Aircraft Observations and Surface-based Retrievals of Summertime Sub-cloud Drizzle Evaporation Rates over the Eastern North Atlantic

Abstract:

Marine boundary layer (MBL) clouds over the Eastern North Atlantic (ENA) are largely maintained by the flux of aerosols and water vapor from the ocean surface that are transported to cloud base by turbulence. Clouds in this region often undergo a latitudinal transition in morphology from a single cloud layer to complex broken cloud structure that exhibits mesoscale organization, and this transition is associated with MBL decoupling and is frequently observed in the wake of cold fronts. Simulations suggest that the cloud transition over the ENA is initiated by increases in the surface fluxes and vertical turbulent transports. Clouds in the ENA often produce drizzle that has significant impact on the sub-cloud layer turbulence and decoupling in the regions where the transition from single layer stratocumulus to cumulus-coupled MBL occurs.

Aircraft data were collected in the vicinity of the Department of Energy’s Atmospheric Radiation Measurement (ARM) ENA observatory located on Graciosa Island in the Azores during the Aerosol and Cloud Experiments in the ENA (ACE-ENA) field campaign. The ARM Aerial Facility Gulfstream-159 (G1) aircraft was deployed during two periods: June through July of 2017 and January through February of 2018. The G1 measured a range of variables including the cloud and drizzle droplet size distribution from 10 to 3,000 µm using a 2-dimensional stereo probe. In addition to these aircraft data, the ENA observatory is equipped with a K-a band Zenith Radar (KAZR), which can measure the effective reflectivity factor, Doppler velocity, and spectral width with a resolution of 30-m from near-ground to 20 km in altitude.

We use G1 aircraft measurements to establish a robust relationship between radar effective reflectivity factor, which can be computed from the cloud and drizzle droplet spectrum, and rainfall rate (Z-R relationship). We apply this aircraft-measured Z-R relationship to the KAZR cloud radar profiles and compute rainfall rates for the entire ACE-ENA period. We extend the ACE-ENA results by applying the Z-R relationship to compute rainfall rate profiles and evaporation rates over longer periods and we classify these evaporation rates according to decoupling and cloud structural measures.