Extratropical stratosphere-troposphere exchange in an AGCM with the horizontal grid size of 20 km

摘要

The transport and exchange process across the extratropical tropopause is investigated using an atmospheric global climate model with a horizontal grid size of about 20 km, which can depict filamental structures near the tropopause. The advection of an idealized passive tracer, which is initialized to have a nonzero value only above the tropopause, is calculated in January and July in the Northern Hemisphere. A version of the model with coarse resolution is used to examine the horizontal resolution dependence. The exchange between the stratosphere and the troposphere has its maximum amounts at 300 hPa in January and at 200–250 hPa in July. Above 400 hPa, the amount of exchange in the 20-km model is about one-half of that of the 180-km model, in accordance with the existence of a sharp tropopause. In contrast, the amount in the 20-km model below 400 hPa is more than that in the 180-km model, partly due to the inability of simulating filamental structures along with the storms in the 180-km model. However, the net transport across the tropopause does not have large dependence on the horizontal resolution. The net transport below 400 hPa is from the stratosphere to the troposphere and is large over the Pacific and Atlantic storm track in January. The net troposphere to stratosphere transport is seen above 300 hPa and is large near the subtropical jet over the Eurasian continent. A large part of this transport is attributed to the temporal change of the potential vorticity estimated from a vertical difference in the longwave radiation.