6 Radiative transfer contribution

It is known that the boundary layer (BL) has a different evolution during the daytime and the nighttime (André 1978). We can distinguish two phases: during the day we see, especially in clear atmosphere conditions, the development of a convective layer whose thickness is of the order of 1 km, during nighttime, a strong thermal inversion process can decrease this layer to 30 m.

In order to describe the convective process we need a sophisticated treatment of the turbulence (Wyngaard et al. 1971). To describe the thermal nightly inversion the principal difficulty is represented by the turbulence-radiative transfer interaction and the description of turbulence in a strongly stable thermal stratification (Mahart 1985).

It is known that many difficulties are encountered if one tries to use the same model to simulate daily and nightly conditions (Wyngaard & Coté 1971). The radiative contribution in the analysis of the boundary layer has been tested and confirmed by many authors. It has not yet been proved, on the contrary, that the radiative transfer is relevant for our simulations that take care of the whole atmosphere. Besides that, the Meso-Nh model has been conceived to simulate daytime and nighttime conditions as well but using a radiative scheme one needs a description of the ground characteristic: air temperature above the ground, ground temperature, humidity and roughness. Having no detailed measurements of ground parameters, following climatological assumptions, we tested the model with a temperature difference $\Delta{T}=1$ degree between the ground and the upper air. Figure 20 shows the seeing evolution over 4 hours simulation time during the 16 May night obtained with and without radiative transfer. We can conclude that the simulation with radiative scheme resolves more turbulence and this contribution seems not negligible.

We thus recommend, in the future, a systematic study using this configuration.

 

Figure 20: Temporal evolution of the seeing over a 4 hour simulation time, with a radiative scheme (full line) and without it (thin line) during the 16 May 1993