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Droughts and heat waves deeply interact with the exchange of energy and matter between land surface and atmosphere. The conditions associated with a combined heat wave and drought can have positive or negative effects on the sensible heat flux (H), latent heat flux (LE), and net flux of CO2 (NEE). In return, while each of these fluxes can exert different local feedbacks on temperature, atmospheric humidity and soil moisture, all of them result in bulk heating of the troposphere (H and LE through direct local and indirect non-local heat transfer, and NEE through radiative forcing). H is positively affected by increased solar irradiation, but can be supressed by advection of warm air diminishing the gradient between surface and air temperature. Potential LE is positively affected by irradiation, temperature and water vapour pressure deficit, but actual LE can be suppressed by stomatal closure of plants and by reduced soil moisture from low rainfall and high past LE. NEE is the result of plant photosynthesis (GPP) and plant and soil respiration (R), both of which can be enhanced by high temperatures or irradiation, and suppressed by drought. Focusing on direct measurements mostly by a network of Eddy-Covariance (EC) stations of the ICOS (www.icosri.eu), TERENO (www.tereno.net) and other networks, we hypothesize that the net effect of the 2018 event at a site, and thus its feedback on global warming, depends on the balance between co-existing positive and negative effects of the combined heat wave and drought on the respective fluxes. Variables such as albedo, growing degreedays (GDD), soil moisture and ecosystem-level water use efficiency help to separate these co-existing positive and negative effects from each other. Preliminary results indicate different degrees of heterogeneity between sites fordifferent variables. As expected, reduced precipitation and soil moisture, as well as increased GDD, could be found at almost all sites in the affected region. NEE was mostly less negative, indicating a weaker sink or even a source forCO2, as expected from past studies on earlier events. The network density and number of site-years available now confirms that this was true for all major ecosystem types - forest, grassland and (rainfed) cropland, and resultedin increased atmospheric CO2 concentrations. However, notable exceptions occurred at elevated low mountain range sites and during early stages of the event, supporting the hypothesis of a balance between positive andnegative effects, where the former may prevail at strongly energy-limited sites with a usually large water surplus. H was mostly above-average, indicating that local heat production contributed to the event and was typically notsuppressed by warm air advection. LE as well as inferred GPP and R reacted most heterogeneously across sites, demonstrating the large discrepancy between potentially high fluxes due to high irradiation and temperature on theone hand, and suppression by water shortage on the other hand. Despite the variability in LE, its relation to NEE was such that ecosystem-level water use was less efficient than usual at the majority of sites through reduced CO2uptake.

Wind energy is relevant to self-sufficiency in urban areas, but the accuracy of wind assessment is a barrier to allowing wind energy development. The aim of this work is to test the performance of the Griggs-Putnam Index of Deformity of trees (G-PID) over urban areas as an alternative method for assessing wind conditions. G-PID has been widely used in open terrains, but this work is the first attempt to apply it in urban areas. The results were compared with CFD simulations (ENVI-met), and finally, with the linear model WAsP to inspect if deformed trees can offer acceptable wind power assessments. WAsP (meso-) and ENVI-met (micrometeorological model) showed similar results in a test area inside the University of Lisbon Campus. All trees showed a deformation with the wind direction (S and SE). The mean G-PID wind speed for all trees was 5.9 m/s. Comparing this to the ENVI-met simulations results (mean speed for all trees was 4.25 m/s) made it necessary to adapt the index to urban terrains by reducing each Index Deformation class by about ~2 m/s. Nevertheless, more investigation is needed, since this study is just a first approach to this integrated methodology. Also, tree species and characteristics were not taken into account. These questions should be addressed in future studies, because the deformation of trees depends also on the tree species and phytosanitary conditions.

Sylwan 148 (2): 17-25

Data processing methods; Energy balance closure; Intra-annual temporal dynamics of ET; Tables S1-S2; Figures S1-S2

Data processing methods; Energy balance closure; Intra-annual temporal dynamics of ET; Tables S1-S2; Figures S1-S2

Philosophical Transactions of the Royal Society B: Biological Sciences, 375 (1810), ISSN:0962-8436, ISSN:1471-2970, ISSN:0080-4622