Mauna Loa Daily Meteorology

Guest Post by Willis Eschenbach

As a confirmed data junkie, I’m fond of hourly data. The interesting processes in the climate system unfold on the scale of minutes and hours, not years. So I picked up a project I’d started a while ago, but as is too often the case I’d gotten sidetractored by … oooh, shiny … and I’d forgotten about it until I stumbled across my code again.

This project was looking at the hourly averages of various meteorological variables measured at the observatory on Mauna Loa, Hawaii. This is the same place that the CO2 data has been measured since 1959. The data is available here.

To start with, here is the daily temperature at three different altitudes—2 metres, 10 metres, and 35 metres.

Figure 1. Daily cycle of average temperatures at three different altitudes above the ground.

There were some interesting parts of this to me. One is that the surface temperature peaks at about 1 PM … but as you go up in altitude, the peak occurs earlier. Hmmm …

Also, I was surprised that ten metres up in the air the daily variation is less than half of that down at two metres.

Because the atmosphere is heated from the bottom, it is unstable during the day, and overturns. During the night, on the other hand, the atmosphere is coolest at the bottom, so it stabilizes and stratifies. You can see the timing of the onset and the end of the daytime period of turbulence, which starts just before nine am, and lasts until just after dark.

Next, here is the average precipitation rate hour by hour:

Figure 2. Daily cycle of average precipitation rates, millimetres per hour.

Here, we see the typical sequence of weather around a tropical island. The big peak in thunderstorms occurs in the afternoon around three or four o’clock. You also get a much smaller number of early morning thunderstorms.

Next, I looked at the winds:

Figure 3. Daily cycle of average wind speed, metres per second.

This shows something interesting. The “terminator”, in addition to being a series of increasingly bad movies, is the name for the line between light and dark on the surface of the planet. On one side of the terminator, the light heats the air near the surface. This makes the air rise on the lighted side of the terminator. The existence of warm lighter air on the lighted side, plus cool heavier air on the dark side, leads to the “terminator wind”. This is a wind created by the temperature difference across the terminator.

This plot shows the difference between the dawn terminator wind and the dusk terminator wind. The terminator wind always blows from dark to light, which means it always blows toward the sun. Now, the trade winds in the tropics always come from the east and blow towards the west. So at dawn, the terminator wind opposes the trade winds, because it is blowing out of the darkness in the west towards the sun rising in the east. This leads to the drop in wind speed after dawn that you can see in Figure 3.

But at dusk, the terminator wind blows in the same direction as the trade winds, and this increases the average wind speed after the end of the day. Can’t say I understand the rest of the variation, though. I do note that the wind picking up and dying down occurs at the same time as the onset and dying out of the daytime overturning.

(Curiously, I found out about terminator winds by spending lots of time at sea. The sweetest terminator wind is on a dead calm night, not a breath of air … and then the moon rises, and if you are lucky, you can feel the moon wind sweep across the ocean, always blowing towards the moon … but I digress.)

I next looked at the absolute humidity. This one was a surprise.

Figure 4. Daily cycle of absolute humidity, in grams per cubic metre.

The reason that this was a surprise to me was that I had not expected it to vary that much. From a low of two grams per cubic metre at dawn, it more than doubles when it rises to a peak of five grams per cubic metre at three pm. Why is this important?

Water is the dominant greenhouse gas. Because it is an “L-shaped” molecule, water vapor has many ways to absorb radiation. The molecule can flex and twist and stretch in various combinations, so it absorbs thermal radiation (longwave infrared) of a wide variety of frequencies. The important point is this:

The change in the amount of longwave infrared absorbed by atmospheric water vapor is approximately proportional to the log of the change in the amount of water vapor. 

And the amount of water vapor in the air varies during the day by a factor of about two and a half to one … I’d never realized how much greater the afternoon longwave absorption is compared to the absorption at dawn. Who knew? Well, I’m sure some folks knew, but I didn’t.

So I fell to considering the effect of this daily variation. The increase in atmospheric absorption will warm the afternoons, and decreased absorption will cool the early mornings as compared to the average. Now, one corollary of Murphy’s Law can be stated as:

Nature always sides with the hidden flaw.

In terms of the climate system, the poorly-named “greenhouse effect” works to increase the surface temperature. Murphy’s Law means that all related emergent, parasitic, and other losses in response to that surface warming will tend to oppose this effect. In other words, we expect the natural response to elevated surface temperature to be one of cooling of the surface.

For one example among many, when the desert surface gets hot, “dust devils” emerge out of nowhere to cool the surface by means of increased evaporation and convection. They pipe the warm surface air aloft, increasing surface heat loss. But there are no “anti-dust-devils” that act to decrease surface heat loss … Murphy’s Law in action.

Now, the radiative loss varies as the fourth power of the temperature. This means that if the temperature varies around some average value, the radiative losses will be larger than if the temperature were steady. As a result, since the variation in absolute humidity warms the afternoons and cools the early mornings, to that extent it will increase the overall surface radiative losses … Murphy at work again.

Anyhow, we’ve now finally gotten to my reason for writing this post. The figure below shows one more meteorological variable measured at Mauna Loa—the daily cycle in air pressures.

Figure 5. Daily cycle of atmospheric pressure, hectopascals. Note that because of the high altitude of the observatory, the pressure is much lower than the ~1000 hPa pressure at sea level.

I was, and I remain, puzzled by this variation. Why should the pressure peak at both eleven o’clock in the morning and eleven at night, and be at its lowest just before both sunrise and sunset? And why would the two peaks and the two valleys be about the same amplitude? That question is why I’m publishing this post.

All contributions gratefully accepted …

w.

Further Info: The procedure used for the Mauna Loa CO2 measurements is here. For those who think Mauna Loa is a bad choice for CO2 measurements because it is an active volcano, give it a read.

My Usual Request: If you disagree with me or anyone, please quote the exact words you disagree with. I can defend my own words. I cannot defend someone’s interpretation of my words.

My Other Request: If you think that e.g. I’m using the wrong method on the wrong dataset, please educate me and others by demonstrating the proper use of the right method on the right dataset. Simply claiming I’m wrong doesn’t advance the discussion.