182 SYNOPTIC RADIO METEOROLOGY 



in the troposphere and suggest that moisture and temperature stratifica- 

 tion is modified principally by changes in vertical velocity. This latter 

 work was extended [32] by a study of mesoscale centers of horizontal air 

 mass convergence and divergence in the troposphere. Horizontal con- 

 vergence takes place in a low-pressure area, where the winds around the 

 low have a predominant component toward a local vortex at the center. 

 Upward vertical motion results from the pile-up of air in the vortex region. 

 In a high-pressure area, winds have a predominant component away from 

 the center of a high and subsiding air descending from higher levels takes 

 the place of air transported outward from the center of the high. Local 

 convergence, then, implies upward vertical motion in the lower levels of 

 the troposphere, while local divergence implies downward vertical motion 

 in the lower levels. It follows that local convergence centers (small- 

 scale low-pressure cells) produce updrafts in the atmosphere that result in 

 considerable mixing and the destruction of atmospheric layers. Centers 

 of divergence (small-scale high-pressure cells) create strong temperature 

 and humidity inversions by the motion of subsiding air. Such inversions 

 produce large vertical refractive index gradients that partially reflect 

 microwaves traveling through this meteorological environment [33]. In 

 a well-mixed atmosphere, on the other hand, the primary propagation 

 mechanism is believed to be scattering by turbulent fluctuations of the 

 refractive index [34, 35]. 



Sea-level measurements showed N s to be essentially invariant during 

 the experiment, yet signal levels ranged over more than 60 dB, a power 

 factor on the order of 10^ Since scattering theory would account for a 

 rise in signal level of only about 13 dB, Moler and Holden [32] conclude 

 that refractive layering and thermal stability over the oceans are princi- 

 pally functions of vertical wind velocities. These conclusions bear out 

 earlier ones of Saxton [36] and later ones of Flavell and Lane [30] stating 

 that high signal levels from a distant transmitter may well be the con- 

 sequence of refractive layering and subsequent reflection of radio waves. 

 In the same article Saxton also considered both the scattering of radio 

 energy by turbulent eddies and the effects of superrefraction of radio 

 waves produced by departures from normal of the height variation of the 

 tropospheric refractive index. 



A knowledge of the vertical motion of the atmosphere becomes at this 

 point central to the problem of refractive layering. A brief resume of the 

 Moler-Holden method for estimating the relative magnitude and direction 

 of the vertical component of the wind velocity follows. 



Moler and Holden postulate a model atmosphere bound by the follow- 

 ing conditions: 



