The flux data collected at 5 m above the ground at 7 towers in CASES 99 have been analyzed to investigate the spatial variability of surface fluxes in nocturnal conditions. Small-scale heterogeneity is expected to be enhanced in the nighttime stable boundary layer since vertical mixing becomes weak and is carried by small eddies.

For this data, small topographical features such as slopes of a few percent can be clearly detected in nocturnal surface fluxes and that such an influence becomes especially large when the wind speed is low and the net radiation is large negative.

The spatial scale of intermittent turbulence was generally smaller than the width of the network (300 m in radius). Turbulence events that covered the entire array of the towers rarely occurred. The computed time-lag cross correlation coefficients show that turbulence is more likely generated locally at each tower instead of being advected across the network.