Liquid sublayer, evaporation

The free boundary layer has a sub-layer structure. In the evaporative layer from (z , z+), see Figure 7.2, the liquid water volume fraction is greater than the immobile volume fraction, the gas phase is under-saturated, and the amount of phase change and the associated jump in fluxes are determined. In the infinitesimal sublayer which has left and right limits z and z, respectively, the water volume fraction reaches the immobile volume fraction. 

In some of the rollovers, when the amplitude of the convective penetrative oscillations increased so that the oscillations penetrate through the surface sublayers to the liquid surface, the BOR rose very rapidly by 10-50 fold. This behaviour is called Mode 2 Rollover in which the convective oscillations are believed to break up the morphology of the surface sub-layer, particularly the high impedance thermal conduction region. This results in the unimpeded and much higher molecular rate of surface evaporation to be realised (see Sect. 4.6.2 on molecular evaporation rates). 

For Mode 2 events, the amplitude of the oscillating plumes was observed to increase very quickly and almost immediately penetrate through the surface sublayer structure into the molecular evaporating region at the liquid-vapour interface. 

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