April 1975 T. Yonetani 139 Characteristics of Atmospheric Vertical Structure on Days with Thunderstorms in the Northern Kanto Plain A statistical analysis- - By Tsuneharu Yonetani National Research Center for Disaster Prevention, Tokyo (Manuscript received 7 September 1974, in revised form 10 March 1975) Abstract Vertical conditions on 248 days during two summer seasons are investigated to study the distinctive characteristics of the stratification on days with thunderstorms in the northern Kanto plain. For the sake of convenience atmosphere is divided into two layers, upper layer and lower layer. It is recognized that high degree of conditional instability of an upper layer is not the sufficient condition for the occurrence of thunderstorms. However, there are distinct differences of vertical stabilities of a lower layer between on days with thunderstorms and on days without a thunder- storm when an upper layer is conditionally unstable. Namely, it is shown that vertical movement of an air parcel in a lower layer on the former is easier than on the latter. It is also shown that the ratio of the degree of conditional instability of an upper layer to the degree of stability of a lower layer is an important factor affecting the occurrence of thunderstorms rather than their values. These conditions are interpreted into suggesting an interaction between a convective cloud and its subcloud layer, and an instability index is obtained by assuming the interaction. It is shown that this instability index has higher association with the occurrence of thunderstorms than Showalter stability index has. Therefore, the stronger relation is suggested between the occur- rence of thunderstorms and the stability of the whole atmosphere than it has been pointed out. Thus, the degree of over-all stability may be the 1. Introduction most important factor in the occurrence of con- The synoptic conditions favorable for the occur- vective storms among the conditions described rence of severe thunderstorms and tornadoes above. have been described by many authors (e.g. When we compute the stability of air masses Fawbush, et al. 1951; Means, 1952; Miller, 1959). for use in analysis and forecasting, a statical and Reviewing these, Newton (1963) summarized the macroscopical viewpoint is usually taken. Show- conditions as follows : alter stability index (Showalter, 1953) and energy (a) Conditional and convective instability; stability index (Darkow, 1968) are determined (b) Availability of abundant moisture in lower from 850- and 500-mb data. Lifted index (Galway, levels; 1956) is similar to Showalter stability index except (c) Bands of strong winds in lower and upper for the determination of the level where the parcel levels; is lifted. These indicies do not represent structural (d) Some dynamical mechanism which can characteristics like the existence of inversion. On cause the release of instability. the other hand, the presence of strong inversion Analysis done by Ninomiya (1971) shows that has been shown as one of significant features these are not phenomena occurred independently prior to the occurrence of tornadoes or large hail but moisture and wind fields are modified to (Fawbush and Miller, 1953). Discussing the evolu- intensify convections through the interactions tion of convective clouds dynamically, the energy between thunderstorms and large scale fields. supply may be a basic factor. The effects of the 140 Journal of the Meteorological Society of Japan Vol. 53, No. 2 energy supply on the behavior of clouds systems The days with thunderstorms are days when over the sea are analyzed by several authors thunderstorms or a thunderstorm occurred in (Matsumoto and Ninomiya,1966; Ninomiya, 1968 the northern Kanto plain during 09 to 21 JST and 1972). The purpose of this paper is to and the other days are regarded as the days investigate the characteristics of atmospheric verti- without a thunderstrom (refer to Fig. 1). Numbers cal structure on days with thunderstorms in the northern Kanto plain. The viewpoint will be rather dynamical and atmosphere will he divided into two layers, i.e. upper layer and lower layer. It will be simplified that an upper layer is environ- ment in which convective clouds are embedded and a lower layer is a supplier of energy. We have many days with thunderstorms in the northern Kanto district in summer season and almost all intense thunderstorms are initiated in the mountainous area and move into the Kanto plain. Omoto (1971) showed the locations of preferred routes of severe convective storms in the Kanto plain which originated from several moun- tains. The upper layer is temporarily set as a layer above 1,800 m, noting the position of thunderstorms initiations relative to the mountains and their heights in the area under consideration. According to Kurihara (1962), the diurnal varia- tion of temperatures and heights of isobaric surfaces at levels over 850 mb are fairly small Fig. 1. Topographic map of Kanto district and and the magnitudes are less than the probable location of weather stations used in the errors of upper air observations at that time. One analysis. Dotted area is northern Kanto of water vapor pressure is also small unless the Plain and shows a station whose exchange of air-mass does not occur. Thus, daily reports are used to know days assuming that meteorological elements except for with showers. Blacked triangles are the temperature at low levels do not change their peaks which is pointed out to be located values abruptly, we use aerological data taken at in the places from where preferred Tateno (646, 27 m MSL) at 09 JST as represent- routes of hailstorms start (Omoto 1971). ative of the stratification of atmosphere over the Kanto plain in day time, though it is desirable to of days with thunderstorms in 1967 and 1968 are use sounding data taken just before the occurrence 40 and 39, respectively. The following publica- of thunderstorms if we could. tions of the JMA and Tokyo Meteorological District Office are used: Aerological Data of 2. Data used Japan, Monthly Report of the Japan Meteorologi- The JMA (Japan Meteorological Agency) and cal Agency, Ijo-Kisho Chosa Houkoku (report on its cooperators hold a series of thunderstorm abnormal weathers) and Chijo-Kisho Nippo (daily observations in which about 20 regular weather weather report). stations and about 35 cooperative stations in Kanto district join and the results of the observa- 3. An instability index to an upper layer tions are published yearly as Denryoku Kisho We pursue a parcel method modified slightly Gaiho (weather report concerning electrical power). as described below to discuss the vertical stability It was carried out from 15 May to 15 September of an upper layer. Namely, we consider an in 1967 and 1968 and on these 248 days the ascending air parcel embedded in an environment investigation is made. A classification of days which is in hydrostatic equilibrium. Following into a day with thunderstorms and a day without assumptions are made. a thunderstorm is based on these observations. (i) The parcel is saturated. April 1975 T. Yonetani 141 (ii) The forces acting on it are gravity, buoy- velocity and zero liquid water content at zi. The ancy and no frictional force. equations (1)-(4) are essentially similar to those (iii) In its ascending, all excess water vapor over of one dimensional cumulus model used by saturation condenses to liquid which is Squires and Turner (1962), Simpson and Wiggert retained in the parcel. et al. (1969). In one dimensional cumulus model, (iv) It has no ice phase process. it is assumed that entrainment rate is in inverse (v) It entrains environment air at a rate of 100 proportion to the updraft radius R, namely percent per 5,000 m ascent. The last corresponds to that environment air is entrained into the growing cumulus at a rate of approximately 100 percent per 400 mb, that is So we could take this instability index as the doubling of the mass for a rise of 400 mb (Byers maximum vertical velocity in an imaginary and Braham, 1948). The vertical motion of this convective cloud with 1,400 m radius of updraft. parcel can be expressed by a set of equations (1)-(4) (for example, see Mason and Emig, 1961). 4. Vertical structure on days with thunderstorms and on other days The correlation coefficients between Wm(0.0, 1800) and Wm(0.5,1800) and between Wm(1.0,1800) and Wm(0.5, 1800) are 0.98 and 0.99, respectively. The regression equations of Wm(0.0, 1800) and Wm(1.0, 1800) on Wm(0.5, 1800) are Wm(0.0, 1800) =1.00 • Wm(0.5, 1800)-2.8 and Wm(1.0, 1800) = 0.97 • Wm(0.5, 1800)+2.7. These show that the variation of Wm(d T, 1800) is in propor- tion to AT. However, we have a few exceptions. The relation between Wm(d T, 1800) and d T on such days with thunderstorms is shown in Fig. 2. where Cpm is specific heat of moist air at constant pressure, q acceleration of gravity, L liquid water content, Lh latent heat of condensation, Q specific humidity of a parcel, Qe specific humidity of environment air, R gas constant of air, T tem- perature of a parcel, Te temperature of environ- ment air, Tv virtual temperature of a parcel, Tve virtual temperature of environment air, t time, W vertical velocity of a parcel, z height and p entrainment rate. The entrainment rate is give by Fig. 2. Dependence of maximum vertical velocity on temperature excess at 1,800 m (AT). The dashed line is a typical one and the others Noting the nature (v) of the parcel and assuming are exceptional ones on days with that the entrainment rate is independent of height, thunderstorms.