HALO

Introduction 

Insufficient knowledge about clouds, the physical (and chemical) environment that shapes their behaviour, and their interaction in the atmosphere contributes to large uncertainties in climate understanding and modelling. To advance our understanding of clouds and moist processes in general, and to help take advantage of a new generation of remote sensors, the MPI-M has played an active role in the procurement and configuration of the new HALO (High-Altitude Long-endurance) aircraft.
During the next years we anticipate actively using this new, state-of-the-art research-platform, and the instrumentation developed specifically for it.

HALO is based on a production G550 business jet from Gulfstream Aerospace Cooperation modified in light of the necessities of airborne scientific research. This research aircraft can fly into the lower tropical stratosphere (55,000 ft), and an endurance approaching 12 hours. The only comparable platform is the new HIAPER aircraft of the United States NSF (managed by NCAR), which is based on the G500, the predecessor to the G550.  Joint with colleagues at the University of Hamburg, the MPI-M developed the HAMP (Hamburg Microwave Package) to be mounted on a belly pod of HALO.  

HAMP consists of a highly sensitive cloud radar and passive microwave remote sensors, and was designed to bridge in situ measurements and satellite remote sensing.  Cloud Remote Sensing Instrumentation (HAMP) for HALO  MPIfM is providing an advanced set of microwave remote cloud sensing instrumentation to be operated on board of HALO in collaboration with the University Hamburg and the German Aerospace Center. The HAMP (HALO Microwave Package) consists of a cloud radar and a suite of passive radiometers in different frequency bands. We expect that this unique combination of active and passive sensors will help to remove many of the ambiguities inherent in approaches employing pure passive or active remote sensing techniques. While radars provide excellent spatial resolution but only ambiguous information on the microphysical conditions in the sampled volume, radiometers have only poor or no spatial resolution but deliver integral constraints in the observed column as for example the liquid water path or ice/water partitioning. Thus the information provided by these active and passive sensors is largely complementary.

Cloud Radar 

The radar MIRA36 that is planned to be installed on HALO operates at 36.5 GHz. The cloud radar, which is provided by the University Hamburg. The most important difference to CloudSat (which is in orbit as part of the A-train satellite constellation since 2006) is the higher sensitivity, which is further aggravated by higher attenuation at 94 GHz. While CloudSat can only detect drizzling or raining clouds, the air borne measurements will also capture a significant fraction of non-precipitating cloud types. The finer spatial resolution and polarization measurements is another aspect justifying air borne cloud observations.  

Radiometers 

Microwave radiometers sampling at a selected variety of attenuated and window-frequencies have also been procured for HAMP. Over ocean lower frequencies are used for the observation of clouds and rain.   Thereby information can be gained from the emission signal of the liquid hydrometeors in contrast to the low emissivity of the ocean background. Due to the high emissivity of land surfaces, emission based methods are not applicable for hydrometeor observations over land.  There higher frequencies (> 85 GHz) sensitive to scattering at frozen hydrometeors need to be applied. To perform observations over ocean and land HAMP includes frequencies between 22 and 183 GHz.  The specific frequencies were to match those of the AMSU A and B sounders.   The high frequency channel, for the purpose of rainfall retrieval over land was centered at the 118 GHz O2 absorption following a suggestion of Bauer and Mugnai (2003).