If you think surface and air temperature are basically the same thing, think again. Or read our new paper.

In meteorology, air temperature measurements are typically taken 2m above the surface.  It is a routine measurement at weather stations, and this temperature is the basis for analyzing trends, such as global warming.  The temperature of the surface is not so often measured, but it can be inferred by satellites from how much radiation is being emitted by the surface.  Being only 2m apart, one may think that the temperatures basically reflect more or less the same, given their close proximity.

We actually found out that this is not the case: surface temperature responds much more strongly to a lack of water than air temperature.  This finding was just published in our article in the journal Geophysical Research Letters.

Our starting point was to look at the diurnal variation in temperature in a different way.  Typically, variations are plotted against time.  But we know that the strongest forcing of the diurnal cycle is the Sun.  It defines the night (no sun), and solar noon (strongest sunlight).  So what we did is that we plotted temperature variations against solar radiation, rather than time (see Figure).  What we then found is an interesting hysteresis, showing an almost linear increase in air temperature in the morning to noon, but then the temperature stayed almost the same over the course of the afternoon.  We already found this characteristic pattern in earlier studies (Renner et al 2016, Renner et al 2019) in Luxembourg, but did not quite know what to do with it.


Figure: The air temperature hysteresis measured at the rooftop weather station at our institute on 19 July 2016.

We then looked at the rate by which the temperature increased in the morning in response to an increase in solar radiation.  Mathematically this corresponds to the derivative of temperature to solar radiation, and we called this the warming rate.  We then looked at how this warming rate is affected at different levels of water availability.  For this, we used observations of the turbulent heat fluxes, and calculated the evaporative fraction, which expresses how much of the turbulent heat flux is composed of evapotranspiration.  It is a measure of water availability.  No water, and the evaporative fraction is 0.  A value of 1 is typically not found for the evaporative fraction because there needs to always be some sensible heat flux.  In any case, the more water is available, the higher the evaporative fraction is.

What we then found is that the warming rate for surface temperature decreases quite substantially with water availability.  This is what one would expect.  It is the phenomenon known as evaporative cooling.  So the more water is available, the less the temperature rise during the day.  This is no surprise.  But when we looked at the warming rate of air temperature, we found that this rate is almost independent of water availability.  It showed practically no variation with evaporative fraction.  We did not expect this, and so we learned something quite interesting about land-atmosphere exchange.

The lack of response in the warming rate of air temperature has a relatively simple, physical explanation.  Hysteresis behavior, as seen in the variation in air temperature, results from storage effects, and the hysteresis in air temperature results from the heat that is stored during the day in the lower atmosphere.  The size of this heat storage is, however, not fixed, but it depends on the height to which the boundary layer has grown.  And it is well known that boundary layer growth depends on the sensible heat flux.  So this is the key to understand the lack of response.  When water becomes more limiting, evaporation is reduced and so is the evaporative fraction.  This reduction is associated with an increase in the sensible heat flux.  The greater sensible heat flux then results in greater boundary layer growth, so the size of the heat storage is increased, so the air temperature actually does not increase.  In other words, the lack of response in air temperature to evaporative fraction is due to the compensation of boundary layer growth.

Why this is important to know?  Well, it tells us, for instance, which temperature to look at if we want to infer evaporation from the surface.  We should look at surface temperature, not air temperature.  On the other hand, air temperature includes information on the boundary layer, which I think is pretty cool.  It also relates to our work on the thermodynamics of land-atmosphere exchange, starting with our paper on the contrasting climate sensitivities of ocean and land surfaces (Kleidon and Renner, 2017) , where we already dealt with the boundary layer as being the buffer of the strong diurnal variation of solar radiation and that is clearly different from an open water surface (where the buffering takes place below the surface in the water, see ).  It also fits with the insight gained from our thermodynamic perspective that the magnitude of turbulent heat fluxes is set by how much power can be gained from solar radiation (Kleidon and Renner, 2018).  It fits to what we found here, that the sensible heat flux compensates the reduction in evaporation.

So our next steps from here is to look to see how general this behavior is.  So far, we only looked at one station, and Annu is currently extending this analysis to other stations.  Also, I have been working on a theoretical derivation for the warming rates that looks very promising.  So there is certainly more to come from this line of analysis of land-atmosphere interactions.


Making sense of German wind energy

I thought it might be a good idea to meet with wind energy critics to hear what they have to say. But my experience told me otherwise. Perhaps I should have been warned. They are associated with a German organization called „Vernunftkraft“, which has „sensible“ in its name. How presumptuous! It should have been a red warning flag for me.

We recently published a paper that evaluated wind energy generation over the last 15 years in Germany. It was published by PLOS ONE, an open access journal so that everyone can get access to it (here is the link). I was already anticipating some responses to it before it got published. After all, wind energy is a highly polarizing subject. So we decided to issue a press release, as it helps us to explain what the background of this paper was and what the results mean (the press release is here). After its release, I got contacted by a few media outlets, they published it as news, and then I got some responses, by e-mail, or by phone. It is quite a typical line of events.

It is also quite typical that after we publish something on wind energy I get contacted by wind energy critics. In the past, we have published work on limits to wind power that are much lower than what others have said. There are good, physical reasons for us to say this, and these low limits become important at large scales, although we are currently nowhere close to such limits. But I got plenty of phone calls of people who want to use these findings to discredit wind power in general. Either because there are wind farms planned near their village, or because they find nuclear power cooler, or god knows why.

What I feel quite uncomfortable with is that by making physical arguments of lower wind energy potentials, I am being put into a wind critics box. I am not a critic of wind energy. In fact, I am all for renewable energy, as it is the only way out of the global warming mess that we are currently in. This is a logical response to counter global warming. And not just that, it is the logical next step into a sustainable future if human societies want to sustain their quality of life. The pressing question for me is how we can make the transition to a renewable energy future the most effective, and this involves, to some extent, questioning resource potentials for wind energy. Questioning the need for this transition to renewable energy does not even come close to my mind.

With regards to questioning wind resource potentials, in our most recent paper, for instance, we found out that we can explain the produced wind energy fairly well using wind fields from the German weather service and turbine information. Yet, what was produced was about 30% less than what was expected from the wind fields. Some of this we could attribute to turbine aging or to shading effects within wind farms. But the largest part we could not explain. I think this is quite important to know that actual wind energy generation is at least 20% less than the resource potential.

Another aspect we found is that wind speeds in Germany declined over the time period we evaluated at rates that are similar to those reported in the global stilling literature (e.g., see Wikipedia entry here). I do not know whether this trend continued in Germany in recent years, and the reasons for global stilling are also mostly unknown, at least in my opinion. But it is a trend that certainly does not help wind energy in the future to stay productive, and this is important to know. (Actually, we only noticed this because one of the reviewers specifically asked about such trends.)

Also, we could not find statistically significant indicators for decreased efficiencies of wind energy generation due to large-scale wind speed reductions that we would have expected based on our previous work. But this is mostly due to the fact that too many variables correlated with each other, so we could not identify the installed capacity of wind turbines in a region as a significant factor that led to reduced yields. On a qualitative level, it seemed to us that one may find such negative effects already in the northern and northeastern parts of Germany, but this would need more work.

So there is certainly more questioning, more work, and more research to be done to inform the best way to shift to renewable energy.

When the two wind energy skeptics then came recently to my office, they had something entirely different in mind. First, they pointed out that by being retired physicists from industry, they were very well qualified for the topic. But then, after they sat down on the sofa in my office, they demonstrated their “qualifications” by quickly turning into preaching mode. I felt like I had Jehova‘s witnesses sitting in my office. Instead of sins, they talked about the evils of renewable energy, and instead of paradise, they talked about the glory of brown coal. I just could not believe what I heard, stunned by the lack of science and rational thinking! It was hopeless. It did not take long for me to decide that it was time for them to leave my office.

It left me with a sense of frustration and that a transition to renewable energy does not just involve rational thinking, science and technology. It also involves using science to open up human minds beyond to what they are used to and to be able to envision the future. But on that challenge, it seemed that I have failed miserably.

Starting a blog

Why do I start writing a blog? Well, I see a few reasons:

  1. From time to time I get e-mails with questions about my research or regarding my opinion. Sometimes, these are quite good and valid questions, yet I feel that it would be useful to document and/or share the answers. A blog would be a great place to do so.
  2. Sometimes, I feel like explaining why I think a certain paper from our research group is cool, and explain more broadly what it implies. There is not really space in a scientific, peer-reviewed manuscript to do so. So a blog may do well in such added information.
  3. Sometimes I come across something curious that I like to share. Just a bit of insight, not enough for a paper. But enough for a blog post.
  4. I’ll also use it to post some activities regarding my field of research, like when we organize another workshop or a block course on thermodynamics. A blog may work well for this as well.
  5. There may be other reasons too, but I cannot think of it right now.

So I’ll try this out and see how it goes. My focus here is to be objective and scientific, not opinionated. My approach, just as my research, is from physics and thermodynamics.

Any feedback is very welcome! I may need some time to get back to it though, so I apologize for any delays in advance.