But today I wanted to cover a more fundamental topic that I've been asked a lot of questions about. Three readers of this blog have sent me messages asking this fundamental question--
Why does horizontal divergence aloft lead to rising motion? Why can't air also come down from above?
This is a very good question--admittedly one that also bothered me when I was first learning about these patterns during my undergraduate days. So today I thought I'd try to explain what's going on aloft when we see divergence and convergence.
One area everyone seems to be able to agree on is what happens at the surface when we have horizontal divergence or convergence. Take divergence at the surface, for example. If air is spreading apart, it creates a semi-vaccuum, or area of lower pressure in the middle of the area where the air is spreading out. As the saying goes, nature abhors a vacuum and tries to fill it. It's simply a matter of air moving from higher to lower pressure. Since the ground is below and air doesn't come out of the ground, the only way to fill that semi-vacuum left by the diverging air is to bring more air down from above. So divergence at the surface leads to downward motion.
|Fig 1 -- Divergence at the surface leads to downward motion. Simple conservation of mass.|
|Fig 2 -- Horizontal convergence at the surface leads to upward motion.|
Most of our primary jet streams (and, consequently, most of our convergence and divergence aloft) occur at or just below the tropopause level (for reasons that would make another good blog post sometime later). The tropopause is the boundary between the lowest level of the atmosphere, the troposphere, and the next highest layer, the stratosphere. We see this level all the time on our vertical soundings as a kink in the temperature profile.
|Fig 3 -- Sample sounding from KILX at 00Z, Sunday, Feb. 27, 2011.|
Why does the temperature stop cooling off at that level? Above the tropopause lies the stratosphere and within the stratosphere is the ozone layer. Most of us learn in elementary or high school that the ozone layer helps block the harmful UV radiation from the sun (hence why the ozone hole is/was such a big issue). Ozone blocks that UV radiation by absorbing it. As the layer absorbs radiation, it gets warmer. So all that ozone up in the stratosphere is why temperatures don't cool off with height up there. Kind of amazing that we can see that so clearly on our temperature profiles.
Also, notice on the above sounding that I've circled the maximum wind speed in the wind barbs on the right. Note that the maximum winds occur right around the tropopause level or just below it. So, as I said, when we talk about the jet stream aloft, we're usually talking about right up under that tropopause level.
Now this abrupt change in the lapse rate at the tropopause signifies another property of the tropopause and the stratosphere above it. Because temperatures tend to stay the same or even warm a bit with height, this layer is what we call statically stable. Static stability is directly linked to vertical motion. Vertical motion tends to be dampened or inhibited in areas of high static stability. In areas of low static stability (or, what we usually would call relatively unstable areas), it's a lot easier for there to be vertical motions. Because the air tends to warm a bit with height above the tropopause, it tends to be far more stable than air throughout the troposphere below where the air tends to cool off with height. I could spend another entire blog post explaining why this is, so I'm just going to leave it at that for now. The key link is--stratosphere=>stable=>limited vertical motion. And also troposphere=>less stable=>more possibility of vertical motion.
One proxy that meteorologists use to measure stability is a quantity called potential temperature (another good topic for another blog post). It turns out as air moves around in the atmosphere, it likes to keep the same potential temperature. Therefore, if the potential temperature gradient with height is very strong (if the contours are very close together), as air moves around it is forced to stay very close to the same potential temperature value it started with. If the gradient of potential temperature is weaker (if the contours are further apart), the air has a bit more freedom of how it can move around.
So hopefully by now I've managed to convince you that at and above the tropopause the air is very stable. Because the air is so stable, vertical motions are inhibited. But you've probably actually seen this particular phenomenon yourself. Ever look at a big thunderstorm with its anvil cloud shape? Why is it shaped like an anvil? Because as all that air rushes upward in the thunderstorm, eventually it hits the tropopause. When it hits the tropopause, the strong stability reduces the vertical motion. There's still all that air lifting in the updraft below, though. So what does the air do? It spreads out.
|Fig 6 -- Schematic of a thunderstorm anvil cloud overlayed on a photo taken by the author in Anchorage, Alaska, during the summer of 2010.|
But this gets us back to our fundamental question of what happens with divergence and convergence in the jets aloft. Take divergence for example. The original question posed was why divergence aloft didn't mean downward motion from above to fill the void being left by the diverging air.
|Fig 7 -- Why can't we see downwward motion and upward motion if we have divergence in the jet stream winds aloft?|
|Fig 8 -- Vertical motions of any kind are dampened or inhibited by the stable air above the tropopause. The vertical motion of air in the troposphere below is much greater.|
|Fig 9 -- Final model showing why we expect upward motion from below as the main compensator for divergence aloft.|
|Fig 10 -- Final model for what happens in response to convergence aloft. Downward motion below is the favored response.|
Thanks to my blog readers who asked me questions about this topic! I'm very grateful for your interest and I have a lot of fun trying to compose these responses. So, keep your questions coming!