Rainfall Tendency in Winter Sugadairakogen Highlands, Nagano Prefecture

Abstract

The temporal tendency of winter rain events in the Sugadairakogen Highlands was investigated using 33 years daily records of rainfall/snowfall discrimination at the Sugadaira Montane Research Center, University of Tsukuba, and the characteristics of atmospheric circulation in relation to the precipitation-phase changes were analyzed for 12 precipitation events by simple laser-type disdrometer observations with numerical weather simulations. The frequency of rainy days was 12 % from December to February, and 70 % of the rainy days were accompanied with traveling extratropical cyclones. A long-term increase of the snowfall frequency by extratropical cyclones was observed from November to April. The rainy days did not show a long-term trend. However, the large year-to-year variability of rainy days was significantly correlated with the index of the Pacific Decadal Oscillation. The year with an abrupt increase in the number of rainy days corresponded with the warm winter with less accumulated precipitation amount when the snow cover structure in high elevation areas is likely to be affected by the occurrence of rain. In the five rainfall-only cases and seven rainfall and snowfall cases identified in three winter seasons, rainfall dominant events were accompanied with broad warm-air mass expansion covering Honshu area with various courses of extratropical cyclones. On the other hand, rainfall with snowfall events were accompanied with a cyclone passing along the south coast of Japan or long-ranged cold front, and the precipitation phase changed mostly from rain to snow. In Sugadaira, southerly winds were suppressed before the phase change, and the air temperature stayed around 0 °C. A numerical simulation confirmed that weakening of the wind was due to the barrier effect behind the Echigo mountain range with prevailing east-west component winds by the traveling cyclone. A numerical model successfully simulated the rainfall-only events because the synoptic-scale circulation controlled the broad warm air migration. However, cases of a precipitation shift from rainfall to snowfall were not fully simulated. The causes of the discrepancies between the observation and simulation were discussed from the viewpoint of the low-level air-mass exchanges in the large valleys running in the south-north direction and the occurrence of Foehn in the lee-sides of mountain ranges accompanied with a traveling cyclone.