A Parameterization Scheme of Orographic Gravity Wave Drag with Two Different Vertical Partitionings

Part II: Zonally Averaged Budget Analyses Based on Transformed Eulerian Mean Method

Abstract

The effects of orographic gravity wave drag (GWD) on zonal-mean fields of medium-range forecasts are analyzed by means of the transformed Eulerian-mean (TEM) method. Results show that the geostrophic adjustment to GWD behaves very differently between the stratosphere and troposphere.In the troposphere, both the tropospheric and stratospheric GWDs significantly change Eliassen-Palm (EP) flux divergence due to large-scale (model-resolvable) waves. The change in the EP flux divergence is much larger than the net change of tonal wind and GWDs themselves and it is almost balanced with the change in the Coriolis acceleration term due to meridional flows. Among wave activities, transient gravity waves resolved in the model are considered to play important roles in the vertical redistribution of additional wave moments due to GWD.
In the stratosphere, the stratospheric GWD induces a hemispheric single cell meridional circulation. The vertical motions in this cell cause significant temperature changes. The Coriolis acceleration due to GWD-induced meridional flows is almost balanced with the GWD itself. In contrast with the troposphere, EP flux divergence is less affected by GWD.
From the view of Lagrangian-mean meridional circulation, diabatic heating in the tropics and wave, mean-flow interaction due to planetary-scale waves have been recognized mainly to drive a hemispheric single cell circulation (the so-called Brewer-Dobson circulation) in the lower-stratosphere. Our results indicate that the stratospheric GWD contributes to the maintenance of the single cell circulation as well. Especially in the midlatitudes of the northern hemisphere, the GWD can be regarded as an important forcing and considerably enhances lower-stratospheric downward motions in the polar side of the subtropical jet stream. It might affect significantly the transport of trace constituents in the stratosphere.