The use of potential vorticity in tropical dynamics

Maps of the potential vorticity (PV) field on isentropic surfaces are widely used in studies of the midlatitude troposphere and stratosphere. For adiabatic, frictionless flow, parcels stay on their original isentropic surface and PV is materially conserved. In addition, the PV field carries all the essential information about the balanced dynamics via the invertibility principle. The situation in the tropical troposphere is more complicated. Because of diabatic heating due to the condensation of water, parcels do not stay on their original isentropic surface and PV is not materially conserved. It has not even been so clear how to generalize the PV principle for a moist, precipitating atmosphere. In spite of these difficulties, PV is very useful in understanding such problems at the ITCZ, the Hadley circulation, the formation of easterly waves, and the intensification of hurricanes. The paper will review these various problems of tropical meteorology and present a new generalization of Ertel's PV to a moist, precipitating atmosphere.