Long-term Variation of the Baiu Rainy Season Duration: Comparison of Daily Weather Records in Old Diaries and Observed Meteorological Data

Abstract

 This study analyzed the relation between weather descriptions in diaries and meteorological observed data to assess historical daily weather records as a tool to reconstruct hydrological and climatological environments in historical times. We collected and compiled weather records from eight diaries written in Kyoto and its surrounding areas in the late 19th to early 20th centuries. We compared their accounts with precipitation and cloud amount records at the nearest meteorological station on a daily basis. Although the observed values corresponding to each weather category are distributed over a broad range, the diversity of their average for each diary was found to be small. Results show that referring to plural diaries and using climatic indices derived by accumulating weather descriptions for months or seasons (such as number of rainy days) can enhance the reliability and objectivity of climatic reconstruction using daily weather records. Moreover, by applying the results of analyses of the relation between weather record and observed data, we reconstructed a temporally consistent time series of the duration of the Baiu rainy season for about 240 years from the late 18th century by application of the same estimation method of the date of the beginning and the end of the Baiu season to both the historical and the observation period. Analysis of this time series revealed that the positive trend of the length of the Baiu season during the 20th century is the most significant in the whole analyzed period. This result is consistent with earlier studies.