An LES model study of marine stratocumulus-topped boundary layer evolution-CCN impacts, diurnal contrasts and thermodynamic differences

One of the major challenges in studying the stratocumulus-topped boundary layer (STBL) is how marine stratocumulus clouds evolve under various different environmental circumstances, e.g., aerosol, radiation and thermodynamic conditions. We studied the overall evolution and diurnal contrasts of the STBL using the CIMMS (Cooperative Institute for Mesoscale Meteorological Studies) 3D LES model, which explicitly resolve clouds and turbulent eddies of the marine boundary layer. Examined are the microphysical and dynamical evolutions of the STBL under three different initial CCN loadings (maritime, continental and polluted; the concentrations at 1% supersaturation of 163, 1023, and 5292 cm^-3, respectively) for four different thermodynamic conditions (the key differences are in moisture distribution and inversion height). This model uses bin microphysics and grid spacing is 75 m in the horizontal and 25 m in the vertical, to make the total domain size of 3×3×1.25 km. Total simulation time is 6 hrs. The large-scale subsidence is prescribed by w= –Dz, where the large-scale divergence D = 5×10^-6 s^-1 is assumed.

For both daytime and nocturnal conditions, when the boundary layer is moist, drizzle is heavy in the maritime cloud that leads to the dissipation of cloud and therefore cloud depth is thicker in the polluted cloud. To the contrary, under the dry condition the maritime cloud is thicker than the polluted cloud because the maritime cloud does not drizzle while cloud droplets are smaller and evaporate more effectively near the cloud base in the polluted cloud. However, COD and albedo are higher in the polluted cloud regardless of moisture condition, mainly due to the much smaller cloud droplet effective radii in the polluted than maritime cloud but insignificant differences in LWP between the two clouds. Therefore, cloud radiative forcing (CRF) is larger in the polluted cloud. The anthropogenic CRF (polluted – maritime) is larger when the clouds form under the moist and high inversion condition and therefore more developed. The cloud and sub-cloud layer are often decoupled due to shortwave warming of the cloud layer during the daytime. The ratio (I) of the sum of buoyancy flux in the cloud layer to that in the sub-cloud layer can serve as an index of decoupling strength. Comparisons of the I values indicate that the STBL are more easily decoupled when the boundary layer is moist but the inversion height is low. Under the moist but high inversion height condition, decoupling strength is weaker, mainly because LWC near cloud top is much larger and therefore stronger cloud top long wave radiative cooling compensates efficiently for shortwave warming. Under the dry condition, decoupling strength is somewhat smaller than those for the moist condition. Much more detail will be discussed in the conference.