Idealized model for changes in equilibrium temperature, mixed layer depth and boundary layer cloud over land in a doubled CO2 climate

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Thursday, 21 January 2010: 9:15 AM
B216 (GWCC)
Alan K. Betts, Atmospheric Research, Pittsford, VT

An idealized equilibrium model for the undisturbed partly-cloudy boundary layer (BL) is used to explore the coupling of the energy, water and carbon cycles over land and show the change in the BL equilibrium and cloud cover for a warmer, doubled CO2 climate. Reduced stomatal conductance amplifies the background 2K ocean temperature rise to a 5K increase in near-surface temperature over a forest, with a corresponding drop of RH at the base of the ML of about 15%, a 60 hPa rise of cloud-base, and a 10% fall of BL cloud albedo. Transpiration decreases by 16 Wm-2 for forest and by 9 Wm-2 for grassland, while the sensible heat flux increases by 25 Wm-2 for forest and by 20 Wm-2 for grassland. These changes are similar in structure to those seen globally over land in climate models, but they are larger by a factor of four or five. The fundamental asymmetry of increased evaporation over the oceans and reduced transpiration over land increases. Equilibrium evaporative fraction (EF) and ML depth are almost uniquely related, independent of background forcing climate and canopy conductance, which suggests that it may be possible to infer EF for heterogeneous landscapes from ML depth. Similarly, given lower tropospheric subsidence, ML CO2 can be used to estimate net ecosystem exchange. The threshold between net carbon uptake and carbon loss occurs at a surface temperature of 33.5oC in both low and high CO2 climates, for both forest and grassland with our model vegetation parameters.