Coupling between air and surface temperature in the atmospheric surface layer

Buoyancy is one of the dominant mechanism driving turbulence in a convective boundary layer. Such turbulence is not completely random but is often organized into identifiable structures, such as thermals and plumes. Even though buoyancy is the dominant force to drive the plume, wind shear also modifies its structure by tilting it in the flow direction. When a plume flows past a temperature sensor, the recorded trace can be characterized by a characteristic ramp structure. The momentum transport process during a single ramp is thought to be comprised of sweeps, ejection and interaction between sweep and ejection superimposed over the mean flow. For the unstable case within a single ramp air temperature gradually increase followed by a sharp drop which results in large amount of heat flux from the ground surface during ejection and sweep events. Thus, one can expect the intermittency of the heat flux to manifest itself in the surface temperature by some coherent structures.

To correlate the surface temperature fluctuations with turbulent coherent structures, we have employed a thermal infrared camera to measure surface temperature at 1 Hz and turbulence measurements (two 3-D sonic anemometers and fine wire thermocouples) at 10 Hz on clear days over grass and artificial turf surfaces. Experiments were conducted in unstable conditions for Richardson numbers ranging from -1 to -10. Distinct patterns were detected in the surface temperature fields. These measurements are then used to investigate the coupling between surface temperature fluctuation and air temperature ramp structures. Wavelet analysis is employed to detect ejection events and their timescales. Spectral analysis indicates that the vertical velocity has smaller inertial range compared to horizontal velocity components, air temperature and surface temperature. By cross correlating turbulence data and surface temperature from its footprint, we found that horizontal velocity components and air temperature lag surface temperature by the same amount. Also, the renewal events over artificial turf occur over smaller time scales compared to grass since artificial turf will give more unstable condition.