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The seasons

October 7, 2011 Leave a comment

…or, how not to analyze temperature and cloud cover.

A recent post on Watts Up With That by Erl Happ, titled “High clouds and surface temperature“, attempts to use the NCEP reanalysis data set to examine such a relationship.

First, as Erl is (presumably) not a scientist or researcher, I cannot fault him for sourcing his cloud data from a reanalysis product. Reanalysis products are quite appealing because they offer all sorts of variables – temperature, wind, moisture, and “value added” items like cloud fraction – in a gridded data set. The problem is that reanalysis data are not “real” data.

In general, most reanalysis products are assimilations of data from many sources – weather balloons, satellites for periods post-1979, surface stations, etc. – into the first time-step of a sophisticated, high-resolution weather system model.

Don’t get me wrong. Reanalysis products are extremely useful if they are used correctly. I have used them in a paper I co-authored and they are a major part of my current research. For “basic” variables, like temperature and the horizontal wind field, they are fairly good representations of the “real world”. The problem comes when you try to use these value-added things like cloud cover. I will not go into great detail, but suffice to say, you are better off using actual cloud observations from, for example, MODIS.

Okay, moving on to the real heart of the matter, Erl’s basic misunderstanding of the earth’s seasonality cripples his ability to perform any valuable analysis. I will first quote the relevant text:

“The minimum [in surface temperature] is experienced when the Earth is closest to the sun. The Earth is coolest at this time because the atmosphere is cloudier in January. January is characterized by a relative abundance of high ice cloud in the southern hemisphere. Relative humidity peaks in April (figure 6) when tropical waters are warmest. I suggest the variation in the minimum global temperature is due to change in high altitude cloud…”

A schematic of earth's orbit. Perihelion, the time when earth is closest to the sun, occurs in January - NH winter and SH summer.

Earth is closest to the sun in January and farthest in July – absolutely true. Now, the axial tilt of the earth determines the season each hemisphere is in at a particular time of year by directly altering the distribution of solar radiation. In northern hemisphere winter, say January, the northern hemisphere is tilted away from the sun while the southern hemisphere is tilted toward the sun, relative to earth’s orbital plane. This delivers more total radiation (and more radiation per unit area) to the southern hemisphere, and less to the northern hemisphere. However, the earth is closer to the sun and hence receiving more total solar radiation than it does in, say, northern hemisphere summer (July).

What we would expect then, all things being equal, is that the globally-averaged surface temperature would be highest in January when the earth is closest to the sun and receiving the most solar radiation.

Erl just asserts the temperature actually peaks in July without providing an illustrative figure. I hate assertions, so let’s look at some data. I’ve used the 2-meter temperature field from the Modern-Era Retrospective Reanalysis from NASA to generate the following plot of the global average surface temperature. As with all reanalysis data from US government agencies, it is free for anyone to download – atmospheric science enjoys a high degree of data sharing that many fields do not, and hey, your tax dollars paid for this so you should be able to use it! The time period is 1979-2010 and I’ve used area-weighting by the cosine of latitude.

Global average 2-meter temperature. MERRA data is courtesy of NASA's GMAO.

Whoah, it’s higher in July, when the earth is farthest from the sun! Erl was right, what gives?

Well, there is a crucial difference between the hemispheres – there is very little land in the southern hemisphere, and quite a lot of land in the northern hemisphere, especially in the midlatitudes. I’m going to claim that the northern hemisphere land masses are dominating this signal – despite the fact that the earth is 70% ocean! Why? Because they have such a high-amplitude seasonal cycle of temperature. This is due to the fact that the land surface has a much smaller heat  capacity per unit area than the oceans, so for a given amount of radiation absorbed, the land surface will increase its temperature more than the ocean.

Let’s apply a land/ocean mask to the same 2-meter temperature data to try to isolate the two surfaces. To make the comparison, I’ll compare their anomalies relative to their annual average.

2-meter temperature annual cycle for land and ocean. MERRA data courtesy of NASA's GMAO.

Bingo. As you can see, the amplitude of the seasonal cycle is dominated by the land surfaces. Indeed, the average temperature over the oceans barely changes at all, peaking in the late southern hemisphere summer/early fall as we would expect due to the thermal inertia of the oceans. This part of the signal is completely lost when you examine the global average temperature without separating land and ocean data – the global annual cycle has an amplitude of around 4C, while the ocean surfaces have an amplitude of less than half of a degree. Compare that to a nearly 12C amplitude for the land surfaces!

Erl could have saved himself a lot of work and the rest of us a lot of trouble if he would have just used a land/ocean mask, as suggested in the comments at WUWT, to prove to himself that the “continentality” he so derides is in fact the case. Continents heat up a lot in summer and cool down a lot in winter because of their low heat capacity, relative to the oceans. Because there is much more land in the northern hemisphere, the seasonal cycle of global average temperature is dominated by the northern hemisphere seasonality over land. That is, global average temperature tends to peak in July and reach a minimum in January because the northern hemisphere land surface reaches its maximum and minimum temperatures in those same months. This is not a surprise to anyone who has taken a basic course on earth’s climate.

Yes, it’s counter-intuitive that the earth’s global average temperature is highest when it is farthest from the sun, but the explanation is very simple.

“Variation in cloud cover should be the first hypothesis to explore when the Earth warms or cools over time. You would have to be very naive to think that the inter-annual change in temperature that is most obvious between November and March could be due to something other than a change in cloud cover.”

Naive, indeed. This is a striking example of the Dunning-Kruger effect.

Categories: Uncategorized

Cloud chamber demonstration

September 27, 2011 Leave a comment

This is just too cool to pass up.

Despite it being common knowledge in the scientific community, many people don’t know that there is radioactive decay going on all around them – and it’s perfectly safe. Background radiation comes from many natural sources, including the radioactive potassium-40 in the bananas you eat.

Radioactivity has three common “flavors”, though there are a number of other decay processes. Alpha decay occurs when an atom expels a helium 2+ atom and decays to a lighter element, beta decay is the release of an electron, and gamma radiation is the release of a highly energetic photon. Alpha particles can be blocked by a piece of paper, while gamma radiation is only attenuated by a slab of lead. Luckily, background radiation tends to be mostly of the alpha variety.

In the video below, a cloud chamber is used to detect background radiation, as well as to illustrate the radioactivity of Americium and radon gas.

A cloud chamber is a sealed container which usually contains supersatured alcohol. Supersaturation means that the relative humidity of the vapor is greater than 100%. While this seems impossible, it’s in fact a common occurrence in clouds (you can get supersaturations in excess of a few percent in the vigorous updrafts of a thunderstorm). Water cannot spontaneously condense without the aid of a condensation nucleus – a particle like sodium chloride, for example – due to the energy requirements necessary to overcome surface tension, among other things. If you supersaturate a chamber of water and then inject condensation nuclei, a cloud will form instantly.

A cloud chamber operates on a similar principle. Without condensation nuclei in the isolated chamber, you can reach high supersaturations without producing condensation droplets. As an atom undergoes radioactive decay, the radiation ionizes the supersatured vapor. These ions act as condensation nuclei and essentially trace the path of the emitted particle through the chamber with little cloud streaks.

Very cool.

Categories: Uncategorized