Planetary Boundary Layer Feedback and Its Role in Climate Change

Manabe and Wetherald (1975) have noted that the amplification of the surface temperature response in high latitudes on CO2 doubling is probably in part due to restrictions on the vertical turbulent mixing in more statically stable atmosphere. Recent advance in theoretical understanding of stably stratified planetary boundary layers, turbulence-resolving simulations and observational data allows quantification the noted planetary boundary layer (PBL) feedback. The new insight suggests the global significance of the PBL-feedback as shallow (e.g. nocturnal) PBL, where the temperature change is amplified, are frequently observed at every latitude. Recent publications have revealed that the state-of-the-art climate models are probably not sensitive enough to observed changes in the surface air temperature. PBL-feedback and GABLS results suggest an explanation of this fact as PBL is a way too deep in the models. Geographically the expected PBL-feedback also correlates with tropical areas of air subsidence where the PBL depth is restricted. A hypothesis, proposed in this study, relates the higher climate sensitivity to physics of the turbulent exchange in the PBL. Assuming certain simplifications, this hypothesis leads to analytical relationships for the climate sensitivity due to the PBL-feedback. The PBL-feedback has its own distinct signature in routine meteorological data. It predicts that the mean daily minimum temperature, and especially temperatures in long-lived stably stratified PBL, are more sensitive to the global warming than the mean daily maximum temperature. This fact explains, at least partially, empirically observed asymmetry in the diurnal temperature trends and almost global reduction of the diurnal temperature range.