Cloud Adiabaticity and its Relationship to Marine Stratocumulus Characteristics over the Northeast Pacific Ocean

Cloud adiabaticity (α) is defined as the ratio of the actual liquid water path (LWP_measured) in a cloud to its corresponding adiabatic value (LWP_ad). Processes such as drizzle and entrainment can lead to sub-adiabatic LWP_measured. This work examines α and its relationship to microphysical properties for 86 cloud events over the Northeast Pacific Ocean based on data collected during four separate summertime airborne campaigns. For the study region, α was found to be 0.766 ± 0.134. For most cases, clouds with a low value of α were found to have lower droplet number concentration (N_d), higher droplet effective radius (r_e), higher relative dispersion (d), and higher rain rate (R). The sub-cloud aerosol concentration was often lower for the low α cases. The relationship between α and the vertical profiles and cloud top characteristics for both the cloud droplet-only spectrum and full spectrum (cloud and rain droplets) is also examined. Inclusion of rain droplets produced a larger change in d for the low α clouds as compared to the high α clouds. On average, R increased at cloud top for high α clouds, but decreased at cloud top for low α clouds. Accounting for α when estimating N_d from MODIS retrievals results in better agreement with in situ N_d values. Results of this work motivate the need for additional focus on the factors governing α and implications of its value, especially at cloud top where satellite retrievals provide valuable data.