Atmospheric Lapse Rate Regimes and Their Parameterization

Lapse rates, moist adiabatic lapse rates and the critical lapse rate for baroclinic adjustment are calculated and compared for the mean annual, January and July states in the Northern Hemisphere. In the troposphere above the planetary boundary layer zonal mean lapse rates are within 20% of the moist adiabatic lapse rate from the equator up to about 30°N in January and 50°N in July, but are appreciably more stable in higher latitudes. The latitudinal distribution of tropospheric mean lapse rates clearly delineates two regimes in the atmosphere—a low-latitude regime where the lapse rates are essentially moist adiabatic, and a high-latitude regime where the lapse rates are essentially the critical lapse rate for baroclinic adjustment. The dividing point between the two regimes shifts from 28°N in January to 47°N in July, and the transition is less sharp in July than in January. The absence of appreciable seasonal changes in lapse rates in midlatitudes can be attributed to counterbalancing seasonal changes in the strength of moist convection and baroclinic eddies. Hemispheric mean lapse rates in the mid and lower troposphere are within 0.4 K km−1 of the moist adiabatic lapse rate in July, but are as much as 1.9 K km−1 less in January. Implications for simple climate models are discussed. A principal conclusion is that the vertical temperature structure could be well approximated by a radiative-convective equilibrium model with two critical lapse rates—the moist adiabatic lapse rate and the critical lapse rate for baroclinic adjustment.