On the application of mixed -layer theory to the stratocumulus-topped boundary layer
In this dissertation, we explore the applicability of mixed-layer theory to represent stratocumulus-topped boundary layer (STBL).
Mixed-layer theory is used to study the STBL diurnal cycle. Our results show that the diurnal evolution of cloud thickness is sensitive to the entrainment efficiency. Specifically with low entrainment efficiencies, the cloud thickness evolution is in a better agreement with observations. We explain these effects through a consideration of the equilibrium state of cloud boundaries and their adjustment timescales. The susceptibility of cloud albedo to droplet number density dominates the entrainment effects. This suggests that estimates of aerosol indirect effects from stratocumulus clouds will not be particularly sensitive to the way entrainment is represented in large-scale models.
The low-cloud amount (LCA) is diagnosed based on the equilibrium solutions of the mixed-layer model (MLM). ECMWF Reanalysis (ERA-40) data serve as large-scale boundary conditions. Results are compared to the International Satellite Cloud Climatology Project D2 data, especially in light of the relationship between the LCA and the lower-troposphere stability (LTS). Our results show that the synoptic variability in divergence contributes to LCA climatology. This climatology reproduced from MLM is more sensitive to processes that redistribute the mass field as compared to heat and moisture. Other large-scale conditions contribute to LCA depending on their correlation with the LTS and the strength of the LTS signal in individual regions.
An autoregressive noise model is proposed to represent the synoptic variability in divergence based on analysis of ERA-40 data. Using this model, the equilibrium cloud fraction is shown as a function of the mean divergence value, the noise level, and the noise autocorrelation time scale. Mixed-layer model with such noise produces a reasonable comparison to observations in LCA climatology. An interaction rule is specified based on the effects of drizzle. Two specific conditions, random initial cloud depth and random droplet number density, are examined as the possible triggering mechanisms for pockets of open cells (POCs). In both cases, interactions reduce cloud fraction and promote the effect of noise, which lead to a new mixed-layer equilibrium. This can be explained by the mixed-layer multiple equilibria behavior with respect to the large-scale divergence.
With these work, we hope to make contributions to the STBL parameterizations in the large-scale models.