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.