APPLICATION OF FORECASTING TECHNIQUES AND CLIMATOLOGY 



129 



8.3.S Pirect Indications of Nonstandard 

 Conditions in the Western Pacific 



Geneeal Conclusions 



Evidences of superrefraction have frequently been 

 found at Guadalcanal and in the general vicinity of 

 New Guinea and New Zealand. Data obtained on the 

 radio-meteorology of the western Pacific in the region 

 from New Guinea to Sai^jan, from soundings made 

 there in 1944 and 1945/'^"^^ indicate the following con- 

 ditions in l)oth the equatorial and trade wuid Ix-lts of 

 the Pacific. 



1. Unmodified winds of long sea trajectory pro- 

 duce ducts which attain suflicient height and intensity 

 to trap microwaA'es in the 3,000- to 10,000-mc range. 



2. At the relatively low wind speeds characteristic 

 of the equatorial belt (4 to 10 knots), the duct height 

 increases with wind speed from about 20 to about 40 

 ft. X-, and possibly S-, band radars with properly 

 placed antennas may be expected to show marked in- 

 crease in range on surface craft and on aircraft flying 

 within the duct. 



3. At the higher wind speeds typical of the trade 

 wind belt (10 to 20 knots), ducts 50 to 60 ft wide 

 occur regularly. Properly sited S-, as well as X-, band 

 radars may be expected to show marked and persistent 

 increases of range on surface targets. The persistence 

 of the duct in these regions suggests its use in micro- 

 wave communications. 



4. Unless modified by passage over nearby land 

 masses, the atmosphere is approximately standard 

 from 60 to 1,000 ft. 



0. The results are so similar to earlier measure- 

 ments taken in the Caribbean on northeast trade wind 

 air of long sea trajectory as to warrant the conclusion 

 that this type of duct formation is general, at least in 

 tropical and subtropical regions, throughout the world. 



6. From the experimental results around Saipan 

 and the Marianas it is concluded that the coverage of 

 radars operated at frequencies much lower than 3,000 

 mc will probably not be affected l)y the oceanic ducts. 



7. The coverage of microwave radars (3,000 mc 

 and higher) which are sited above 100 ft may not be 

 affected by low-level conditions. 



8. If coverage on surface craft or ultra-low-flying 

 aircraft beyond the range of existing facilities is re- 

 quired and if prevailing wind speeds exceed 10 knots, 

 S-band radars sited 10 to 30 ft above sea level may 

 give better results than high-sited ones. 



9. X-band radars sited at 10 to 20 ft should be 

 useful down to wind speeds of the order of 6 knots. 



10. In microwave comnuinication links, the use of 

 low-sited antennas may increase I'ange beyond that 

 attainable by siting at the highest available altitudes. 

 (From the standpoint of water vapor attenuation and 

 duct utilization, X-band frequency appears to be the 

 optimum for this purpose.) 



11. From a series of ship-based kite soundings tak- 

 en northeast of Saipan in the two distinct weather 

 regimes, (1) a typical fair weather period, with steady 

 10- to 20-knot trades blowing and (3) a stormy period 

 with variable 4- to 15-knot southerly winds and fre- 

 quent rain squalls, it has been found that all sound- 

 ings yield simple surface trapping curves exclusively, 

 the average duct Midths being respectively (1) 44 ft 

 and (2) 37 ft. 



12. Although measurements were not taken under 

 the conditions mentioned in paragraph 11 (2) above, 



.soundings immediately after the squalls showed the 

 usual duct. (Sea surface temperatures were constant 

 at 84 to 85 F. Close to the surface the temperature 

 lapse rate was superadiabatic, but it was dry adia- 

 batic above 100 ft.) 



Obsekvations upon Ducts other than 

 Simple Sueface Trapping 



Although the emphasis in this report is placed upon 

 simple surface trapping, nonstandard propagation is 

 found imder other distributions of atmospheric mois- 

 ture and temperature, l^^levated inversions are found 

 up to around 5,000 ft, due to subsidence in anti- 

 cyclones over the Pacific'; the western and the 

 southern portions of this area usually exhibit the 

 higlier elevations. Illustrative of one of the other fac- 

 tors in nonstandard propagation is the Fohn wind 

 which often produces an elevated S-shaped duct and 

 frequently is superimposed upon a sea breeze. Figure 

 21, plotted from data taken in the vicinity of the 



700 



600 



" 500 



K 400 



I 



C 300 



X 



200 

 100 



74 78 62 66 



17 21 25 



MIXING RATIO 



G/KG 



29 370 375 360 365 390 395 

 M 



Figure 21. Data from Geelvink Bay, New Guinea, 

 showing surface duct as well as elevated S-shaped duct 

 due to Fohn effect from 10,000-ft mountains 100 to 150 

 miles to windward (SW). 



