Cloud transition between high-coverage stratocumulus and low-coverage cumulus over the Eastern Pacific Ocean during MAGIC

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Description

The transition from the stratocumulus (Sc) regime with high cloud coverage, to low-coverage shallow cumulus (Cu) over the Eastern Pacific Ocean underlines one of the most challenging problems to date faced by the modeling community. During the recent ship-based field experiment MAGIC, the marine boundary layer (MBL) clouds were sampled over 21 round trips between California and Honolulu. Sc are ubiquitous off coast of California and a two-layer-cloud pattern (associated with decoupling MBL) can always been seen with Cu forming below before Sc starts to break, such decoupling MBL sometimes still exit for several days before developing into deep convective clouds near Honolulu. Utilizing the observational data, a couple of cases are selected with the transition between the single-layer Sc (associated with coupled MBL) and the decoupled MBL. We develop objective criteria to define the coupled/decoupled MBL from the observational point of view. A detailed analysis of macroscopic properties of the clouds with substantial controlling factors such as precipitation, sea surface temperature (SST), estimated inversion stability (EIS), large-scale subsidence, lifting condensation level (LCL), radiative flux, aerosol effects, as well as in-cloud dynamics are performed relating to the thermodynamic profiles. Our emphasis is placed on identifying the critical parameters that trigger the transition and/or that have impacts on the transition timing.