412 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

A Procedure For Studying Mountain Effects at Low Levels

UWE RADOK

Department of Meteorology, University of Melbourne, Carlton N. 3, Victoria, Australia

ABSTRACT

A representative picture, from the aeronautical point of view, of vertical currents above mountainous country is obtained by letting these currents act on an aircraft set to fly horizontally in still air. Pressure and temperature traces recorded by such an aircraft give the effective vertical velocities and some idea of the temperature lapse rate in the undisturbed stream. Two sets of results are given for illustration. One shows low-level lee waves which were caused presumably by a temperature inversion; the other is a case of strong turbulence side by side with a smooth wave, in which downdrafts reached 1600 ft/min in an 18-knot wind.

1. INTRODUCTION of tracks across the region to be studied. During HE study of mountain effects on wind has these traverses the airspeed and throttle setting tended in recent years to become almost are held constant corresponding to level flight in entirely the domain of sailplanes. The still air. The changes in level experienced by the T aircraft then reflect the effective vertical currents wealth of information now available e.g. for lee waves, especially at high levels, could hardly have which are determined from a continuous record been obtained by any other means, if only for of the aircraft's level and intermittent observa- reasons of economy. However, in the face of these tions of its progress. Temperature and humidity achievements some of the drawbacks of the sail- records serve as subsidary information. plane as a meteorological probe have come to be The measurements just listed could clearly be overlooked. Sailplane measurements inevitably made with widely different degrees of precision. give a lopsided picture, with more detail for up- In the investigations made by the writer it was draft than downdraft regions. This bias becomes necessary to use only the simplest means capable particularly serious at low levels where sailplanes of bringing out substantial vertical velocities, say cannot operate in downdrafts for any length of of the order of 1 m/sec or more. Thus as a sub- time. These low-level downdrafts, which form stitute for the height-record, changes in pressure one of the principal threats to aircraft safety, must were recorded by means of a Friez meteorograph therefore be studied by other means, such as no- attached to the wing of the aircraft. While the lift balloons, smoke puffs, or powered aircraft. accuracy obtained in this way for the absolute The present paper originated from the experi- heights is small it has been shown in another con- ence the writer gained while investigating the mag- text [2] that all but excessive vertical velocities nitude and distribution of vertical currents near can be determined with great precision directly an aerodrome in mountainous country (Cam- from the slopes of the pressure trace. bridge, Tasmania). The effects of such currents The foregoing is of course valid only if there on aircraft cannot be assessed very readily from exist no large distortions in the pressure field of balloon or smoke puff data which in any case are the mountainous region. Visual comparisons of difficult to interpret [1]. For aeronautical pur- actual and indicated heights made by the writer poses a procedure employing a slow-powered air- when flying approximately level with different hill craft offers the best prospects. Details of such a tops suggested that any pressure errors could not procedure, parts of which were evolved by the have exceeded a few millibars. They are there- writer for a previous investigation [3], are dis- fore unlikely to have affected the order of magni- cussed in the next section, while the last section tude of the rate of pressure change along the air- deals with a sample of results interesting from the craft path during the flights in question. viewpoint of lee-wave theory. A crucial point in the procedure is the accurate location of the aircraft with respect to ground 2. METHOD features at short intervals. Following a suggestion Basically the method consists of flying a slow air- by Mr. E. Desmond a small reflector gun sight craft up- and down-wind along each of a number was attached aiming downwards to the side of

Unauthenticated | Downloaded 10/06/21 09:01 AM UTC VOL. 35, No. 9, NOVEMBER, 1954 413 the aircraft. This permitted the moments of cross- duces the temperature deviation ing railway tracks, coast lines, and roads, to be precisely recorded on the meteorograph chart by * = -z(r -7) = -z(r-7)^, (2) means of an electric time marker. The same time marks were also conveyed to a second meteoro- where r is the dry adiabatic lapse rate. Thus the graph, equipped with accelerometer and rigidly aircraft will record along its path the temperature attached to the aircraft near the observer's seat. changes The pressure and temperature traces of this in- strument, although somewhat affected by vibra- T = -7Z-Z(r-y)^t^l tions, proved extremely useful in showing the trend u of developments not otherwise apparent during a flight. -z[-r-(r-7)f]. (3) Apart from giving directly the vertical velocities the pressure record when taken in conjunction Equation (3) shows that when the aircraft flies with the temperature trace provides extra infor- in the direction of the wind (c > 0) higher tem- mation regarding the temperature lapse rate, an peratures will be recorded in the troughs of the important quantity in lee-wave theory. Consider aircraft path than near the crests for all (sub- the temperature changes the aircraft would re- adiabatic) lapse rates. On the other hand, when cord in an arbitrarily disturbed stationary field of the aircraft flies against the wind (c < 0) higher flow, assuming that all changes are dry adiabatic, temperatures could be recorded near the crests of that the aircraft velocity c and the wind velocity u the path than in the troughs if the lapse rate are constant, and that both the temperature lapse satisfied the condition rate y and the deviation z of the streamlines from their undisturbed level depend only on the hori- 7 < (1 - \u/c\)T. (4) zontal coordinate. Then it is easy to see that the Given sufficiently regular traces the lapse rate of height changes of an aircraft flying through the the undisturbed flow can be determined from the disturbed field of flow will be given by relation

7 = r + («/c)(r + t/z), (5) provided always that the flow disturbances are the At any level the streamline displacement z pro- same at all levels in the height range explored.

FIG. 1. Tracks of an aircraft set to fly horizontally in still air (11K, 19th August 1952). The wind is from left to right in the plane of the diagram. Figures are vertical velocities in m/sec (updrafts positive). Heavy lines are up-wind flights; broken lines are down-wind flights.

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FIG. 2. Aerological data for Hobart and Cambridge, 19th August 1952. The temperatures and dewpoints recorded during the flight shown in FIGURE 1 appear as heavy broken lines on left.

3. TYPICAL RESULTS tive) corresponding to the tracks shown are given by the figures attached to the latter. The observations to be discussed here were The meteorological conditions cannot be dis- made at levels between 400m and 1200m in the cussed in detail, but for reference are fully shown vicinity of the aerodrome at Cambridge, Tasmania. in FIGURE 2. The temperatures recorded during This aerodrome is situated on the eastern side of the flight in question appear as a heavy broken line a chain of hills running Northwest to Southeast on the left and indicate a dry-adiabatic lapse rate and rising in places beyond 300m. On their west- through almost the entire height range studied. ern side, 13 km in a direct line from the aerodrome, The beginnings of an inversion appeared at the lies Hobart and beyond it the Mount Wellington highest point of the flight. No strictly simul- range exceeding 1300m in height. Disturbed flow taneous wind observations were made, so that the conditions over the Hobart-Cambridge region winds prevailing during the aircraft flight have to regularly occur with winds from westerly direc- be deduced from a number of balloon flights made tions. on the day in question, shown in FIGURE 2 right. A set of typical flight paths under such condi- Some point checks, obtained by timing the air- tions is given in FIGURE 1 which shows a vertical craft over pairs of up- and downwind traverses, cross section running east-west through Mount are indicated by crosses. The most striking and Wellington. The wind was from left to right in definite feature emerging from these data is the the plane of the diagram. Up- and down-wind strong wind shear in the lowest kilometer. traverses appear as full and broken lines re- The flight paths in FIGURE 1 give clear evidence spectively. The ground profile shown is repre- of lee waves 8 to 10 km long. It is noteworthy that sentative of the region traversed rather than the during each of the four traverses descending mo- exact intersection with the surface contours. The tion was found upwind from the chain of hills be- vertical velocities (in m/sec, updrafts being posi- tween Hobart and Cambridge and ascending mo-

Unauthenticated | Downloaded 10/06/21 09:01 AM UTC VOL. 35, No. 9, NOVEMBER, 1954 415 tion in its lee. As is often unavoidable all the me- over the area both temperatures and winds changed teorological observations were made in the dis- all the time—a very characteristic difficulty in the turbed field of flow which is reflected e.g. in some practical study of mountain effects. Sections of of the pilot balloon winds. Under such circum- a flight record obtained between two showers are stances there exists no firm basis for applying the reproduced in FIGURE 4. The temperature trace perturbation theory of lee waves [4] in a strict has been reversed photographically and placed manner. Nevertheless, it is fairly obvious that the between the pressure and acceleration traces to observations of temperature and wind at higher facilitate comparisons. levels given in FIGURE 2 cannot be reconciled with FIGURE 4 shows initially a weak wave with the prerequisites for lee waves in the case of a pressure and temperature in phase. Then, about dry-adiabatic surface layer laid down by that 3 km from the mountain (cf. FIG. 1), a distinctly theory [4, sect. 4]. It seems therefore necessary more intense wave motion set in and finally to seek the cause of the waves in the fact that there dragged the aircraft down at the rate of 1600 ft/ was a temperature inversion at low levels; its min in the immediate neighbourhood of the moun- relevance was already suggested in the first dis- tain. Together with the aircraft velocity of 86 mph cussion of the lee-wave phenomenon [5] and has against a wind of 20 mph that downdraft velocity recently again received support [7]. shows that the air followed the steep mountain The other set of observations to be discussed slope down to at least the lowest level reached by here was obtained with much weaker winds and the aircraft, 400 meters below the summit, with- a lapse rate of 8.5C°/km in the surface layer. The out breaking away from the surface. meteorological conditions are illustrated in FIG- In the intense wave near the mountain pressure URE 3, but with a series of snow showers passing and temperature were in antiphase. By means of

FIG. 3. Aerological data for Hobart and Cambridge, 24th August 1952. The temperatures and dewpoints corre- sponding to the flight record in FIGURE 4 appear on left as heavy full lines on which aircraft ascents and descents are indicated by arrows.

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fected; it contained no sign of the large vertical velocities previously found near the mountain. The existence of smooth and turbulent waves of quite different intensities as well as of irregular heavy turbulence, all within close range of one another, is not easily explained in terms of the stable lee waves considered by the theory of small perturbations. These observations thus underline the importance of a new attack on the lee wave problem made recently by Long [6] who considered unsteady finite, as well as infinitesimal, disturb- ances. Laboratory experiments based on that theory [7] have produced lee disturbances similar to the phenomena illustrated in FIGURE 4, viz. a smooth downflow in the lee of the obstacle fol- lowed either by slightly turbulent waves or by a hydraulic jump. The method of observation de- scribed here thus promises to be of value for con- FIG. 4. Sections of a flight record obtained at 15K on firming the relevance of such laboratory results for 24th August 1952. Traces are (top to bottom) vertical the atmosphere. acceleration, temperature, pressure. ACKNOWLEDGMENT equation [5] the lapse rate near the mountain The data discussed in this paper were obtained works out, from the temperature and height am- for, and with the support of, the Commonwealth plitudes and the velocities given above, at 6C°/km. Department of Civil Aviation. At Cambridge The "inversion" shown by the aircraft tempera- members of that department, of the Meteorological tures in FIGURE 3 left thus was not genuine but Service, and of the Aero Club of Southern Tas- reflected horizontal temperature contrasts in the mania contributed to the work in innumerable disturbed field of flow. A genuine inversion prob- ways. Some valuable suggestions for the paper ably existed at a somewhat higher level. came from Miss A. M. Grant. All this help is The turbulence changes during this flight also gratefully acknowledged. deserve notice. The acceleration trace as well as the appearance of the other traces in FIGURE 4 14 th January 1954. show the intense wave near the mountain to have REFERENCES been distinctly smoother than the weak wave fur- [1] Hoehndorf, F., and Marquardt, R., Beitr. Phys. fr. ther away. That however was not the full story. Attn., Vol. 12, pp. 147-168 (1934). After turning back within 100 yards of the moun- [2] Radok, U., Austral. J. Sc. Res., A, Vol. 2, pp. 550- 563 (1949). tain side the aircraft flew downwind on a track [3] Radok, U., Weather Dev. Res. Bull., no. 15, pp. 28- parallel and close to that of the upwind traverse. 39 (1950). On this run violent turbulence was encountered [4] Scorer, R. S., Q. J. R. Met. SocVol. 79, pp. 70-83 (1953). in place of the smooth wave. FIGURE 4 shows that [5] Kuettner, J., Beitr. Phys. fr. Attn., Vol. 25, pp. 251- the wing meteorograph traces suffered complete 299 (1938). obliteration. However, here the second meteoro- [61 Long, R. R., Tellus, Vol. 5, pp. 42-57 (1953). [7] Long, R. R., Bull. Am. Met. Soc., Vol. 34, pp. 205- graph trace proved its worth by remaining unaf- 211 (1953).

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