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Land-surface-atmosphere coupling in observations and models.

This review is a synthesis of my understanding of the physical processes involved in the land-surface-atmosphere interaction, illustrated using figures adapted from my published work, covering the past 15 years (so they reflect both my idiosyncrasies and as well as many aspects of the ECMWF model). I believe synthesis is always valuable, because of the complexity of the earth system. In section 2, I will discuss the surface energy balance, and the role of the surface and cloud albedos in the shortwave and longwave fluxes, and comment on the radiative scaling of the diurnal temperature range and the night-time boundary layer. Section 3 will deal with the role of water in the surface energy partition, vector methods for understanding mixed layer evolution, the coupling between surface and boundary layer and the impact of clouds on the surface energy partition. Finally, section 4 will discuss evaporation-precipitation feedback, the coupling of column water vapor, clouds and precipitation to vertical motion and moisture convergence over land in the tropics and compare the relation of surface shortwave cloud forcing and precipitation forcing of the atmosphere in ERA-40 with observations. My frame is that of land-surface climate; not the details of synoptic weather, but the systematic features of the diurnal and seasonal cycle, and also the way processes are coupled on the daily timescale. How these are represented in the model, what is observable in nature and what are the underlying fundamental interrelated physical processes. Where possible, I will contrast relationships that are observable with how well they are simulated in the ECMWF model. Our observational datasets are however always incomplete; and key parameters, like soil moisture and the surface fluxes, are not measured on the scale of the model grid.

Plain English Discussion

This semina paper was the basis for the review published a year later as

Betts, A. K. (2009), Land-surface-atmosphere coupling in observations and models. J. Adv. Model Earth Syst., Vol. 1, Art. #4, 18 pp., doi: 10.3894/JAMES.2009.1.4 [Published Online 1 Jul. '09] http://james.agu.org/index.php/JAMES/article/view/v1n4/JAMES.2009.1.4

This is an illustrated review explaining what determines the climate during the day and at night over forest and grasslands. Sunlight warms the surface but some is reflected, especially is there is snow cover on grass. Clouds and water vapor in the atmosphere trap the earth's heat and reduce the cooling of the surface at night. Rainfall refills the reservoirs of water in the soil, which increases evaporation, giving a cooler surface, more clouds and the likelihood of more rain.

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Betts, A. K. (2008), Land-surface-atmosphere coupling in observations and models. ECMWF Semina on 'The parametrization of sub-grid processes and their interaction with the dynamics', Sept. 1-4, 2008. http://www.ecmwf.int/publications/library/ecpublications/_pdf/seminar/2008/Betts.pdf