Seasonal and Diurnal Characteristics of Rainfall at Eastern North Atlantic

The extensive coverage of persistent, but diverse marine low clouds over the subtropical Eastern North Atlantic (ENA) Ocean plays a pivotal role in the global climate system by affecting radiative budgets and promoting heat and moisture exchange between the sea-surface, the boundary layer, and the overlying troposphere. The marine low clouds are poorly represented in global climate models (GCMs), partly due to inadequate long-term observations of cloud macrophysical and microphysical structures, radiative effects, and associated aerosol conditions. A permanent ENA site was established by the U.S. Department of Energy (DOE)’s Atmospheric Radiation Measurement (ARM) Climate Research Facility on Graciosa Island in the Azores, providing unique information on marine low clouds properties. The site also experiences a wider variety of synoptic intrusions that promote cloud diversity and enhanced rainfall rates.

This study investigates the variability in precipitation in subtropical marine low clouds on diurnal to seasonal timescales, as measured continuously at this ENA facility. Three years (from Nov. 2014 to Nov. 2017) of data collected at ENA from multiple instruments, including an optical rain gauge, collocated 2DVD and Parsivel2 disdrometers are analyzed. Good agreement between these three instruments enables higher confidence in their use to investigate and summarize the precipitation properties of the observed raindrop size spectra (DSD) for different precipitation regimes (e.g., drizzle, stratiform and shallow convective precipitation). Cloud regimes are classified by using collocated ENA vertically pointing cloud radar (KAZR) observations and associated cloud boundary/depth analyses. A more diverse particle size distribution and precipitation range is observed during spring and winter months, with ENA precipitation peaking in May and December (around 7 am local). Distinct clusters are found when consulting various DSD parameters, including higher concentration of smaller drops from shallow convective clouds and variations therein to deeper (non-boundary layer) and/or thicker stratiform cloud regimes.