66 DEACON AND WEBB [CHAP. 3 



G. Effect of Rainfall 



Five of Van Dorn's measurements shown in Fig. 10 were taken during a 

 period of moderately heavy rainfall (~0.5 cm/h). They show that rainfall can 

 appreciably augment the transfer of momentum between atmosphere and sea, 

 and analysis of the problem shows that, for large drops, the increment of stress 

 so produced will approximate to the product of the rate of rainfall (g cm~2 sec~i) 

 and the wind speed at some such height as 5 or 10 m. 



4. Transfer of Heat and Water Vapour 



A. Introductory 



One approach to evaluation of the coefficient da in the evaporation bulk 

 formula (8) has been from annual heat budget considerations over the oceans 

 [see the accounts by Jacobs (1951, 1951a) and Sverdrup (1951)]. This line of 

 attack, valuable as it has proved to be in giving the broader features of the 

 interchanges between ocean and atmosphere, is not capable of much refinement 

 since it is difficult to evaluate all the forms of heat transport with sufficient 

 accuracy. However, for lakes the heat budget over periods of sufficient length — 

 a fortnight and upward — is more reliable [see applications by Anderson (1954) 

 and Webb (I960)]. Other approaches are by eddy fluctuation measurements and 

 by profile observations. Since these two methods are applicable over periods 

 as short as 15 min or so, it is reasonable to expect that they will eventually lead 

 to fully adequate knowledge of the dependence of the bulk coefficients on wind 

 speed and thermal stratification. 



There is no doubt that, in the near future, the approach by profile measure- 

 ments will prove fruitful, and it is with this method that most of the following 

 discussion is concerned. The measurements demand accurate instrumentation, 

 since, over the open sea, the vertical gradients are small. The small wind 

 gradient is a consequence of the smaller roughness parameter than over land ; 

 while, in the case of temperature and humidity, the small gradients are charac- 

 teristic of an air-mass which has become well modified over a sea surface of 

 almost constant temperature — the diurnal variation of sea-surface tempera- 

 ture is only slight owing to the large thermal capacity of the upper layers of 

 the sea. 



It is to be remembered that, over the ocean, small gradients of potential 

 temperature do not imply negligible thermal stratification since the wind 

 gradient, which appears in the denominator of the Richardson number, is also 

 small. To gain some idea as to the importance of stratification, it may be noted 

 that the air minus sea temperature difference observed by German weather 

 ships in the Atlantic is — 3°C nearly half as frequently as it is zero (Brocks, 

 1956). Even with a wind as strong as 6 m/sec such a value corresponds to a 

 Richardson number at 6 m height of about —0.17 (estimated from (34), p. 68) 

 and reference to Fig. 2 shows that under these conditions the thermal stratifica- 

 tion has a marked effect. 



