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The helicopter-borne ACTOS for
small-scale cloud turbulence observations
Holger Siebert
, Katrin Lehmann , Manfred Wendisch, and Raymond Shaw
Leibniz-Institute for Tropospheric Research (IfT), Leipzig, Germany Dept. of Physics, Michigan Technology University, Houghton, MI, USA Besides a technical overview of ACTOS the pos- sibilities of using a helicopter for cloud research are Clouds play a major role in the Earth system and are discussed by showing measurement examples which relevant to many aspects of climate and daily weather clearly demonstrate the unique capabilities of the new forecast. The dynamics of clouds span a wide range system.
of spatial scales from the macroscopic cloud extension itself down to the Kolmogorov microscale (typically in the mm range for atmospheric conditions). Since cloud 2. Experimental Setup
microphysical properties on larger scales are controlled by processes taking place on smaller scales, measure- ACTOS is an autonomous measurement payload which ments with high spatial and temporal resolution are es- can, in principle, be carried by different platforms such as sential for a better understanding of cloud processes.
balloons, blimps, Zeppelin, or helicopters. The system is The majority of airborne in-situ observations of clouds equipped with sensors to perform high-resolution mea- have been made by fast-flying research aircraft which surements of meteorological standard parameters such limits the spatial resolution of most parameters to the as wind vector, air temperature, and humidity but also meter scale or so. To overcome this limitation the Air- cloud and aerosol microphysical properties such as liq- borne Cloud Turbulence Observation System (ACTOS) uid water content (  ) and number concentrations of has been developed which was originally designed for interstitial aerosol particles in boundary layer clouds.
the use beneath a tethered balloon (Siebert et al. 2003, To keep the load for the carrier platform as low as pos- 2006b). Due to the low true airspeed (TAS) of such a sible, a light-weight frame made from carbon-fiber and balloon-borne system the spatial resolution of the mea- aluminum was designed. The total weight of ACTOS in- surements is much higher compared with aircraft data. cluding the instruments is 200 kg. ACTOS is equipped In this paper the new helicopter-borne version of ACTOS with an autonomous power supply and data acquisition is introduced (see also Siebert et al. (2006a)). However, to be completely independent from its carrier platform. A compared to balloon–borne measurements, which also data link between ACTOS and the helicopter cabin was meet the slow–flying criterion, a helicopter is even more installed to ensure on-line monitoring of standard param- advantageous due to its longer cruising range and pos- eters during the flights.
sible ceiling. A helicopter is more flexible in time and space than a balloon and can be chartered at different 2a. Sensor Equipment
airfields. Furthermore there are fewer limitations with re- ACTOS was designed to provide collocated measure- spect to possible payload (weight, size, available electri- ments of several types of parameters. All sensor outputs are sampled with a joint real-time data acquisition sys- tem to ensure precise temporal correlation between the

different measured parameters. In the following a short Cloud droplet microphysical properties are mea- list of all devices is given, for a more detailed discussion sured with a modified version of the Fast For- of sensor performance the reader is referred to Siebert ward Scattering Spectrometer Probe (M-Fast-FSSP, et al. (2003, 2006b).
Schmidt et al. 2004; Lehmann et al. 2006). The M- Fast-FSSP counts each individual droplet and mea- The three-dimensional wind vector and the vir- sures its size and the inter-arrival times. The size tual air temperature are measured with an ultra- distribution, number concentration, and the   of sonic anemometer/thermometer (Solent HS, see the droplets is derived from the measurements.
also Siebert and Muschinski 2001).
A combination of differential Global Positioning Sys- tem (dGPS) and inertial sensors (optical fiber gyros and accelerometers) measure the attitude angles, angular rates, position, and the velocity vector of ACTOS with different time resolution and precision.
A navigation computer combines all measurements and gives all parameters with 10 Hz resolution (100 Hz are optional) and with best precision.
A special version of the so-called UltraFast Ther- mometer (UFT, Haman et al. 2001) allows air tem- perature measurements with 500 Hz resolution. The UFT is based on temperature-dependent resistance measurements of a thin wire which is protected against droplet impaction with a shielding rod in front of the wire.
Figure 1: Schematic sketch of ACTOS with numbered de- vices. The system can roughly be divided into three parts (i) the Humidity fluctuations are measured with a Lyman- tail unit (9), (ii) the main body consisting of five covered 19"-inch absorption hygrometer. A pre-impactor inlet pre- racks including electronics, data acquisition, power supply, and vents cloud droplets to enter the optical system dGPS antenna (8) and (iii) an outrigger made of carbon-fiber which would lead to an irreversible bias of the sig- tubes with all sensor and sampling inlets: 1) Sonic anemome- ter, 2) M-Fast FSSP, 3) PICT, 4) Impactor Inlets, 5) UFT, 6) PVM-100A, and 7) Nevzorov probe.
For air temperature and relative humidity measure- ments standard PT-100 resistance-wire thermome- ters and a capacitive hygrometer based on the In addition to the standard equipment a second cloud Vaisala Humicap sensor serve as calibration stan- spectrometer called PICT (Phase-Doppler Interferome- dard for the UFT and Lyman- , respectively.
ter for Cloud Turbulence) was installed beside the M- Fast-FSSP for this experiment. The PICT instrument The   is measured with a Particle Volume Mon- itor PVM-100A (airborne version, Gerber 1991).
measures the radius, longitudinal velocity component, and time of arrival of droplets that enter its sampling vol- The number concentration of interstitial aerosol par- ume. The sampling volume is defined by the region in ticles is measured with two Condensation Particle which two laser beams intersect: light scattered from Counter (CPC). Since the two CPCs have a differ- the two beams by a droplet is detected at several an- ent lower cut-off characteristics, the concentration of gles, providing temporal interference patterns (Doppler ultrafine particles in the size range between 6 and bursts) related to the droplet size and speed (Bachalo 12 nm is derived which is used as an indicator of and Sankar 1996). The sampling frequency is sufficiently freshly nucleated particles (Siebert et al. 2004).
high that there is no dead time and therefore inter-droplet

distances as small as the beam cross section can be de- termined. An advantage of PICT is that droplet sizes ranging from newly activated cloud droplets to drizzle can be detected with a single instrument (  m). Furthermore, at low , such as in the experi- ment described below, the instrument provides a mea- sure of the longitudinal turbulent velocity component.
Figure 1 depicts a schematic sketch of the current deflected downwash version of ACTOS. The setup is divided into three major parts: i) the 1.5 m long outrigger on which all sensors and inlets are attached, ii) the main body consisting of five covered 19" standard racks including the sensor electronics, data acquisition and power supply, and iii) the tail unit which keeps ACTOS in the mean flow direction. A rough skid system with shock absorbers ensure a safe take-off and landing procedure. The side covers can easily be removed for final settings before 2b. ACTOS Carried by Helicopter
Figure 2: Sketch of ACTOS suspended from the helicopter of Using the Helipod system Bange and Roth (1999) and type BELL LongRanger. The rope is 140 m long, several flags Muschinski et al. (2001) demonstrated that state-of- are attached to the rope to increase the visibility. The down- the-art turbulence measurements with a payload car- wash from the main rotor blades is deflected backwards, thus, ried by helicopter as external cargo are possible. With turbulence measurements on ACTOS are safely unbiased. The sketch is not in scale.
the appropriate combination of and length of the rope, measurements are unbiased from the influence of the helicopter downwash which is deflected back- wards. However, these measurements were done un- der cloud-free conditions and no experience was avail- 10.000 ft (3.000 m) able concerning helicopter flights with an external cargo Minimum Cloud Base 3.000 ft (1.000 m) in clouds. After clarification of the flight regulations under such conditions first test flights with a mock-up version of ACTOS were successfully performed in 2004 and 2005.
Hereby, ACTOS was dipped into the clouds from above whereas the helicopter remains outside the clouds such Table 1: Overview of flight parameters and conditions for the that it can be flown under visual flight regulations (VFR). helicopter-borne ACTOS.
is the true air speed during For this reason a 140 m long rope was used which is measurement flight conditions, whereas much longer than the 15 m rope used for the Helipod mum for carrier flights with ACTOS to the measurement but has also the advantage of allowing lower without being influenced by the downwash. Figure 2 shows a sketch of the com- For this study a single-engine helicopter of type Bell bination of ACTOS and the helicopter.
Long Ranger (BELL 206III LR) was used. For VFR The conditions and requirements for measurement flights one pilot and three scientists/operators can be on flights with ACTOS are summarized in Tab. 1.
board the helicopter. Flights with the helicopter inside the clouds have to be performed under Instrument Flight Regulations (IFR) which are planned for 2006. The he- licopter will be flown by two pilots and ACTOS will be operated by two scientist. This allows measurements in extensive stratiform cloud sheets which would not be possible under VFR and which will significantly extend the capabilities of ACTOS.
3. Measurement Example
The first field campaign with the helicopter-borne AC- TOS was conducted in April 2005 at the airfield Koblenz/Winningen, Germany. Here, one example is shown to illustrate the unique possibilities of using a slow-flying helicopter-borne payload for cloud investiga- tions. The data were taken on 27 April at a height of 2100 – 2300 m above ground level. Since the helicopter was not allowed to penetrate into the cloud, a constant flight level could not be maintained due to varying cloud top heights. Figure 3 depicts a 70 s long record of selected parameters for this flight leg. The   with maximum indicates a cloud penetration close to cloud top. The vertical velocity shows strong downdrafts at cloud edges and updrafts with the same order of mag- nitude in the cloud core region. It is noteworthy that both Seconds of the day [UTC] downdraft regions at the cloud edges are still inside the cloud with a non-vanishing   . The temperature de- creases rapidly when entering the cloud and drops from Figure 3: Measurementsof liquidwatercontent C. Whereas inside the cloud core the fluctu- wind velocity , static air temperature , and aerosol particle are comparable small, number concentration in the size range 6 to 1000 nm ( fluctuations between the cloud regions.
red curve) and 12 to 1000 nm ( , black curve). The true An interesting feature is contained in the record of the airspeed of the helicopter was 15 aerosol particle number concentration in the size range between 12 and 1000 nm is relatively constant over the entire record the second data set in- cluding the size range from 6 to 1000 nm, shows sev- 4. Summary
eral significant peaks after the cloud penetrations with up to four times higher particle concentrations. The oc- The approach of using a helicopter-borne measurement currence of so-called ultrafine particles in the size range system for cloud research was introduced. The unique between 6 and 12 nm is an indication for freshly nucle- possibilities and limits of such a system for high resolu- ated particles. The cloud edges are a preferential region tion measurements were discussed and technical infor- for new particle formation.
mation of the payload ACTOS were given. A measure- ment example of a short cloud penetration illustrates the Schmidt, S., K. Lehmann, and M. Wendisch, 2004: Mini- unique capabilities of the new system for high resolution mizing instrumental broadening of the drop size distri- bution with the M-Fast-FSSP. J. Atmos. Oceanic Tech- nol., 21, 1855–1867.
Acknowledgement We thank R. Maser, H. Franke, and
D. Schell from enviscope GmbH Frankfurt/M, Germany Siebert, H., H. Franke, K. Lehmann, R. Maser, E. W.
for technical support during the experiment. The heli- Saw, R. A. Shaw, D. Schell, and M. Wendisch, copter was chartered from Rotorflug GmbH, Friedrichs- 2006a: Probing fine-scale dynamics and microphysics dorf, Germany and flown by M. Flath.
of clouds with helicopter-borne measurements. Bull. R. Shaw's participation was part of a sabbatical leave Am. Meteor. Soc., submitted.
supported by IfT and the U.S. National Science Founda- Siebert, H., K. Lehmann, and M. Wendisch, 2006b: Ob- tion (grant ATM-0320953). Part of this work was funded servations of small scale turbulence and energy dis- by the Deutsche Forschungsgemeinschaft (WE 1900/7- sipation rates in the cloudy boundary layer. J. Atmos. Sci., 63, 1451 – 1466.
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and J. Heintzenberg, 2003: A new tethered balloon- Appl. Opt., 30, 4824–4831.
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Haman, K. E., S. P. Malinowski, B. D. Stru´s, R. Busen, and A. Stefko, 2001: Two new types of ultrafast aircraft thermometer. J. Atmos. Oceanic Technol., 18, 117–
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