Antarctic cloud diary

I spend a lot of time in my PhD thinking about the effect of Antarctic clouds on the surface energy balance of Larsen C – that is, how clouds interact with incoming and outgoing energy flows, and therefore how they affect melt over ice shelves. Particularly, I’m interested in understanding how their composition (i.e. whether they’re made of liquid or ice and the shape, number and water content of ice crystals and cloud droplets) affects the way they control the energy reaching the surface of the ice shelf below them.

BUT. That’s all very dry and academic. It’s hard to relate computer model output and raw data from aircraft campaigns to the real deal.

I’ve always been a cloud nerd, but I’ve been absolutely blown away by the clouds down at Rothera. Given that there are only a few types of cloud that can form in the cold polar atmosphere (convection can’t really happen because the sun can’t warm the ground enough, which means that cumulus clouds can’t form, except on the warmest of summer days), there’s a huge amount of variety.

I’ve been impressed by the array, so I thought I’d share some of the best and most interesting examples here, in no particular order – this one is for all the sky watchers. Hol’ tight the cloud physics gang.

  1. Altocumulus lenticularis (aka lenticulars)

What looks like a UFO, glues itself to mountains, and flips aircraft upside down? That’s right – lenticulars.

lenticular_sml

These clouds form in the lee (downwind) of mountains, and are generated by gravity waves formed when air is forced over steep topography. Gravity waves basically just describe the way that air moves when it is pushed over an obstacle like a mountain: as the air moves up it becomes colder and denser than its surroundings, and when it passes over the mountain this makes it sink. However, because it has been forced upwards, when it descends it has inertia, and therefore overshoots the level it would normally settle at (its ‘equilibrium level’). That means it oscillates around the equilibrium level, each time getting a little bit closer to that goldilocks point where it has just the right temperature and density to stay at the same height.

You can see it really clearly in this picture:

lenticular wave crest

Lenticulars form in the crest of the waves, so you can see them far downwind of mountains. In fact, we saw some pretty spectacular ones in Punta Arenas while we were waiting for our flight South. However, they are best when they’re plonked right on top of the mountain, crouched like creepy cloudy little goblins.

I saw this one on a totally beautiful, calm, still day, when it was virtually the only cloud in the sky. It stayed there for a really long time because of the absence of any wind.

dingle lenticular - sml

 

2. Low cloud

Low clouds don’t typically accompany the kind of weather that is useful for flying or outdoor activities, so they’re not anyone’s favourite, but they do look pretty amazing when they swallow up huge mountains.

vanishing ramp_sml

Besides, it’s super cool to be able to see the distinct layers against the flanks of the slopes – cloud layers are often very thin and comprised of liquid droplets here, so they almost make stripes. It’s hard to see them when you’re in them, but this sort of shows the gist:

stripy cloud layer

 

3. Everyday clouds

Sc over mountains - Copy

Even the clouds we get most often at Rothera can be pretty gorgeous. Most days when I’m on met duty (observing the weather, which literally means I get to just stare out the window at the sky – dreamy), there’s stratocumulus. The word literally means layer-heap, which is pretty instructive really. That’s the lower cloud you can see in the picture below – it’s not uniform enough to be stratus (the sky-enveloping, homogeneous grey mess you get in the UK that’s so pathetic it can’t even produce real rain, only drizzle) because it has some vertical extent and texture (which is easier to see when it’s above you), so it’s strato-cu.

Ci and Sc over Rothera

The layer above is cirrus (meaning ‘feather’). Cirrus clouds form much higher in the atmosphere: around 20,000 ft. That means they are really cold clouds, made up of ice crystals that evaporate more slowly than water, which gives them that wispy, undefined edge. They often have more interesting patterns too.

It’s particularly spectacular when they co-occur, like in this picture.

 

4. Helmholtz clouds

helmholtz

I saw these while doing an air ob (met observations for the aircraft operations) and rushed out to take a picture of it, but it had already started to disperse. BUT you can still sort of see the general idea of it.

Helmholtz waves are what we’d classically imagine when we think of ocean waves – the regularly repeating crests like you see in the sea. They’re caused by shear, or velocity differences, at the interface between two layers with varying characteristics, i.e. where those two layers meet. In the sea that’s the boundary between air and water, and waves are generated when the wind blows over the ocean surface, but in the atmosphere it might just be two different layers of air moving at different speeds.

This was a particularly windy day, with lots of complicated variations in the winds at different heights. It was an especially difficult day for the weather forecasters, who had a lot to focus on, getting the winds at different levels correct for the pilots.

 

5. Orographic clouds

These are a particlarly nerdy fave of mine. I find myself writing the phrase ‘orographically forced cloud’ quite a lot (translation: ‘turned into cloud by mountains’), so actually getting the chance to SEE this in real life was a special treat.

These clouds are produced when the air flows directly over the mountains that run down the spine of the peninsula – so we had to fly over them to see them. It’s amazing – on one side of the peninsula it’s perfect flying weather – not a scrap of cloud in the sky and the sun beating down. As soon as you hit the crest of the mountains however, there’s a solid bank of cloud obscuring the surface beneath, and extending almost as far as the eye can see.

Orographic cloud formation

That’s because the air is forced up and over the mountains from a lower level. At warmer temperatures, all the water in the air is present in gaseous form, so you can’t see it. As it rise, the air cools (because it’s higher up, and temperature tends to decrease with altitude). Once it gets cold enough, the water contained within the air as water vapour changes into liquid water droplets, and forms a cloud. Those clouds form from the very apex of the mountains, and extend downwind quite a way. So, this type of cloud is generated by air getting lifted straight up and over by a cross-peninsula flow (which is very common, actually – westerlies blow pretty frequently round ‘ere.

 

 

 

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