The potential vorticity budget of an atmospheric general circulation model

The Bur form of the potential vorticity (PV) equation in isentropic coordinates is used to examine the effects of advection and the effects of parameterized mechanical and thermal forcing on the PV budget of the second generation Canadian Climate Centre general circulation model (CCC GCM). Model data corresponding to Northern Hemisphere winter are used. The simulated PV flux contains significant nonadvective contributions in the planetary boundary layer, in the gravity wave drag regions of the Northern Hemisphere, and in the tropical midtroposphere in regions of intense latent heat release associated with persistent moist convection. The model advective PV flux is compared to the advective PV flux calculated from a National Centers for Environmental Prediction observational dataset. Large discrepancies are seen where parameterized gravity wave drag dominates the mechanical forcing field in the model. The zonally averaged model PV flux in the upper troposphere and lower stratosphere is characterized by a balance between the meridional transport of isentropic absolute vorticity and dissipation from parameterized gravity wave drag. A simulation not including gravity wave drag shows stronger poleward transport of relative vorticity and stronger equatorward transport of planetary vorticity in the Northern Hemisphere, compared to a run including this parameterization. The PV budget along two isentropic surfaces, one in the ''overworld'' and the other in the ''middleworld'' as defined by Hoskins, is examined. On the overworld (lower stratospheric) isentrope, the effect of parameterized gravity wave drag in the Northern Hemisphere is a predominantly southward transport of PV. This is balanced by northward advection of PV by the lower-stratospheric meridional circulation. Assuming a similar balance in the atmosphere, an estimate of the observed mean mechanical forcing field is obtained by calculating the advective PV flux on the 390 K surface from assimilated data. On the middleworld isentrope, the PV budget exhibits an approximate three-way balance between the advective, mechanical, and thermal parts of the PV flux in midlatitudes. The implications for stratosphere-troposphere exchange are discussed. The sign of the meridional component of thermal PV flux is used to deduce that on average, radiative cooling (diabatic heating) is located in regions of positive (negative) vertical wind shear in the model.