[MUSIC] Hello, we have seen that the mean velocity profile inside the boundary layer could often be described with the logarithmic law. And that the bottom roughness is a crucial parameter that drives the scales of the turbulent fluctuations. However, if we measure the wind profiles at the same location, but at different hours of the day, we can get very different results. These two wind profiles correspond to one hour average measurements taken at daytime for the black dots or during nighttime for the open circles. We use here a linear log graph, hence, when the velocity profile follows a algorithmic law, we get a straight line. On one hand, the wind profile taken at 1 PM seems to fit correctly the logarithmic law. But on the other hand, the wind profile taken at 8 PM deviate from the logarithmic fit. The roughness length associated to these two profiles is given by the intersection of the straight line with the vertical axis. And we do find here a very large variation of this roughness length during the same day. Search measurements within the atmospheric boundary layer indicate that that the logarithmic profile is not universal. And that the bottom roughness is probably not the single source of turbulent fluctuations. Indeed, the atmospheric layer is strongly affected by solar radiations, which warm up the land and increase the surface temperature. The increase of the temperature reduces the air density close to the surface and leads to rising motions of air parcels. This unstable configuration induces an intense convection within the surface layer and generates strong turbulent fluctuation, even if there is no wind and the ground is perfectly flat. Hence, the solar heating is an additional and independent source of turbulent motion at the bottom of the atmospheric layer, which is not the case for the rough bottom of coastal weather layers. If now we consider a stable stratification with dense fluid layers located at the bottom, the bottom heat source will increase the temperature, reduce the density, and convective plumes will emerge from the bottom boundary. We could easily visualize such convective motions in a laboratory experiment. Here, a green fluorescent dye was spread at the bottom to visualize the vertical ascent of the warm and light fluid parcels. The vertical ascent and descent of the fluid parcels driven by the convection break the stratification and lead to a well mixed layer. In this convective boundary layer, the density and the temperature are fully mixed and almost uniform along the vertical. The height of this mixed layer depends on the initial stratification and the heat flux intensity. The signature of such convective phenomena in the atmosphere is visible on the clouds. When the water vapor rises in altitude, the pressure and temperature drops. Condensation occurs and small cumulus clouds appear at the top of the mixed layer. This cotton like pattern of cumulus clouds is a typical signature of a convective boundary layer. To follow the development of the atmospheric boundary layer, usually called ABL, we can use an Aerosol Lidar, which quantify precisely the concentration of small particles or water droplets in the atmosphere. Inside the mixed layer, the concentration of aerosols is high and uniform. Therefore the backscatter signal of the Lidar is strong and we can measure the height of the ABL. We have plotted on this image, the backscatter intensity along the vertical as a function of time. We can see in red, the vertical development of the atmospheric boundary layer and how it increases during the day as a response to the solar heating. Wind Lidar could be also very useful to quantify the dynamical response of the boundary layer. On this image, the Wind Lidar measurements of the vertical velocity are plotted as a function of time during the day. We can see that strong velocity fluctuations could reach one or two meters per second. This high velocity occur after midday when the convection is fully developed. During the night, there is no heating source for thermal convection, and the atmospheric boundary layer is stable and stratified. The potential temperature increases with height. The light air is above the denser one. Note that the potential temperature theta is not equivalent to the air temperature. The potential temperature of an air parcel is the temperature that the parcel would acquire when this parcel is adiabatically brought down to the ground level. Under normal stability stratified condition, the potential temperature increases with height. Hence, on this example, the atmospheric boundary layer is stably stratified at midnight. However, after midday, when the thermal convection occurs, the potential temperature is uniform above the ground. Here again, the constant and uniform value of the potential temperature is the signature of unstable convective motions inside the atmospheric boundary layer. This constant profile of potential temperature coincide with the measurements of strong vertical velocity fluctuations. Hence, we have seen here how the diurnal cycle, which is induced by the solar heating, affects the turbulence dynamics of the atmospheric boundary layer. How this diurnal cycle will affect the wind profiles? Here is the mean wind profile during the night at 1 AM for a typical summer day. At 1 PM, the wind profile is less shear, with stronger winds velocity close to the ground. Then, at 8 PM, we recover a higher velocity shear with low velocity at the ground level. This evolution of the wind profile during the day is typical for a summer month. To sum up, the diurnal evolution of the atmospheric boundary layer affects the wind profile. During the night, there is no heat convection, and the ABL is stably stratified. This stratification inhibits the vertical motion and we can therefore have a strong velocity shear between the ground and the upper atmosphere, why? In the middle of the day, when the solar heating triggers intense convective motions, the horizontal momentum is also well mixed by the turbulent fluctuations. And the mean velocity profile tends to be uniform along the vertical. The vertical velocity shear is therefore, strongly reduced. Later, in the early evening, the solar heating is switched off and the vertical velocity fluctuation induced by the convection vanishes. We therefore slowly recover a stably stratified boundary layer with a strong vertical shear of the mean velocity, thank you.
