The CLARE '98 airborne and ground-based campaign at the Chilbolton Observatory (5 - 23 October 1998) was undertaken in the context of the ESA Earth Radiation Mission; ERM later merged with the Japanese ATMOS-B1 mission and evolved into EarthCARE (Clouds, Aerosol and radiation Explorer) that has been selected as an ESA Earth Explorer Core mission.

The scientific objective of the Earth Radiation Mission (ERM) was to provide a set of highly resolved cloud profiling observations that are needed to increase understanding of the energy transport, and the phase changes of water, between the Earth's surface and the top of the atmosphere.
The ERM components were to:

• measure vertical structure of cloud and aerosol fields in connection with their horizontal distribution, and over all climate zones;
  
  
• correlatively measure the radiation budget components at the top of the atmosphere;
  
  
• derive the vertical structure and horizontal distribution of radiation budget components, cloud water and cloud ice content, aerosol optical thickness, and other geophysical parameters, using the ERM measurements in synergy with other simultaneous data.

CLARE '98 campaign objectives

The CLARE '98 campaign was part of ESA's Earth Observation Preparatory Programme (EOPP). The objectives were to collect and analyse radar and lidar as well as in-situ data to support the development of retrieval algorithms and to consolidate the scientific requirements of the future ERM.

CLARE '98 measurements

Three aircraft were employed; one performed in-situ measurements while the other two made remote measurements from different altitudes. Complementing the airborne measurements, an extensive set of ground-based measurements was made at the Chilbolton Observatory, Hampshire, UK.

Aircraft measurements:

• UKMO-MRF Hercules C-130, performed in-situ measurements of temperature, wind, humidity, particle size spectra (FSSP, 2D-C and 2D-P), and bulk water (Johnson-Williams probe). In addition, the C-130 performed radiative measurements (broad-band, narrow-band plus microwave).
  
  
• INSU/IPSL Fokker 27 (ARAT), performed measurements with a 94 GHz cloud radar ('Kestrel' from the University of Wyoming), a lidar ('Leandre') and an array of radiometers.
  
  
• DLR Falcon, carried out high-altitude measurements with the 'Alex' lidar and the 'Fubiss' spectrometer, as well as short- and long-wave radiometers.

Ground-based measurements at the Chilbolton Observatory:

• 3 GHz scanning 'CAMRa' radar
• 35 GHz scanning 'Rabelais' radar
• 94 GHz vertically-pointing 'Galileo' radar
• 95 GHz vertically-pointing 'Miracle' radar
• 22.2, 28.8 and 94 GHz zenith-pointing radiometers
• 35 GHz radiometer
• 21.3, 23.8, 31.65, 51.25, 53.85 and 54.85 GHz multi-frequency radiometer.
• IR radometers (2)
• UV Raman lidar
• CT-75K Lidar ceilometer

Rain rate, temperature and humidity measurements:

• VIS-IR sensor
• Video camera
• Cloud camera

Summary of CLARE '98 achievements

1. There were seven successful aircraft missions probing a variety of clouds.
2. It was shown that a combination of radar and lidar spaceborne sensors should be able to detect virtually all radiatively significant ice clouds and the majority of water clouds together with their tops and bases, and any multiple cloud layers.
3. Techniques were developed and validated for deriving the vertical profiles of liquid water cloud characteristics.
4. The combination of radar and lidar measurements was shown to a powerful way of identifying layers of supercooled cloud.
5. It was shown that the combination of radar on lidar on the same space platform will be a uniquely powerful tool for quantifying cloud ice water content, and the effective radius of particles in ice cloud.

Further details of the CLARE '98 campaign are available in the International Workshop Proceedings of the CLARE '98 experiment. ESA Publication WPP-170 (1999)