Coherent eddies and temperature structure functions for three contrasting surfaces .1. Ramp model with finite microfront time

Air temperature time series within and above canopies reveal ramp patterns associated with coherent eddies that are responsible for most of the vertical transport of sensible heat. Van Atta used a simple step-change ramp model to analyse the coherent part of air temperature structure functions. However, his ocean data, and our own measurements for a Douglas-fir forest, straw mulch, and bare soil, reveal that even without linearization his model cannot account for the observed decrease of the cubic structure function for small time lag. We found that a ramp model in which the rapid change at the end of the ramp occurs ina finite microfront time can describe this decrease very well, and predict at least relative magnitudes of microfront times between different surfaces. Average recurrence time for ramps, determined by analysis of the cubic structure function with the new ramp model, agreed well with values determined using the Mexican Hat wavelet transform, except at lower levels within the forest. Ramp frequency above the forest and mulch scaled very well with wind speed at the canopy top divided by canopy height. Within the forest, ramp frequency did not vary systematically with height. This is in accordance with the idea that large scale canopy turbulence is mostly generated by instability of the mean canopy wind profile, similar to a plane mixing layer. The straw mulch and bare soil experiments uniquely extend measurements of temperature structure functions and ramp frequency to the smallest scales possible in the field.