Polar Mesospheric Cloud Measurements from HALOE
Mark Hervig, GATS Inc., Driggs, Idaho
Polar mesospheric clouds (PMCs) occur in both hemispheres
near the summer solstice at high latitudes and altitudes near the mesopause
(~82 km) [e.g., Alpers et al., 2000]. From the ground these
clouds appear in reflected sunlight against the twilight sky. These
sightings can be brilliant, leading ground-based observers to call
them noctilucent or ”night-shining” clouds (NLCs) (Figure 1). Wegener
 was the first to suggest that these clouds could be composed of
water ice, and this theory has persisted because PMCs have been associated
with ice supersaturations and because their growth appears to be an exponential
function of temperature [e.g., Rusch et al. 1991; Lübken
et al., 1996]. Until now, this assertion has remained unconfirmed
by observation because PMCs had only been measured at ultraviolet to near-infrared
wavelengths where particulate spectra are void of unique absorption features
or “fingerprints.” PMCs were measured in the infrared for the first
time by the Halogen Occultation Experiment (HALOE). PMC extinctions
retrieved from HALOE measurements at eight wavelengths show remarkable
agreement with model spectra based on ice particle extinction (Figure 2).
The infrared spectrum of ice has a unique signature, and the HALOE-model
agreement thus provides the first physical confirmation that water ice
is the primary component of PMCs [Hervig et al., 2001].
|Figure 1. Noctilucent clouds
photorgaphed by Pekka Parviainen in Finland (top) and Peter Dalin in Russia
||Figure 2. HALOE PMC extinctions
compared to modeled extinction spectra. The HALOE data are averages
based on 16 PMC measurements from July 25 to August 4, 1997,
between 62°N and 72°N latitude. Vertical bars on each HALOE
point represent the standard deviation of these measurements. Model
spectra were calculated using the average PMC size distribution from von
Cossart et al.  with ice refractive indices from Bertie et
al.  for 100 K temperature, and from Toon et al.
 for 163 K temperature. The model spectra were scaled to match
the HALOE extinction at 6.62 mm wavelength.
These scale factors were 0.76 and 0.65 for results based on the Bertie
et al. and Toon et al. indices.
Alpers, M., M. Gerding, J. Hoffner, and U. von Zahn, NLC particle properties
from a five color lidar observation at 54°N, J. Geophys. Res., 105,
Bertie, J. E., H. J. Labbe, and E. Whalley, Absorptivity of ice I in
the Range 4000-30 cm-1, J. Chem. Phys., 50, 4501-4520, 1969.
Hervig, M. E., R. E. Thompson, M. McHugh, L. L. Gordley, J. M. Russell
III, and M. E. Summers, First confirmation that water ice is the primary
component of polar mesospheric clouds, Geophys. Res. Letters, 28, 971-974,
Lübken, F-J., K. H. Fricke, and M. Langer, Noctilucent clouds
the thermal structure near the Arctic mesopause in summer, J. Geophys.
Res., 101, 9489-9508, 1996.
Rusch, D. W., G. E. Thomas, and E. J. Jensen, Particle size distributions
in polar mesospheric clouds derived from solar mesosphere explorer measurements,
J. Geophys. Res., 96, 12,933-12,939, 1991.
von Cossart, G., J. Fiedler, and U. von Zahn, Size distributions of
NLC particles as determined from 3-color observations of NLC by ground-based
lidar, Geophys. Res. Lett., 26, 1513-1516, 1999.
Wegener, A., Die Erforschung der obersten Atmospharenschichten, Gerlands
Beitr. Geophys., 11, 102, 1912.
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