Experimental study of Aerosol-Cloud Interactions using a 5-hole Probe with Remotely Piloted Aircraft

Enhancements in Remotely Piloted Aircraft Systems (RPAS) have increased their possible uses in many fields for the past two decades. For atmospheric research, lightweight RPAS (< 2.5kg) are now able to fly at altitudes greater than 3 km and even in cloud, which opens new opportunities to understand aerosol-cloud interactions. The European project BACCHUS (Impact of Biogenic versus Anthropogenic Emissions on Clouds and Climate: towards a Holistic Understanding) focuses on these specific interactions.

The RPAS are being utilized in he BACCHUS project to study aerosol-cloud interactions in climatically different environments. The purpose of this study is to conduct aerosol-cloud closure studies by characterizing the vertical distribution of aerosol, radiative fluxes, 3D wind vectors and meteorological state parameters, complemented by ground-based observations of Cloud Condensation Nuclei (CCN). Cloud microphysical properties such as cloud drop number concentration and size can be predicted directly from the measured CCN spectrum and the observed updraft, the vertical component of the wind vector.

On the RPAS, updraft measurements are obtained from a 5-hole probe synchronized with an Inertial Measurement Unit (IMU) to obtain 3D wind measurements. The 5-hole probe has been first calibrated in a wind tunnel, then comparison of flight results has been done with sonic anemometer located on a 60m meteorological mast. Error analysis has been conducted to estimate the accuracy of 3D wind measurements. Comparison of Power Spectral Density function (PSD) and Turbulent Kinetic Energy values (TKE) from in flight RPAS wind and static sonic anemometer wind also strengthened the validity of RPAS measurements. During BACCHUS field experiments, the RPAS were programmed to fly at a level leg just below cloud base to measure updraft. Cloud radar data were also available to compare RPAS updraft with ground-based measurements. Flights in different wind conditions and comparisons with anemometer and cloud radar validated 5-hole probe measurements, which can be implemented in studies to obtain cloud microphysical properties.