Fine-Scale Structure of the Marine Stratocumulus Cloud Top over the Azores

The fine-scale structure of the Entrainment Interface Layer’s (EIL) and the influence of the EIL on the entrainment process in Stratocumulus (Sc) clouds was one of the main focus of the measurement campaign ‘Azores stratoCumulus measurements Of Radiation, turbulEnce and aeroSols’ (ACORES). ACORES took place in July 2017 on the Azores, a remote archipelago in the North-East Atlantic Ocean, which frequently experiences persistent Sc cloud layers. This makes the Azores an ideal location for studying the physics of Sc. Measurements were carried out using the combination of the two sensor packages ‘Airborne Cloud Turbulence Observation System’ (ACTOS) and ‘Spectral Modular Airborne Radiation measurement sysTem - HELIcopter-borne Observations of Spectral Radiation’ (SMART-HELIOS).

Both sensor systems are carried as external payloads below a slow-moving (typically 20 m/s) helicopter: SMART-HELIOS is carried by means of a 20 m long rope and below this ACTOS is attached by a second, 150 m long, rope. This setup achieves cloud in-situ measurements performed by ACTOS, using a temporal resolution of typically 100 Hz, whereas simultaneously SMART-HELIOS remains outside the cloud layer and observes it from above. The resulting high spatial resolution of the dynamic, thermodynamic, cloud microphysical, and radiative observations are ideal for this kind of studies and will provide a new look into the fine-scale structure of the EIL.

This contribution will give an overview of the ACORES-campaign and discuss the measurement setup and sampling strategy in more detail.

One feature of the sampling strategy was the subsequent series of profiles around the Sc top region (Dolphin-like flight patterns). The slow true airspeed of the helicopter combined with climb rates similar to aircrafts (2 to 5 m/s) results in steeper profiles and therefore, compared to aircraft measurements, averaging over less cloud cells during one individual profile. Individual profiles of local turbulence parameters with vertical resolutions on the centimeter to meter scale (e.g., local energy dissipation rates and similar parameters for temperature and liquid water content) will be presented in combination with parameters describing the static-stability (Brunt-Väisälä-frequency), wind-shear and gradient Richardson number.

One measurement highlight of this campaign is the first direct in-cloud observation of the virtual temperature, as a measure of buoyancy. This parameter has been derived from the speed of sound with an ultrasonic anemometer/thermometer and is unaffected by sensor wetting, which is a common hazard for in-cloud temperature measurements. Therefore, the buoyancy flux can be estimated directly. Together with heating/cooling rates measured with the radiation setup and infrared pictures of Sc top taken on-board by SMART-Helios a new insight on the structure of the Sc top will be presented.

Another measurements highlight was the successful sampling of temperature using the fast-response UltraFast Thermometer 2nd generation (UFT-2). Taking wetting-effects not into account, the UFT-2 can reach frequency responses of 1 kHz, which relates to horizontal and vertical resolutions in the centimeter and millimeter range, respectively. Thus, the UFT-2 is ideal for detecting small-scale temperature fluctuations, further enhancing the capability for studying the fine-scale structure of the EIL.

For selected flights, the holographic System HALOHolo was installed on ACTOS in addition to the standard instrumentation for cloud microphysical properties. The instrument allows for the investigation of the cloud microphysical response to turbulent mixing and entrainment at Sc cloud top. With ACTOS low true airspeed, about every 3 m of the flight path a robust estimate of the droplet size distribution is available. First data of this instrument will be presented and compared to results from the Cloud Droplet Probe (CDP-2).