Abstract
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.
Highlights
The understanding of the Polar Mesospheric Cloud (PMC) particle size distribution has been developed in close synergy between various observational techniques and sophisticated numerical modeling, immensely increasing our insights in the complex dynamical and microphysical processes that affect the life cycle of the clouds (Rapp and Thomas, 2006; Merkel et al, 2009; Chandran et al, 2012; Wilms et al, 2016; Baumgarten et al, 2010)
Using a common volume approach, we showed that Cloud Imaging and Particle Size (CIPS) PMC albedo and ice water content (IWC) and Optical Spectrograph and InfraRed Imager System (OSIRIS) tomography PMC volume scattering coef ficient and ice mass density can be made into comparable quantities in a common volume, and that these quantities agree within the specified errors of each instrument
We extend our previous study between CIPS and OSIRIS and focus on particle sizes and espe cially the effect of the assumed shape of the size distribution using the most recent CIPS PMC data version 5.20
Summary
The understanding of the Polar Mesospheric Cloud (PMC) particle size distribution has been developed in close synergy between various observational techniques and sophisticated numerical modeling, immensely increasing our insights in the complex dynamical and microphysical processes that affect the life cycle of the clouds (Rapp and Thomas, 2006; Merkel et al, 2009; Chandran et al, 2012; Wilms et al, 2016; Baumgarten et al, 2010). Some of the very first studies of particle sizes were reported by Witt (1960) and Tozer and Beeson (1974), measuring linear polarization of scattered light from PMC using ground based ob servations and rocket experiments These early findings indicated par ticle sizes smaller than 200 nm. The first particle size estimates from satellite experiments were reported by Thomas and McKay (1985) using data from Solar Mesosphere Explorer (Barth et al, 1983) satellite measuring a combination of UV and visible light to derive color ratios of PMC scattering Their findings suggested that the effective particle size is smaller than 70 nm. The limited information in the optical signal for size esti mates is a problem that inherently makes it hard to compare size results from different instruments
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