20 M.J. BLACKWELL 



working models of atmospheric turbulence, will soon be determined on a 

 more quantitative basis. 



2. TURBULENT TRANSFERCONSIDERATIONS 



An attempt is made here to indicate the elements of atmospheric turbulence 

 theory applicable to the particular problem of evaporation, and the hmits 

 of practical appHcation of this theory. At the outset it should be made clear 

 that there are two distinct concepts of the mechanism of atmospheric 

 turbulent transport, one being that of continuous mixing by eddies and 

 the other being the classical theory of transfer in terms of the product of a 

 transfer coefficient and the gradient of the property concerned. Regarding 

 the first concept, evaporation is derived from the simple equation : 



E=pw'q' (i) 



where £ is the eddy flux of vapour and w' and q represent the fluctuations of 

 vertical velocity and specific humidity. Its simphcity suggests that other 

 methods are hardly necessary. Yet it is only in the last few years that 

 instrumental techniques and data-processing equipment have begun to 

 allow the potentialities of the method to be reahsed, e.g. by Swinbank 

 (1955) and Taylor and Dyer (1958). 



The second concept was followed by Pasquill (1949) in taking the 

 laboratory established wind-profile law for aerodynamically rough 

 surfaces : 



where k is von Karman's constant (0-4), tq is the surface drag or shearing 

 stress, and Zq is a parameter based on the characteristic roughness length of 

 the surface. This has been shown to apply in the atmosphere in conditions 

 of neutral stabihty (i.e. when there is no gradient of potential temperature). 

 It is convenient at this stage to introduce the stabihty parameter known as 

 the Richardson number : 





R<=lr''^^-^ <5) 



which takes values Ri<o for unstable, Ri = o for neutral and Ri>o for 

 stable conditions. It is found that u, logz profiles are convex to the H-axis 

 for Ri<o and concave for R/>o. 



