Planetary boundary layer feedbacks in climate system and triggering global warming in the night, in winter and at high latitudes

TitlePlanetary boundary layer feedbacks in climate system and triggering global warming in the night, in winter and at high latitudes
Publication TypeJournal Article
Year of Publication2009
AuthorsZilitinkevich, S, Esau, I
JournalGeography, Environment, Sustainability
PublisherMoscow State University
KeywordsClimate change, Daily maxima and minima in SAT, Daily temperature range (DTR), Global warming, Sensitivity of SAT and DTR to the PBL height, Stable and convective planetary boundary layers (PBLs), Surface air temperature (SAT)

A remarkable feature of the ongoing global warming is the asymmetry in trends of the daily minima, θmin , and maxima, θmax , of the surface air temperature (SAT): θmin increases faster than θmax , so that the daily temperature range (DTR), θmax –θmin , basically decreases. The state of the art general circulation and climate models (GCMs) do not reproduce it and predict approximately the same change rates for θmin and θmax . We propose that the difference in trends of θmin and θmax is caused by the strong stability dependence of the height, h, of the planetary boundary layer (PBL). Indeed, the daytime warming is associated with deep convective (C) PBLs (with the heights hC ~ 103 m), in contrast to the nocturnal and/or wintertime cooling associated with shallower mid-latitudinal nocturnal stable (NS) PBLs (with hNS ~ 200 m) and even shallower high-latitudinal long-lived stable (LS) PBLs (with hLS ~ 30-50 m) developing during longer than night periods of the persistent surface cooling. As a result, one and the same increment, δQ0 , in the surface heat flux leads to only minor increment in θmax in deep C PBLs, but essential increments in θmin in shallow NS and especially NS PBLs. The latter type of the PBL has been discovered only recently and is not yet accounted for in modern GCMs. In the present paper, we derive theoretical estimates of the variations, δθmin and δθmax , in the SAT minima and maxima associated with the stable and convective PBLs, respectively, and by this means explain the observed asymmetry in the growth rates of θmin and θmax . To characterise the role of PBLs in the climate system, we introduce the concepts of local and general PBL feedbacks. Besides the strengths of feedbacks, we propose to take into account the reaction times of different mechanisms. The proposed concepts could be applied to different climate-change problems from global (as in this paper) to local, in particular, to those caused by the land-use modification.

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