Formation Mechanism of an Intensified Cold Air Mass in the Middle Troposphere over East Siberia in December 2005 Using a Regional Climate Model

摘要

The formation mechanism of an intensified cold air mass around East Siberia in December 2005, which brought a heavy amount of snow over northern Japan, is investigated using a regional climate model. The strong cold air mass is simulated by the CTL run with full diabatic processes. However, the experiment with the adiabatic process (ADIBIATIC run), which is conducted to investigate the effects of the atmospheric response and the advection of large-scale atmospheric circulation from the outside of the calculation domain, does not simulate the intensified cold air mass below 235 K at 500 hPa. Another sensitivity experiment with diabatic heating processes, except for surface heat fluxes (NO-SFH run), reproduces the intensified cold air mass due to the diabatic heating process in the middle to upper troposphere. The intensification and maintenance of the cold low in the typical events cannot be reproduced by the experiment without radiation process (NO-RAD-R run), while the NO-SFH run can simulate those important features of the cold low. These results indicate that the radiation process in the middle to upper troposphere is indispensable to intensify the cold air mass below 235 K at 500 hPa. From the satellite images, it is evident that the formation of clouds is accompanied by a cold low in the middle troposphere. It is speculated that the clouds are the source of the cooling due to the infrared cooling process, which may intensify the cold low. Therefore, the infrared cooling of the clouds is supposed to be the primary process in the intensification of the cold air mass in December 2005.