So what lies at the heart of this mechanism? The fact that the terrain of the Great Plains has a long, gradual slope.
Let's go back to the situation I was describing in my last post during the daytime. During the day, the sun heats the earth's surface, which in turn warms the air above it. It stands to reason, then, that (absent other effects) the closer air is to the surface, the warmer the air will be (with respect to air at the same horizontal level elsewhere).
Let's think about how this works with respect to a gradually sloping terrain like over the Great Plains. Say that you start sitting up in the air at a mile above sea-level over Saint Louis. Then, start heading west along a horizontal line, staying at a mile above sea-level. As you head west, the terrain is slowly going to rise beneath you, and you'll end up closer and closer to the ground. You'll finally run into the ground around Denver.
But, remember what I just said about the temperature. So along that same horizontal line, the closer you got to the surface, the warmer the air should have become. Therefore, the further west along that horizontal level, the closer that level is to the surface and the warmer the temperature. Therefore, there exists a temperature gradient along that level, with relatively warmer air in the west and cooler air in the east. This is simply a function of the sloping terrain.
I'm going to redraw the above graphic, showing shadings to separate relatively warm air from relatively cool air--remember these are along horizontal lines, though. Furthermore, there's a limit to this effect in the vertical. This only makes sense in regions of the atmosphere that are directly effected by air rising from the surface. This layer of air where warm air bubbles up from the surface and cold air sinks to compensate is called the mixed layer. I draw a dotted line at the top of this layer in the drawing below.
This is all well and good during the day, but what happens when the sun goes down? As soon as the radiation away from the surface becomes greater than the incoming absorbed radiation from the sun, the surface will begin to cool. This in turn cools the air right above the surface. Just like we saw in my last post, this surface cool layer expands in depth as slightly warmer air from above entrains in and heat continues to be radiated out by the surface.
But now, let's remember the effect of the slope of the terrain again. Now the opposite is happening--the closer you are to the surface, the colder the air should be. If you draw a horizontal line through this new near-surface layer, then the air to the west along that level (being closer to the ground) is cooler than the air to the east. So, we now have the opposite temperature gradient below the level of the nocturnal inversion.
Let's go back to these thermal wind arguments. Remember above, with the cold air to the east, the thermal wind (that is, the way (geostrophic) winds change with height) points from north to south. Below the level of the nocturnal inversion, the temperature gradient is reversed. Now, with colder air to the west, the thermal wind vector points in the opposite direction--from south to north.
Let's assume, like in my last blog post, that the ambient (geostrophic) wind direction happens to be out of the south. So, the wind is blowing from south to north. What happens when we apply the thermal wind vectors to this background wind?
You can see in the diagram above that in combining these two, we have a wind that increases with height below the nocturnal inversion, then decreases with height above the inversion. The result? A maximum in the wind speed right at the level of the nocturnal inversion. Just what we were expecting to see.
You can see now that the maximum of this low-level nocturnal wind phenomenon is indeed slightly above the surface--in fact, more specifically, at the level of the nocturnal inversion. Furthermore, you can see the strong wind shear implied in the near-surface layer--an essential ingredient for tornadic storm development. It's no wonder that this nocturnal low-level wind maximum is a critical phenomenon to worry about when looking for severe weather evolution late in the day.
So, after the last post and this one, we've seen a lot about the reasoning behind that statement about the "low-level winds really kick up around sunset". We naturally expect this because of the "shutting off" of surface friction and mixing with the establishment of a cool, stable surface layer right around sunset. This frees the wind from the forces holding it back, causing a temporary imbalance in forces that accelerates the wind just above the ground.
However, in the central US, the sloping terrain further enhances this phenomenon. The slope of the terrain allows horizontal temperature gradients to set up during the day (and reverse near the surface at night) which alters the structure of the wind in the vertical. This structure works to focus the wind maximum right at the level of the nocturnal inversion.
So that was a somewhat technical description of my understanding of this "nocturnal boundary-layer wind maximum" phenomenon. Hopefully it was helpful, or at least gave you something to think about. At least now whenever you hear someone on the Weather Channel or the news talk about the "low-level winds picking up at nightfall," you might have a better idea of just what's going on.