Cloud Turbulence Measurements with ACTOS
Contact: Dr. Holger
Siebert
High-Resolution Cloud Measurements
Turbulence in clouds covers a large range of spatial
scales, which contribute to the different cloud processes. Starting with
scales comparable to the cloud dimension itself (~ 100 – 1000 m) down
to the dissipation scale (~1 mm), where kinetic energy dissipates into heat,
yields a broad range of spatial scales of roughly six orders of magnitude.
This makes turbulence measurements quite difficult and prevents that all
scales can be resolved in a single experiment at the same time. The influence
of atmospheric turbulence on the different cloud processes is a key topic in
the current discussion on cloud physics. For example, on large scales
unsaturated environmental air is mixed into the clouds at their edges and at
cloud top (so-called “entrainment”). On smaller spatial scales
further turbulent mixing finally leads to the homogenization of the affected
cloud areas. The mixing processes have significant influence on the
thermodynamic properties of the cloudy air in which cloud droplets develop
and grow to larger sizes finally ending as drizzle or rain drops. On even
smaller scales comparable with the size of cloud droplets itself turbulence
interacts directly with the droplets which results in an increased droplet
collision efficiency and has, therefore, consequences on further droplet
growth and the onset of precipitation. Since most turbulence measurements in
clouds are done by aircraft with typically high true air speed (TAS), the
spatial scales which can be resolved are in the order of meters. Assuming a
TAS of 100 m/s, sampling frequencies in the order of 100 kHz are needed for
turbulence sensors to resolve the dissipation scale. This is, even for
laboratory experiments, a challenge and currently not possible on aircraft.
Furthermore, often the different sensors are spread on the aircraft instead
of being collocated, which makes correlations of different parameters on
small scales questionable. To overcome this limitations the cloud turbulence
measurement system ACTOS (Airborne Cloud Turbulence Observation System) has
been developed for high-resolution measurements in clouds. ACTOS is an
autonomous measurement payload including sensors for turbulence
(three-dimensional wind vector, temperature, and humidity) but also for cloud
microphysical parameters such as droplet number concentration and droplet
size distribution. The photo shows ACTOS in its current version in front of a
helicopter. In this combination measurements in clouds are possible up to a
height of about 3000 m. ACTOS is carried by the helicopter with a 140 m long
rope fixed at the external cargo hook and can be dipped into the clouds from
above. Therefore, with a TAS of 15 m/s (in this special configuration)
high-resolution turbulence measurements unbiased from the helicopter downwash
are possible. During the measurement flights most devices can be monitored
on-line from the helicopter, besides ACTOS has its own real time data
acquisition and power supply and is, therefore, completely autonomous. ACTOS
can be carried by any other platform as balloon or Zeppelin.
The Turbulence Measurement Payload ACTOS
ACTOS is an autonomous measurement payload for
high-resolution cloud turbulence measurements on the decimeter scale and was
first deployed and tested in 2000. The system includes several devices to
measure cloud turbulence and cloud microphysical parameters. An overview of
all sensors is given in the sensor
table. Furthermore, a navigation unit is included to measure position,
velocity, and attitude of the payload. By means of these parameters the wind
vector can be corrected for ACTOS motion. A real-time data acquisition system
and power supply complete the setup. A telemetry or fiber-optic based
connection ensures on-line monitoring during the flight. In the current
helicopter-borne version, ACTOS consists of five 19" racks including the
electronics, data acquisition, and power supply. In Fig.1 and in the sketch
of Fig. 2 this is the red body part of ACTOS. A tail unit (with
anti-collision light) keeps ACTOS in the mean flow direction. All turbulence
sensors are fixed at a 1.5 m long carbon-made outrigger to minimize flow
distortions by the solid body of ACTOS. The lower part of ACTOS is designed
as a rough skid system to ensure a safe landing. The complete length of ACTOS
is 5.5 m, the total weight is 200 kg.
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| Fig. 1: ACTOS shortly before take-off |
Fig. 2: Sketch of ACTOS with main sensor
equipment |
ACTOS beneath different platforms
Within the last years ACTOS was consequently enhanced in cooperation with
the enviscope GmbH Frankfurt/Main and deployed in several international field
campaigns. Since ACTOS is an autonomous payload it can be carried by
different platforms. By 2004 ACTOS was carried by the tethered balloon MAPS-Y
(see Fig. 3); several experiments concerning boundary layer clouds were done
with this configuration. The balloon is 24 m long and has a volume of 400 m3.
MAPS-Y was operated by the German Bundeswehr (Technical Center for Ships and
Naval Weapons, WTD-71). ACTOS was fixed about 25 m beneath the balloon to
minimize flow distortions created by the balloon. The ceiling was about 1600
m, the maximum wind speed was 15 m/s, and the total weight of ACTOS was 120
kg.
In 2005 ACTOS was carried the first time by a helicopter (BELL 206III LR
"LongRanger", see Fig. 4), which was operated by Rotorflug GmbH. In this new
configuration ACTOS was fixed with a 140 m long rope at the external cargo
hook, the ceiling was 3000 m. The first flights were performed under VFR
(Visual Flight Regulations) and ACTOS was only dipped into the clouds from
above but the helicopter has to remain outside. Flights under IFR
(Instrumental Flight Regulations) are planned for the near future, which
allows experiments under overcast conditions. The combination of ACTOS with a
helicopter is much more flexible than the balloon version since the ceiling
is increased by a factor of two and individual clouds can be probed.
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| Fig. 3: Tethered-Balloon MAPS-Y |
Fig. 4: ACTOSin front of the BELL
LongRanger |
Projects and Results from Field Experiments
Technical Details for individual Sensors
Last Change: 2005-12-07
