(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 59, No. 1, pp. 33-46, 2012
On the evening of January 29, 2003, when the upper cold air left the Sea of Japan, a broad cloud band along the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) decayed. The southern edge of the broad cloud band consisted of several snowbands with lengths ranging from tens of kilometers to 100 km. These snowbands were generated in the south off the broad cloud band, or were separated from the snowfall regions in the broad cloud band. The structures of a snowband, which formed the broad cloud band, were studied. The snowband contained convective cells having a maximum echo top of more than 5 km. Most convective cells developed over the sea and near the coastal region. The interaction of the convective cells was not necessary for the generation of new convective cells. In the lower levels, a mesoscale convergence line was present, which was important for the generation of new convective cells. In the coastal region, the northerly relative wind extended far north of the snowband and the local convergence layer was thicker than that over the sea. This thick convergence layer contributed to the maintenance of the existing convective cells, which moved from the sea, and to the generation of new convective cells. This resulted in the local intensification of the snowband.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 59, No. 5, pp. 333-350, 2012
This study analyzed long-term changes and the causes of the frequency of heavy snowfall, which was defined by a daily snowfall event over the 2-year return period, in regions where the daily snowfall of the 2-year return period is greater than 5 cm in Japan, by using daily snowfall data provided by Japan Meteorological Agency (JMA) and observed by East Japan Railway Company. The frequency of heavy snowfall shows a significant decreasing trend from the 1950s to the 2000s and a significant downward jump in the late 1980s in the Hokuriku region, but no significant trend or jump in other regions. In the late 1980s, a significant jump is detected in the winter mean temperature in all regions of this study, the East Asian Winter Monsoon Index (MOI), and the Arctic Oscillation Index (AOI). The winter mean temperature has a significant correlation with MOI and AOI in all regions of this study. This fact indicates that the jump in the winter mean temperature is related to the jump in MOI and AOI. The frequency of heavy snowfall has a significant correlation with the winter mean temperature, MOI and AOI in many regions of this study. These correlations are high in the Hokuriku region, but low in other regions. The correlation between the frequency of heavy snowfall and the winter mean temperature tends to be higher than the correlation between the frequency of heavy snowfall and MOI or AOI. Furthermore, the frequency of heavy snowfall in the Hokuriku region is sensitive to the change of temperature, because most of the heavy snowfall events occur at temperature slightly below the threshold between rain and frozen precipitation. As a result, in Hokuriku region, the downward jump in the frequency of heavy snowfall is strongly affected by an upward jump of the winter mean temperature.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 59, No. 7, pp. 549-561, 2012
A radar echo population of 179 thunderstorms generated in the Tokyo metropolitan area on 5 August 2008, when the Zoshigaya-rainstorm occurred at the center of Tokyo, is presented. The analysis was made using the 3-dimensional radar data of Japan Meteorological Agency. One third of the total convective cells had diameters less than 3.5 km and the average diameter was 5.5 km. The mode of the lifetimes of the cells was from 20 to 40 minutes, and 88% of the cells diminished up to 60 minutes after their initiation. The echo top of the half of the cells reached 15 km, which was the limit of the radar observation. Although the rainfall amount estimated from radar echo was less than 40 mm for the half of the cells, and 1/3 of the total cells counted more than 60 mm. Vertically integrated liquid water (VIL) ranged from 1.4 to 42.4 kgm-2. The maximum VIL was equivalent to 70％ of the precipitable water estimated from the upper sounding on the day. The traveling speed of the cells was less than 2 ms-1 in accordance with weak wind velocities in the lower to middle troposphere. The time from echo initiation to rainfall peak was as short as 10 to 30 minutes for almost all the cells. The thunderstorms consisting of the Zoshigaya-rainstorm ranked to the highest-class in horizontal size, lifetime and total rainfall amount among 179 thunderstorms. The horizontal size of cells in our case was nearly equal to the cases reported for other areas in the world, whereas the echo top height was larger than those of the other cases.
TENKI, Vol. 59, No. 7, pp. 563-577, 2012
Many air-mass thunderstorms were generated in the Tokyo metropolitan area on 5 August 2008, on which a severe local rainstorm caused a flash flood near the center area of Tokyo. Using three-dimensional radar reflectivity data of Japan Meteorological Agency (JMA), nowcastings concerning time and rainfall intensity of rainfall peak of thunderstorms were examined. Five nowcasting methods: formation and descending of precipitation cores aloft, time-changes of vertically integrated liquid water, time-change of echotop height, lightning activity and quantitative forecasting of rainfall intensity using three parameters, were adopted to eight thunderstorms which were related to the heavy-rainfall warnings being issued from JMA on the day. Some of the five nowcasting methods were effective for every thunderstorm, whereas common methods available for all the thunderstorms were not determined.