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Diurnal Cycle of Land-Atmosphere Interactions

Speaker: Pierre Gentine, Columbia University
Series: CEE Departmental Seminars
Location: Bowen Hall Auditorium
Date/Time: Monday, November 29, 2010, 4:30 p.m. - 5:30 p.m.

Abstract:

In the first part of this presentation, we will demonstrate how a simple analytical model of the land-atmosphere interactions can be used to comprehend the response of surface heat fluxes (sensible, latent and soil heat flux) to radiative forcing at the land surface. We developed a mathematical model of the heat flux in the continuum extending from subsurface soil up to the boundary layer, solving the problem analytically using spectral methods that are based on the summation of many periodic signals with different wavelengths over a 24-hour period.

We report the counterintuitive finding that the soil responds faster than the boundary layer to changes in surface radiation, such as when clouds passing overhead momentarily reduce solar radiation. The previous frameworks for this problem erroneously assumed that soil responds very slowly to heat input. The findings also offer new guidelines for positioning field instruments for more precise information on the phasing and amplitude of the heat flux.The new soil-atmospheric boundary layer model solution provides the framework for estimating surface heat flux through the assimilation of soil and air temperature observations. Realistic heat flux at the land surface boundary affects the accuracy of short-term weather prediction out to two weeks. The solution also provides insight into emergent behaviors at the interface of the boundary layer and soil that are important to understanding land-atmosphere feedbacks, which are essential to assessing the response of regional water and energy cycles to climate variability and global change on seasonal and inter-annual time scales. This model is used to demonstrate that current soil heat flux methodologies are inaccurate since they are missing an important part of the spectrum of soil heat flux, with corresponding errors in the surface energy balance closure.

In a second part of this work, we will using the North American Reanalyses that land-surface turbulent heat fluxes only control the frequency of summertime convective precipitation but cannot impact the quantity of rainfall. Moreover the modification of the frequency of rainfall is only observed in the Eastern Mississippi region. This result has important consequences for constraining regional climate models and the soil moisture-precipitation feedbacks in such models.