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The diurnal cycle over land.

Note this talk was published in 2003 as Betts, A.K. (2003), The diurnal cycle over land in “Forests at the Land-Atmosphere interface” [ISBN: 0-85199-677-9], Eds.,.M. Mencuccini, J. Grace, J. Moncrieff and K..McNaughton, pp. 73-93, CABI Publishing, Wallingford, Oxon OX10 8DE, UK.

This talk will review from primarily an observational perspective the nature of the diurnal cycle over land, with illustrations from high latitudes to the tropics. Understanding the coupling between different processes at the land surface is of fundamental importance, because in global models, many processes are parameterized, and are poorly constrained by routine observational inputs. Yet the diurnal cycle is observed synoptically, so it is an excellent indicator of whether the surface processes and their interaction with the boundary layer (BL) are modeled correctly. In addition, the diurnal range of surface temperature and humidity are important prognostic variables for society. First I shall review some basic concepts, using some illustrative examples (based on Betts, 2001), and then discuss additional controls on the diurnal cycle at high latitudes. Finally a tropical example (over Rôndonia in the southern Amazon basin) will be used to illustrate the difficulty in getting the diurnal cycle of precipitation right in a forecast model, because of the interaction of many processes. Near the earth’s surface, many variables have a characteristic diurnal or daily cycle, driven by the diurnal cycle of the incoming solar radiation, which is zero at night and peaks at local noon. The atmosphere is relatively transparent to the short-wave radiation from the sun and relatively opaque to the thermal radiation from the earth. As a result, the surface is warmed by a positive net radiation balance in the daytime, and cooled by a negative radiation balance at night. The surface temperature oscillates almost sinusoidally between a minimum at sunrise and a maximum in the afternoon. This is referred to as the diurnal cycle of temperature. In warm seasons, the daily net radiation balance is positive, and the daily mean temperature is determined by the daily mean surface energy balance, which involves not only the short and long-wave radiation components, but also heat transfers to the atmosphere.

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Betts, A. K., (2001): The diurnal cycle over land. ECMWF Semina, Sept. 3-7, 14pp http://www.ecmwf.int/publications/library/ecpublications/pdf/seminar/2001/sem01betts.pdf