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The role of clouds and surface processes on local temperature variability : analysis of multi-variables observations

Abstract : Large-scale dynamics dominate the surface temperature variations and atmospheric conditions in Western Europe to the first order. However, this large-scale air mass circulation alone does not explain all the temperature and precipitation variability. At the second order, this variability depends on small-scale processes via the atmospheric boundary layer and the surface energy balance (SEB), which itself depends largely on radiation and thus on cloud properties.The objective of this thesis is to better understand local processes and their influence on local climate variability, with a particular focus on the role of clouds.To do so, the first objective is to quantify the specific local contribution of the main SEB terms acting on short-term (i.e. hourly) temperature variations in Ile de France, and to determine their importance and the conditions under which one or the other of these terms will be preponderant. The four terms acting on the temperature variations are radiation (which can be separated into clear sky and cloud contribution), heat exchange with the atmosphere, heat exchange with the ground, and temperature advection. We develop the OTEM model that allows us to estimate these terms almost exclusively from observations, using the SIRTA-ReOBS dataset. We show that the sum of these four terms gives a good estimate of the hourly temperature variations. The weight of each term of the SEB on the hourly temperature variations is analyzed using the random forest method, whose main advantage is its ability to handle thousands of input variables and identify the most significant ones. This analysis showed that regardless of the season, clouds are the main modulator of the sun's effect on hourly temperature variations during the day, and they completely dominate during the night.The second objective is to study the specific role of clouds in temperature variations. For this purpose, other observations including lidar profiles have been used, exclusively under cloudy conditions. Several cases were created from the radiative effect of clouds during the day and night to (i) better understand how they affect the state of the atmosphere and thus other variables at the surface, and (ii) characterize the type of predominant clouds according to their radiative effect.Finally, we investigate the spatio-temporal variability of the previously obtained results: (1) spatial variability by applying the same method to the Meteopole site in Toulouse to understand how local specific conditions affect each of the terms involved in the surface temperature variations; (2) variability as a function of large-scale air circulation conditions by separating our results as a function of North Atlantic weather patterns.
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Submitted on : Thursday, January 13, 2022 - 1:25:08 PM
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  • HAL Id : tel-03524679, version 1

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Oscar Javier Rojas Muñoz. The role of clouds and surface processes on local temperature variability : analysis of multi-variables observations. Ocean, Atmosphere. Université Paris-Saclay, 2021. English. ⟨NNT : 2021UPASJ016⟩. ⟨tel-03524679⟩

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