Vertical atmospheric mixing (H. Michael Mogil, CCM, CBM, DMS)
Most of us have seen what happens when a pot of water, sitting on a heated stove top, starts to boil. Bubbles form on the bottom of the water, and when they get big enough, they start to rise. But, if all the bubbles were to rise without any water replacement, there would be a hole in the bottom of the water. Hence, water from the top must come down to take its place. This is called convection (upward and downward movement of fluids) and involves mixing the water (warmer water rises and cooler water sinks to take its place). When the water boils vigorously, it is easy to see the mixing.
This process also happens in many atmospheric and oceanic settings, although not as quickly and not as visible. For the purposes of this article, I’ll concentrate on daytime heating of the ground (which is akin to heating the bottom of the pot). Hence, as sunlight heats the ground, rising “bubbles” of air (referred to as air parcels, in weather circles) develop. As this happens, air from above must come down to take the place of rising air parcels.
In the pot, horizontal motions were not present. In the atmosphere there are horizontal air movements (i.e., winds). And, typically, what happens at night is that the winds several thousand feet above ground are decoupled from winds nearer the ground. As a result, winds above ground level may be blowing quite strongly, while near the ground, winds are much lighter to calm.
As daytime heating, convection and mixing develop, higher wind speed air from above the ground is brought down to the ground. This mixing process increases winds near the ground and reduces winds above the ground. It also acts to equalize air temperatures in the mixed layer.
Fig. 1 (a meteogram for Portland, ME – PWM) is a prime example of the effects of this process. In the wake of Matthew moving away from the U.S. mainland and the passage of a cold front, winds at Portland, ME were relatively light before sunrise on Oct. 10, 2016. Once sunlight kicked in, winds and wind gusts picked up dramatically. As sunset occurred, winds dropped off, just as quickly and dramatically.
Not shown are two soundings (atmospheric balloon probes) from Portland, ME from Oct. 10, 2016. In the morning sounding, winds a few thousand feet above the ground were blowing at about 35 miles per hour, while winds near the ground were blowing between 10 and 15 miles per hour. At around six thousand feet above ground, winds were blowing at 55 miles per hour!
By early evening, winds had mixed throughout the lowest 10 thousand feet. Except for near the ground (which had already decoupled from the winds higher up in the atmosphere), winds were blowing a more or less uniform 30 miles an hour.
This effect happens on many days across the Nation (and around the world), especially during the chillier months of the year. Sometimes, however, with moving weather systems and other factors, it’s just harder to see what is happening.
The next time you experience a windy day and a calm morning and/or evening, think about vertical mixing. You can see what a nearby sounding looks like by visiting the Plymouth State University web site or another reliable weather web site that posts real-time sounding graphics.
© 2016 H. Michael Mogil
Originally posted 10/12/16