Introduction

The SOAR Cheyenne II research aircraft is an airborne platform for atmospheric research consisting of instrumentation that has the capability of measuring in situ microphysical properties of clouds and their thermodynamic environment, documenting the composition of clouds and diagnosing the physical processes within them. For adequate sampling of particles in and around clouds, the research aircraft has the capability of measuring the size distribution of particles ranging from 0.1µm to 1.55mm in diameter. This dynamic range is achieved by the permanent platform of the SOAR research aircraft which consists of the Passive Cavity Aerosol Spectrometer Probe (PCASP SPP-200), the Cloud Droplet Probe (CDP) and the Cloud Imaging Probe (CIP). This range gives the scientists a spectrum of measurements in the temporarily suspended aerosol range and in the cloud hydrometeor range. In addition, inferences on the cloud composition and the particles that act as surfaces or embryos for water to condense on can be achieved by an airborne Cloud Condensation Nuclei (CCN) counter.

Aircraft performance

The Piper Cheyenne II is a twin-turboprop aircraft capable of over 6 hours of endurance when operated at the maximum range power setting. Designed for long range and high altitude flight, the aircraft has a cabin that is pressurized for maximum safety and comfort. The Cheyenne is FAA certified for flight into known icing conditions. The aircraft's performance in icing conditions has been proven in numerous projects. The de-icing systems include engine inlet anti-ice, pneumatic de-icing boots that protect the empennage and wing leading edges, windshield anti-ice and electric propeller de-ice. The Cheyenne II is powered by two 620shp Pratt & Whitney PT6A-28 turboprops driving three blade constant speed propellers. In the instrumented configuration, the aircraft has research speeds between 65 and 100m/s, with a rate of climb varying from 1000 to 2800ft/min and a service ceiling of 25,000 to 30,000ft. Range at these research speeds is around 1500nm. The aircraft is modified to accommodate various cloud physics and aerosol instrument packages.

Avantech Research developed the Aircraft Integrated Meteorological Measurement System (AIMMS-20AQ) gives up-to-the-second real-time three-dimensional wind conditions evaluated using the Kalman Filter Digital Signal Processing (DSP) technique. The AIMMS-20AQ consists of four components: an Air Data Probe (ADP), a carrier-phase Geostationary Positioning Satellite (GPS) measurement module, an Inertial Measurement Unit (IMU) and Central Processing Module (CPM).

The Passive Cavity Aerosol Spectrometer Probe (PCASP SPP-200) is configured with a sampling cone that extends into the direction of flight. All particles entrained in the sample flow are aerodynamically focused to interact with the laser beam in a sampling zone location in the center of the beam. Light scattered by a particle entering the laser beam is collected and focused onto a photodetector where it undergoes 3 stages of amplification to cover the range between 0.1 and 3.0µm.

The DMT Cloud Imaging Probe (CIP)  is very similar in technical operation to the 2D-OAP developed by PMS. A linear array of laser beams is focused on a sampling area where the particles’ shadows are optically magnified to provide the imaging data. The data output is distributed in 62 channels making the minimum detectable particle size at 12.5 µm and the largest particle at 1562.5 µm. Particles that shadow the two end diodes (1 and 64) are end rejected. The CIP incorporates a Liquid Water Content (LWC) detector.

The DMT Cloud Droplet Probe (CDP) is similar in technical operation to the FSSP. The instrument counts and sizes individual droplets as they traverse a laser beam. Droplets that hit the laser beam within the sample volume scatter light and the forward scattered light is collected onto an optical beam splitter and then onto a pair of photo detectors, the sizing detector and the qualifying detector. The range of the CDP is fixed and the output is distributed in 30 size channels in the range of 2 µm to 50 µm.

The Forward Scattering Spectrometer Probe (FSSP) is an aircraft mountable particle size spectrometer designed for in situ particle size measurements of water droplets typically found in clouds, fogs, and other droplet spray situations.  It is also used to collect size distributions of haze, dust, pollution, and other airborne particulates.  The FSSP has a size ranges covering 0.5 - 47µm.  The probe is configured in a rugged 7-inch OD cylindrical housing.  Two hollow tubes with a hollow sampling tube between them extend out from the forward end cap that is always oriented into the direction of flight when mounted on an aircraft to allow in-situ sampling of ambient air flowing through the sampling tube during flight. The probes are equipped with anti-icing heaters around the sampling tube and on each of the extension tips to allow continuous flight operation during icing conditions.  For ease of service the probe mounts in the cylinder as a plug-in assembly with two internal connectors at 90° to each other allowing the probe light tubes to be oriented either perpendicular or parallel to the mounting pad.

The DMT CCN counter samples aerosols from outside the aircraft to measure their capability to act as cloud condensation nuclei. An air sample is introduced in the CCN chamber at the  top center of a vertical cylindrical column and is surrounded by an aerosol-free humidified uniform supersaturation flow environment. As the air sample flows down through the chamber, CCN activate in response to the exposed supersaturation and grow to droplets. An optical particle counter at the base or outlet of the chamber detects all particles with diameters larger than 0.5µm. The heart of the instrument is the 50 cm long cylindrical column which provides the environment to activate and grow aerosol particles. The column is mounted vertically with the ambient aerosol entering at the top, and the increase in supersaturation takes place down the column. The unit operates at a single supersaturation. The supersaturation depends on the temperature difference between the top and bottom of the column as well as the flow rate in the column.  The supersaturation can be varied between 0.1 % and 1.0 %. The column has three temperature control zones, for rapid shifting between supersaturation. Approximately 30 seconds are required for a shift from one supersaturation to another, although operation in the field shows that shifting from a high supersaturation (1.0 %) to a low supersaturation (0.1 %) may take more than a minute since the temperature controllers are more efficient at warming the column than at cooling. The data output is distributed in 20 bins of resolution over the sizing range of 0.75 µm to 10 µm. The particle sizing data is updated at 1-second intervals. At a sample flow rate of 60 vccm, 6000 particles per cubic centimeter can be counted with a maximum of a 10 % coincidence. The CCN counter is mostly operated at supersaturation steps of 0.1, 0.25, 0.5 and 1.0 %. Particles that exit the base of the column and are in bin 1 through bin 20 comprise the measured CCN concentration.

Summary of parameters and measuring devices used on the SOAR Piper Cheyenne II. See table.

Crew

The SOAR Cheyenne II research aircraft is supported by a crew of professional pilots and flight scientists that are dedicated to this aircraft and its deployment.

Flight meteorologist/Project Director

Duncan Axisa

Research pilots

Gary Walker - David Prentice