Test of the volume-of-fluid method on simulations of marine boundary layerclouds

The impact of using grid-averaged thermodynamic properties (i.e., neglectin g their subgrid variability due to partial cloudiness) to represent forcing s for condensation or evaporation has long been recognized. In particular, numerical difficulties in terms of spurious oscillations and/or diffusion i n vicinity of a cloud-environment interface have been encountered in most o f the conventional finite-difference Eulerian advection schemes. This probl em is equivalent to the inability of models to accurately track the cloud b oundary within a grid cell, which eventually leads to spurious production o r destruction of cloud water at leading or trailing edges of clouds. This p aper employs a specialized technique called the "volume-of-fluid" (VOF) met hod to better parameterize the subgrid-scale advection process that account s for the transport of material interfaces. VOF also determines the actual location of the partial cloudiness within a grid box. Consequently, relevan t microphysical parameterizations in mixed cells can be consistently applie d in "cloudy" and "clear" regions. The VOF technique is incorporated in a t wo-dimensional hydrodynamic model to simulate the diurnal cycle of the mari ne stratocumulus-capped boundary layer. The fidelity of VOF to advection-co ndensation processes under a diurnal radiative forcing is assessed by compa ring the model simulation with data taken during the ISCCP FIRE observation al period as well as with results from simulations without VOF This study s hows that the VOF method indeed suppresses the spurious cloud boundary inst ability and support, a multiday cloud evolution as observed. For the case w ithout VOF, the spurious instability near the cloud top causes the dissipat ion of the entire cloud layer within a half of a diurnal cycle.