CONDITIONAL STATISTICS OF VERTICAL HEAT FLUXES IN LOCAL ADVECTION CONDITIONS

Turbulent fluctuations have been investigated in the internal boundary layer (IBL) which forms after a dry-to-wet surface transition. The IB L is defined as that part of the atmospheric surface layer where the i nfluence of the downstream surface is noticeable. The results of the a pplication of three different quadrant analysis techniques are present ed. The three techniques, in increasing order of the amount of informa tion supplied, provide: (1) the diurnal variation of quadrant contribu tion (Ci), number fraction (Ti) and conditional average ((w's')(i), wi th s = T or q) of vertical sensible and latent heat fluxes, (2) the qu adrant contribution and number of samples of different sizes depending on the relative magnitude of each sample, and (3) the distribution of the nondimensional probability density function. The results show tha t in the IBL the vertical flux of sensible heat is maintained by (i) a small fraction of large samples with warm air carried upwards, and (i i) a larger fraction of small samples with cool air carried downwards. Both processes are almost equal in importance. In the morning and nea r the top of the IBL negative temperature fluctuations are limited by the near-uniform temperature conditions upstream and above the IBL. Th is limitation reduces, at that location, the conditional average of th e sinking motions of cool air. Closer to the wet surface the negative temperature fluctuations are less susceptible to the above mentioned l imitation. As a consequence contributions from all four quadrants are almost equal leading to a very small vertical heat flux. In the presen ce of a temperature inversion over both the upstream and the downstrea m terrain, shear-generated turbulence appears to be the cause of the r elative abundance of sinking motions of warm air and rising motions of cool air, leading to a reversal of the sensible heat flux. The latent heat flux is positive (i.e. directed away from the surface) at all ti mes and is maintained in almost equal amount by (i) a small number of large magnitude samples with moist air carried upwards, and (ii) small magnitude samples with sinking motions of dry air. These sinking moti ons of dry air are far more numerous, especially in the morning, but t heir conditional average is very small. The abundance of sinking motio ns of dry air is attributed to the fact that over the downstream terra in evaporation is greatly enhanced, leading to a skewed w'q' signal. T his skewness is clearly visible in the w'q'-probability density distri bution of the morning runs. In the evening the asymmetry between these two different contributions disappears. This is because evaporation i s greatly reduced and large positive humidity fluctuations no longer o ccur.