Studies relating to the influence of topographical features upon surface air flow and incident radiation

Abstract

The present studies aim to contribute towards a solution of one of the central problems of agricultural meteorology, viz. the specification and classification of climate and weather on a scale intermediate between that adopted in regional analyses - where areas measured in many hundreds of square miles are under consideration -, and the now vigorously explored realm of micrometeorology with its main emphasis upon the energy exchanges in, and the physical properties of, a layer of atmosphere some tens of metres deep lying over a plane surface with or without a uniform cover of low vegetation. Terms such as "local -", "meso -" and, latterly, "topo -" climatology (Thornthwaite, 1954) embrace the subject matter now under consideration, and in spite of the etymological claims of the prefix "meso ", "topo" is probably the most illuminating as it forces attention upon the role of topographical features, especially the geometrical form of the earth's surface, in influencing air -flow and the exchange of radiant energy.The writer's particular interest has been in connection with the design and siting of shelter -belts and shelter -screens for reducing certain environmental stresses upon agricultural crops and farm livestock. This is not the place to consider the evidence for the existence of such stresses nor the desirability of reducing them in order to increase production - for a discussion of these aspects reference might be made to other papers by the writer (Gloyne, 1955, 1957). What is relevant is that reliable advice is -2- now dependent upon more knowledge of the detail of "local" climate and weather in the absence of any imposed shelter, particularly in hilly areas; for, as emphasised in a recent authoritative treatise on the subject, "Shelter Belts and Micro- climate" (Caborn, 1957), "a comprehensive scheme of research is desirable (in which) meteorology can contribute valuable information on the structure of the climate near the ground and especially on the pattern of air -flow in regions of broken relief ".To realise fully the potential value of such investigations, it is obviously necessary to seek ways of placing the knowledge obtained in a general framework, and it is here suggested that any synthesis will involve a study of weather and climate in relation to surface geometry in the widest sense. This, if true, implies that methods are needed for defining the geometry of an area in such terms and on such scales as are meaningful in relation to certain climatic parameters, which in turn have been formulated in the light of the sensitivity of some operation or body to the atmospheric environment. In view of the obvious importance of the quantitative description of ground contour, reference will be made to certain techniques utilised by geographers (see Appendix I(a)).To prevent any misunderstanding,it must be emphasised that there is no suggestion that a general theoretical solution can be reached concerning the interaction of the atmosphere with the small and medium -sized topographical features of the earth's surface. All that is envisaged is that, given tolerance ranges for wind, temperature, humidity etc., it might be possible progressively to limit the uncertainty in the estimation or "forecasting" of these variables by increased attention to surface geometry. Further, should more precise information be required in any given case, a prior examination of the problem from the standpoint outlined above might well minimise the amount of on -site field work required, and maximise the value of information obtained from field surveys, both singly and collectively. At the very least, attention to the topic might be expected to point the way to ad -hoc methods of codifying the increasing accumulation of climatological data.