A new airborne tandem platform for collocated measurements of microphysical cloud and radiation properties

Abstract

A new airborne tandem measurement platform for cloud-radiation interaction studies is introduced in this paper. It consists of a Learjet 35A research aircraft and the AIRcraft TOwed Sensor Shuttle (AIRTOSS), which is an instrumented drag-body towed by the Learjet. Currently, the AIRTOSS is instrumented with a Cloud Imaging Probe (CIP) for measuring cloud microphysical properties and an Inertial Navigation System (INS) for measurements of flight attitudes. The cable dragging AIRTOSS can be as long as four kilometres. Thus, truly collocated measurements in two altitudes above, in, and below clouds can be obtained. Results from first test flights with Learjet and AIRTOSS are reported here. The flights were performed from Hohn Airport, Germany. Specific manoeuvres were flown to test the aerodynamic behaviour of the drag-body and to investigate the suitability of AIRTOSS for high-precision irradiance measurements which require a stable flight attitude of AIRTOSS. The flight attitude data show that AIRTOSS is sensitive to several flight manoeuvres such as curves, altitude and airspeed changes, and also to changes of towing cable length. The effects of these manoeuvres on the attitude angles of AIRTOSS have been quantified. Maximum roll angle deviations were observed during curve flight. Even small changes in heading can lead to high roll angles (one degree change in heading causes a change in roll angle of about eight degrees). The pitch angle varies during climb or dive periods, extending or retracting of towing cable, acceleration or deceleration, and even when flying at too low or too high true airspeed depending on altitude. Values of pitch angle between −5° (dive) and 8° (climb and retracting towing cable) have been observed. While change in attitude is not problematic for cloud particle property measurements it is for radiation measurements. Here, the deviation from the horizontal should be no more than 3° to avoid large errors. When keeping the above mentioned flight parameters constant, sufficiently stable flight conditions can be maintained to perform high-quality irradiance measurements with AIRTOSS in future experiments. During this test campaign also observations of cloud microphysical data as for example droplet number concentrations and size distributions with the AIRTOSS in stratocumulus clouds were performed to prove the compliance with scientific needs. Simultaneous radiation measurements of the clouds have been made. The measurements of internal operational data of AIRTOSS as well as the first atmospheric data demonstrate the suitability of this tandem platform for detailed cloud microphysics and radiation interaction studies.

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W. Frey et al.
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Atmos. Meas. Tech. Discuss., 2, 1–35, 2009
www.atmos-meas-tech-discuss.net/2/1/2009/
© Author(s) 2009. This work is distributed under
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A new airborne tandem platform for
collocated measurements of
microphysical cloud and radiation
properties
W. Frey1, H. Eichler1, M. de Reus1, R. Maser2, M. Wendisch1, and S. Borrmann1,3
1Johannes Gutenberg University Mainz, Institute for Atmospheric Physics, Mainz, Germany
2Enviscope GmbH, Frankfurt, Germany
3Max-Planck-Institute for Chemistry, Particle Chemistry Department, Mainz, Germany
Received: 17 December 2008 – Accepted: 22 December 2008 – Published: 8 January 2009
Correspondence to: W. Frey (freyw@uni-mainz.de)
Published by Copernicus Publications on behalf of the European Geosciences Union.
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AMTD
2, 1–35, 2009
The AIRTOSS
(AIRcraft TOwed
Sensor Shuttle)
W. Frey et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
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??
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Abstract
A new airborne tandem measurement platform for cloud-radiation interaction studies
is introduced in this paper. It consists of a Learjet 35A research aircraft and the AIR-
craft TOwed Sensor Shuttle (AIRTOSS), which is an instrumented drag-body towed
by the Learjet. Currently, the AIRTOSS is instrumented with a Cloud Imaging Probe
(CIP) for measuring cloud microphysical properties and an Inertial Navigation System
(INS) for measurements of flight attitudes. The cable dragging AIRTOSS can be as
long as four kilometres. Thus, truly collocated measurements in two altitudes above,
in, and below clouds can be obtained. Results from first test flights with Learjet and
AIRTOSS are reported here. The flights were performed from Hohn Airport, Germany.
Specific manoeuvres were flown to test the aerodynamic behaviour of the drag-body
and to investigate the suitability of AIRTOSS for high-precision irradiance measure-
ments which require a stable flight attitude of AIRTOSS. The flight attitude data show
that AIRTOSS is sensitive to several flight manoeuvres such as curves, altitude and
airspeed changes, and also to changes of towing cable length. The effects of these
manoeuvres on the attitude angles of AIRTOSS have been quantified. Maximum roll
angle deviations were observed during curve flight. Even small changes in heading can
lead to high roll angles (one degree change in heading causes a change in roll angle
of about eight degrees). The pitch angle varies during climb or dive periods, extending
or retracting of towing cable, acceleration or deceleration, and even when flying at too
low or too high true airspeed depending on altitude. Values of pitch angle between −5◦
(dive) and 8◦(climb and retracting towing cable) have been observed. While change
in attitude is not problematic for cloud particle property measurements it is for radia-
tion measurements. Here, the deviation from the horizontal should be no more than 3◦
to avoid large errors. When keeping the above mentioned flight parameters constant,
sufficiently stable flight conditions can be maintained to perform high-quality irradiance
measurements with AIRTOSS in future experiments. During this test campaign also
observations of cloud microphysical data as for example droplet number concentra-
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AMTD
2, 1–35, 2009
The AIRTOSS
(AIRcraft TOwed
Sensor Shuttle)
W. Frey et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
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tions and size distributions with the AIRTOSS in stratocumulus clouds were performed
to prove the compliance with scientific needs. Simultaneous radiation measurements
of the clouds have been made. The measurements of internal operational data of AIR-
TOSS as well as the first atmospheric data demonstrate the suitability of this tandem
platform for detailed cloud microphysics and radiation interaction studies.
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1Introduction
Clouds constitute an important factor in the global climate since they affect the ra-
diation balance of the Earth-atmosphere system in complicated but significant ways
(cooling and warming of the atmosphere, as discussed in the IPCC report (Solomon
et al., 2007)). Widely varying cloud micro- and macrophysical properties lead to high
variability in the radiative impact of clouds which makes it hard to parameterise their
influence on Earth’s radiation budget. In order to investigate the link between cloud
micro- and macrophysical properties by in-situ measurements it is highly desirable to
assure simultaneous observations of microphysical particle properties within the clouds
and radiation measurements above and beneath the clouds. So far this has been at-
tempted by using several aircraft in stack during numerous airborne experiments such
as the Cirrus Regional Study of Tropical Anvils and Cirrus Layers – Florida Area Cir-
rus Experiment (CRYSTAL-FACE, Jensen et al., 2004) or the Tropical Composition,
Cloud and Climate Coupling (TC4) mission (Toon, 2007). These attempts of collocated
airborne sampling have only partly been successful.
There are serious practical difficulties encountered when using more than one air-
craft in cloudy atmospheres: Tight flight safety regulations complicate the coordination
of the aircraft, especially for close proximity of aircraft. It seems almost impossible to
obtain truly synchronised measurements when aircraft are flying at different speeds. In
addition, flying with several aircraft is rather expensive.
Therefore, a new tandem measurement constellation has been developed, consist-
ing of two instrumental carriers: a Learjet 35A research aircraft and the AIRcraft TOwed
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Sensor Shuttle (AIRTOSS). AIRTOSS is a sensor pod which is attached to a winch
under the aircraft wing. It can be detached from, towed by, and retracted onto the
Learjet. The Learjet is presently mainly equipped with radiation instruments, while the
AIRTOSS currently carries instrumentation to measure cloud microphysical properties.
Towing cable length and airspeed can be varied to adapt for different vertical profile
measurements. The position of the AIRTOSS can be controlled such that it stays away
from the exhaust or contrail of the Learjet. With this novel tandem setup truly collocated
measurements in and around clouds can be performed for the first time.
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2AIRTOSS/Learjet tandem: implemented instrumentation
The sensor locations of the instruments within the tandem measurement constellation
is shown in Fig. 1. The instruments and their technical specifications are listed in
Table 1.
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2.1AIRTOSS
The empty drag-body of AIRTOSS (see Fig. 1a) is 2.57m long, has a diameter of
0.24m and a weight of 27kg. With the Cloud Imaging Probe (CIP) installed the length
is 2.85m. AIRTOSS has a maximum payload of 43kg and thus, maximum weight of
70kg. During the campaign the weight of the fully equipped AIRTOSS was 54.2kg,
see Table 2 for details. This table also shows the power consumption of the different
devices (see at the end of this section) and the rough centres of gravity for the devices.
The centre of gravity of the fully equipped AIRTOSS is located 50mm behind the hook
to keep the AIRTOSS in a horizontal position during flight. In Fig. 2 a sketch of AIR-
TOSS with the positions of the instruments, batteries, and computer is presented. An
Inertial Navigation System (INS) is installed in the rear part of the drag-body to mea-
sure the attitude angles (roll, pitch, and heading) and accelerations. For measuring
the exact position a Global Positioning System (GPS) is placed in front of the INS. The
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CIP is located at the tip of AIRTOSS. It delivers 2-dimensional shadow images of cloud
particles in a size range of 25–1600µm in diameter (Baumgardner et al., 2001). Thus,
particle shapes, particle size distributions, and number concentrations are obtained
from the CIP measurements. Power supply for the AIRTOSS devices is provided by
batteries. During the test campaign 22 batteries with a nominal voltage of 1.2V and
capacitance of 15Ah which are connected in series are used. With an estimated con-
sumption of 26V at a current of 8.35A the batteries are running for about one hour and
48min. Table 2 shows how power consumptions of AIRTOSS devices are distributed.
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2.2Learjet instrumentation
The winch for the AIRTOSS is installed beneath the right wing of the Learjet as shown
on Fig. 1b. An instrumented wingpod is attached to the left wing (see Fig. 1a), fur-
ther instruments are installed on top of the fuselage. Here, the Stabilized Platform for
Airborne Radiation Measurements (SPARM) is used for horizontal stabilisation of the
spectral irradiance sensor which, in the current configuration, measures downwelling ir-
radiance (F↓
platform as well as the radiation sensors is given in Wendisch et al. (2001). A sen-
sor to measure spectral upwelling radiances (I↑
of 350–2200nm is installed in the wingpod. A digital, multispectral 2-D CCD camera
(DuncanTech, 2002) is installed in the middle of the wingpod and looks downward with
a viewing angle of 58.1◦. It measures upwelling radiances in the green (550nm), red
(660nm), and near-infrared (880nm) wavelength ranges. At the front of the wingpod
a Forward Scattering Spectrometer Probe (FSSP-100) is installed to measure cloud
particle size distributions in the size range of 2–47µm (Dye and Baumgardner, 1984).
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λ) in a wavelength-range of 350–2200nm. A description of the stabilisation
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λ) also covering the wavelength-range
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