Which factors make forests cooler: Evaporation or their high aerodynamic conductance? Our paper just published in HESS suggests that it is the latter.

Trees and plants moderate the Earth’s surface temperature. Generally, the cooling effect of vegetation is mainly attributed to the process of evapotranspiration. In our paper just published in HESS, we used observations to unravel the importance of evaporative cooling for short vegetation and forest in shaping diurnal variations in temperatures and found that, actually, it is not only evaporation that keeps the forests cool.

Temperature variations during the day are mainly shaped by the diurnal rhythm of the Sun. To what extent solar radiation heats the surface depends on the energy partitioning at the surface. For instance, a wet surface consumes energy via evaporation and leaves less energy for warming or, in other words, it results in evaporative cooling. In our previous work (Panwar et al., 2019) we showed that for a grassland in the central US, this effect of evaporative cooling is evident only in the diurnal variation of surface temperature, but not in air temperature (~ 2m above the canopy, see also this blogpost). But do we also find this response in forests?

Figure 1: Short vegetation and forests behave quite differently regarding their diurnal temperature variation (orange: surface temperature; blue: 2m air temperature).  Why is this so?

A quick look at the diurnal course of temperatures at a short vegetation site and a forested site suggests that these behave quite differently (see Figure 1). We characterize this difference using the warming rate in the morning, which is the increase in temperature in response to an increase in solar radiation. We then looked at how this warming rate responds to different evaporative conditions, which we characterized by the so-called evaporative fraction (the ratio of the latent heat flux to the total turbulent heat flux). Our study used observations from 51 FLUXNET sites covering short vegetation, savanna, and forest (Panwar et al., 2020).

What we found is quite surprising: the warming rate of air temperature is similar in magnitude for all vegetation types and has negligible response to the evaporative fraction (Figure 2, right). What this means is that the diurnal variation in air temperature is unaffected no matter whether the surface is wet or dry, or whether it is covered by grasses or a forest.  Contrarily, the warming rate of surface temperature for short vegetation is lower when the surface is wet, and higher when it is dry. This results in a broader distribution of the warming rate (red in Figure 2, left).  In forests, the warming rate of surface temperature is similar to air temperature and carries no signal of the evaporative fraction. So for forests, even the diurnal temperature variation at the canopy surface does not show signs of evaporation.

Figure 2: The diurnal variation of (a) surface temperature is greater for short vegetation (red) than for forest (blue) but (b) their diurnal variation of air temperature is similar. In our paper, we provide an explanation for this response to evaporation.

Why doesn’t the warming rate respond to evaporation in forests? It is not just evaporation that matters to temperature, but also how easily heat and mass is transported from the surface to the atmosphere, a property referred to as aerodynamic conductance. Forests have a much higher aerodynamic conductance, facilitating a stronger transfer of heat to the atmosphere and which lowers the canopy’s temperature. We hypothesize that the cooling effect of the high aerodynamic conductance overshadows the evaporative cooling effect and demonstrated this using a formulation of the surface energy balance. The higher aerodynamic conductance of the forest alone explained 56% of the reduced warming rate of surface temperatures compared to that of short vegetation.

Diurnal temperature variations are a basic pattern of our environment, and it plays a central role in understanding global warming, extreme events, ecological functioning, and human health. Through this study, we show how evaporation and vegetation shape this variation. As a next step, I am currently using ERA5 reanalysis data to explore how well this approach works to understand global patterns of diurnal temperature variations. We hope that this will broaden our knowledge of the different factors shaping diurnal temperature variations, which will hopefully improve our interpretation of the spatial pattern of global warming.

References:

Annu Panwar, Axel Kleidon, and Maik Renner (2019) Do Surface and Air Temperatures Contain Similar Imprints of Evaporative Conditions?, Geophys. Res. Lett., 46, 3802–3809, https://doi.org/10.1029/2019GL082248. 

Annu Panwar, Maik Renner, and Axel Kleidon (2020) Imprints of evaporative conditions and vegetation type in diurnal temperature variations. Hydrol. Earth Syst. Sci., 24, 4923–4942, https://doi.org/10.5194/hess-24-4923-2020.

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