazuke, where the typhoon center passed most close, are constructed. Horizontal wind components are divided into tangential radial(vr )components, and the positive signs are taken in the case of (vo) and cyclonic rotation and direction toward the center respectively. The components vo and v2.are computed from observing wind components of the tangential and the radial direction of an imaginary circle whose center agrees with the typhoon center. On this occasion, the tilts of the typhoon axis are neglected except in the neighbourhood of the typhoon center. The wind observation at 0 h on 14 Sep. nearest to the typhoon center shows abrupt variation of wind directions from NW to W at 5 km height. From this fact and pressure anomaly for Itazuke, it is deduced that the axis of the typhoon tilts toward the rear abruptly above about the 5 km height. Therefore the components vo and v,. above the 5 km height at this time are computed from the imaginary circle having the center on the point about the 200 km behind the surface center (the mark (3 in Fig. 14). The vertical wind components are computed by the method proposed by one of the authors. That is, if we put the vertical velocity by v, cm/sec, the dry adiabatic lapse-rate by rd.-- 10°C/1 km, the lapse-rateby T°C/1 km, and specific humidity by gr/1 kg, the vertical velocity v, is obtained by the equation

After completicn of this manuscript, we learned that M. E. GRAVE-;[181 had treated ti e tropoi auf-es of what we call the typhoon type in this retort in thee subtropics, Puerto Bice. Therefore, the " low latitudes type" may be a better 'aim e iylcn e. Fig. 18 Vertical distributions of three components of wind v, (a), vo (b) and vz (c), for Itazuke in Kezia. In these figures, dashed areas represent areas the of positive sign, and in figures (a) and (b), solid lines represent isoTels for each 5 in/see, and in figure (c), those are isovels for each 5 cm/Sec. 1951 An Aerological Investigation of the Structure of the Typhoon253 where ii/4z=1 km and 4i4t-=1/12 hr and we assume the wet adiabatic change and neglect the advective terms. Figs. 18 (a, b and c) show the vertical distributions of the three-components of wind vo, v, and vz. In these figu,res, the dashed areas represent the areas of the positive sign of these three components. In figures (a) and (b) , solid lines are isovels drawn by each 5 m/sec and in figuve (c) by each 5.cm/sec. Further, in the latter figure, values having suffix 4, are the vertical velocities when the denominators of the computing equation of vertical velocity became smaller than (Y2 and in these cases, we considered only their signs. From these figures the following results are obtained. The signs of vo are positive in the area whose radius is 8° in latitudes at the surface. However, this length of radius decreases with the heights gradually, becomes about 4° in latitudes at the 7 km height and above this level it increases again till 50 in latitudes at the 10 km height, where it begins to decrease once more until it at last disappears at about 15 km. Accordingly, we may consider that the extent of the typhoon decreases with the heights, reaches once the minimum at about the 7 km height, above this level increases with height again, above about the 10 km height dereases with height once more, and disappears at about 15 km height. This fact almost coincides with the result obtained from the analysis of the pressure field. The velocities of the components v®increase towards the center, but not increase uniformly, and maximum velocity areas exist in several places. Though circumstances near the center are not clear because of scarcity of data, the strongest wind area is located about 10D-200 km in front of the center. Con- cerning the rear of the typhoon center, as Kezia entered into the westerlies, no distinct result is obtained because of effects of the westerlies, The components are positive near the ground from the distance 6° in latitudes to the center and are the stronger, the nearer to the center. However, the height of this positive area near the ground is considerably low and everywhere between about 2 km and 5 km height, the signs of these components are negative and furthermore, above about the 7 km, those are almost negative also. Therefore the air converging towards the center near the ground below 2 km height seems to diverge outwards from the center everywhere above this level. In the neighbourhood of the typhoon center, strong negative velocities are observed. This strong outflow of the upper air seems to be related to the decrease of the pressure gradients with height. Circumstances in the rear of the typhoon are also not clear because of effects of the westerlies. The distribution of the components rt/ is not so systematic, but below about 5 km height near the cegater,ascending currents seem to exist almost everywhere. Besides in the upper layers near the center remarkable descending currents are observed. (b) Analysis of the horizontal stream lines As the observation data are not sufficient to make the analysis of the stream lines at the same time, in this report the method of relative positition Is taken, Figs, 19 and 20 represent stream lines at several levels in Jane (at the 3 km, 6 km 254 Y. Masuda and M. TakeuchiVol. 11 Nos. 3-4 and 10 km height) and in Kezia (at the 3 km, 6 km and 12 km height). In these figures, heavy solid lines with arrows in front show the stream lines, areas surrounded by thin solid lines divergent areas, dashed areas surrounded by thin dotted lines convergent areas, and full barbs 10 m/sec. Here the stream lines drawn according to the SandstrOm's method, and intervals 1-,pi:woon stream lines are inversely nronortional to the numbers of barbs. Areas of divergence and convergence are determined by the boundaries where stream lines begin or end and by the areas where intervals between stream lines become broad or narrow. In the method of relative position, changes in the stages of the typhoon are neglected. As, in reality, considerable changes occur, wind velocities change remarkably, though wind directions are almost conserved through a certain period. Therefore, in these figures, the directions of stream lines seems to have considerable certainty, but areas of divergence and convergence are probably uncertain. The fact that the center of the typhoon rotation, obtained 1951An Aerological Investigation of the Structure of the Typhoon255 from these stream line figures, seems to tilt towards the front with height, contrary to the result obtained from the analysis of the pressure field, may be also caused by the scarcity of data. However, from these figures the essential features of the wind field are probably seen qualitatively. The extent of the closed cyclonic rotation decreases with height, and disappears at 10 km height in Jane and at 12 km height in Kezia. And above these levels stream lines show cyclonic curva- ture only. Outside of the area of cyclonic rotation, anticyclonic rotation predominates, and this area of anticyclonic rotation spreads with height. The fact that the height of Kezia is heigher than that of Jane, suggests that Kezia came into Japan at a maturer stage than Jane. According to the Handbook of Meteorology [19], the mature stage is characterized by the intensity of the typhoon being steady or diclining, with the area of high winds still increasing. The radiuses of the 1000 mb isobar are about 220 km (at 0000 L. M. T. 3 Sep. ) in Jane and about 256 Y. Illasuda and M. TakeuchiVol. ll Nos.3-4

340 km (at 0000 L. M. T. 12 Sep. ) in Kezia, but the pressures at the centers are approximately 940 mb and 945 mb respectively. Therefore, we may suppose that Jane landed on Japan at a less mature stage than Kezia. The fact that it took five days from formation to landing on Japan in Jane, and seven days in Kezia, also supports the above consideration. Now, let us examine the experimental law of "steering" considered as the most useful method for forecasting the moving direction of the typhoon. As known from the diagrams of stream lines, the wind fields except for cyclonic rotation are never uniform, so we can not determine what level is the steering level, or what winds are steering currents. According to Hurricane Note [20], steering currents are found at the level on which cyclonic rotation disappears. In our examples also, currents in front of the typhoon at 10 km in Jane and at 12 km in Ke2ia seem to almost agree with the moving directions. However, in Jane this agree- ment is good for a considerable part of the front of the center, on the other hand in Kezia for only the immediate front of the center. Therefore, we may approximately consider the winds in front of the center as the steering currents, but can not determine them so uniquely that they can be used for the forecasting the moving direction. This problem must be studied in detail after accumulation of many data in future. Areas of convergence and divergence are distributed somewhat irregular, and it seems to be difficult to grasp their general characters. This is probably because of the scarcity of data and the method of relative position. However, in outline, we may indicate the properties of the divergence field common to Jane and Kezia as follows, that is, (i) at the level on which cyclonic rotation disappears in front of the center the convergent area is located left of the moving direction and the divergent area right of it and (ii) below this level convergent and divergent areas are distributed almost reciprocally around the center. (c) Analysis of the vertical stream lines At present, it is almost impossible to draw in detail the stream lines in the vertical cross section in the typhoon. Therefore, in this report, the vertical stream lines are determined by the radial components v,, and the vertical components vz_ for Itazuke which happens to be the nearest to the typhoon center (see Fig. 18). Fig. 21 shows the vertical stream lines obtained in this manner. In this figure; arrows represent vectors of wind velocities whose horizontal components are v,. and vertical components are vz in Fig. 18 and the solid lines with arrows in front , are stream lines in this cross section. However, vertical components in this figure are enlarged 200 times. Moreover, dotted arrows are the horizontal components at the height at which the vertical velocities could not be computed. As this figure is the stream lines in the 9.-z, cross section, it must be noted that the real motions of air particles in the typhoon are superposed by cyclonic rotation and the above motions in v-z cross section. Fig. 21 Stream lines in vertical cross-section for Itazuke in Kezia. Arrows represent vectors of wind velocities hose horizontal compcnents arc v, and vertical components are v, and solid lines with arrows in front are sfrean-i lines.

From this figure, many interesting results are obtained. In the thin layer below 2 km height every wind within the area of distance 6° in latitudes from the center, is directed towards the center, and stream lines divided into three branches. The first branch ascends at about the distance 3° in latitudes from the center and goes away from the typhoon system while drawing the convective system of about the height 4 km. The second branch ascends near the center, once directs itself outwards at about the height 4-5 km and furthermore ascends a little away from the center, and draws a very high convective system (about 12 km height). More- ' over, this convective system divides into two convective systems having centers at about the height 8 km, The third branch ascends near the center a:ong the minimum pressure axis up to about the height 12 km, meets with the stream lines 258V. Masuda and M. TakeuchiVol. 11 Nos. 3--4

divided from the second branch, and forms remarkable descending currents at the distance 1° in latitudes from the center. Therefore, we may conclude that the air converging in the thin layer near the surface (below about the height 2 knn ) maintains the large three convective systems. As for the rear of the typhoon, because of scarcity of data and effects of westerlies, clear decriptions are not obtained. Results obtained from this figure are not so trustworthy. It is perhaps not too much to say that they are imaginary. However, the positions of ascending and descending currents and heights of the convective systems somewhat coincide with the results obtained by the above analyses of the pressure and the temperature fields.

7. The transformation of the typhoon into an extra-tropical cyclone

From the above descriptions, it is thought that the typhoon has a dynamical structure rather than a thermal structure. What mechanism is necessary for the transformation of a typhoon with its dynamical structure into an extra-tropical cyclon- e ? As mentioned above, Jane was transformed into an extra-tropical cyclone near Ho- kkaido and Kezia in the midePe of the Japan Sea. As suitable data of this paragraph for ormation Kezia are not obtained, we shall consider the situation of transf- into an extra-tropical cyclone for Jane. Fig. 22 is the surface weather chart at the time when the typhoon was located in the middle of Hokkaido and shows that the form of isobars became somewhat asymmetrical by the incursion of the cold air mass from the southwest. Fig. 23 represents the vertical cross section at 12h on 4 sep.1950 through the rear of the typhoon. In this figure, the incursion of a cold dome is recognized between Akita and Yonago. The situation near the surface of this figure closely resembles that of an extra-tropical cyclone. There- fore, it is suggested that a typhoon which was approximately symmetrical with respect to the center, begins to have the thermal character as an extra-tropical cyclone. Does the dynamical structure of the typhoon then, disappear, com- pletely ? Fig. 24 represent the N-S vertical cross section o! the heights and temperatures on the standard pressure levels through the rear of the typhoon at 12h on 4 sep. In this figure, solid lines represent the forms of the standard pressure levels, dotted lines isothermsl on those levels, and dashed and dotted lines boundaries of the cold and warm air mass in the lower layers respectively. In the lower layers, similarly with Fig. 23, the situation approximately resembles that of an extra tropical cyclone. However, in the layer from 700 mb to 300 mb level, two heigh pressure areas exist above Sendai and Misawa. This fact indicates that thedynamical!stru- Fig. 24 The vertical cross section of heights and temperature of the standard levels at 1200 1,111.T. 4 Sep. 1950. 260Y. Masuda and Ni. TakeuchiVol. Nos. 3-4

cture of the typhoon still remains. However, contrary to the mature stage, as phases of the pressure and the temperature field do not agree so well with each other. It is seemsthat the thermal effect of the lower layers reaches up to considerably upper layers, and the dynamical structure is destroyed gradually. Ther- efore, we may conclude that when the typhoon comes into the middle latitudes, at first it begins to have the thermal asymmetry in the lower layers, then its dynamical structure is destroyed gradually and so it transforms itself into an extra-tropical cyclone.

8. The model of the typhoon

On the basis of the analytical results obtained above we may establish the model of the typhoon in outline. Fig. 25 shows the model of the typhoon at a mature stage established after considerations of the characteristic features of the pressure, the temperature and the stream line fields. In this figure, the horizontal coordinate is mea.- sured by the distance from the center in latitudes and vertical -coordinate by the height in KM. And solid lines represent representative anomalies of pressure, dotted lines those of temperature, the dash rirvt-f.r1 - - popause, the heavy dotted line the axis of the minimum pressure, and arrows directions of stream lines in this cross section respectively. Furthermore, the letters H and L indicate the centers of the maximum and minimum pressure anomalies, and W and C those of the warmest and the coldest areas. As this figure represents the vertical cross section, it must be noted that in reality a uniform cyclonic rotation superposes on this motion.

9. Conclusion

It must be noticed that as the analysis performed in this report are restricted to two paticular examples, Jane and Kezia, and the data are not sufficient the results obtained in this study may be modified in future. However, we may at least summarize the results obtained by our analyses in this way : the typhoon is 1951An Aerological Investigation of the Structure of the Typhoon261

not simple convective system but a system superposed by a few secondary dynamical convective systems. This is supported by following facts : i) In the upper layers, the secondary pressure field in which the relatively high and low pressure areas are distributed at about 200 km intervals reciprocally, superposes on the primary pressure field in which pressure decreases towards the typhoon center uniformly. ii) The axis of the minimum pressure is almost vertical up to about 500 mb level, and above this level steeply tilts towards the rear. The mean gradient of this axis is about 1/30. iii) The pressure distribution of the upper layers is not symmetrical with the typhoon center of the earth surface, but with the cyclonic center of the upper levels. iv) The pressure gradient near the typhoon center decreases with height. v) In the upper levels the warmest areas are located under the relatively high pressure areas and coldest areas above the relatively low pressure areas. vi) The height of the warmest areas become lower with the distance from the center. vii) The shape of the tropopause in the case of typhoon is the same as the "tropopause funnel " and the " t ropopause ridge " in the case of extra-tropical cyclone. viii) The relation between the tropopause height and its temperature in typhoon is mostly different from that of the middle latitudes, and a low and cold or a high and , warm tropopause is observed. ix) The cyclonic rotation of the typhoon decreases with height, and disappears at about 10km height. The height at which the cyclonic rotation disappears seems to represent the specific stage of the typhoon. x) The wind in front of the typhoon center at the level where the cyclonic rotation disappears are considered as the steering currents, but they are not determined so uniquely as to warrant its use for the forecasting of the moving direction, xi) The air which maintains the convective systems of the typhoon is gener- ally the air which converges in the thin layers near the surface below 2krn height. xii) When typhoon transforms into an extra-tropical cyclone, the dynamical structure tends to disappear and a thermal character begins to predominate.

Acknowledgment The authors wish -to express their hearty thanks to Dr. H. Arakawa and the members of the First Forecasting Research Laborato'ry, Mete- orological Research Institute, for their encouragement and helpful advices throughout this work. Sincere thanks are also due to Messrs. M. Hashimoto and J. Sampei for their many helpful discussions they had with us, and especially to Miss Y. Oka() for her indispensable aid in locating the desired data and figures. 262Y. Masuda and M. TakeuchiVol . II Nos.3",4

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