A hydraulic jump
17th December 2004
At Casey Station this morning the sun was shining, the temperature was only a degree or two below freezing and there was barely a breeze. The weather could hardly have been more benign but up on the plateau edge a few kilometres to our east strange things were happening.
A long thin 'sausage' of cloud lay along the lip where the ice dome begins to flatten out above the coastal slopes, with puffs of dusty blown snow rising in front of it. Parts of the cloud were sunlit and parts in shadow providing sharp visual texture, and as we watched it became clear that the entire cloud, all fifty or so kilometres of it, was rolling. The near side was descending, giving the impression of a vast steamroller starting its helter-skelter progress down on top of us. Yet it didn't move, in fact it even began to pull back despite appearing to roll towards us.
We have six meteorological personnel here for the summer, forecasters as well as weather observers. They became quite excited, and two of them asked to be allowed to ride up on quads as far as the skiway ten kilometres from Casey to take photographs and pressure measurements. They called the feature a hydraulic jump.
Another meteorological colleague guided me to the International Antarctic Weather Forecasting Handbook. There I discovered that an official name for this effect is Loewe's Phenomenon. It is described as 'the most spectacular phenomenon occurring during katabatic periods', the times when winds pour off the Antarctic plateau as down-slope drainage of cold, dense air often bringing blizzards to coastal regions. To quote further, 'Loewe's phenomenon includes a sudden slowing down of the wind speed and a change in the depth of the cold air layer. This 'jump' in the wind is always associated with a 'jump' in the pressure… drifting snow is also always observed upstream of the jump, as the wind is strong.'
Basically, as I understand it, what is happening is that the screaming katabatic wind collides with a mass of nearly stationary air, and gets deflected upwards. Sometimes this leads to a rolling line of cloud, as today, at other times or in other places to a wall of blowing snow rising almost vertically upwards. I'm not sure that other descriptions and explanations have got very much further than this simplistic story. The Handbook discusses observations during the sixties by the French at Dumont D'Urville (down the coast to our east) and by Nils Lied behind Davis (to our west), and Franco-American experiments in the eighties, but still it had to admit that 'Loewe's phenomena have a frontal character and… downstream of them the flow loses its simple behaviour in such a manner that Ball's assumptions cannot be used any more, explaining the failure of the hydraulic theory'.
Another visit to the Australian Antarctic Meteorological Centre (across the corridor from my office) led me to be shown today's satellite image on which our own Loewe's Phenomenon was clearly visible as paired white and dark lines (the cloud and its shadow), with mainly clear skies to both east and west of it. I suppose all clouds are as visible on satellite images if one has access to them, but the clarity of this one, and the proximity of the real thing looming above us, struck me as particularly impressive. The cloud itself was quite awesome, especially in its rolling movement.
All in all, it has to be said that Antarctica can be quite an odd place.