COHERENT EDDIES AND TEMPERATURE STRUCTURE FUNCTIONS FOR 3 CONTRASTINGSURFACES .1. RAMP MODEL WITH FINITE MICROFRONT TIME

Air temperature time series within and above canopies reveal ramp patt erns associated with coherent eddies that are responsible for most of the vertical transport of sensible heat. Van Atta used a simple step-c hange ramp model to analyse the coherent part of air temperature struc ture functions. However, his ocean data, and our own measurements for a Douglas-fir forest, straw mulch, and bare soil, reveal that even wit hout linearization his model cannot account for the observed decrease of the cubic structure function for small time lag. We found that a ra mp model in which the rapid change at the end of the ramp occurs ina f inite microfront time can describe this decrease very well, and predic t at least relative magnitudes of microfront times between different s urfaces. 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 mu lch scaled very well with wind speed at the canopy top divided by cano py height. Within the forest, ramp frequency did not vary systematical ly with height. This is in accordance with the idea that large scale c anopy turbulence is mostly generated by instability of the mean canopy wind profile, similar to a plane mixing layer. The straw mulch and ba re soil experiments uniquely extend measurements of temperature struct ure functions and ramp frequency to the smallest scales possible in th e field.