A Study of the Monsoon Snowfall at Wajima

[摘要] Observations of precipitation elements were made with actual-size photography and microphotography during two winter seasons at Wajima, situated on the northern coast of the Noto peninsula which juts out into the Japan Sea, and the relation to the meteorological conditions was studied. Obtained results may be summarized as follows:(1) The typical precipitation under winter monsoon conditions at Wajima is of the showery type of short duration. Continuous precipitation with nearly uniform intensity occurs only when a depression approaches or a local discontinuity exists in the vicinity.(2) At the beginning of a showery precipitation, a fall of temperature, _??_ rise of barometer, gusts, and a cyclonic shift of wind direction are observed usually, in like manner as in an air mass thunderstorm. A shower cloud may be regarded as a small scale thundercloud.(3) The showery precipitation by winter monsoon is nothing else than an instability shower, which is characteristic of cold air that travels over a warm sea surface. The occurrence of showers is found to be the more frequent, the lower the temperature at 700 mb level (Fig. 14).(4) At the beginning of a shower, large grains of graupel fall first usually, which last some 5 to 10 minutes but gradually decrease their sizes, and are f_??_ally replaced by large snowflakes. The total duration of a shower amounts generally from 20 to 30 minutes. In some cases, only graupel falls from beginning to end; in other cases, thickly rimed. crystals fall at the beginning which are gradually replaced by bare crystals. Frequently somewhat smaller graupel pellets or snowflakes precede the larger ones.(5) Most crystals which fall in a shower are of the stellar or the spatial-dendritic: form. These crystals are considered to be fallen after sufficient growth in relatively short time because of their large growth rate.(6) Soft hail is nothing but soft rime which is formed in the free atmosphere. Its external form may be classified into 4 types: conical, hexagonal, granular, and rime-like Each of these 4 types may, however, approach to conical (or truncated-conical) form after sufficient development, so the cone-type may be the fundamental form of soft hail. The crystalline appendages, frequently observed at the top of a cone or on the upper surface of a truncated cone, are inferred to be formed while falling through water-saturated atmosphere. The density of soft hail is found to be about 0.40, nearly agreeing with that of soft rime already known to be 0.20_??_0.60.(7) Small hail is precipitation corresponding to hard rime. Its external form is conical, similar to soft hail, but the internal structure is hard and semi-transparent. It falls usually intermingled with rain accompanying the passage of a strong cold front with vigorous convective activity. Such small hail as is defined in most textbooks never fell under winter monsoon conditions at Wajima.(8) For a few examples of continuous snowfall, the correspondence between crystal forms and the upper air conditions was investigated.(9) A rise of temperature is frequently observed at the cold-front passage in the nighttime. This is considered to be caused by the destruction of a ground inversion layer (masked front of H. v. Ficker). The same cause may account for the temperature rise occurring with showery precipitation in night.In order to confirm the above conclusions, it is desirable to make further observations concerning the horizontal distribution of monsoon showers and its variation with time, with radar or a dense surface network, and to make more detailed observations relating to their precipitation elements.

[学科分类] 大气科学