#vEGU21 Next week we’ll present our work on precipitation scaling, diurnal temperature range, offshore wind, and limits to vegetation productivity based on our thermodynamic Earth system view

Thermodynamics rules the world, as well as the science that we present at this year’s EGU General Assembly, which is, alas, virtual rather than in Vienna. It may not be obvious, and our contributions are spread across different sessions. But in the end, we follow the solar energy as it passes through the Earth system, seeking simple, physics-based explanations to simple phenomena: precipitation scaling with temperature found in observations, the diurnal temperature range across regions and vegetation types, also in observations, limits to offshore wind energy in the North sea and what these imply for renewable energy scenarios, and how the really low efficiency of photosynthesis fits to the notion of vegetation being optimal.

Sarosh, our latest addition to the group, presents in the session HS7.9 “The atmospheric water cycle under change: feedbacks, land use, hydrological changes and implications“. His work focuses on estimating the sensitivity of extreme precipitation with temperature from observations in India. Extreme precipitation is expected to increase with temperature at the same rate at which air can hold moisture. This is set by thermodynamics and is known as the saturation vapor pressure curve, or the Clausius Clapeyron (CC) equation, yielding a relative increase by about 7%/K. However, observations in tropical regions typically show precipitation – temperature scaling rates that are largely negative with significant deviations from the CC rate, meaning that precipitation decreases with temperature. Why is this so, and what can we learn from this for the response to global warming? Sarosh shows in his work that this negative scaling arises from the cooling effect of clouds on surface temperature. Scaling rates derived from observations thus not only represent how precipitation changes with temperature, but also how precipitation affects temperatures through changes in the radiative forcing. Sarosh uses an energy balance model constrained by thermodynamic limit of maximum power to separate the effect of clouds on surface temperature. He then shows that extreme precipitation actually shows a positive scaling with temperature consistent with the CC rate once the effect of clouds is accounted for. He presents on 28 April 2021 at 09:29 CEST and is available for discussion 09:35-10:30 CEST.

Annu presents an update on her work on the diurnal temperature range, expanding the scope from her previous papers (Panwar et al. 2019, Panwar et al. 2020) to the continental scale. One might assume that because surface and air temperatures are so close together (being only 2m apart from each other!) that they carry the same information on evaporation. In her presentation, however (in session CL4.17 “Land-Atmosphere Interactions and Climate Extremes”), she shows that the diurnal variations of surface and air temperatures respond rather differently to changes in evaporative conditions. Using FLUXNET observations and the ERA5 reanalysis she found that the diurnal surface temperature range in short vegetation reduces strongly (by up to 20 K) in response to changes in the evaporative fraction but weakly (by up to 10 K) in forests. The diurnal air temperature range, however, responds similarly across both vegetation types. Using a simple atmospheric boundary layer model, she then shows that the weaker response in air temperature is due to differences in boundary layer growth, which is similar across vegetation types. Using a surface energy balance model, she shows that the diurnal temperature range of surface temperature depends primarily on evaporative fraction, aerodynamic conductance, and solar radiation. This work conveys that although surface and air temperature are measured just 2 m apart, their diurnal variations are governed by rather different physical constraints. Her analysis also revealed systematic biases in ERA5 temperature products linked to evaporative conditions. She presents on 27 April 2021 at 09:23 CEST in session CL4.17 and is available for discussion from 09:38-10:30 CEST.

Jonathan will present a snapshot of his ongoing analysis of the results from a recent revaluation of offshore wind energy scenarios for the German Bight. Numerical weather forecasting simulations and estimates from the KEBA model, which represents a simple formulation of the kinetic energy budget of the lower atmosphere, showed that projected offshore wind turbine deployments in 2050 may only yield ~3300 to 3000 full load hours per year rather than > 4000 h/a that are currently expected in many future energy scenarios. Jonathan looks at which atmospheric variables primarily shape this potential reduction in yield. He is investigating the role of variables like wind directions, atmospheric stability, boundary layer heights and surface friction in shaping the yield from very large wind parks. His analysis will also test the validity of assumptions inherent within the KEBA approach and therefore the limits of its applicability. His work will lead to an improved understanding of how large wind parks interact with the atmosphere and how these interactions affect the yield from the same wind parks. This understanding is critical for developing KEBA into an easy to use tool for policy analysis. You can hear and watch his updates in session ERE 2.1 “Energy Meteorology”. His presentation will take place on 30 April 2021 at 09:16 CEST followed by a breakout text chat from 09:42 – 10:30 hrs CEST.

Axel presents in the session HS10.3 “General organizing principles and optimality in ecohydrological systems”, aiming to link thermodynamic limits and vegetation optimality. His contribution is motivated by the apparent contradiction between the well-known, very low energy use efficiency of vegetation, utilizing merely 1-2% of the energy contained in sunlight, and the notion of optimality on the other hand, stating that vegetation makes best use of its environment. If vegetation is optimally adapted, why did it not figure out how to better use sunlight? Axel addresses this discrepancy by arguing that the primary limitation of vegetation productivity is its gas exchange with the atmosphere, rather than light availability. This gas exchange is driven by the heating of solar radiation but is limited by thermodynamics. The evaporation rates estimated from this approach, which also result from gas exchange at the surface, match observations very well, indicating that this gas exchange is the bottleneck and can explain the low conversion efficiency. What this interpreation implies is that vegetation may have far less control over the evaporation process than what some model predictions have suggested, and that the response to global change is dominated by physical constraints, rather than vegetation characteristics. And that we may need to look at vegetation optimality differently. His vPICO session is scheduled for 29 April 2021 at 14:15 CEST, with the breakout chat scheduled for 14:35-15:00 CEST. His presentation is based on a recent publication (ArXiv version) that is summarized in a recent blog post as well.

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