The impact of plant stomatal control on mesoscale atmospheric circulations

Avissar, R. and Pielke, R.A., 1991. The impact of plant stomatal control on mesoscale atmospheric circulations. Agric. For. Meteorol., 54: 353-372. Only a few numerical studies with mesoscale atmospheric models have been undertaken to explore the influence of vegetation on the generation and modification ofmesoscale atmospheric circulations. Nevertheless, these few studies have demonstrated the importance of a correct representation of vegetation in the simulation of atmospheric features. In the present study, a mesoscale atmospheric model with a sophisticated land surface parameterization is used to emphasize the major role of plant stomata on the control of the Bowen ratio and on mesoscale atmospheric circulations. This study demonstrates that refinement of the land surface parameterization and stomatal mechanism, including subgrid-scale processes, is required in order to simulate confidently land-atmosphere interactions. This is particularly important when plants experience relatively strong stress and a small change in the stomatal behavior generates large variations in sensible and latent heat fluxes at the Earth's surface. I N T R O D U C T I O N In atmospheric models, the Earth's surface is the only boundary that has a physical significance. Moreover, it is the differential gradient of the dependent variables along this surface that generates many mesoscale circulations (i.e. terrain induced mesoscale systems) such as sea and land breezes, mountain-valley winds, urban circulations and forced airflow over rough terrain. This gradient also has a pronounced influence on the remaining mesoscale flows (i.e. synoptically induced mesoscale systems) such as squall lines, hurricanes and traveling mesoscale cloud clusters. Pielke (1984) reviewed these circulations and flows in detail. Changes in this lower boundary over time (i.e. from anthropogenic activity or overgrazing by animals) can cause substantial climatic changes such as desertification (Otterman, 1975; Idso, 1981 ). Thus, because of the crucial importance of this boundary for the atmospheric systems, it is essential to represent it as accurately as possible in models. 0168-1923/91/$03.50 © 1991 Elsevier Science Publishers B.V. 354 R. AVISSAR AND R.A. PIELKE The representation of land surfaces as a bottom boundary requires different types of parameterizations than those required to represent the surface of water bodies (e.g. lakes, oceans, etc. ). This is because water is translucent to solar radiation and overturns easily, in contrast to the ground which is opaque and does not readily overturn. Also, owing to the complexity of the physical and biological processes involved in the representation of vegetation, it is much easier to represent bare soils than plant canopies in the parameterization of land surfaces. Mesoscale models have become more sophisticated in the parameterization of bare soils. These models evolved from the prescription of potential temperature as a periodic heating function (e.g. Kuo, 1968; Neumann and Mahrer, 1971; Pielke, 1974; Mahrer and Pielke, 1976) to the solution of a surface energy budget (e.g. Physick, 1976; Mahrer and Pielke, 1977a, b; Estoque and Gross, 1981; McCumber and Pielke, 1981; Ookouchi et al., 1984; Mahfouf et al., 1987; Avissar and Mahrer, 1988a; Segal et al., 1988; Avissar and Pielke, 1989; among many others). However, since much of the world is vegetated, it is necessary to represent correctly the contribution of plants as well as the soil surface to the surface processes. Vegetation has been introduced in only a few mesoscale models and relatively little evaluation of the influence of vegetation on mesoscale circulations has been published in the research literature. An interactive atmosphere-vegetation-soil model was applied by McCumber (1980) who evaluated, among other aspects, the effect of vegetation on the development of the summer sea breeze over South Florida. This study also indicated that significant induced mesoscale circulations can be related to sharp horizontal changes in the character and type of the vegetation cover. Garrett (1982) included a vegetation module in his model while studying the interactions between convective clouds, the convective boundary layer and forested surfaces. Yamada ( 1982 ) incorporated vegetation in a planetary boundary-layer model in order to study air circulations in the lower atmosphere. However, these studies did not attempt to explore quantitatively and systematically the impact of vegetation on the generation and modification of mesoscale systems. One of the first attempts to evaluate expected circulations between a vegetated area contrasted with bare soil was reported by Anthes (1984). That study provided an evaluation of the anticipated thermal contrast in subtropical latitudes. In addition, it provided scaling by means of a linear analytical model, illustrating the possible characteristics of mesoscale circulations generated by such contrasts. Avissar and Mahrer ( 1988a, b) demonstrated the importance of different types of vegetations on the development and modification of small local circulations during radiative frost events. Mahfouf et al. ( 1987 ) studied the influence of soil and vegetation on the development of mesoscale circulations. They found that the juxtaposition of a well-transpiring vegetated area with a IMPACT OF STOMATAL CONTROL ON MESOSCALE ATMOSPHERIC CIRCULATIONS 3 5 5 dry bare land can generate circulations as strong as sea breezes. Probably the most complete study on the interactions between vegetation and mesoscale circulations was recently presented by Segal et al. (1988 ) who analyzed the influence of various plant covers and densities on different mesoscale features such as thermally induced upslope flows and sea breezes, as well as the generation of circulations induced by the juxtaposition of vegetated areas with bare dry lands. Parts of their results were supported by observations and infrared surface temperatures obtained from the Geostationary Operational Environmental Satellite (GOES). It is widely accepted that these vegetation effects on mesoscale systems are the result of the fact that when the plants transpire effectively, the radiative energy received at the Earth's surface from the sun and the atmosphere is mostly redistributed into latent heat flux in contrast with the redistribution into sensible heat flux over a dry surface. Thus, the heating of the atmosphere during daytime hours above a dry surface is usually more intense than that above vegetated areas. This generates horizontal gradients of temperature (and, therefore, pressure) in the atmosphere which initiate circulations between the two areas. However, when the plants are under strong stress and do not transpire, the sensible heat flux produced over the vegetation is almost identical to that obtained over a bare surface and the vegetation does not contribute significantly to the mesoscale systems. Transpiration is biophysically controlled by the plant stomata. It is, therefore, obvious that the contribution and realistic parameterization of the stomatal mechanism in mesoscale atmospheric models is essential. This paper reviews the different parameterizations of plant stomata that have been adopted in mesoscale atmospheric models and demonstrates their contribution to regional atmospheric circulations. PARAMETERIZATION OF PLANT STOMATA IN ATMOSPHERIC MODELS In the parameterization of vegetation in mesoscale atmospheric models, the plant stomatal control is obtained by introducing a resistance (or conductance) in the equation which describes the latent heat flux from the vegetative surface. For instance, in Avissar and Mahrer ( 1988a; also described in Segal et al. (1988) and Avissar and Pielke (1989) ) this equation is ,~Ev = a ' f p2u .q . ( 1 )