Common volume satellite studies of polar mesospheric clouds with Odin/OSIRIS tomography and AIM/CIPS nadir imaging
Authors: Broman Lina , Benze Susanne , Gumbel Jörg , Christensen Ole Martin , Randall Cora E. 
Published: October 2019

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.