Ice crystal characterization in cirrus clouds: a sun-tracking camera system and automated detection algorithm for halo displays
Halo displays in the sky contain valuable information about ice crystal shape and orientation: e.g., the 22° halo is produced by randomly oriented hexagonal prisms while parhelia (sundogs) indicate oriented plates. HaloCam, a novel sun-tracking camera system for the automated observation of hal...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/10/2499/2017/amt-10-2499-2017.pdf |
| Summary: | Halo displays in the sky contain valuable information about ice
crystal shape and orientation: e.g., the 22° halo is produced
by randomly oriented hexagonal prisms while parhelia (sundogs)
indicate oriented plates. HaloCam, a novel sun-tracking camera system
for the automated observation of halo displays is presented. An
initial visual evaluation of the frequency of halo displays for the
ACCEPT (Analysis of the Composition of Clouds with Extended
Polarization Techniques) field campaign from October to mid-November
2014 showed that sundogs were observed more often than
22° halos. Thus, the majority of halo displays was produced
by oriented ice crystals. During the campaign about 27 % of
the cirrus clouds produced 22° halos, sundogs or upper tangent
arcs. To evaluate the HaloCam observations collected from regular
measurements in Munich between January 2014 and June 2016, an
automated detection algorithm for 22° halos was developed,
which can be extended to other halo types as well. This algorithm
detected 22° halos about 2 % of the time for this
dataset. The frequency of cirrus clouds during this time period was
estimated by co-located ceilometer measurements using temperature
thresholds of the cloud base. About 25 % of the detected
cirrus clouds occurred together with a 22° halo, which implies
that these clouds contained a certain fraction of smooth, hexagonal
ice crystals. HaloCam observations complemented by radiative transfer
simulations and measurements of aerosol and cirrus cloud optical
thickness (AOT and COT)
provide a possibility to retrieve more detailed information about ice
crystal roughness. This paper demonstrates the feasibility of
a completely automated method to collect and evaluate a long-term
database of halo observations and shows the potential to characterize
ice crystal properties. |
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| ISSN: | 1867-1381 1867-8548 |