Case study on the formation of snowfall in a basin covered by the stratiformed precipitation system with an extratropic cyclone in winter

1. Introduction

In winter, a stratiformed precipitation system often extends widely with an extratropical cyclone moving on the Pacific coast of Japanase archipelago. In the stratiformed precipitation system which has cold and warm regions with the boundary of 0 degree Celsius at the lower layer, snowfall and rainfall regions distribute. Discriminants of snowfall and rainfall, however, don't depend on such cold and warm regions in a simplistic form. To enable the discriminant of snowfall or rainfall, we need the knowledge of the formation of snowfall brought by the stratiformed precipitation system.

Kofu basin about 300 m above the sea level (ASL) surrounded by mountains with the elevation of 1 to 2 km is near the Pacific coast of Central Japanese archipelago (Figs. 1 and 3), which is one of the regions where snowfall or rainfall is often brought by the stratiformed precipitation system associated with an extratropical cyclone. From December on 2012 to February on 2013, 17 extratropical cyclones moved on the Pacific coast of Japanese archipelago. In Kofu basin, 6 cases of them were snowfall and sleet events, while 11 cases of them were rainfall events.

In the present study, we discuss the formation of snowfall in a basin through the case study of the structural variation of the stratiformed precipitation system associated with the extratropical cyclones and the meteorological conditions with the variation on 14 January 2013, which brought snowfall on Kofu Basin. For the analyses, we use the X-MP radar of University of Yamanashi (UYR) installed on Kofu Basin, C-band radar data operated by Japan Meteorological Agency (JMA), surface meteorological data at observatories of JMA and Sounding data.

2. Results

The stratiformed precipitation system extended abroad on the northern part of the extratropic cyclone at sea off the south of Japan Island at 0900 Local Standard Time (LST = UTC + 9 hours) (Fig. 1). At the level of 850 hPa, cold and warm regions with the boundary of 0 degree Celsius distributed in the precipitating system, respectively. From the sounding observation at Hamamatsu, the isothermal layer ranged from 0 to 0.5 degree Celsius appeared from 2.0 km ASL to 1.4 km ASL (Fig. 2). At the time, snowfall was observed at Kofu observatory of JMA (Fig. 3), while rainfall was observed at the observatories on Shizuoka coastal region on the south of Kofu Basin.

Figure 4a shows time-height sections of radar reflectivity (Z_H) and correlation coefficient (ρ_HV) observed by UYR at point ea' in Fig. 3, namely over Kofu Basin. A particle looks like a large drop when surface layer of the particle starts melting; Z_H becomes large with initial melting particles. ρ_HV depends on the dispersion in the ratio of the vertical-horizontal size of individual hydrometeors; ρ_HV is sensitive to the presence of large wet particles such as melting aggregates.

At 0330 LST, precipitating system began to fall on Kofu Basin. Below 0.6 km ASL, large Z_H and small ρ_HV were maintained from 0430 LST to 0530 LST. From 0530 LST to 1430 LST, Z_H and ρ_HV maintained almost large although they fluctuate slightly at moments. Then, ρ_HV and Z_H were small and large, respectively from 1430 LST to 1530 LST.

At Kofu observatory of JMA, weather situations were rainfall at 0300 LST, snowfall at 0600 LST, 0900 LST and 1200 LST, and rainfall at 1500 LST and 1800 LST (Fig. 4b). Precipitation occurred from 0450 to 1500 LST. Snow depth reached 10 cm from 0600 LST to 1100 LST. Although snow depth decreased at 1200 LST, it reached 10 cm again. Then, snow depth decreased at 1500 LST; it was constant from this point forward.

Temperature decreased moderately with increasing water vapor mixing ratio from 0400 LST to 0530 LST (Fig. 4c). Until 0610 LST, temperature decreased about 0 degree Celsius abruptly and water vapor mixing ratio decrease about 4 g kg^-1; Relative humidity was about 90 %. Furthermore, wind became calm. Then, above-mentioned situations maintained with calm or very weak wind until 1400 LST. After 1400 LST, temperature increased, water vapor mixing ratio also increased slightly and wind developed.

3. Discussion

When the stratiformed precipitation system covered on Kofu Basin from 0300 LST to 0600 LST, we consider that rainfall with raindrops and melting snowflakes changed to snowfall with snowflakes and melting snowflakes in Kofu Basin. For the period, it is suggested the cooling and humidification with the evaporation of raindrops and the sublimation and melting of snowflakes made temperature drop to about 0 degree Celsius from about 4 degree Celsius and water vapor mixing ratio become about 4 g kg^-1 from about 3 g kg^-1 in Kofu Basin.

Then, snowfall with snowflakes and melting snowflakes with temperature of about 0 degree Celsius and water vapor mixing ratio of about 4 g kg^-1 in calm continue from 0600 LST to 1400 LST. We suggested that the sublimation and melting of snowflakes continued to cool and humidify the air in Kofu Basin. As the result, the cold and humid air remained in Kofu Basin and snowfall was maintained. Finally, rainfall with drops and melting snowflakes dominated when wind began to blow and temperature increased in Kofu Basin.

From the above result and discussion, we suggest that the important things for the formation of snowfall in Kofu Basin associated with an extratropical cyclone moving on the Pacific coast of Japanase archipelago were as follows: 1) the air capable of abundant cooling and humidification in the basin below the level of the about 0 degree Celsius isothermal layer, 2) the cooling and humidification by the evaporation of raindrops and the sublimation and melting of snowflakes falling in the basin and 3) the residence of the cold and humid air in the basin.

Acknowledgement

This study is supported by Grants-in-Aid for Scientific Research of Japan Society for JSPS and Social System Reformation Program for Adaption to Climate Change of NIED under JST/MEXT. Then, this study was also supported by the Global COE Program of University of Yamanashi directed by MEXT.