Use of Holography for Airborne Cloud Physics Measurements

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  • 1 Meteorological Office, Bracknell, Berkshire, England
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Abstract

The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well below the practical limit for the 2-D Cloud probe. Holographic ice concentrations appear to be systematically larger than those from the 2-D, typically by a factor of about half an order of magnitude. Some possible sources of error in each system have been examined but the exact cause of the discrepancy remains unproven. The relative unambiguity of the holographic sample volume suggests that this system will give the most reliable results, particularly for columns.

Abstract

The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well below the practical limit for the 2-D Cloud probe. Holographic ice concentrations appear to be systematically larger than those from the 2-D, typically by a factor of about half an order of magnitude. Some possible sources of error in each system have been examined but the exact cause of the discrepancy remains unproven. The relative unambiguity of the holographic sample volume suggests that this system will give the most reliable results, particularly for columns.

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