Marine Boundary Layer Cloud Observations in the Azores

Abstract

he recent deployment of the Atmospheric Radiation Measurement Program (ARM) Mobile Facility at Graciosa Island, Azores, in the context of the Clouds, Aerosol and Precipitation in the Marine Boundary Layer (CAP-MBL) field campaign added the most extensive (19 months) and comprehensive dataset of marine boundary layer (MBL) clouds to date. Cloud occurrence is high (60%?80%), with a summertime minimum. Liquid precipitation is frequently present (30%?40%), mainly in the form of virga. Boundary layer clouds are the most frequently observed cloud type (40%?50%) with a maximum of occurrence during the summer and fall months under the presence of anticyclonic conditions. Cumulus clouds are the most frequently occurring MBL cloud type (20%) with cumulus under stratocumulus layers (10%?30%) and single-layer stratocumulus (0%?10%) following in frequency of occurrence. A stable transition layer in the subcloud layer is commonly observed (92% of the soundings). Cumulus cloud bases and stratocumulus cloud tops correlate very well with the top of the transition layer and the inversion base, respectively. Drizzling stratocumulus layers are thicker (350?400 m) and have higher liquid water path (75?150 g m?2) than their nondrizzling counterparts (100?250 m and 30?75 g m?2, respectively). The variance of the vertical air motion is maximum near the cloud base and is higher at night. The updraft mass flux is around 0.17 kg m?2 s?1 with 40%?60% explained by coherent updraft structures. Despite a high frequency of stratocumulus clouds in the Azores, the MBL is almost never well mixed and is often cumulus coupled.