High frequency ground temperature fluctuation in a Convective Boundary Layer

To study influence of the turbulent structures in the convective boundary layer (CBL) on the ground temperature, during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) observational campaign, high frequency ground temperature was recorded through infra-red imagery from 13 June - 8 July, 2011. The 2-D ground temperature characteristics were then studied and compared to surface layer turbulence data for clear days. The ground temperature standard deviation follows the power-law exponent similar to surface layer similarity theory. The high correlation region between the air and the ground temperatures is aligned with the wind direction; the spanwise spread of the correlation region depends on the wind direction variability. The upwind part of the high correlation region is comparable with the flux footprint function by Hsieh et al. (2000). Conditional averaging shows that when a cold air parcel descends through the atmospheric surface layer (ASL) during a sweep event, the ground cools down. As the air parcel remains near the ground, it warms up and the heat transport from the ground to the air decreases. Thus the ground warms up during this period. After attaining a certain buoyancy the warm air parcel ascends, creating an ejection event. During the initial period of ejection, the ground temperature peaks, and subsequently the ground starts to cool down as the ground heat flux starts to decrease. Animation of ground temperature fluctuations reveals that the imprints of these ejection and sweep events move along with the wind. The estimated speed of these coherent structures is of the same order as the wind speed at 8 m above ground level or higher. With increasing stability of the CBL, the mean size of these structures increases and they become streakier.