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The role of the land surface in
the climate system
April 2002
European Centre for Medium-Range Weather
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4 . Impact of land surface on weather: A brief literature survey
Betts et al. (1996) review the impact of land surface in the context of global numerical weather prediction: Diurnal and seasonal feedback loops are discussed as well as feedback loops controlling the BL evolution. We will highlight here typical mechanisms controlling the interaction between land surface and the atmosphere, over the US, Europe, and the tropics.
Early modelling efforts (Benjamin and Carlson 1986; Lanicci et al. 1987) have shown the sensitivity of precipitation in the US Great Plains to evaporation upstream, in the Mexican Plateau. The characteristic storm environment, leading to heavy precipitation over the Midwest, involves the breakdown of a capping inversion formed by an overlying pre-existing boundary layer from the Mexican plateau, which overlies the cool moist BL originating in the Gulf of Mexico. This complex pattern of differential advection impacts on the strength of the capping inversion, and the strength of the inversion is controlled by evaporation upstream of the precipitation area. Lower values of evaporation lead to a stronger capping inversion, and the low level flow from the Gulf will not break through the inversion until much further north. The location and extent of the heavy precipitation associated with the July 1993 US floods was found to be highly sensitive to the correct representation of these mechanisms in the ECMWF model (Beljaars et al. 1996; Viterbo and Betts 1999a; Section 5.1).
Spatial gradients of soil moisture can also enhance the differential heating maintaining and reinforcing the surface front in a pre-storm environment and intensifying the thermally direct (ageostrophic) circulation (Chang and Wetzel 1991; Fast and McCorcle 1991). A similar mechanism is active in a cold front associated with a severe squall line developing explosively (Koch et al. 1997).
The seasonality of leaf area index impacts on the systematic errors of US Midwest lower tropospheric temperature in summer (Xue et al. 1996; Yang et al. 1994). At a smaller scale, there are many observational and modelling studies demonstrating the importance of mesoscale fluxes and smaller-scale heterogeneity, specially at low wind speeds.
Over Europe, Rowntree and Bolton (1983) demonstrated the role of local and non-local response of medium-range rainfall forecasts to anomalies in the initial soil. The mechanisms relevant to this soil moisture-precipitation feedback were scrutinised in Schär et al. (1999) in a study demonstrating the impact of idealised (and large) anomalies of soil water on the European summer circulation. Unlike the desert-albedo feedback hypothesis (Charney 1975), the European soil-precipitation feedback is not of a large-scale dynamical nature, i.e., it is not associated to changes in large-scale flow. Precipitation recycling has also a small role in Europe. Three main feedback loops have been identified. Wet soils, associated with low Bowen ratios, lead to the build-up of a shallow BL, concentrating moist entropy at low levels and giving higher values of convective available potential energy (CAPE). Additionally, lower Bowen ratios lead to higher relative humidity, lowering the level of free convection. Finally, a positive feedback of radiative origin, with increased cloud cover, but larger net radiative flux, leads to larger moist entropy and convective instability.
All the examples above refer to extratropics spring and summer examples, in snow free situations. Section 5.2 below presents spring examples in the presence of snow. There is little or no impact of land surface on the atmospheric circulation in winter (see Giorgi (1990) and Section 5.3 below).
Despite an extensive list of publications on the role of land-surface in the tropical climate, there is scanty evidence of its impact for short- and medium-range forecasts. A notable exception is the work of Walker and Rowntree (1977) who demonstrated the role of enhanced soil moisture gradients on the short-range (1-2 days ahead) forecast of the generation of easterly waves, using a simplified model over West Africa.
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