THE POTENTIAL VORTICITY BUDGET OF AN ATMOSPHERIC GENERAL-CIRCULATION MODEL

The Bur form of the potential vorticity (PV) equation in isentropic co ordinates is used to examine the effects of advection and the effects of parameterized mechanical and thermal forcing on the PV budget of th e 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 contribu tions in the planetary boundary layer, in the gravity wave drag region s of the Northern Hemisphere, and in the tropical midtroposphere in re gions of intense latent heat release associated with persistent moist convection. The model advective PV flux is compared to the advective P V flux calculated from a National Centers for Environmental Prediction observational dataset. Large discrepancies are seen where parameteriz ed gravity wave drag dominates the mechanical forcing field in the mod el. The zonally averaged model PV flux in the upper troposphere and lo wer stratosphere is characterized by a balance between the meridional transport of isentropic absolute vorticity and dissipation from parame terized gravity wave drag. A simulation not including gravity wave dra g shows stronger poleward transport of relative vorticity and stronger equatorward transport of planetary vorticity in the Northern Hemisphe re, compared to a run including this parameterization. The PV budget a long two isentropic surfaces, one in the ''overworld'' and the other i n the ''middleworld'' as defined by Hoskins, is examined. On the overw orld (lower stratospheric) isentrope, the effect of parameterized grav ity 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 forcin g field is obtained by calculating the advective PV flux on the 390 K surface from assimilated data. On the middleworld isentrope, the PV bu dget exhibits an approximate three-way balance between the advective, mechanical, and thermal parts of the PV flux in midlatitudes. The impl ications 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 region s of positive (negative) vertical wind shear in the model.