FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0153.PDF
4 February 1955 153 Mountain Waves: Theory and Practice Scientists and Pilots Discuss "Airflow Over Mountains" ;^ . The difficulties ofgliding: Camphill's ridge-top wind-reversal. | j1 XCEPT at gliding contests, opportunities for organized joint discussion between pilots and meteorologists on lee-wave phenomena have been rare in the past. The full attendance at the Royal Meteorological Society on January 19th showed how wel- come was such an opportunity, for the Society was holding a "free-for-all" dis- cussion on Airflow over Mountains. The first basis of the discussion was asummary given by MR. G. A. CORBY of the Meteorological Office, Dunstable. Earlyvisual evidence that mountains could influence airflow (apart from an increase inturbulence) he said, had been seen in cap clouds over and downwind of mountains.Mother-of-pearl clouds over Norwegian ranges had been observed at heights of7O-8O,OOOft. Glider pilots in the 1930s had been thefirst to report rising air in the lee side of mountains, undoubtedly caused by someform of atmospheric lee-wave. From their accounts had come the conception of thenature of such waves: typical wavelengths were three to five miles; the wave amplitudeincreased with height up to certain level and then decreased; and in the waves the air was remarkably smooth.Power pilots, too, had shown a growing awareness of these phenomena. During the past few years, B.E.A. pilots hadreported many instances of flying through alternate smooth regions of rising and sinking air. Normal rates of ascent anddescent varied from a few hundred to 1,000 ft/min, with a maxi- mum of 2,500 ft/min over Europe. On one occasion, overNorthern Spain, an aircraft flying at 11,000ft had been lifted to 14,000ft and forced down to 8,000ft three times, in spite of theuse of maximum power in the regions of sink and throttling back in the rising air.A study of pilots' reports had shown that, for the formation of waves, the wind must be fairly strong. Normally, the windvelocity increased with height and remained constant in direction. The occurrence of waves was more probable in winter than insummer, and the evening was the most favourable time of day. Most theoretical explanations of wave phenomena had in thepast aimed at analytical steady-state solutions by pertubations theory, Mr. Corby continued. Linear differential equations couldexpress wind variation in space, and the solutions of these equations depended on the scale of the mountains over whichthe air was assumed to flow. Diagrams based on Queney's calculations were next exhibited.The flow of a uniform airstream over a small ridge showed a single wave at the ridge, with no lee-waves. For a ridge ofabout 100 km width, geostrophic forces (in addition to stability considerations) entered the calculations, and lee-waves were dis-cernible, increasing in height with distance downwind. These did not give a realistic picture, however, for these waves werenot the type encountered by powered aircraft and gliders. The speaker next turned to the theoretical work of Dr.R. S. Scorer. According to Scorer, he explained, an airflow either could or could not have lee-waves; if it could, then it wasvery sensitive to wavelength. A ridge or mountain could cause disturbances of all wavelengths, and the airflow in effect selectedthe one to which it was sensitive. A diagram of the airflow over a ridge under conditions favour-able for the occurrence of lee-waves, based on Scorer's theory, showed lee-waves increasing in amplitude with height up to acertain level, above which the amplitude decreased. The varia- tion of Is, and the agreement of this theory with observation, wasnext described in detail. Mr. Corby concluded by showing slides of the various wave systems propounded by Dr. J. Forchtgott.A summarized translation of a paper by Forchtgott, The Measurement of Airflow Deformation behind Mountain Ridges,was presented to the meeting by MR. A. H. YATES. The author, he said, had put forward a model based on ground and airobservations as a basis for theoretical comment. Three types of observed waves were listed.The first was a simple lee-wave, or downwind wave. Here lenticular clouds were present over the downwind crests; there was no inversion; and near the ground there were "rotatingclouds" at the wave-crests, with up-currents at the edge nearer the ridge and down-currents at the downstream edge.The second type was termed an "inversion wave," and possessed lenticular cloud at the wave-crest above the ridge anda standing rotating eddy in the lee of the ridge. Thirdly there was a composite wave, a combination of the first two types com-prising a more complex pattern. According to Forchtgott, the appearance of the rotating clouds was of a gradual drift down-wind and decrease in size followed by a rapid movement upwind. Forchtgott's paper next referred to the four sections of waveairflow—descending air, trough, ascending air, and crest—and to the fact (previously reported in this country by Ludlam) thattemperature soundings taken in only one of these regions would be misleading. To obtain the correct temperatures, and hencethe real lapse-rate in undisturbed air, an average of regions 1 and 3 should be taken. Many previously reported inversions,the author claimed, might not have been actual inversions, due to the effect of waves. The general discussion was opened by MR. C. E. WALLINGTON,in charge of the met. forecasting at last year's World Gliding Championships, who pointed out that some frontal zones assistedthe formation of waves. He described a glider flight he had made from Camphill last July, on a day when a warm front wasapproaching. Although on this occasion a low, turbulent flight along theridge had been anticipated, smooth wave-lift was encountered, and the speaker had afterwards attempted to discover whetherobservations made on this flight would support Dr. Scorer's wave theory. Downstream the waves were of decreased strength: Mr.Wallington submitted that they were there of a composite nature which, rather than turbulence, might account for this dissipation. The observations did support the theory. After suggestingthat waves ahead of warm fronts be further investigated, the speaker gave a suggested model for the pre-frontal region. DR. R. S. SCORER of Imperial College next described die con-ditions under which eddies could form on either side of a ridge. He gave details of the effect of lee-waves, observations madeduring last year's Isle of Man expedition, and catabatic effects. A photograph showing the ridge-top eddy at Camphill, in whichtwo windsocks a few yards apart pointed in opposite directions, brought enthusiastic applause from his audience. MR. FRANK FOSTER, glider pilot and B.E.A. captain, contributedaccounts of waves he had experienced during airline flying over Europe. On one occasion near a 30-mile ridge of mountains inFrance the aircraft had risen at the rate of 1,500-2,000 ft/min for two minutes or more in spite of the engines being throttledright back. Another wave, experienced south-east of Nice, had produced a regular series of clouds, spaced at half-mile intervals,which curled over at their ends; turbulence had been associated with these curl-overs. The orographic effect of airflow on rainfall was discussedby MR. J. S. SAWYER (Meteorological Office), who quoted an example where the positions of rainfall maxima along and justinland from the Dutch coast had implied a wave system. In general, however, this was a complicated problem. That waves associated with fronts were uncommon was sub-mitted by MR. H. S. TURNER (Meteorological Office, Northolt).The most usual time for waves was when the edge of an anti- cyclone coincided with ridges. One hundred reports of waves hadbeen collected at Northolt from B.E.A. pilots, and a possible relationship between standing waves and high-level turbulencewas being studied. Further details of the airline reports at Northolt were given by MR. R. K. PILSBURY (Meteorological Office, Northolt). Of thehundred reports, he said, 66 were definite cases of waves. In most cases there existed a layer of unstable air up to about 2,000ft,then a stable layer 800-10,000ft deep, and a third unstable layer. The wind velocity increased with height in every case at leastup to die top of the second layer. The shortest wavelength experienced was two miles, with themajority between three and eight milev. In support of Scorer's theory, the strongest vertical currents did occur near the top ofthe stable layer and, in addition, the up-currents were in general stronger than the down-currents. Examples had been given of1,200 ft/min at night and 1,900 ft/min at 8 a.m. In eight out of 10 of these strong cases, there had been aninversion, and in six out of 10 a double inversion. The instances of turbulence had apparently been encountered at times whenwaves were either forming or dissipating.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
