POLAR MESOSPHERIC CLOUD MODELING AND SEASONAL FORECASTS WITH
Ryan Aschbrenner, James Grifﬁn, Hilary E. Snell
Atmospheric and Environmental Research, Inc., Lexington, MA
Polar mesospheric clouds (PMC) are thin ice clouds that form near the mesopause (at 80-85 km altitude) during summer months
at both poles. This atmospheric region is dynamically driven to temperatures persistently below 150 K for much of the summer
season. Additionally, dynamical and photochemical processes are responsible for producing a local water vapor maximum. The
low temperature and available moisture, when combined with the presence of cosmic dust, provide for a supersaturated zone
hospitable to the nucleation and sustained development of icy particles. Past studies have indicated that the ice persists at polar
latitudes for much of the midsummer . Furthermore, both ground and space-based observations have indicated an increasing
PMC frequency of occurrence and more common extensions of PMC beyond the polar regions , and long-term observations
by ultraviolet sensors have indicated an increased PMC brightness over recent decades . This trend is a proposed indicator
of global climate change, linked in part to increasing concentrations of carbon dioxide and methane .
As PMC become increasingly widespread, their potential impact on earth limb-viewing satellite sensors must be considered.
This amounts to (1) predicting the increased radiance (either due to thermal emission or scattered solar radiation) relative to
that encountered in the cloud-free mesopause region, and (2) providing nowcasts and forecasts of the spatial extent of the PMC
region throughout the summer months. We describe a system that addresses the above two points.
2. PMC MODEL
2.1. Radiance Model
Prediction of PMC limb radiance enhancement requires careful treatment of the cloud and viewing geometries and accurate
simulation of cloud emission and scattering. We employ a shell model to represent the three-dimensional extent of the PMC
ﬁeld. The shell delineates the atmospheric region expected to contain PMC ice, and it is deﬁned in terms of a cloud top and base,
along with latitudinal extent. Given detailed information on temperature and humidity, the shell may exhibit spatial variability,
or “patchiness” often associated with PMC. Within the shell, ice absorption and scattering efﬁciencies are computed from Mie
theory, which, in turn, determine the absorption and scattering coefﬁcients needed for computing radiative impacts. Ice particles
are assumed to obey a lognormal size distribution, with the largest particles concentrated near cloud base due to sedimentation.
Sensor lines of sight are traced through this shell to allow for calculation of the path-integrated PMC radiance.
To provide a nowcast/forecast of PMC formation and sustained development, we rely on the spatial distribution of the supersat-
urated zone. This distribution is computed using 3D ﬁelds of the saturation ratio, which is deﬁned as the ratio of water vapor
partial pressure to the saturation ratio of water vapor over ice. Since the saturation ratio depends exponentially on temperature
(and linearly on humidity), accurate knowledge of upper mesospheric temperature is critical. We report results obtained using
temperature and humidity retrievals from the Microwave Limb Sounder (MLS) on board the NASA Aura satellite.
Our model provides the PMC spectral radiance contribution, which may be combined with radiance from a clear sky background
model to infer the enhancement attributed to the PMC. Our results demonstrate that PMC emission is signiﬁcant in the infrared
windows near 8 and 13 μm. In particular, the emission exceeds the clear sky background in the 13 μm window by approximately
a factor of ten, and by a factor of 100 in the 8 μm window. The calculations for 13 μm agree with limb-viewing observations of
PMC by the SPIRIT III radiometer on board the Mid-Course Space Experiment (MSX) satellite.
We have obtained three years worth of MLS temperature and humidity data (2005-2007) and have computed ﬁelds of the
saturation ratio for complete PMC seasons, which generally extend from May through September in the northern hemisphere,
with the peak of the season occurring in early July. Our results show continuous high levels of supersaturation at high latitudes
(above approximately 60◦ ) for approximately a month surrounding summer solstice. Equatorward of this region, our model
provides a patchy PMC ﬁeld that makes excursions below 50◦ during the peak of the PMC season. The patchy appearance
is consistent with observations of noctilucent clouds, which is the term applied to PMC when viewed from the ground in the
visible. Examination of an image provided by the Cloud Imaging and Particle Size Experiment, which is an instrument on
the NASA Aeronomy of Ice in the Mesosphere (AIM) mission, shows encouraging agreement with the PMC ﬁeld computed
using MLS data. As additional AIM data becomes available, we plan to further assess the utility of the saturation ratio in
both outlining regions of the mesopause region affected by PMC and in identifying the extent to which PMC ice would impact
 U. von Zahn and U. Berger, “Persistent ice cloud in the midsummer upper mesosphere at high latitudes: Three-dimensional
modeling and cloud interactions with ambient water vapor,” J. Geophys. Res., vol. 108, no. D8, pp. 19–1 – 19–15, 2003.
 E.P. Shettle, G.E. Thomas, J.J. Olivero, W.F.J. Evans, and D.J. Debrestian, “Three-satellite comparison of polar mesospheric
clouds: Evidence for long-term change,” J. Geophys. Res., vol. 107, no. D12, pp. 2–1 – 2–9, 2002.
 M.T DeLand, E.P Shettle, G.E. Thomas, and J.J. Olivero, “Solar backscattered ultraviolet (SBUV) observations of polar
mesospheric clouds (PMCs) over two solar cycles,” J. Geophys. Res., vol. 108, no. D8, pp. 12–1 – 12–18, 2003.
 G.E. Thomas, “Global change in the mesosphere-lower thermosphere region: Has it already arrived?,” J. Atmos. Terr.
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