CCSR/NIES 5.4g WATER ISOTOPE GENERAL CIRCULATION MODEL : SENSITIVITY EXPERIMENTS FOR IMPROVEMENT

Abstract

The stable water isotopes (H₂¹⁸O, HDO) have been included into the CCSR/NIES Atmospheric General Circulation (AGCM) 5.4g. This model reproduced global scale distribution of δ¹⁸O in precipitation and the temperature effect roughly, but δ¹⁸O in precipitation in arid region and in cold region was overestimated and temperature effect was underestimated. Then in this study we conducted three sensitivity experiments focusing on isotope parameterizations and horizontal resolution. In the first experiment, to control the kinetic effect in re-evaporation of precipitation below cloud base, we used the effective relative humidity. As a result, δ¹⁸O in precipitation in arid region become lighter and similar to observation. In the second experiment, we focused on the treatment of ice crystals, but in this experiment, there is not strong sensitivity globally. In the third experiment, we used the high horizontal resolution with the effective relative humidity. In this experiment, in addition to arid region, δ¹⁸O in precipitation in cold region become lighter and similar to observation and temperature effect is also simulated more realistically.