SEC:T. 2J 



LARt;K-SCALE INTERACTIONS 



209 



Concentrating for the moment upon the layer below cloud base, we see that 

 its most important feature is that it is almost completely mixed. Throughout 

 its lowest two-thirds, the specific humidity falls oif (from ship's deck level) 

 an average amount comparable to its small fluctuations, or only about 0.3- 

 0.5 g/kg out of a mean value of 15 g/kg. This well-mixed region has thus been 

 christened the "homogeneous layer". Its potential temperature also is almost, 

 but not quite, constant with elevation. Weakly unstable in the lowest strata, 

 the temperature lapse rate becomes less than adiabatio above about 200 m 



g/kg 



Fig. 53. Typical aircraft run at 550 ft elevation over the sea in the western Atlantic trade- 

 wind region. Aircraft flying up-wind. (By courtesy of A. F. Bunker.) 



Wind from 070°, 8 m/sec. February 25, 1958 ; ITN, 60°W. Sky coverage, five-tenths 

 cumulus. Vertical velocity (cm/sec), departure of horizontal velocity from mean at 

 the level (cm/sec), departure of temperature from mean at the level (°C), and departure 

 of specific humidity from mean at the level (g/kg) shown as function of time. Aircraft 

 flying at about 60 m/sec so 10-sec interval represents a horizontal distance of approxi- 

 mately 600 m. Instrumentation was such that velocities and temperatures could be 

 read at one-fifth second intervals. Due to slow response of humidity element, moisture 

 values are one-second averages. 



elevation. Careful statistical studies of numerous profiles like that of Fig. 52 

 (Bunker et al., 1949) show that under normal and strong trade conditions the 

 upper two-thirds of the sub-cloud layer are slightly statically stable in all but 

 the upstream and poleward fringes of the trades. 



The effect of this stabilization upon the important scales of motion is brought 

 out in Fig. 53, a typical aircraft run at the 550-ft level. ^ While the moisture 

 fluctuations and their correlations with up-drafts are nearly unchanged from 

 the low levels, the temperature fluctuations are now mainly out-of -phase with 

 the rising motions on the 50-300 m scale as well as in the longer "eddies". This 



1 A much faster-responding humidity element is now in use on the aircraft but no 

 reduced records are yet available at the time of going to press. 

 8— s. I 



