Observations Pertaining to Precipitation within the Northeast Pacific Stratocumulus-to-Cumulus Transition

Three stratocumulus-to-cumulus transitions sampled during the Cloud System Evolution over the Trades (CSET) campaign are documented, with a focus on the Lagrangian evolution of the in-situ precipitation characteristics. The three Lagrangian transitions, all initially sampled within northeast Pacific stratocumulus and resampled two days later, include the ultra-clean clouds (cloud droplet number concentrations (Nd) <1 cm-3) sampled on 19 July. This shallow stratocumulus deck was thermodynamically coupled to the ocean surface, with Nd of ~20 cm-3 and near-surface in-situ 10-minute precipitation rates of 0.37-0.54 mm hr-1, exceeding that typically documented for stratocumulus. On 7 August, precipitation from a higher stratocumulus deck reached the surface less frequently, and Nd maintained at ~20 cm3 throughout the transition. A more polluted stratocumulus layer (Nd ~225 cm-3) on 27 July produced little precipitation. Hourly satellite-derived cloud fraction Hovmollers indicate that more quickly-deepening boundary layers tend to transition faster, into more intense but more occasional precipitation. The transitions begin either in the morning or late afternoon, suggesting that preceeding night processes can precondition or delay the inevitable transition. The rain drop-size distributions shift towards larger drop-sizes throughout the transition, while rising cloud bases and drier sub-cloud layers can facilitate overall increases in the sub-cloud evaporation, indicating both microphysical and thermodynamic processes contribute to the transition. The co-association between boundary layer depth and precipitation does not allow for definitive conclusions on the isolated effect of precipitation on the transition. The diversity of the three examples suit them well for further dedicated modeling studies.