Profiles and evaporation

It is suggested that in all conditions of stability either of two slightly different profile equations can be used, both carrying in‐built stability weighting factors for use in transport equations. Setting Λ = In( z—d )/ z 0 the more convenient equation for field use is u = S 0 Λe − n Λ/2 , where n is dependent on stability (+ in lapse, — in inversion): the important assumptions in physics are that d and z 0 are surface constants, and that S 0 = u * / k for all values of n , as it is for n = 0. For other values of n , the standard plot of vn̈ . In( z—d ) has a slope, S ( = du /dΛ), at some average values ū, λ, and backward extrapolation to Λ = 0 gives an intercept u i such that u i /ū = nλ /2 = y . The weighting factor for the shear, f = S 0 / S , thus becomes e y /(1— y ). To find y , u/ū is plotted against U , where U is the velocity measured in a neutral period. No knowledge of temperature gradient is needed – other than ability to recognize when it was near enough to zero to be able to pick out U , z profiles. Because λ appears in y, the roughness length, z 0 , does not vanish: in effect, it replaces the extra ‘length’ that is put into other stability weighting factors – not needed here. Profiles of wind, temperature and humidity above and within two large plots of kale (one irrigated) are studied in detail for short periods of strong lapse and strong inversion, first to show the high quality in anemometry needed to exploit the profile equation (barely achieved on the north plot, and not on the other); second, to demonstrate a way of correcting for zero errors in the thermometry: and, third, to show that above the crop the profiles of wind and water vapour pressure have the same shape, and the Bowen ratio is invariant with height. Within the crop, in both lapse and inversion, the top of the crop is the source or sink for sensible heat transfer. In a humidity inversion, the top is also the sink for latent heat transfer, but in a lapse the source is at, or close to, the virtual sink for momentum at z = d + z 0 . Evaporation from the north (unirrigated) plot was calculated for 44 days (27 June to 8 August 1971), each day's total being the sum of six estimates for 4‐hour periods. Two weighting factors were used: first, a factor ϕ, based on Richardson number; second, the factor f 2 . The 44‐day totals were 128mm (using ϕ; identical with calculated potential evaporation), and 137mm (using f 2 ). More detailed study of 13 days in the period (using f 2 ) showed that in the diurnal cycle, evaporation, on average, is zero in the eight hours of darkness, very small from 04 to 08h, large from 08 to 16h, and moderate from 16 to 20h. The energy balance is satisfactory. Comparison of Bowen ratios over the two plots indicated that E (irrigated)/ E (unirrigated) was about 1.06. A direct comparison (neutron moisture meter) gave a ratio near 1.10. (The absolute values are suspect.)