Wednesday, August 31, 2011

Understanding diurnal cycles

One of the simplest and most predictable patterns in meteorology is what is called the "diurnal cycle", meaning the cycle of what happens every day.  Our weather is governed by the sun, so naturally the sun's rising and setting creates a repeated cycle of events every day.  Here's a 24-hour meteogram for several important weather variables at my weather station from several days ago:
Nighttime hours (between sunset and sunrise) are shaded in light gray.  This meteogram shows a time when we had quiet synoptic-scale conditions here in Seattle--no real fronts or troughs or anything like that coming through.  Just quiet weather.  Let's look at each of these variables individually.

Temperature -- Temperature is shown in the red curve of the top panel.  You can see that the high temperature was reached at about 4:30 PM, after which the temperature began falling pretty steadily.  This steady fall continues overnight until we hit our low temperature of 51.3 degrees Fahrenheit.  Notice when this temperature occurs--right about at the time of sunrise.  It's a common misconception (or, at least I've heard it from many people) that the middle of the night is the coldest time of the night.  Not so!  If you think about the sun as our source of heat, as long as there is no sun, we'll continue to cool.  So temperatures won't warm up again until after the sun has started to come up.  In the meantime, the earth continues to radiate away energy, continuing to cool us all throughout the night.  In fact, even after the sun comes up, it still takes an hour or so for us to be receiving enough energy from the sun to outweigh the energy the earth is radiating away.  As you can see, the temperature doesn't really start increasing again until about an hour or so after sunrise.

Dewpoint--Dewpoint temperature (in green in the top panel of the chart) is one of the few variables that really doesn't have much of a diurnal cycle.  If I had to give it a diurnal effect, I'd suspect that during the day the dewpoint might increase a little because more heat means more water evaporates, increasing the moisture content of the air.  Overnight, though, unless we cool down to the dewpoint and start condensing water out of the air, there's really nothing for the dewpoint to do.  So, overall, we do see a general, gradual increase in the dewpoint throughout the period.  However, I don't see much of a diurnal cycle here.

Wind--The wind speed is shown by the shaded light blue area in the second panel down.  The wind direction is given by the little triangles (with the legend on the right side).  Now, my weather station is very poorly sited to get accurate wind measurements (more on that in another blog post).  So I don't trust these wind measurements much, particularly the wind direction.  But in looking at wind speed, we do see part of a typical diurnal cycle.  During the daylight hours, we generally do see a light breeze--in this case, between 1-2 mph.  However, notice that right after the sun sets, the wind speed drops to zero and stays there all night.  This is a very typical diurnal cycle for the wind.  In a previous blog post, I described how a low-level inversion develops overnight as the surface cools.  This stabilizes the near-surface air and inhibits vertical mixing.  While above this inversion the winds tend to increase in speed as night falls (see my previous post on the nocturnal boundary layer wind maximum), below the inversion the winds drop to near calm.  We see this above.

Another, more subtle way of seeing this nocturnal inversion developing is in the temperature curve.  Look closely at the difference between how the temperature curve looks during the day and during the night.  See how during the day the temperature curve is rather "squiggly" with little bumps going up and down?  In contrast, during the evening and overnight, the temperature curve is very, very smooth without those bumps or wiggles.  The bumps and wiggles of the temperature curve during the day show vertical mixing with the air above. Typically, during the day, the temperature cools off with height as you move away from the surface.  This makes the atmosphere more unstable.  The more unstable the air becomes, the easier it is for air to move up and down.  Little pockets of cooler air from slightly above the surface are sometimes brought down to the surface in gusts of wind.  This causes the temperature to drop a little bit before later recovering.  So those bumps and wiggles mean vertical motion is going on.

However, at night, we cool off at the surface faster than the air cools off above.  This creates a temperature profile that warms with height--a very stable profile (the nocturnal inversion).  As the air becomes more stable, it becomes much harder for there to be any vertical motion.  As such, the occasional intrusions of air from above with a different temperature basically stop.  Without these intrusions, there are no wiggles in the temperature trace--the temperature can cool evenly.  It's pretty neat that you can see this difference.

Pressure--This is another little-known diurnal cycle.  It turns out that our air pressure, which is usually considered to be a rather robust variable, also has a pronounced diurnal cycle.  It's sometimes hard to see this cycle during periods of very active weather synoptically--when a strong surface low is nearby, the falling pressure signal from the approaching surface low overwhelms this diurnal cycle.  But, it's easy to see when the weather is quiet.  It's also quite easy to see in the tropics.

So what makes the pressure diurnal cycle?  Notice that we have a minimum in pressure around 5:00 PM--slightly after we reached our high temperature during the day.  Why is this?  As air warms during the day, it expands and its density decreases.  As the air molecules get further apart and the density decreases, the pressure also tends to decrease.  So, the heating during the day causes the air to expand and the pressure to tend to fall.

During the night, the opposite happens.  As the near-surface air cools, it compresses and becomes more dense.  As you might expect, this tends to increase the pressure.  We see that in the diagram above--starting at sunset and continuing through the night, the pressure rises rather significantly.  It actually continues rising into the next day after the sun comes up.  Only after the air has started really heating up does it start falling again (at around 9:00 AM in this diagram).  So the pressure diurnal cycle kind of mirrors the temperature diurnal cycle, only lagged by a little bit.

Now when you're looking at weather patterns during the day, things should make a little more sense.  We can see how the winds calm and the pressure starts rising during the evening.  We can see how the winds start picking up and the pressure starts falling by late morning.  We can also see evidence of an increasingly unstable or stable layer near the surface by what the winds are doing or how the temperature profile looks.  Pretty good for a quiet weather day.

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