180 SYNOPTIC RADIO METEOROLOGY 



index and bending from normal for each air mass and emphasizes the 

 close relation between the two. This affords the synoptic radio meteor- 

 ologist a set of standard reference profiles for the study of a given air mass 

 or the confluence of contrasting air masses at a frontal zone. 



Arvola [24] discussed the changes in refractive index profiles caused by 

 migratory weather systems. Examining a series of synoptic situations in 

 the midwestern portion of the United States during November 1951 that 

 gave rise to greater-than-normal refraction, Arvola found that ridges and 

 accompanying subsidence effects generally gave rise to strong A^ gradients 

 and enhanced signal strength over a 200-km link broadcasting at 71.75 

 Mc/s. Refractive gradients were stronger in the warmer air masses and 

 at times when moist air was present below the inversion created by the 

 subsidence mechanism. Strong gradients which appeared behind a squall 

 line later weakened with the approach of a cold front. After the passage 

 of this front stratification in the cold air again increased the gradient. 



Subsequent investigations of polar continental air across central North 

 America make use of reduced-to-sea-level forms of the radio refractive 

 index as synpotic parameters. The reduced forms are sensitive indica- 

 tors of synoptic changes and afford a clearer picture of storm structure 

 than that obtained using analyses in terms of unreduced N or B units 

 (defined in chapter 1). Later portions of this chapter consider in detail 

 these two synoptic parameters. 



Jehn [25], at the University of Texas, used a form of potential refractive 

 modulus, K, developed by Lukes [26] and Katz [27] to account for the 

 height dependence feature of the refractive index. Articles by Jehn 

 [28, 29] on synoptic climatology use composite analysis techniques to 

 study the synoptic properties of the Texas-Gulf cyclone and the central 

 United States type of cold outbreak. 



In another application of the potential refractive modulus, Flavell and 

 Lane [30] have utilized Katz's K unit (see chapter 6) to study tropospheric 

 wave propagation. Field strength measurements on VHF-UHF trans- 

 horizon radio links over the British Isles are analyzed in terms of cross 

 sections in terms of K and regional charts of AK, the difference between 

 K at the surface and /^gsombar- These charts show features similar to 

 those obtained by use of No or A (see chapter 1). 



The authors cite a series of measurements on a 500-km path at 877 

 Mc/s on which the received signal was ordinarily below the noise level. 

 The singular occasions on which the received signal could be measured at 

 the normal times of radiosonde ascent all exhibited a symmetric variation 

 of AK over the transmission path. These results lend credence to the 

 hypothesis that synoptic disturbances play an important role in trans- 

 horizon telecommunicat ions. 



Moler and Arvola [31] advanced the hypothesis that the vertical 

 gradient of the refractive index is affected by broad-scale vertical motion 



