The role of planetary waves in the formation of polar stratospheric clouds

Several recent works attribute the formation of polar stratospheric clouds (PSCs) to the occurrence of localized orographic waves. Using ECMWF analyses, we investigate the large scale stratospheric flow conditions in a number of cases where PSCs have been detected both in the Arctic and in the Antarctic. We show that PSCs appear within strong planetary scale uplifts of isentropic surfaces. The adiabatic cooling of air parcels travelling within such planetary scale uplifts while conserving their humidity and trace constituents, seems to be the main mechanism for PSC formation. The PSC distribution would then follow a planetary structure, even though local orographic waves could still play an additional role when planetary scale conditions are met.