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THE 1979 MORNING GLORY EXPEDITION By ROGER K. SMITH and JONATHAN GOODFIELD Geophysical Fluid Dynamics Laboratory, Monash University, Melbourne, Australia ORNING GLORY is the name given to a type of wind squall, or succession of wind M squalls, which occurs early in the morning, mainly in spring, at places around the southern coast of the Gulf of Carpentaria in northern Australia. The name derives from a spectacular roll cloud, or series of roll clouds, which frequently accompanies these squalls and whose form is often accentuated by the soft light of the rising sun. The squalls are also accompanied at the surface by a sharp pressure jump, sometimes in excess of a millibar. The Gulf region is sparsely populated, remote and access to much of it is difficult, even by four wheel drive vehicles, so it is not surprising that published descriptions of the Morning Glory have been few. Nevertheless, one excellent account is given in a short article in Weather by Neal et al. (1977). These authors assemble what scant meteorological data are available from a relatively sparse network of surface observation stations around the Gulf; most of it in the form of weekly barograph traces, but including a few anemograph traces as well. In particular, they establish a broad climatology of occurrences, based on the incidence of nocturnal or early morning pressure jumps, but the data are insufficient to do more than speculate on the origin and nature of this seemingly unique phenomenon. It had been suggested earlier by Clarke (1972) that the Morning Glory is a type of undular bore, propagating on the radiatively cooled nocturnal inversion, and that it might be caused by katabatic drainage over the western slopes of Cape York Peninsula, which rise gently (gradient approximately 1 in 500) to the Dividing Ranges bordering the Peninsula’s east coast. Clarke’s ideas were supported by calculations using a simple numerical model, and it had been known for some time that katabatic winds on the ice slopes of Antarctica could develop vigorous hydraulic jumps (Ball 1957), but the observational data were insufficient to appraise them adequately. Following Clarke’s retirement as Officer-in-Charge of the Australian Numerical Meteorology Research Centre in 1977, he and the first author decided to mount an expedition to the Gulf in an effort to acquire sufficient data to resolve the mysteries surrounding the Morning Glory. The expedition was planned for late September/early October 1979, a time of year when, according to the climatology by Neal et al., one could expect a maximum frequency of occurrence, with, on average, about four Morning Glories every ten days. We were fortunate to interest Roger Merridew, the owner and pilot of a light aircraft, in the venture and he offered his services, asking only that the expedition cover the operating costs for the plane. We were joined also by Derek Reid of the CSIRO Division of Atmospheric Physics in Melbourne and an experienced veteran of several large meteorological expeditions in Australia. The team was completed by three applied mathematics undergraduates (Karen McAndrew, Peter Watterson and the second author) and a laboratory technician (Terry Long) from Monash University; Richard Hagger, a keen amateur meteorologist and photographer; and Reg Clarke’s wife, Elsje. The plan was as follows. The Clarkes would aim to establish a series of sensitive recording barometers and Woelfle anemographs at sites spaced along the road linking Chillagoe, just west of the Dividing Range, to Kowanyama on the Gulf coast, a straight line distance of about 360 km (Fig. I). The other eight members of the expedition would be stationed at Burketown, an isolated township just south of the Gulf, where one could expect Morning Glories, propagating from the north-east, to arrive just after sunrise, facilitating the observations. Part of this group, operating from the airfield at Burketown, would determine upper winds by tracking pilot balloons, measure surface pressure with a sensitive digital aneroid barometer, and take photographs; the others would fly through the disturbance at various levels to determine its thermal and moisture structure, and would make similar soundings in the environment ahead of it. This exercise required some caution since, as far as we knew, no one had flown through the squall before and neither 130

THE 1979 MORNING GLORY EXPEDITION

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THE 1979 MORNING GLORY EXPEDITION

By ROGER K . SMITH and JONATHAN GOODFIELD Geophysical Fluid Dynamics Laboratory, Monash University, Melbourne, Australia

ORNING GLORY is the name given to a type of wind squall, or succession of wind M squalls, which occurs early in the morning, mainly in spring, a t places around the southern coast of the Gulf of Carpentaria in northern Australia. The name derives from a spectacular roll cloud, or series of roll clouds, which frequently accompanies these squalls and whose form is often accentuated by the soft light of the rising sun. The squalls are also accompanied at the surface by a sharp pressure jump, sometimes in excess of a millibar.

The Gulf region is sparsely populated, remote and access to much of it is difficult, even by four wheel drive vehicles, so it is not surprising that published descriptions of the Morning Glory have been few. Nevertheless, one excellent account is given in a short article in Weather by Neal et al. (1977). These authors assemble what scant meteorological data are available from a relatively sparse network of surface observation stations around the Gulf; most of it in the form of weekly barograph traces, but including a few anemograph traces as well. In particular, they establish a broad climatology of occurrences, based on the incidence of nocturnal or early morning pressure jumps, but the data are insufficient to do more than speculate on the origin and nature of this seemingly unique phenomenon. It had been suggested earlier by Clarke (1972) that the Morning Glory is a type of undular bore, propagating on the radiatively cooled nocturnal inversion, and that it might be caused by katabatic drainage over the western slopes of Cape York Peninsula, which rise gently (gradient approximately 1 in 500) to the Dividing Ranges bordering the Peninsula’s east coast. Clarke’s ideas were supported by calculations using a simple numerical model, and it had been known for some time that katabatic winds on the ice slopes of Antarctica could develop vigorous hydraulic jumps (Ball 1957), but the observational data were insufficient to appraise them adequately.

Following Clarke’s retirement as Officer-in-Charge of the Australian Numerical Meteorology Research Centre in 1977, he and the first author decided to mount an expedition to the Gulf in an effort to acquire sufficient data to resolve the mysteries surrounding the Morning Glory. The expedition was planned for late September/early October 1979, a time of year when, according to the climatology by Neal et al., one could expect a maximum frequency of occurrence, with, on average, about four Morning Glories every ten days. We were fortunate to interest Roger Merridew, the owner and pilot of a light aircraft, in the venture and he offered his services, asking only that the expedition cover the operating costs for the plane. We were joined also by Derek Reid of the CSIRO Division of Atmospheric Physics in Melbourne and an experienced veteran of several large meteorological expeditions in Australia. The team was completed by three applied mathematics undergraduates (Karen McAndrew, Peter Watterson and the second author) and a laboratory technician (Terry Long) from Monash University; Richard Hagger, a keen amateur meteorologist and photographer; and Reg Clarke’s wife, Elsje.

The plan was as follows. The Clarkes would aim to establish a series of sensitive recording barometers and Woelfle anemographs at sites spaced along the road linking Chillagoe, just west of the Dividing Range, to Kowanyama on the Gulf coast, a straight line distance of about 360 km (Fig. I). The other eight members of the expedition would be stationed at Burketown, an isolated township just south of the Gulf, where one could expect Morning Glories, propagating from the north-east, to arrive just after sunrise, facilitating the observations. Part of this group, operating from the airfield at Burketown, would determine upper winds by tracking pilot balloons, measure surface pressure with a sensitive digital aneroid barometer, and take photographs; the others would fly through the disturbance at various levels to determine its thermal and moisture structure, and would make similar soundings in the environment ahead of it. This exercise required some caution since, as far as we knew, no one had flown through the squall before and neither

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Fig. 1. Map of north-eastern Australia including locations mentioned in the text and topographic contours. The road linking Chillagoe with Kowanyama is shown also

the vigour of the organised motion, nor the intensity of turbulence, could be easily anticipated.

The Burketown team commenced operations at 0500 each morning, one and a half hours before sunrise, continuing observations for several hours. One or two single theodolite wind soundings were made before sunrise, followed by a few more with two theodolites when it became light. All the time a vigilant eye was focused on the eastern horizon for signs of a Morning Glory. Six mornings passed with no sightings and local inhabitants told us that the last occurrence had been some weeks before - obviously the statistics encompassed large standard deviations! At last, on 29 September, half way through the expedition, we hadjust arrived on the airfield to be greeted with the awesome sight of a long, low cloud line stretching the full length of the eastern horizon; there was still over an hour to go before sunrise and the cloud was approaching rapidly. The squall arrived before we had set up both theodolites and the wind data obtained during its passage were poor, but we were fortunate to obtain a good sounding prior to passage. Nevertheless, as soon as it was light, the plane was airborne and in pursuit, and we were hopeful that valuable data could still be obtained.

This first flight was as exciting as it was, perhaps, historic! After take off, a spiral ascent was made to 1600 m and then a course was set towards and normal to the cloud line, which at this stage was about 20 km distant. Moderate turbulence was encountered along this track, but as we neared and overflew the cloud, which had a top at about 1200 m, the motion became very smooth. Despite the poor light, some unique photographs of the cloud, showing the considerable internal organisation of the disturbance, were obtained (Figs 2 and 3). A second traverse was made just below the cloud top and, again, the flow was smooth until we were several kilometres to the rear of the cloud line, where light to moderate turbulence was experienced. At this stage, the pilot felt that a lower traverse would be safe and we descended to about 600 m, aiming to penetrate the cloud from the rear, a little below its middle level. Tension mounted as we approached the cloud, but so far only moderatd turbulence had been encountered. The pilot endeavoured to fly at constant speed (about 100 kn) and constant attitude on traverses so that vertical displacements of the aircraft would give an indication of vertical motions in the disturbance. He was calling out altitude and rate of climb at frequent intervals, and Derek

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Photograph by R. K. Smith

Fig, 2. Aerial view ofrhe Morning Glory about 0630 local time on 29 September 1979

Photograph by R. K. Smith Fig. 3. As,for Fig. 2

Reid, the navigator, was recording them. Karen McAndrew was noting temperature and humidity every ten seconds, and the first author was trying to hold the camera still enough to take photographs! On entering the cloud, the aircraft plunged over 200 m in a period of 40 seconds or so, and the rate of climb indicator moved off the scale; there was a sudden jolt and in the next forty seconds the plane was carried upwards, recovering its penetration

altitude, and we were through the cloud into the smooth air ahead. After this excitement, our task was to make a temperature sounding in the air ahead of the squall a t altitudes from 100 m up to two kilometres. As we did this, the cloud roll began to dissipate rapidly, presumably as the squall moved into drier air, and further penetrations were abandoned.

The data obtained from the aircraft were as good as we could have hoped for, but without useful wind soundings it was insufficient to determine the nature of the disturbance. The following few days seemed quite unfavourable for Morning Glories, with gusty south-easterly winds blowing, but another chance came on 4 October, two days before we returned to Melbourne. We had begun operations as usual at 0500 and conditions seemed very favourable; a light westerly wind was blowing at the surface and humidity was high. However, no squall appeared and no signs of a n approaching disturbance were observed during a short flight a t 0630. At 0800 it was decided to return to the hotel for breakfast, but a regular watch was kept on the sky. About 0830 a magnificent roll cloud was sighted in the east (Fig. 4) and we hurried back to the airfield. The theodolite

Photograph by R. K. Smith Fig. 4. Morning Glory at about 0845 local time on 4 October 1979 seen from Burketown

observers were soon in action and the plane airborne and on its way to intercept the squall. Time lapse movie cameras were positioned on the ground and a movie camera was operated in the aircraft. Two impressive photographs of the leading cloud line taken during the flight are shown in Figs5 and 6. This particular Morning Glory was unusually late, the leading squall arriving at Burketown at 0935. Altogether, five cloud lines were observed, but the first two were best formed; these are shown in Fig. 7 when the leading cloud was almost overhead at Burketown. Each cloud roll was accompanied by a strong surface wind gust lasting two or three minutes, and cloud spacing was about 10 km; the whole entourage took over an hour t o pass. By 1100, the surface wind had backed from north-north-east to a northerly as the sea breeze became well established.

Excellent data wereDbtained on this squall, both from the air and from the ground, and, on analysis, confirmed the picture suggested by Clarke that the Morning Glory is essentially an undular bore propagating on the low level inversion. A construction of the airflow streamlines, relative to the disturbance and normal to the cloud lines, is shown in Fig. 8; this is based on the balloon wind soundings and clearly illustrates the wavelike

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Photograph by D. G. Reid

Fig. 5. Aerial view of the Morning Glory of 4 October 1979; leading edge offirst cloud line

Photograph by D. G. Reid Fig. 6. As for Fig. 4: trailing edge

character of the Glory. While there is probably a small re-circulation region in the first two waves, the apparent rolling motion in the clouds is due to their continual generation in the ascending portion of the waves, followed by re-evaporation in the subsiding portion as wave crests move on. At Burketown, surface pressure rose abruptly by more than one millibar with the passage of the leading squall, followed by smaller amplitude oscillations as subsequent waves arrived. Significantly, the mean post-Glory surface pressure remained higher than that before arrival, even when account was taken of the normal diurnal pressure variation in that region; this is a characteristic of a bore.

The data gathered so far suggest that the Morning Glory generally originates over Cape York Peninsula, especially on the western side. It appears mostly to be the product of interaction between a sea breeze front, surging from the east coast of the Peninsula and aided by a prevailing north-easterly wind, and the developing nocturnal inversion. Pressure jumps recorded on 4 October and on the previous evening at stations around the

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Photograph by R. K. Smith Morning Glory at about 0930 local time on 4 October 1979 seen from the airfield at Burketown.

Thefirst cloud roll is almost overheadand is apparently decaying as the disturbance moves into drier air. At this time the clouds were much better formed over the sea, 30 k m to the north

Fig. 7.

x (km)

Cross-section of the relative streamlines normal to the cloud line in the Morning Glory of 4 October 1979 as deduced from double theodolite wind data. Flow is from left to right and x = 0 marks the leading edge of the disturbance at the surface. Balloon trajectories are shown by dotted lines: note that vertical motions are sufficiently intense in certain parts of the wave system to cause balloons with a natural rare of rise of about 2 ms" to descend. Numbers labelling contours denote values of the relative streamfunction in m2s-'

Gulf are consistent with the hypothesis that the Morning Glory we observed at Burketown was a straight line, hundreds of kilometres long, moving towards 237" at a fairly uniform speed of 9 ms-'. In comparison, the Glory observed on 29 September had a speed of about 10.8 ms-' and an almost identical orientation. On both occasions, the speed of movement is faster than the wind at any level and the disturbance is truly propagating.

Much remains to be learned about the Morning Glory, especially with regard to its origin and its demise, and other expeditions are planned. A more detailed discussion of the 1979 expedition results are contained in a paper by Clarke ef al. (1981).

Fig. 8.

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ACKNOWLEDGMENTS

Many Gulf residents, ranging from managers of cattle stations on Cape York Peninsula to local police, gave generous assistance to the expedition, and valuable observations, including times of passage, were made by Mr Scotty Maxwell of Mornington Island and Mrs Lee Kehoe of Burketown. Anne Allingham, from the University in Townsville, happened to be researching in Burketown on 4 October, and joined the ground team on that crucial day giving much help. To all these people, and to our expedition colleagues, we record our thanks.

The expedition was made possible by grants from Monash and Melbourne Universities, the CSIRO Division of Atmospheric Physics, the Ian Potter Foundation and the Royal Meteorological Society Australian Branch, to whom we are indebted. Finally, we wish to thank the Bureau of Meteorology, and many of its staff, for the loan of equipment and the provision of data.

REFERENCES

Ball, F. K. (1957) The katabatic winds of Adilie Land and King George V Land. Tellus, 9,

Clarke, R. H. (1972) The Morning Glory: an atmospheric hydraulic jump. J. Appl. Met. ,

Clarke, R. H., Smith, R. K. and Reid, D. G. (1981) The Morning Glory of the Gulf of

Neal, A. B., Butterworth, I. J. and Murphy, K. H. (1977) The Morning Glory. Weather,

pp. 201-208

11, pp. 304-31 1

Carpentaria: an atmospheric undular bore. Mon. Wea. Rev., 109, to appear

32, pp. 176-183

A MATHEMATICAL CLIMATE STUDY ON THE EFFECT OF DUST FROM THE MOUNT ST. HELENS VOLCANO ERUPTION

By A. M. GADIAN and D. R. DAVIES Department of Mathematics, University of Exeter

UCH attention has been paid to the recent eruption of Mount St. Helens in the western M USA. Climatic effects of volcanic dust in the upper atmosphere have been discussed by Lamb (1970), but by using a numerical model it is now plausible to simulate the effects of the dust’s attenuation of short wave incoming solar radiation on the model’s dynamics. Hunt (1977) considered the effects on a large general circulation model of an amount of volcanic debris similar to that released by Krakatoa in 1883. Although mathematical models cannot accurately predict atmospheric weather patterns more than a few days in advance, they include the main large scale physical processes which are likely to affect climate trends. Applying a numerical model to the recent Mount St. Helens eruptions provides very interesting results, some of which are described below.

A modified quasi-geostrophic Beta plane model is used (Phillips 1956), since it produces flow patterns and wind velocities, etc. which are similar in global scale characteristics to those observed on mid-latitude flow charts and since the model is relatively efficient in computing time. The simulated ice edge is determined by the surface temperature, interactions with the atmospheric dynamics and the incoming solar radiation cycle. These results are compared with those obtained by a control experiment with no dust.

The Mount St. Helens eruption, although not very large, produced relatively large

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