Differences in Aircraft-Observed Cloud Microphysical Properties between Along-Wind and Cross-Wind Flight Paths during ACE-ENA

The Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign was conducted over the Azores during 2017-2018 with a total of ~160 hours of aircraft in situ measurements in combination with ground-based observations. On July 18, 2017, the aircraft flew horizontally on a repeating “L” shaped pattern at 5 nearly constant altitude segments ranging from below cloud base to cloud top. The vertex of the “L” was located near the radar site on Graciosa Island with one arm toward the northwest (along the prevailing wind direction) and the other arm toward the northeast (across the prevailing wind direction). Distinct cloud microphysical properties were observed along each arm. The along wind direction has lower droplet number concentration, larger mean diameter and liquid water content, and increased drizzle formation compared with the cross wind direction. Observations of below cloud aerosol and CCN are similar along both directions and likely not responsible for the differences in microphysical properties. Available radar and satellite imagery show elongated cloud elements that line up with the wind direction. The along wind flight paths at different altitudes sample near the center of one of these cloud elements along its wide dimension, while the cross wind flight paths cross cloud elements along their narrow dimensions. Studies of cloud-aerosol interactions and cloud microphysics using aircraft observations need to consider the flight direction relative to the prevailing wind and the orientation of cloud features. After discussing the July 18 case in some detail, we will summarize observed differences between along and cross wind microphysical observations on other days and highlight seasonal differences (summer vs winter) and cloud-type differences (drizzling vs non-drizzling clouds).