A Load of Hot Air
How does air move between atmospheric layers? Ben Valsler spoke to Alison Ming, a PhD Student at the Cambridge University to find out more.
Alison - I'm working on the problem of tropical upwelling and what drives it. We know that air in the troposphere, so the lowest layer of the atmosphere we live in, air of the tropics rises up by convection because it's really warm there. And it carries on going, but at a certain point, it hits the tropopause and the tropopause is sort of that notional boundary between the troposphere (the layer below) and the stratosphere (the next layer above). So, when it hits the tropopause, it carries on going into the stratosphere. What causes that air to carry on going into the stratosphere is still a question that is being debated and that's what I'm working on.
Ben - So, what is the structure of the atmosphere above us?
Alison - The layer we live in is called a troposphere and the temperature really decreases upwards in the troposphere then the temperature sort of stabilises and increases in the stratosphere - so there's an inversion of temperature. And people use that to therefore define a tropopause which is that sort of notional boundary that sits between the troposphere and the stratosphere. And the tropopause is lower in the midlatitudes. So above us, it'll be about 10 kilometres high. In the tropics, it's much higher. It will be about 15 kilometres.
Ben - So, you've mentioned the troposphere and the stratosphere, but what's the difference and where's the boundary?
Alison - There are many ways in which you could separate the two layers of the atmosphere. So, they're not obviously separated. There isn't a fixed boundary between the two, but you could use different properties of them to define that tropopause. So, one way of defining it would be using the temperature, there are other dynamical quantities that you could pick a value of and say, "We'll take this value to be roughly where the tropopause sits."
Ben - Why do you think there needs to be a different mechanism? Why can't it just be more convection?
Alison - Some people think it is just more convection, but other people think that waves from just outside the tropics travel up to that region break and then tops up the air. So, that's a different pumping mechanism to air just rising because it's hot and there's various other mechanisms that people think might be driving that upwelling and I'm trying to figure out which one it is.
Ben - So, why is it important to us? Why do we need to know what the process is that actually pushes this air into the stratosphere?
Alison - Understanding the dynamics of a climate means that we then understand better how our models work. Also, it means that our weather models will also get better and understand what's going on in there.
Ben - So, in terms of understanding our weather and understanding our climate, why is it important that we know what's going on up in the stratosphere?
Alison - In the past, people didn't really think that the stratosphere mattered to what happens to us in the troposphere and weather modelling didn't include stratospheric levels. But then we found that there was a hole in the ozone layer, and discovered that if the ozone layer wasn't there then we'd all be bombarded with UV radiation. But then also, there is a very slow circulation in the stratosphere. So, air that goes up over the tropics then travels polewards very, very slowly. It takes up to 5 years to reach the poles and then it falls back down. And that circulation has an indirect effect on the troposphere and both the stratosphere and the troposphere are coupled together. And people found that by including the stratosphere into weather models which is something that's only been done very recently, we could improve our predictions of weather.
Ben - So, even though we in the UK are a very long way from the tropics where this upwelling is actually happening, there is an important dynamic process going on, thousands of metres above us from thousands of miles away that can then impact, not just the weather that we have, but the long term weather trends the climate.
Alison - Yes. Most of the stuff that gets into the stratosphere gets there through that tropical pipe. So, things like CFCs, aerosol, pollutants, everything gets up to there via that tropical pipe. Those CFCs and other aerosols will eventually affect the (radiative) balance in the stratosphere and then eventually affect the climate in the troposphere. So, one example I'd like to take is in 1991, there was a volcanic eruption - the Pinatubo eruption and that volcano managed to inject quite a lot of dust and aerosols into the stratosphere. The few years following Pinatubo, the whole world was slightly cooler, mostly because there were lots aerosols in the stratosphere. It's not effect that was felt right after the eruption, but was an effect that was also felt for many years after.
Ben - So, it's obviously a very important thing for us to understand this global transport and the transport between different layers of the atmosphere. How are you actually going about it? What are you doing to study it?
Alison - What I do is, I take a very simplistic model, two layers of the atmosphere and what that model does is, it models the very basic circulation around the planet and then I look for specific features in my model results to see whether I can understand what drives the upwelling in that model. And then I'd look at observation data set and see whether I can find similar features and if they do exist, then it's very likely that the dynamics that's happening in my model is similar to the dynamics happening in the real world.
Ben - Once we know that, what sort of new questions will we be able to ask or hopefully answer?
Alison - One of the other questions I'm working on is just above the tropical tropopause, there is an annual cycle in temperatures. If you think of it, the sun goes over the tropics twice a year, so there really should be a semi-annual cycle in temperatures, but as it happens, there's a very, very strong annual cycle and that's the one other question that's also of interest to me and I'm working on. So hopefully, by better understanding tropical upwelling then I'll be able to have a good stab at what is also causing that annual cycle.