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Massachusetts Institute of Technology (2010)

Spectral behavior of the coupled land-atmosphere system

Gentine, Pierre

Titre : Spectral behavior of the coupled land-atmosphere system

Auteur : Gentine, Pierre

Université de soutenance : Massachusetts Institute of Technology

Grade : Thesis (Ph. D.) 2010

Résumé
The main objective of this thesis is to understand the daily cycle of the energy coupling between the land and the atmosphere in response to a forcing of incoming radiation at their common boundary, the land surface. This is of fundamental importance as that the initial/ boundary conditions of the land-surface state variables (e.g. soil moisture, soil temperature) exert strong control at various temporal scales on hydrologic, climatic and weather related processes. Hence diagnosing these state variables is crucial for extreme hydrological forecasting (flood/ drought), agronomic crop management as well as weather and climatic forecasts. Consequently in this thesis, the daily behavior of a simple land-atmosphere model is examined. A conceptual and linearized land-atmosphere model is first introduced and its response to a daily input of incoming radiation at the land surface is investigated. The solution of the different state and fluxes in the Atmospheric Boundary Layer (ABL) and in the soil are expressed as temporal Fourier series with vertically dependent coefficients. These coefficients highlight the impact of both the surface parameters and the frequency of the radiation on the heat propagation in the ABL and in the soil. The simplified model is shown to compare well with field measurements thus accounting for the main emergent behaviors of the system. The first chapter of the thesis describes the theoretical background of the equations governing the evolution of temperature and humidity in the ABL and in the soil. In the second chapter, the pioneering work of Lettau (1951), which inspired our approach is summarized. In his work Lettau studied the response of a simplified linearized land-atmosphere model to a sinusoidal net radiation forcing at the land surface. The third chapter of the thesis describes the SUDMED project, which took place in Morocco in 2003. During this project a wheat field was fully instrumented with continuous measurements of soil moisture, radiative fluxes, turbulent heat fluxes and soil heat flux. This site will be taken as a reference for model comparison. The fourth chapter of the thesis presents the three studies with distinctive goals. In these studies our linearized land-atmosphere model is first introduced. Then the propagation of the land-surface diurnal heating is presented and the model is compared to observations from the SUDMED project. Finally the repercussion of a land-surface energy budget error noise is investigated. Finally in the last chapter of the thesis we discuss possible evolution and improvements of the analytical coupled model presented in this thesis. In particular, it is emphasized that the non-linearity of the the boundary-layer height is of great importance for the predictability of the ABL state.

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