Time and Spatial Variations of Heavy Snowfalls in the Japan Sea Coastal Region

Transcription

1 August 1981 T. Akiyama 591 Time and Spatial Variations of Heavy Snowfalls in the Japan Sea Coastal Region Part II. Large-Scale Situations for Typical Spatial Distributions of Heavy Snowfalls Classified by EOF By Takako Akiyama Meteorological Research Institute, Tsukuba 305, Japan (Manuscript received 22 November 1980, in revised form 8 June 1981) Abstract In Part I of the present study, spatial distributions of daily snowfall (precipitation) over Niigata Prefecture (the Japan Sea coastal side of Japan) are classified into three types (Mountain-, Normal- and Plain-type) in terms of EOF. In Part 2, we examine the large-scale situations in the heavy snowfall period (three days) of these three types, basing on composite analysis. One of the important features found commonly for heavy snowfalls of the three types is that the air in the upper trough is much colder than in moderate/light snowfall cases. The intrusion of very cold upper trough to south over the warm Japan Sea results in formation of thick layer of transformed airmass (mixed layer). M-type heavy snowfalls (heavy snowfalls concentrated in high mountain [~1,000m] area) occur when a wedge-shaped deep upper trough propagates fast over the Japan Islands, accompanied with a rapidly developing surface depression. For this type of snowfalls, the formation of the thick mixed layer and its orographic lifting (low-level wind is perpendicular to the mountain range) will be essential. P-type snowfalls occur when a cold vortex slowly propagates to east after protruding southward over the Japan Sea. In this case the surface depression does not rapidly develop. The mixed layer is very deep and active cumulus convection develop there. The orographic lifting is not primarily important because the wind is not perpendicular to the mountain ranges. 1. Introduction Heavy snowfalls frequently occur over the lee side area of the Japan Sea (i.e., the Japan Sea coastal side of Japan) in the winter monsoon season. The precipitation reaches frequently to 50mm*day-1 (snowfall depth: 50*70 cm * day-1) and occasionally to *100mm*day-1 (snowfall depth: 100*150cm*day-1). Several authors have tried to find some relationships between the snowfall and the largescale situations in their climatological or statistical studies. Ishihara (1968) examined correlation coefficient between snowfall amount at each gage station and various meteorological elements observed at some upper stations. He showed the largest correlation coefficient (* -0.45) was found between snowfall amount and 500mb temperature at Wajima (see Fig. 1 for the location). Fukuda (1965) and Kawamura (1968) studied geographical distribution of occurrence frequency of heavy snowfalls in relation to largescale flow pattern. In these three studies, however, daily snowfall distributions were not discussed. The heavy snowfalls tend to concentrate over either mountain area ("mountain snowfall type") or coastal plain area ("plain snowfall type"). Fujita (1966) and Kurashima (1968) stressed, comparing the composite weather maps for the two types of snowfalls, that the large-scale situation of "plain snowfall" was different from that of "mountain snowfall". However, in their studies the classification of snowfall types was somewhat subjective and time variations in largescale situations were not discussed (i.e., they

2 592 Journal of the Meteorological Society of Japan Vol. 59, No. 4 examined the situation in the heavy snowfall day only.). We think that the relationship between snowfalls and large-scale situations is not fully described in the aforementioned studies. In Part I of the present study, we have described the time and spatial variations of daily precipitation (snowfall) in Niigata Prefecture (see Fig. 1) in terms of EOF (empirical orthogonal function). We classified objectively spatial distribution of precipitation into three types of snowfalls (mountain-, normal- and plain-snowfall types) using the 1st and 2nd time variation functions, A1(t) and A2(t). In Part 2, we will study large-scale situations for the heavy snowfalls of these three types. Our attention will be focussed on time and spatial variations in large-scale situations in the heavy snowfall period (i.e., three successive days centered at the heavy snowfall (area-averaged precipitation *20mm*day-1) days. 2. Results, of Part I and outline of the study in Part II Fig. 1 Domain for the analysis of large-scale situation. Fig. 2 Typical examples of daily precipitation maps for M-type (left: Jan ) and P-type (right: Jan ) heavy snowfalls. able to classify objectively daily precipitation patterns into three snowfall types basing on the second time variation function (A2(t)). "Normal snowfall type" was defined by small value of A2(t). In N-type the maximum area of snowfall is at the mountain area of *500m height. "Mountain snowfall type" was defined by large positive value of A2(t). Snowfalls of M-type concentrate on the high (*1,000m) mountain area. As a typical example of M-type Results of Part 1 heavy snowfall, the daily precipitation map of In Part I of the present study, we described Jan. 8, 1968 is presented in the left map of Fig. the time and spatial variation of daily precipitation (snowfall) over Niigata Prefecture (see 2. "Plain snowfall type" was defined by large negative value of A2(t). Snowfalls of P-type are Fig. 1) by using EOF for the winter of 1963 N brought about mainly on the coastal plain area The first and second functions (P1 and P2) (*200m). As a typical example of P-type heavy account for *50% and *15% of the total snowfall, the daily precipitation map of Jan. 15, variance of precipitation. The first spatial function (B1(x)) represents approximately the climato is presented in the right of Fig. 2. The heavy snowfall days of these three types logical precipitation pattern and the second were selected from among the analyzed period, spatial function (B2(x)) represents the contrast according as the criterion (section 5 of Part 1). of precipitation between the high mountain area Then three successive days centered at the and the coastal plain area. Consequently we were selected heavy snowfall days were picked up. The three days were designated as "before day", "in day" and "after day" respectively. The 3-day period was designated "heavy snowfall period". The snowfall type of the "in day" is used to designate the snowfall type of the "heavy snowfall period". We had shown the characteristics of the snowfall distribution in the heavy snowfall periods of P-type and M-type. Outline o f the present study Main purpose of the present study is to elucidate the differences in large-scale situations for the three types of heavy snowfall periods. For the analysis, we select cases from among the period of 8 Januarys and 8 Februarys (total 472 days) of 1963*1970 (i.e., the former subperiod in Part 1). 13*15 cases of heavy snowfall periods

3 August 1981 T. Akiyama 593 for each snowfall type are selected. T500 is the lowest in the category of "heavy To study the large-scale situations we use 12- precipitation". For *80% of "heavy precipitation" days, T500* is lower than -30*. On hourly sounding data at Wajima (see Fig. 1). We also use 500mb, 700mb, 850mb and the the contrary, T500 is the highest in the category surface maps issued by JMA. From these maps, of "light precipitation". For *80% of "light height, temperature and the surface pressure are precipitation", T500, is above -30*. It is noted estimated at 5*5* latitude-longitude grids for that in the day when T500 at Wajima is lower the domain presented in Fig. 1. than -40*, area-averaged precipitation becomes inevitably 10mm*day-1 or more. In section 3, the relation between 500mb temperature and precipitation amount is examined In short, the lower T500 becomes, the more for three snowfall types to find a clue for the snowfall (precipitation) is brought about. This investigation. In section 4, composite time indicates the strong influence of the cold vortex sections at Wajima are analyzed for the three or cold trough aloft on the snowfalls in the types of heavy snowfall periods. In section 5, analyzed heavy snowfall area. The meteorological implication is as follows: As sea surface we will focus our attention to M-type and P-type heavy snowfall periods and examine composite temperature is not widely fluctuated through the weather maps for these periods. winter monsoon season, the variation of air temperature in the lowermost layer is much mb temperature and precipitation amount smaller than that in the free atmosphere. Therefore, the thermal stratification becomes unstable It has been widely accepted in Japan that temperature drop below -35* at 500mb over (or less stable) when the midtropospheric temperature is lowered. Under such situation, convective the Japan Sea coastal area is one of signs of heavy snowfalls in this region. To find a clue activities are enhanced in the unstable stratification (Ninomiya, 1968a, b) and airmass trans- for further investigation on large-scale situations of heavy snowfalls, we will examine 500mb formation over the Japan Sea reaches to the temperature at Wajima (T500) in relation to areaaveraged daily precipitation (p) over Niigata 600mb layer of "heavy snowfall category" is upper level (*700mb). In fact, the SFC* Prefecture. less stable (-*/*p*3k/100mb, see Fig. 7) Fig. 3 presents normalized histograms of T500 as compared with that of "moderate snowfall at Wajima for three snowfall types of three category" (-*/*p*4k/100mb, figure is not categories of p. The three categories are "heavy shown here). The mixed layer (*e uniform layer) precipitation" (p*20mm*day-1), "moderate of "heavy snowfall category" is deeper than that precipitation" (20mm*day-1>p*10mm*day-1) of "moderate snowfall category". The saturated and "light precipitation" (10mm*day-1>p). It (or nearly saturated) layer of "heavy snowfall is evident that the histogram for each category category" reaches up to *700mb while that of is largely different each other. On the average, "moderate snowfall category" reaches to 00mb. *8 Next, we examine the difference among the three "snowfall types" of "heavy snowfall category". As the histograms of T500 for three types of "heavy snowfall category" are not largely different each other, we can not relate directly T500 at Wajima to the snowfall types in the present section. In order to find the character- Fig. 3 Normalized histograms of 500mb temperature at Wajima for the three snowfall types for the three categories of precipitation. In the histogram of P-type heavy precipitation, a secondary peak appears at -15*-20*. When T500 is above -25* (in this case, T SFC is usually higher than 5* in the coastal plain area), rain falls on some part of the analyzed area. The largescale situations for rain-snow mixture will be different from that for snowfall. As we intend to study the case of snowfalls, the analysis in the following sections will be made for the cases when T500 at Wajima is equal to or lower than -25*.

4 594 Journal of the Meteorological Society of Japan Vol. 59, loo. 4 istics of large-scale situations for these snowfall types, we will make more detailed analysis in the following sections. 4. Composite time section analysis for heavy snowfall periods We will investigate the variation of large-scale situation in the heavy snowfall periods by composing the upper data at Wajima for "heavy snowfall period" of three types. The composition for each type is made from *15 cases which are listed in Table 4 of Part I. Fig. 4 presents time variations of composite height (Z) and temperature (T) in the heavy snowfall periods. -24, 0 and 24 hour in the abscissa stand for 12 GMT of "before day", "in day" and "after day" respectively*. Fig. 5 shows composite time section of *Z (local time change of Z in 24 hour), *T (local time change of T in 24 hour) and V (meridional component of wind velocity). Fig. 6 shows composite hodographs for M-type and P-type heavy snowfall periods. Fig. 7 shows vertical profiles of mean potential temperature *, equivalent potential temperature *e, and saturated equivalent potential temperature *e* averaged for the heavy snowfall periods. These figures reveal the following features: a) Vertical structure of airmass Fig. 4 Composite time variations of height Z, surface pressure P5 c, temperature T and wind at Wajima for the 3-day period of the three-type heavy snowfalls. -24, 0 and 24 hour on abscissa stand for 12 GMT of "before day", "in day" and "after day" respectively. * Daily precipitations in this paper were measured for the period from 00GMT of the next day. of the day to 00GMT Figs. 5, 6 and 7 show that the lower troposphere is evidently subdivided into three layers; (1) the lowermost "boundary layer" (SFC* 900mb) characterized by unstable stratification (-*e/*p<0) and strong vertical shear, (2) "mixed layer" (900*800 or 700mb) characterized by vertical uniformity in *e, wind speed and high relative humidity (*saturated) and (3) "stable layer" characterized by stable stratification and westerly wind increasing with height. The "boundary layer" and "mixed layer" are the layer of air-mass transformation, while the "stable layer" is not affected by the air-mass transformation. These three layers are found commonly in the three type heavy snowfall periods, though the thickness or depth of them are somewhat different each other. The top of the mixed layer of P-type reaches up to 700mb, while that of M-type is at 800mb. b) Height and surface pressure fields M-type: Heavy snowfalls occur *24 hour after the passage of surface low and *6 hour after the passage of upper trough. Trough axis evidently tilts to west with height. It is shown in section 5 that the large local time change (*Z*±100gpm/24 hour) is due to the rapid movement of a deep wedge-shaped trough aloft. P-type: Heavy snowfalls occur under the deep cold trough and in the surface low pressure. In this type, the surface depression does not rapidly develop. *Z is small (*Z*±50gpm/ 24 hour) in the whole layer. As shown in section 5, the trough in P-type is not wedge shape but cut-off vortex. N-type: The situations are in the intermediate features between M- and P-types. c) Temperature field M-type: During the period of "before day"* "in day", temperature in SFC*350mb dropped rapidly. *T is *2*/24 hour at the surface and *-4*/24 hour at 600mb. The minimum temperature is observed (T500*-35*) around 0 hour (t=0). Afterwards, temperature above 700mb level recovers rapidly while temperature below 700mb level does slowly. The top of the "mixed layer" is at *800mb. P-type: In the period of "before day"*"in day", temperature in the midtroposphere drops rapidly (*T*-4*/24 hour at 500*600mb, minimum T500 *-37*) while temperature in SFC*700mb drops slowly (*T*-1*/24 hour). In consequence, thermal stratification in the lower layer is more unstable than that in

5 August 1981 T. Akiyama 595 Fig. 5 Composite time sections of local time change of height in 24 hour *Z, local time change of temperature in 24 hour *T and meridional component of wind V for the 3-day period of the three-type heavy snowfalls. Fig. 6 Composite hodographs for M-type (left) and P-type (right) heavy snowfall periods. M-type (see Fig. 7, *e in the lower layer of P- type is higher than that of M-type). In "after day", temperature rises slowly in 500*600mb while it still drops slowly in 850*700mb. This indicates that thick clod airmass spreads widely in the lower layer over the Japan Sea and the Continent. The top of the "mixed layer" is comparatively higher (*700mb) than that in M-type. d) Wind field In the "boundary layer", WNW*NW monsoon prevails in the both M-type and P-type heavy Fig. 7 Composite vertical profiles of potential temperature *, equivalent potential temperature *e and saturated equivalent potential temperature **e averaged for the 3-day periods of M-type and P-type heavy snowfalls. snowfall periods. In the "mixed layer" wind speed is approximately uniform, but wind direction is backing with height (i.e., northerly component decreases with height). The vertical wind shear in the mixed layer is directed SSW to NNE. This vertical shear is due primarily to horizontal gradient of temperature in the mixed

6 596 Journal of the Meteorological Society of Japan Vol. 59, No. 4

7 August 1981 T. Akiyama 597 layer directed SE to NW, though wind is not exactly geostrophic. In the "stable layer" or the layer above the "mixed layer", westerlies increase with height. The wind veers with the passage of the trough. The features mentioned above are commonly viewed for the heavy snowfall periods of three types. In M-type, composite wind speed is significantly large through the whole layer. In "boundary layer" and "mixed layer" wind speed and northerly wind component reaches to the maximum value in "in day". In P-type, wind speed is considerably small through the whole layer. The outstanding feature in P-type is that southerly wind component in 700*300mb reaches its maximum in "in day". The increase of southerly wind component is accompanied with the southward movement of the cold vortex as described in section 5. Since in both "boundary layer and mixed layer" wind speed is large and wind direction (WNW*NW) is nearly perpendicular to the mountain ranges, orographic lifting will be very effective for M-type. The strong WNW-NW winds will stream falling snow flakes into the inland or the high mountain areas. Because wind speed is small and wind direction (WNW*WSW) is not perpendicular to the mountain ranges, orographic lifting will be small for P-type. The relatively weak WNW*WSW wind will bring snowflakes at the plain area adjacent to the Japan Sea. The large-scale situations described in this section are listed in Table 1. As far as we examined in the composite time sections, we found that the large-scale situation of N-type heavy snowfalls is in the intermediate situation between P-type and M-type heavy snowfalls. In the following sections we will focus our attention on P-type and M-type. Fig. 8 Composite 500mb height Z500 maps for the 3-day periods of M-type (left) and P-type (right) heavy snowfalls. S. Composite maps for M-type and P-type heavy Fig. 9 Composite 500mb local time change in 24 hour *Z500 for the 3-day periods snowfall periods of M-type (left) and P-type (right) heavy a) 500mb height field Fig. 8 and Fig. 9 are composite maps of Z500 snowfalls. (500mb height) and *Z500 (local time change of east coast of the Continent in "before day" moves Z500 in 24 hour) for M-type (left maps) and P- eastwards rapidly in "in day" and arrives over type (right maps) heavy snowfall periods. the Sea of Okhotsk in "after day" moving eastnorth-eastwards. Common feature: 500mb low area bounded The movement of the trough is by 5,300gpm contour is located further south more evidently viewed in *Z500 field (Fig. 9). as compared with the normal location in January. M-type heavy snowfall occurs in and after the This is one of the characteristics of the largescale passage of wedge-shaped cold trough. Z500 has situations in the heavy snowfall periods. its maximum meridional gradient around the M-type: Wedge-shaped trough centered at the southern Japan Sea (the analyzed heavy snowfall

8 598 Journal of the Meteorological Society of Japan Vol. 59, No. 4 area). It accounts for strong westerlies there. P-type: Deep trough centered over the east coast of the Continent in "before day" deepens with time, slightly moving southward. In "after day", it moves slowly eastward. Associated trough line was nearly stationary in "before day"*"in day". In "after day", the trough line passes slowly eastward over the analyzed heavy snowfall area. The negative *Z5oo in "before day".*"in day" indicates the deepening or a little southward movement of the trough. The outstanding feature is that the maximum value of negative *Z500 is at the southern portion of the Japan Sea. This well accounts for the increasing of southerly wind component at Wajima in "in day" (see Fig. 6). The meridional gradient of Z500 is small around Niigata Prefecture as comparing with that in M-type. This is consistent with weak westerlies at Wajima. The meridional gradient of Z500 is strong in the south of the Japan Islands. It is evident that P-type heavy snowfall occurs within a cold vortex. b) Surface pressure field Fig. 10 and Fig. 11 show composite maps of PSFC (surface pressure) and *PSFC (local time change of PSFC in 24 hour) for M-type (left maps) and P-type (right maps) heavy snowfall periods. Fig. 10 Composite surface pressure PSFC for the 3-day periods of M-type (left) and P-type (right) heavy snowfalls. Fig. 11 Composite surface pressure local time change in 24 hour *PSFC for the 3-day periods of M-type (left) and P-type (right) heavy snowfalls. Common feature: The Continental anticyclone and the stationary Okhotsk Sea low exist through the 3-day period. The pressure gradient over the Japan Sea is directed principally eastward and NW*WNW monsoon wind prevails around the analyzed heavy snowfall area. M-type: The cyclonically curved 1,008mb contour at the northern portion of the Japan Sea in "before day" indicates the existence of a weak depression, which rapidly develops moving eastward and becomes a strong cyclone over the Okhotsk Sea in "in day"*"after day". This rapid development of depression account for the large negative *PSFC in "before day"*"in day". The anticyclone over the continent spreads eastward over the Japan Sea, as seen from the movement of 1,016mb contour in Fig. 10. In "in day"* "after day", as the Japan Sea area is situated behind the rapidly developed cyclone, the NW monsoon is extremely strong. P-type: Variations in PSFC pattern are slight through the 3-day period. The cyclonically curved contour over the Japan Sea in "in day" indicates the existence of a weak depression. As shown in Fig. 13, the depression, however, does not 'so largely develop nor rapidly propagate as in M-type. The anticyclone over the continent did not spread to east. Therefore the monsoon is rather weak in P-type. c) 700mb temperature field Composite maps of 700mb temperature (T700) for three days of M-type and P-type heavy snowfall periods are presented in Fig. 12. Common feature: The cold core centered at

9 August 1981 T. Akiyama 599 Fig. 12 Composite 700mb temperature T700 for the 3-day periods of M-type (left) and P-type (right) heavy snowfalls. 5*N/125*E is stationary *4for the 3-day period, while the cold trough in 30*40*N moves slowly eastward. Cold core at 850mb is also stationary over the continent throughout the 3-day period (figure is not presented). In short, the massive cold airmass exists over the continent and adjacent sea area. The thermal gradient in the transformed airmass over the Japan Sea accounts for the strong vertical wind shear in the "mixed layer" directed SSW to NNE around the analyzed heavy snowfall area. This wind shear in 900*700mb is one of the characteristic features of heavy snowfall over Niigata Prefecture*. M-type: The 700mb cold trough moves eastward in "before day"*"in day" and then northeastward in "after day". P-type: The cold area bounded by -20* isotherm protrudes southward over the southern part of the Japan Sea in "in day". The cold trough remains at *130*E in "before day"* "in day" and then moves slowly to east in "after day". 6. Remarks and discussion In Part I of the present study, spatial distri- In the case of moderate/light snowfalls, vertical wind shear in the mixed layer is weak and is directed WSW to ENE or W to E. butions of daily snowfall (precipitation) over Niigata Prefecture, the Japan Sea coastal side of Japan, are classified into three types (M-type, N-type and P-type) in terms of the time variation function, A1(t) and A2(t). In Part II, we examine the large-scale situations in the heavy snowfall period (three days) of these three types mainly basing on composite analysis. The results are summarized in Table 1. In order to realize the situations obtained by composite analysis, we pick up the case, 7*9 Jan (M-type) and 14*16 Jan (P-type) from among the composite cases as typical examples. The surface and 500mb maps for these periods are presented in Fig. 13 and Fig. 14 respectively. (see Fig. 2 for the "in day" precipitation maps.) The large-scale situations and their time change noted in items (1), (2) and (3) of Table 1 are well elucidated in Fig. 13 and Fig. 14. M-type heavy snowfall occurs when a wedge-shaped deep upper trough propagates fast over the Japan Islands with a rapidly developing surface depression. P-type heavy snowfall occurs when a cold vortex slowly propagates to east after protruding southward over the Japan Sea. In this case the surface depression does not rapidly develop (see the small depression over the Japan Islands at 12 GMT 15 Jan. 1967). One of the important Fig. 13 Typical examples of surface maps for M-type (left: 7-9 Jan. 1968) and P-type (right: Jan. 1967) heavy snowfall periods.

10 600 Journal of the Meteorological Society of Japan Vol. 59, No. 4 Fig. 14 Typical examples of 500mb maps for M-type (left: 7-9 Jan. 1968) and P-type (right: Jan. 1967) heavy snowfall periods. features found commonly for M-type and P-type heavy snowfalls is that the air in the upper trough is much colder than that in moderate/ light snowfall cases. Items (4) and (5) of Table 1 stress the situations in the lower layer directly related to snowfall amount and distribution. The intrusion of very cold upper trough to the warm Japan Sea results in the formation of the transformed thick layer (mixed layer). For M-type heavy snowfalls, the formation of the thick "mixed layer" and its orographic lifting (low-level wind is *perpendicular to the mountain ranges) will be essential (cf., numerical simulation by Estoque and Ninomiya, 1976). In P-type heavy snowfalls, the "mixed layer" is very deep and active cumulus convections develop there. Sometimes, they are organized into mesoscale convective systems and cause the concentrated snowfalls over the coastal plain area (see Matsumoto et al., 1967 and Miyazawa, 1968). In P-type heavy snowfalls, the orographic lifting is not primarily important because the wind is not perpendicular to the mountain ranges. In the present study we described the largescale situations of mountain- and plain-heavy snowfalls over Niigata Prefecture (*100*150 km2), Japan. It is pointed out that the snowfall spatial distribution in this small domain is controlled by large-scale circulation. At the end of this section, we will compare the results of the present study with those of the previous studies. (Fujita, 1966 and Kurashima, 1968). The large-scale situation in "in day" is similar to that described in the previous studies. We note difference between the previous and present studies: In the previous studies selection of heavy snowfall days and classification of snowfall types were somewhat subjective. They described mainly the large-scale situation on the heavy snowfall days (i.e., "in day" of the present study) and did not examine fully the time change in the large-scale situation and associated snowfalls. The significance of the present study lies in that temporal changes of snowfalls and largescale situations have been described in objective and synthetic ways. Acknowledgements The author is grateful to Dr. K. Ninomiya, Electronic Computation Center, JMA, for his valuable comments. She is also grateful to Dr. S. Yoshizumi, Meteorological College, JMA, for his critical review of the manuscript. References Estoque, M. A. and K. Ninomiya, 1976: Numerical simulation of Japan Sea effect snowfall. Tellus, 28, Fujita, T., 1966: The characteristic of synoptic pattern in heavy snowfall in the coastal and in the mountainous region in Hokuriku district. Tenki, 13, (in Japanese). Fukuda, K., 1965: Synoptic study on the mechanism of heavy snowfall. Geophys. Mag., 32, Ishihara, K., 1968: Study of statistical analysis and forecast of snowfall on the area of the Japan Sea side of central Japan, Geophys. Mag., 34, Kawamura, T., 1968: Microclimatology of snowfall. Tech. Rep. JMA, 66, (in Japanese). Kurashima, A., 1968: Studies on the winter and summer monsoons in East Asia based on dynamic concept. Geophys. Mag., 34, Matsumoto, S., K. Ninomiya, and T. Akiyama, 1967: A synoptic and dynamic study on the three dimensional structure of mesoscale disturbances observed in the vicinity of a cold vortex center. J. Met. Soc. Japan, 45, Miyazawa, S., 1968: A mesoclimatological study on heavy snowfall. Pap. Met. Geophy., 19, Ninomiya, K., 1968a: Heat and water budget over the Japan Sea and the Japan Islands in winter season, J. Met. Soc. Japan, 46,

11 August 1981 T. Akiyama 601 -, 1968b: Cumulus group activity over the vapor convergence in subcloud layer. J. Met. Soc. Japan Sea in wintertime in relation to the water Japan, 46,