Seasonal/Regional Variation of Variability Characteristic of Daily Maximum/Minimum Temperatures in Japan Observed and Reproduced by RegCM Nested in NCAR-CSM

Abstract

Observation data and output of a regional climate model, RegCM2.5, nested in a global climate model, NCAR-CSM, were analyzed to determine the standard deviation and frequency of extreme values, which were used as indices to characterize the variability of the daily minimum (T_min) and maximum (T_max) temperatures in Japan. The frequency of extreme values, which are defined in this paper as deviations greater than twofold the standard deviation, was estimated from data using a new method. The significant results of this analysis of observation data are as follows. With regards to both T_min andT_max, the significant seasonal variations of standard deviation are the annual and semiannual ones, with the minimum value in August and a weak minimum in January. The region where the standard deviation is large drifts northward from February to May, with a maximum standard deviation in Apri1. The central latitude almost agrees with that where the zonal wind is maximum at 500 hPa. As compared with the normal distribution, the extremely high temperature in summer, and the extremely low temperature in winter, are observed less frequently. The indices calculated from the model's output after simulating the current climatic condition were compared with the observation data for evaluation. From this comparison, it is shown that the variability characteristic of T_max was not reproduced well; however, that of T_min was reproduced well by the model. Therefore, it is concluded that T_min. reproduced by RegCM/CSM can be used to predict the change in the frequency of extreme values by correcting the systematic temperature bias, which is within ±1 K.