Cloud Adiabaticity and Its Relationship to Marine Stratocumulus Characteristics Over the Northeast Pacific Ocean

Abstract

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 subadiabatic LWP measured. This study 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 (Nd), higher droplet effective radius (re), higher relative dispersion (d), and higher rain rate (R). The subcloud aerosol concentration (Na) was often less 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 Nd from Moderate Resolution Imaging Spectroradiometer retrievals results in better agreement with in situ Nd values. Results of this work motivate the need for additional focus on the factors governing α, such as cloud type, and implications of its value, especially for remote-sensing retrievals.