Impact of agriculture, forest and cloud feedback on the surface energy balance in BOREAS
We explore the impact of agriculture, forest and cloud feedback on the surface energy budget using data obtained using a research aircraft, mesonet towers and model data. The forest has an order of magnitude larger roughness length, a lower albedo, a much smaller seasonal cycle in surface Bowen ratio (BR) and a weak mid-summer maximum of CO2 uptake compared to agricultural areas, which have much smaller BR and much higher mid-summer CO2 uptake, but a net CO2 release and much reduced evaporation in spring and fall. Higher surface temperatures and the higher albedo over agricultural land reduce Rnet near local noon in the warm season by about 50 W m2 in comparison with the adjacent boreal forest. The annual averaged Rnet, derived from 2 years of tower data, is 14 W m2 less over grass sites than over forest sites. A reanalysis time-series for the BOREAS southern study area shows the coupling on daily timescales between the surface energy partition, the mean boundary layer depth, the cloud field and the long-wave and short-wave radiation fields. The albedo of the cloud field, the cloud short-wave forcing at the surface, varies over the range 0.1–0.8 with decreasing surface BR, and plays a major role in the surface energy budget. We estimate that this cloud feedback may increase albedo by 0.13 and reduce Rnet by 25 W m2 in summer over agricultural land.
Betts, A. K., R. Desjardins and D. Worth (2007): Impact of agriculture, forest and cloud feedback on the surface energy balance in BOREAS. Agric. Forest Meteorol., 142, 156-169, doi:10.1016/j.agrformet.2006.08.020 [Special book issue, “The Contribution of Agriculture to the State of Climate” Eds., R.L. Desjardins, M.V.K. Sivakumar, C. de Kimpe, ISSN 0168-1923, Elsevier, 2007]