Altitude Dependence of Trends of Summertime Precipitation during the Period 1979-2015 in Nagano Prefecture

Altitude dependence of trends of precipitation has seldom been studied, because insufficient long-term observation data have been collected at various elevations in mountainous areas. In order to clarify the altitude dependences of precipitation and trends, rain gauge data collected at 37 meteorological observation stations by Japan Meteorological Agency in Nagano prefecture in summer (June-September) during the period 1979-2015 are analyzed. Horizontal distributions of summertime precipitation and trends are also investigated, and the influences of relocating observation stations on trends of summertime precipitation are verified. As a result, positive correlations between monthly precipitations and elevation are found, as in previous studies. An analysis of horizontal distributions of summertime precipitation indicates more precipitation in the southwestern part than the northeastern part of Nagano prefecture. A significant difference among trends of precipitation between relocated meteorological stations and others is not recognized. Regarding altitude dependence of trends of precipitation, trends of monthly precipitation in June tend to be small at high-altitude meteorological observation stations rather than at low-altitude stations. However, the negative correlation between annual change rates of monthly precipitation in June and elevation was caused by a decreasing trend of monthly precipitation in June at Ontakesan (Elevation: 2195 m) , which is statistically significant with a 5% significance level. No significant relationship between trends of monthly precipitation and elevation was found in July, August, and September. An analysis of horizontal distributions of trends of summertime precipitation indicates that precipitation in June and September tended to decrease in the southern part of Nagano prefecture.