Abstract
We investigated the applicability of records of the first date of snow cap on Mt. Iwaki in Aomori Prefecture to the reconstruction of autumn temperatures. 179 years of dates on the first snow cap on Mt. Iwaki until the 1860s were collected from the diaries of the Hirosaki Domain Office and other historical documents. Since the date on observations with the first snow-cap depends on the viewing location, we selected observed data from Hirosaki City for modern and contemporary periods, and calibrated the relationship with temperature. After analyzing the relationship with upper layer meteorological observations was obtained between the October mean temperature on 850 hPa of geopotential height, corresponding to almost the summit altitude of Mt. Iwaki, and the first date of snow capping on the top of it. Since our target was to reconstruct temperatures in the early modern period when solar activity was weak, we decided to determine the relationship between temperature and the date of first snow cap in years when solar activity was relatively weak. We found that the highest correlation was obtained when the analysis was limited to years with the Wolf sunspot number less than 190, or the sunspot group number less than 9. Smoothing procedures using the 11-year moving median resulted in a reconstruction error of less than 1°C in RMSE. The obtained reconstructions show that temperatures were about 1°C lower than in modern times, during the two grand solar minima in the 17th-19th centuries. There are several issues that remain to be addressed by this study, including how to determine the conditions of independent peaks suitable for temperature reconstructions and how to evaluate the effect of the solar cycle on the temperature-first snowfall date relationship.
