Assessing the response of vegetation energy fluxes to heat-waves using the H-TESSEL land surface scheme
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In literature it is suggested that energy fluxes over forest and grassland have a contrasting response during heat-waves. In order to investigate whether this is reproduced by the H-TESSEL land surface scheme, an 11-year offline run of the model is compared to observations. A selection of heat-waves based on the ECA/E-Obs dataset is performed, after which the surface energy fluxes are compared for the two vegetation types. The number of heat-waves are then correlated to spring-time and summer-time soil moisture depletion. A connection is found for summer-time, but not for spring-time water depletion. Net radiation is compared to observations, and the additional amount received during heat-waves is found to be underestimated by the model, which most probably comes from the low discernibility of the actual observations in the model forcing. Grassland in general receives the same net radiation with forest in contrast with observations, due to the similar albedo prescription in the model. Fluxes are then compared for the two vegetation types. Forests in general compensate for the additional amount of net radiation by increasing sensible heat release, while grasslands increase latent-heat release. The observed differences in the response of forest and grassland fluxes to heat-waves are simulated by the model with a small differentiation in the partitioning of net radiation. When only heat-waves with low soil moisture are selected, there seems to be an improvement to that. Short-duration heat-waves are then filtered out, which brings the results even closer to observations. Latent heat is overestimated during heat-waves, especially for grassland, and in combination with the fact that sensible heat is not differentiated in the same amount, points to the connection with vapour pressure deficit, which should be further investigated. Ground flux in both vegetation types remains high compared to the observations. This is potentially a bias in skin temperature calculation connected to sensible and latent heat through the surface energy balance solver, which was not investigated further. Fluxes are then examined during the evolution of heat-waves. This shows no differentiation from previous results, while extreme water depletion in long-lasting events was not observed. A CART analysis is performed on the dataset which highlights the conservative soil water consumption of forest compared to grassland, and points to a stronger regulation of the evaporation by forests, depending on soil moisture. Finally, the possibility of using artificial neural networks for reproducing statistical relations in the model is shortly discussed.