Observations Of Polar Mesospheric Clouds Research Articles

Formation of ice particles through nucleation in the mesosphere

Abstract. Observations of polar mesospheric clouds have revealed the presence of solid ice particles in the upper mesosphere at high latitudes; however, their formation mechanism remains uncertain. In this study, we investigated the formation process of ice particles through nucleation from small amounts of water vapor at low temperatures. Previous studies that used classical nucleation theory have shown that amorphous solid water particles can nucleate homogeneously at conditions that are present in the mesosphere. However, the rate predictions for water in classical nucleation theory disagree with experimental measurements by several orders of magnitude. We adopted a semi-phenomenological model for the nucleation process, which corrects the evaluation of the molecular cluster formation energy using the second virial coefficient, which agrees with both experiments and molecular dynamics simulations. To calculate the nucleation process, we applied atmospheric conditions for the temperature, pressure, numerical density of dust grains, and cooling rate. The results indicate that homogeneous water nucleation is extremely unlikely to occur in the mesosphere, while heterogeneous nucleation occurs effectively. Dust grains generated by meteor ablation can serve as nuclei for heterogeneous nucleation. We also showed that the ice can form directly in a crystalline state, rather than an amorphous state.

Case Study of a Large Mesospheric Front in Polar Mesospheric Clouds

A large mesospheric front structure was observed on 16 July 2010 in Polar Mesospheric Clouds (PMCs) using common volume observations performed by the Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) instrument and the Odin Optical Spectrograph and Infrared Imager System (OSIRIS) at ∼75°N, 144°E. During the 4.5 hours long observation time, the front structure manifested in the PMC layer as an ice free elongated structure with sharp edges to the surrounding cloud field. A propagation direction from Southeast to Northwest and simultaneous clockwise rotation of 12 deg/hour are observed and the horizontal extent of the structure is found to be about 1800 km long and 190 km wide. Common volume observations of the mesospheric environment in terms of temperatures and water vapor provided by the Odin Sub-Millimetre Radiometer (SMR) indicate an extensive elevated warm air mass during the occurrence of the front structure and colder temperatures at the sharp front edge. The presence of a wave structure with λ<em>_z</em>∼6 km at the altitude of 70–90 km coincides with a sharpening of the front edge. We compare the general characteristics of the current mesospheric front to earlier reports on mesospheric fronts based on ground-based and space-borne airglow and PMC observations.

Detection of Polar Mesospheric Clouds Utilizing Himawari‐8/AHI Full‐Disk Images

AbstractWith the objective of advancing the polar mesospheric cloud (PMC) detection capability by the Advanced Himawari Imager (AHI) onboard the Japanese geostationary‐Earth‐orbit (GEO) meteorological satellite Himawari‐8, a novel two‐step PMC detection technique applied to the Himawari‐8/AHI full‐disk images has been developed. The two‐step approach is dividing the PMC detection into stronger (the first step) and weaker (the second step) signals and enhances the detection capability while significantly decreasing the false PMC detections. The improved PMC sensitivity by Himawari‐8/AHI is comparable with the Cloud Imaging and Particle Size (CIPS) onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite. With this encouraging result, PMC observations from Himawari‐8/AHI provide an additional extensive data set to the aeronomy and space science community.

Horizontal Movement of Polar Mesospheric Clouds Observed From the Himawari‐8 Geostationary Meteorological Satellite

AbstractThe horizontal movement of polar mesospheric clouds (PMCs) is examined with data from Geostationary‐Earth‐Orbit (GEO) meteorological satellite Himawari‐8 using a cloud tracking technique. While previous PMC observations from the ground and Low‐Earth‐Orbit satellites have a limitation on local time coverage or field of view, observations from the GEO satellite provide a great opportunity to study the PMC variability with entire local time coverage and a wide field of view. A pattern matching technique is applied to PMC data from the Advanced Himawari Imagers (AHIs) onboard Himawari‐8, and the horizontal movement of PMC along the transverse direction to the line of sight is derived. At the center of field of view of AHI, the tracking velocity represents the zonal movement of PMCs around the latitude of 81.3°N and the longitude of 140.7°E. The horizontal movement shows large oscillations with amplitudes of 10–30 m/s and spectral peaks at periods of 12, 14.8, and 24 h. The mean zonal velocity for the northern summer season of 2018 is about 8 m/s in the westward direction. The characteristics of the tracking winds resemble those of horizontal wind reported by previous studies. This indicates that the horizontal movements of PMC observed by Himawari‐8 are mainly subjected to wind advection.

New insights on polar mesospheric cloud particle size distributions from a two-satellite common volume study

The particle size distribution of Polar Mesospheric Clouds (PMC) is closely related to the fundamental processes of cloud formation and evolution. Still, despite substantial observational efforts, specific details about the particle size distribution have remained obscure. In this study, we aim at deriving more constraints on PMC size distributions by combining optical measurements from two satellite instruments observing a common PMC volume. We use a special set of 2D tomographic limb observations from the Optical Spectrograph and Infrared Imager System (OSIRIS) on the Odin satellite from 2010 to 2011 in the latitude range 78° N to 80° N and compare these to simultaneous PMC observations from the nadir-viewing Cloud Imaging and Particle Size (CIPS) instrument on the AIM satellite. A key goal is to find the assumption on the mathematical shape of the particle size distribution that should be applied to a vertically resolving limb-viewing instrument to reach consistent size results compared to the column-integrated ice distribution as seen by a nadir-viewing instrument. Our results demonstrate that viewing geometry and sampling volume of each instrument must be carefully considered and that the same size distribution assumption cannot simultaneously describe a column-integrated and a local height-resolved size distribution. In particular, applying the standard Gaussian assumption, used by many earlier PMC studies, to both limb and nadir observation leads to an overestimate of particle sizes seen by OSIRIS by about 10 nm as compared to CIPS. We show that the agreement can be improved if a Log-normal assumption with a broad distribution width around σ = 1.42 is adopted for OSIRIS. A reason for this broad distribution best describing the OSIRIS observations we suggest the large retrieval volume of the limb measurement. Gravity waves and other small-scale processes can cause horizontal variations and a co-existence of a wide range of particle populations in the sampling volume. Horizontal integration then leads to apparently much broader size distributions than encountered in a small horizontal sampling volume.

PMC observations from the OMPS Limb Profiler

We present new observations of polar mesospheric clouds (PMCs) from the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (LP) instrument. The OMPS LP limb scattering measurements provide altitude-resolved PMC observations with full global coverage from 2012 to the present. Relative PMC brightness observed by OMPS LP varies between hemispheres due to changes in viewing geometry, consistent with other limb scattering PMC measurements. LP season start dates for PMC detection are comparable to the CIPS instrument on the AIM satellite. LP also views the same location as nadir-viewing PMC measurements from the OMPS Nadir Profiler with a minimal time difference. These “common volume” measurements can be used to improve the quality of the 40+ year satellite PMC data record by reducing the number of false detections in SBUV-type measurements at mid-latitudes.

Common volume satellite studies of polar mesospheric clouds with Odin/OSIRIS tomography and AIM/CIPS nadir imaging

Abstract. Two important approaches for satellite studies of polar mesospheric clouds (PMCs) are nadir measurements adapting phase function analysis and limb measurements adapting spectroscopic analysis. Combining both approaches enables new studies of cloud structures and microphysical processes but is complicated by differences in scattering conditions, observation geometry and sensitivity. In this study, we compare common volume PMC observations from the nadir-viewing Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) satellite and a special set of tomographic limb observations from the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite performed over 18 d for the years 2010 and 2011 and the latitude range 78 to 80∘ N. While CIPS provides preeminent horizontal resolution, the OSIRIS tomographic analysis provides combined horizontal and vertical PMC information. This first direct comparison is an important step towards co-analysing CIPS and OSIRIS data, aiming at unprecedented insights into horizontal and vertical cloud processes. Important scientific questions on how the PMC life cycle is affected by changes in humidity and temperature due to atmospheric gravity waves, planetary waves and tides can be addressed by combining PMC observations in multiple dimensions. Two- and three-dimensional cloud structures simultaneously observed by CIPS and tomographic OSIRIS provide a useful tool for studies of cloud growth and sublimation. Moreover, the combined CIPS/tomographic OSIRIS dataset can be used for studies of even more fundamental character, such as the question of the assumption of the PMC particle size distribution. We perform the first thorough error characterization of OSIRIS tomographic cloud brightness and cloud ice water content (IWC). We establish a consistent method for comparing cloud properties from limb tomography and nadir observations, accounting for differences in scattering conditions, resolution and sensitivity. Based on an extensive common volume and a temporal coincidence criterion of only 5 min, our method enables a detailed comparison of PMC regions of varying brightness and IWC. However, since the dataset is limited to 18 d of observations this study does not include a comparison of cloud frequency. The cloud properties of the OSIRIS tomographic dataset are vertically resolved, while the cloud properties of the CIPS dataset is vertically integrated. To make these different quantities comparable, the OSIRIS tomographic cloud properties cloud scattering coefficient and ice mass density (IMD) have been integrated over the vertical extent of the cloud to form cloud albedo and IWC of the same quantity as CIPS cloud products. We find that the OSIRIS albedo (obtained from the vertical integration of the primary OSIRIS tomography product, cloud scattering coefficient) shows very good agreement with the primary CIPS product, cloud albedo, with a correlation coefficient of 0.96. However, OSIRIS systematically reports brighter clouds than CIPS and the bias between the instruments (OSIRIS – CIPS) is 3.4×10-6 sr−1 (±2.9×10-6 sr−1) on average. The OSIRIS tomography IWC (obtained from the vertical integration of IMD) agrees well with the CIPS IWC, with a correlation coefficient of 0.91. However, the IWC reported by OSIRIS is lower than CIPS, and we quantify the bias to −22 g km−2 (±14 g km−2) on average.

Initial report on polar mesospheric cloud observations by Himawari-8

Abstract. We provide an initial report on polar mesospheric cloud (PMC) observations by the Japanese Geostationary Earth Orbit (GEO) meteorological satellite Himawari-8. Heights of the observed PMCs were estimated to be 80–82 km. Observed PMCs were active only during summertime in both the northern and southern polar regions. These observations are consistent with known PMC behavior. From its almost fixed location relative to the Earth, Himawari-8 is capable of continuously monitoring PMC every 10 min with three visible bands: blue (0.47 µm), green (0.51 µm), and red (0.64 µm). Thus, Himawari-8 can contribute to PMC research in the near future.

Making limb and nadir measurements comparable: A common volume study of PMC brightness observed by Odin OSIRIS and AIM CIPS

Combining limb and nadir satellite observations of Polar Mesospheric Clouds (PMCs) has long been recognized as problematic due to differences in observation geometry, scattering conditions, and retrieval approaches. This study offers a method of comparing PMC brightness observations from the nadir-viewing Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) instrument and the limb-viewing Odin Optical Spectrograph and InfraRed Imaging System (OSIRIS). OSIRIS and CIPS measurements are made comparable by defining a common volume for overlapping OSIRIS and CIPS observations for two northern hemisphere (NH) PMC seasons: NH08 and NH09. We define a scattering intensity quantity that is suitable for either nadir or limb observations and for different scattering conditions. A known CIPS bias is applied, differences in instrument sensitivity are analyzed and taken into account, and effects of cloud inhomogeneity and common volume definition on the comparison are discussed. Not accounting for instrument sensitivity differences or inhomogeneities in the PMC field, the mean relative difference in cloud brightness (CIPS - OSIRIS) is −102 ± 55%. The differences are largest for coincidences with very inhomogeneous clouds that are dominated by pixels that CIPS reports as non-cloud points. Removing these coincidences, the mean relative difference in cloud brightness reduces to −6 ± 14%. The correlation coefficient between the CIPS and OSIRIS measurements of PMC brightness variations in space and time is remarkably high, at 0.94. Overall, the comparison shows excellent agreement despite different retrieval approaches and observation geometries.

Periodicities of polar mesospheric clouds inferred from a meteorological analysis and forecast system

AbstractThere is currently an ambiguity in what controls polar mesospheric cloud (PMC) periodicities near 83 km altitude. This is primarily because satellite and ground‐based data sets cannot resolve global mesospheric temperature variability over the diurnal cycle. To address this limitation, we employ a global meteorological analysis and forecast system that assimilates mesospheric satellite data with two significant advances. The first is that we use output at a more rapid one hourly cadence, allowing for a quantitative description of diurnal (24 h), semidiurnal (12 h), and terdiurnal oscillations. The second is that the output drives a simple PMC parameterization which depends only on the local temperature, pressure, and water vapor concentrations. Our study focuses on results from July 2009 in the Northern Hemisphere and January 2008 in the Southern Hemisphere. We find that the 24 h migrating temperature tide as well as the 12 h and 24 h nonmigrating tides dominate northern PMC oscillations whereas the 12 h and 24 h nonmigrating tides dominate southern oscillations. Monthly averaged amplitudes for each of these components are generally 2–6 K with the larger amplitudes at lower PMC latitudes (50°). The 2 day and 5 day planetary waves also contribute in both hemispheres, with monthly averaged amplitudes from 1 to 3 K although these amplitudes can be as high as 4–6 K on some days. Over length scales of ~1000 km and timescales of ~1 week, we find that local temperature oscillations adequately describe midlatitude PMC observations.

Comparing nadir and limb observations of polar mesospheric clouds: The effect of the assumed particle size distribution

Nadir viewing observations of Polar Mesospheric Clouds (PMCs) from the Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) spacecraft are compared to Common Volume (CV), limb-viewing observations by the Solar Occultation For Ice Experiment (SOFIE) also on AIM. CIPS makes multiple observations of PMC-scattered UV sunlight from a given location at a variety of geometries and uses the variation of the radiance with scattering angle to determine a cloud albedo, particle size distribution, and Ice Water Content (IWC). SOFIE uses IR solar occultation in 16 channels (0.3–5μm) to obtain altitude profiles of ice properties including the particle size distribution and IWC in addition to temperature, water vapor abundance, and other environmental parameters. CIPS and SOFIE made CV observations from 2007 to 2009. In order to compare the CV observations from the two instruments, SOFIE observations are used to predict the mean PMC properties observed by CIPS. Initial agreement is poor with SOFIE predicting particle size distributions with systematically smaller mean radii and a factor of two more albedo and IWC than observed by CIPS. We show that significantly improved agreement is obtained if the PMC ice is assumed to contain 0.5% meteoric smoke by mass, in agreement with previous studies. We show that the comparison is further improved if an adjustment is made in the CIPS data processing regarding the removal of Rayleigh scattered sunlight below the clouds. This change has an effect on the CV PMC, but is negligible for most of the observed clouds outside the CV. Finally, we examine the role of the assumed shape of the ice particle size distribution. Both experiments nominally assume the shape is Gaussian with a width parameter roughly half of the mean radius. We analyze modeled ice particle distributions and show that, for the column integrated ice distribution, Log-normal and Exponential distributions better represent the range of masses that contribute to the IWC. We further show that agreement between SOFIE and CIPS is further improved with the assumption of either Log-normal or Exponential ice particle size distributions. This improvement suggests that the range of mass bearing particle radii is larger, but not significantly shifted from what is obtained by assuming a Gaussian distribution. The assumption of an Exponential particle size distribution, as shown to be justifiable here, has the attractive benefits of being characterized with a single parameter, the mean radius, which greatly facilitates studies of the spatial and temporal variation of PMC particle size distributions as well as comparisons between observations and models. Overall, our results represent a validation of both the CIPS and SOFIE datasets.

Inter-hemispheric comparison of PMCs and their environment from SOFIE observations

Observations of polar mesospheric clouds (PMC) and their environment from the Solar Occultation For Ice Experiment (SOFIE) are examined to quantify differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH) during summer. The results indicate that hemispheric differences are smaller when using pressure as the vertical coordinate, instead of altitude. The peak in PMC mass density (Zmax) was found to exist 1.2km higher in the SH, but at about the same pressure (∼0.0055hPa) in both hemispheres. This occurs because systematically warmer temperatures in the SH polar summer stratosphere and mesosphere expand the overlying atmosphere to raise the altitudes of pressure levels in the SH mesosphere relative to the NH. For the five southern and five northern PMC seasons observed to date, the primary differences are that PMCs in the Northern Hemisphere are more frequent (24%±32%), have greater ice mass density (65%±26% at Zmax), and exist for a longer seasonal period (10±10 days). These differences are attributed primarily to lower temperatures (4 to 9K) in the north because water vapor differences are small (1.9%±5.4% at Zmax).

The content and composition of meteoric smoke in mesospheric ice particles from SOFIE observations

This work provides the first observational evidence that ice particles comprising polar mesospheric clouds (PMC) contain small amounts of meteoric smoke (0.01–3% by volume), using measurements from the Solar Occultation For Ice Experiment (SOFIE). PMC observations at wavelengths from the ultraviolet through the infrared are inconsistent with pure ice, but can be explained in terms of the extinction simulated for a mixture of ice and meteoric smoke. Simulations of ice–smoke mixtures considered 25 different smoke compositions. The PMC observations were consistent with smoke composed of carbon (C), wüstite (FeO), or magnesiowüstite (MgxFe1−xO, x=0.1–0.6), and inconsistent with the other compositions. A method was established for simultaneously retrieving PMC particle size and the volume fraction of smoke in ice using SOFIE observations at multiple wavelengths. Compared to assuming pure ice, SOFIE retrievals considering ice–smoke mixtures yield smaller ice radii (24%) and higher concentrations (137%).

The composition of meteoric smoke and the smoke content of mesospheric ice from SOFIE observations

This work provides the first observational constraints concerning the composition of meteoric smoke particles (MSP) using measurements from the Solar Occultation For Ice Experiment (SOFIE). Multi-wavelength smoke observations in the polar mesosphere suggest MSP compositions of olivine (Mg 2 x Fe 2−2 x SiO 4, x=0.4 or 0.5), carbon (C), magnetite (Fe 3O 4), wüstite (FeO), or magnesiowüstite (Mg x Fe 1− x O, x=0.1, 0.2, 0.6). The observations exclude smoke compositions of hematite, fayalite, proxene, magnesium silicate, and silica. SOFIE observations further indicate that ice particles comprising polar mesospheric clouds (PMC) contain small amounts of meteoric smoke (0.02–4% by volume). The PMC observations are consistent with smoke compositions of carbon, wüstite, or magnesiowüstite ( x=0.1–0.3). Smoke volume densities determined from MSP observations during Southern polar spring are lower than model results, while smoke volume densities consistent with PMC observations are higher than model results. These differences may be related to the models using a climatological atmosphere since dynamics are a dominant control on MSP abundance. Compared to assuming pure ice, SOFIE retrievals considering ice–smoke mixtures yield smaller ice radii (30%) and higher concentrations (190%).

First lidar observations of polar mesospheric clouds and Fe temperatures at McMurdo (77.8°S, 166.7°E), Antarctica

[1] We report the first lidar observations of polar mesospheric clouds (PMCs) and temperatures made with an Fe Boltzmann lidar at McMurdo, Antarctica in summer 2010–2011. Eighty-five hours of PMCs were observed between 21 Dec 2010 and 15 Feb 2011, giving an overall occurrence frequency of 29.9%. The mean PMC centroid altitude is 84.59 ± 0.17 km, confirming previous reports that clouds in the Southern Hemisphere are ∼1 km higher than in the North. By combining the McMurdo (77.8°S) observations with those obtained at the South Pole (90°S) and Rothera (67.5°S), we find that the mean PMC altitude increases with increasing latitude with a statistically significant ascending rate of 40 ± 3 m/deg. A negative correlation is found between the daily-mean temperatures at 90 km and PMC brightness. The observations provide direct evidence that the cold phase of wave-induced temperature oscillations facilitates PMC formation and Fe depletion, supporting previous modeling results.

Polar mesospheric clouds (PMCs) observed by the Ozone Monitoring Instrument (OMI) on Aura

Backscattered ultraviolet (BUV) instruments designed for measuring stratospheric ozone profiles have proven to be robust tools for observing polar mesospheric clouds (PMCs). These measurements are available for more than 30 years and have been used to demonstrate the existence of long‐term variations in PMC occurrence frequency and brightness. The Ozone Monitoring Instrument (OMI) on the EOS Aura satellite provides new and improved capabilities for PMC characterization. OMI uses smaller pixels than previous BUV instruments, which increases its ability to identify PMCs and discern more spatial structure, and its wide cross‐track viewing swath provides full polar coverage up to 90° latitude every day in both hemispheres. This cross‐track coverage allows the evolution of PMC regions to be followed over several consecutive orbits. Localized PMC variations determined from OMI measurements are consistent with coincident Solar Backscatter Ultraviolet model 2 (SBUV/2) measurements. Nine seasons of PMC observations from OMI are now available and clearly demonstrate the advantages of these measurements for PMC analysis.

SBUV observations of polar mesospheric clouds compared with MLS temperature and water vapor measurements

Earlier studies have shown that Polar Mesospheric Clouds (PMC) occur more frequently in the northern hemisphere (NH) than the SH, consistent with colder NH temperatures. Coincident PMC observations with the Solar Backscatter Ultraviolet instruments on the NOAA‐16 and NOAA‐18 satellite and temperature and water vapor measurements with the Microwave Limb Sounder on the Aura satellite support this result. These coincident measurements also show that, for similar temperatures and water vapor mixing ratios, PMCs occur more frequently and are brighter in the NH than the SH. Possible reasons for these hemispheric differences are discussed.

Modeling the temperature of the polar mesopause region: Part II—Intra-seasonal monthly oscillations

Measurements show that the polar mesospheric clouds (PMC) can vary, in the zonal mean, with periods around 1 month [Bailey et al., 2005. Observations of polar mesospheric clouds by the Student Nitric Oxide Explorer. J. Geophys. Res. 110, D13203, doi:10.1029/2004JD005422]. This observation has been the impetus for the present paper, where we describe corresponding temperature oscillations generated by the Numerical Spectral Model (NSM). Our numerical results are taken from the 3D and 2D versions of the NSM, which produce inter-annual and long-term variations in the polar mesopause region, as discussed in the accompanying paper (Part I). In the NSM, the intra-seasonal temperature variations with periods around 2 months are generated by the meridional winds that in turn are accelerated by the momentum deposition from small-scale gravity waves (GW) propagating north/south. The wave-driven dynamical process underlying the oscillations is intrinsically non-linear like that generating the quasi-biennial oscillation (QBO). Our analysis demonstrates that the seasonal annual and semi-annual variations excite the oscillation frequencies through non-linear cascading.

SOFIE PMC observations during the northern summer of 2007

This work examines the first season of polar mesospheric cloud (PMC) observations from the Solar Occultation for Ice Experiment (SOFIE). SOFIE observations of temperature, water vapor, and PMC frequency, mass density, particle shape, and size distribution are used to characterize the seasonal evolution and altitude dependence of mesospheric ice and the surrounding environment. SOFIE indicates that ice is nearly always present during summer, and that the ice layer is continuous from about 81 km altitude to the mesopause and above. Ice particles are observed to be more aspherical above and below the extinction peak altitude, suggesting a relationship between particle shape and mass density. The smallest particles are observed near the top of the ice layer while the largest particles exist at low concentrations near cloud base. A strong correlation was found between water vapor and particle size with small particles existing when H 2O is low. This relationship holds when examining variability in altitude, and variability over time at one altitude.

Ten years of Southern Hemisphere polar mesospheric cloud observations from the Polar Ozone and Aerosol Measurement instruments

Polar mesospheric clouds (PMCs) were observed in the Southern Hemisphere by both the Polar Ozone and Aerosol Measurement (POAM) II and III instruments. POAM II operated from 1993 to 1996 and its three seasons of Southern Hemisphere PMC observations were previously reported by Debrestian et al. (1997a, 1997b). POAM III operated from 1998 to 2005 and thus observed PMCs during seven Southern Hemisphere seasons. The two POAM instruments are in identical orbits and are very similar in design and measurement capability. These similarities allow for the application of a common PMC detection algorithm to generate a consistent long‐term data record by combining the two POAM data sets. In this paper we present an analysis of this combined data set, which consists of approximately 670 PMC observations. The seasonally averaged PMC occurrence frequency, when adjusted for differences in detection sensitivity of the instruments, is found to be strongly anticorrelated with the solar Lyman‐α flux. The POAM PMC profiles were analyzed using a geometrical cloud model to determine cloud height, vertical thickness, and wavelength‐dependent extinction. Seasonally averaged cloud altitudes range from 82.6 to 83.5 km and the thickness varies between approximately 2 and 3 km. The multiwavelength capability of the POAM III instrument has been used to derive information on PMC particle sizes using the spectral dependence of the measured slant path optical depth. The results of this analysis indicate a mean value of 52 nm for the PMC optically effective radius, which is consistent with a variety of other observations.