Biosphère Climat

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Temperature is a key parameter for characterizing the state of the atmosphere. The diurnal variability of the temperature profile in the atmospheric boundary layer ( CLA ) and the free troposphere ( TL) is the basis of important meteorological, dynamic and radiative processes: stratification of the troposphere, formation of aerosol and cloud layers, initiation of convection, diurnal cycle of the boundary layer, propagation of gravity waves, transport… Closely linked to the potential temperature profile is that of sensible heat flux which allows to account for biosphere-atmosphere and intra-atmosphere exchanges. The profiles of potential temperature and sensible heat flux, as well as the relationship between the two (thermal diffusivity) are among the most important atmospheric parameters to complete the mass and energy balance in the atmosphere and are the basis any atmospheric modeling (LES, regional models).

Today, our understanding of intra-atmospheric exchanges of crucial minority compounds such as H 2 O or CO 2 or of any pollutant (NOx, CO ..) remains partial for lack of observations of the rapid spatio-temporal variability of the profile of temperature and water vapor and also simultaneous observations of the sensible and latent heat flux profiles outside the surface layer. More particularly concerned are exchanges at the level of the training layer ( CLA-TL) or the coupling / decoupling of layers within the troposphere and the quantification of these couplings. Another lack of observation concerns the problematic phases of the morning and evening transition at the crossroads of several diagrams of process representation.

Figure 1 Research objectives on atmospheric dynamics

The TERA (temperature) and COWI (speed, HO2 and / or CO 2 ) and MOBILIS (aerosols) lidar make it possible to combine simultaneous measurements of speed and scalars and therefore to carry out flux measurements using turbulent correlation methods or gradient [Gibert JGR, 2007; Gibert JTECH, 2011] . Ultimately, a restitution of the 3D speed and concentration fields will be possible experimentally and can be compared with the results of LES simulations . A link can be made between small-scale dynamic processes (turbulence diffusivity coefficient) and their medium-scale causes / effects (boundary layer height, jets, waves).

An example of a study of the morning transition and of MOBILIS-COWI instrumental synergy is given below. Convection in the boundary layer forces the formation of gravity waves in the residual layer, identified by the fluctuations in the vertical speed of the air and the polarization of the laser light back scattered by the particles in the atmosphere [ Gibert QJRMS 2011].

Figure 2 Example of a MOBILIS-COWI synergy for a study of the morning transition of the atmospheric boundary layer:
(a) Lidar depolarization at 532 nm  (b) Vertical speed. Local time corresponds to UTC time + 2h