Case study of an ice void structure in polar mesospheric clouds

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

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

Introduction

Five concentric atmospheric gravity wave (AGW) events have been identified in Polar Mesospheric Cloud (PMC) images of the summer mesopause region (~82–84 km) made by the Cloud Imaging and Particle Size (CIPS) instrument on board the Aeronomy of Ice in the Mesosphere satellite during the Northern Hemisphere 2007 and 2009 PMC seasons. The AGWs modulate the PMC albedo, ice water content, and particle size, creating concentric ring patterns. On only one occasion (13 July 2007), the concentric AGWs in PMCs were aligned with AGWs with similar shapes observed in 4.3 µm radiance in the lower stratosphere, as measured by Atmospheric Infrared Sounder (AIRS). Coincident AIRS and Infrared Atmospheric Sounding Interferometer nadir measurements of 8.1 µm radiance reveal a region of deep convection in the troposphere close to the estimated centers of the AGWs in the stratosphere, strongly suggesting that convection is the wave source. The AGWs in CIPS on 13 July 2007 were ~1000 km away from the observed deep convection. Three other concentric AGWs in PMCs were 500–1000 km away from deep convection in the troposphere, while no convection was observed related to the wave on 29 July 2009. We perform a 2‐D ray tracing study for the AGW event on 13 July 2007. The calculated propagation distance is much shorter than the distance between the AGWs in PMCs and the observed convection. The 2‐D ray tracing study indicates that the AGWs in PMCs and in the stratosphere are probably excited by different tropospheric convective systems.

Abstract. The variation trends and characteristics of polar mesospheric clouds (PMCs) are important for studying the evolution of atmospheric systems and understanding various atmospheric dynamic processes. Through observation and analysis of PMCs, we can gain a comprehensive understanding of the mechanisms driving atmospheric processes, providing a scientific basis and support for addressing climate change. Ultraviolet (UV) imaging technology, adopted by the Cloud Imaging and Particle Size (CIPS) instrument on board the Aeronomy of Ice in the Mesosphere (AIM) satellite, has significantly advanced the research on PMCs. Due to the retirement of the AIM satellite, there is currently no concrete plan for next-generation instruments based on the CIPS model, resulting in a discontinuity in the observation data sequence. In this study, we propose a compact and cost-effective wide-field-of-view ultraviolet imager (WFUI) that can be integrated into various satellite platforms for future PMC observation missions. A forward model was built to evaluate the detection capability and efficiency of the WFUI. CIPS and Solar Occultation for Ice Experiment (SOFIE) data were fused to reconstruct a three-dimensional PMC scene as the input background. Based on the scattering and extinction characteristics of ice particles and atmospheric molecules, the radiative transfer was calculated using the solar radiation path through the atmosphere and PMCs. The optical system and satellite platform parameters of the WFUI were selected according to CIPS, enabling the calculation of the number of photons received by the WFUI. The actual detection signal is then simulated by photoelectric conversion, and the PMC information can be obtained by removing detector noise. Subsequently, a comparison with the input background field was conducted to compute and analyze the detection efficiency. Additionally, a sensitivity analysis of the instrument and platform parameters was conducted. Simulations were performed for both individual orbits and for the entire PMC seasons. The research results demonstrate that the WFUI performs well in PMC detection and has high detection efficiency. Statistical analysis of the detection efficiency using data from 2008 to 2012 revealed an exponential relationship between the ice water content (IWC) of PMCs and detection efficiency. During the initial and final durations of the PMC season, when the IWC was relatively low, the detection efficiency remained limited. However, as the season progressed and the IWC increased, the detection efficiency significantly improved. We note that regions at lower latitudes exhibited a lower IWC and, consequently, lower detection efficiency. In contrast, regions at higher latitudes, with a greater IWC, demonstrated better detection efficiency. Additionally, the sensitivity analysis results suggest that increasing the satellite orbit altitude and expanding the field of view (FOV) of the WFUI both contribute to improving the detection efficiency.

Gravity wave observations in the summertime polar mesosphere from the Cloud Imaging and Particle Size (CIPS) experiment on the AIM spacecraft

Evaluation of AIM CIPS measurements of Polar Mesospheric Clouds by comparison with SBUV data

Retrieval of polar mesospheric cloud properties from CIPS: Algorithm description, error analysis and cloud detection sensitivity

The cloud imaging and particle size experiment on the aeronomy of ice in the mesosphere mission: Cloud morphology for the northern 2007 season

Comparison of polar mesospheric cloud measurements from the Cloud Imaging and Particle Size experiment and the solar backscatter ultraviolet instrument in 2007

Phase functions of polar mesospheric cloud ice as observed by the CIPS instrument on the AIM satellite

The effects of atmospheric gravity waves (AGWs) on Polar Mesospheric Cloud (PMC) evolution and brightness are studied using a two dimensional version of the Community Aerosol and Radiation Model for Atmospheres (CARMA 2D). The primary objectives for doing CARMA modeling of AGW effects on PMCs are to address the question of whether AGWs can account for the rapid, orbit by orbit changes in cloud structure and brightness seen in overlapping regions from images of the Cloud Imaging and Particle Size (CIPS) experiment on board the Aeronomy of Ice in the Mesosphere (AIM) spacecraft. We present comparisons of PMC brightness changes between our numerical simulations and observations from the CIPS experiment. Previous modeling studies have indicated a much longer life‐time for PMC than the 90 min between CIPS orbits. We present CARMA 2D results showing dependence of ice particle growth and PMC brightness on AGW perturbation of background temperatures and water vapor concentrations. The model shows differences in brightness of PMCs due to differences in number of large ice particles depending on the scale and periods of the AGWs and also indicates that overall cloud brightness is a function of the wave period. While the maximum rate of change in PMC brightness from the model is still almost a factor of two less than the CIPS observed maximum rate of change in brightness, our study indicates that the variation in PMC brightness is in part due to the upward transport of water vapor into water depleted region by AGWs and the growth of ice particles from sub visual to visual and to larger sizes than they normally would have without AGWs. The presence of short period AGW cause periodic oscillations in cloud brightness about the no‐AGW brightness while long‐period AGW can temporarily increase the brightness of PMCs compared to the PMC brightness under no‐AGW case. However, both the short‐period and long‐period AGW ultimately reduce the domain averaged PMC brightness in the long‐term. This agrees with CIPS observations of generally dimmer PMCs in regions of high AGW activity. The seasonal variation in PMC albedos and the day to day variations seen in CIPS can be reproduced using a spectrum of short and long period AGW.

The cloud imaging and particle size (CIPS) experiment is one of three instruments on board the Aeronomy of Ice in the Mesosphere (AIM) spacecraft that was launched into a 600 km Sun‐synchronous orbit on 25 April 2007. CIPS images have shown distinct wave patterns and structures in polar mesospheric clouds (PMCs), around the summertime mesopause region, which are qualitatively similar to structures seen in noctilucent clouds (NLCs) from ground‐based photographs. The structures in PMC are generally considered to be manifestations of upward propagating atmospheric gravity waves (AGWs). Variability of AGW effects on PMC reported at several lidar sites has led to the notion of longitudinal differences in this relationship. This study compares the longitudinal variability in the CIPS‐observed wave occurrence frequency with CIPS‐measured PMC occurrence frequency and albedo along with mesospheric temperatures measured by the sounding of the atmosphere using broadband emission radiometry instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics spacecraft. Our results for the latitude ranges between 70° and 80° show a distinct anticorrelation of wave structures with cloud occurrence frequency and correlations with temperature perturbations for at least two of the four seasons analyzed, supporting the idea of gravity wave‐induced cloud sublimation. The locations of the observed wave events show regions of high wave activity in both hemispheres. In the Northern Hemisphere, while the longitudinal variability in observed wave structures show changes from the 2007–2008 seasons, there exist regions of both low and high wave activities common to the two seasons. These persistent features may explain some of the observed differences in PMC activity reported by ground‐based lidar instruments distributed at different longitudes. The statistical distribution of horizontal scales increases with wavelength up to at least 250 km. We also discuss the possibility of atmospheric tides, especially the nonmigrating semidiurnal tide, aliasing our observations and affecting the results presented in this analysis.

A gravity wave (GW) model that includes influences of temperature variations and large‐scale advection on polar mesospheric cloud (PMC) brightness having variable dependence on particle radius is developed. This Complex Geometry Compressible Atmosphere Model for PMCs (CGCAM‐PMC) is described and applied here for three‐dimensional (3‐D) GW packets undergoing self‐acceleration (SA) dynamics, breaking, momentum deposition, and secondary GW (SGW) generation below and at PMC altitudes. Results reveal that GW packets exhibiting strong SA and instability dynamics can induce significant PMC advection and large‐scale transport, and cause partial or total PMC sublimation. Responses modeled include PMC signatures of GW propagation and SA dynamics, “voids” having diameters of ∼500–1,200 km, and “fronts” with horizontal extents of ∼400–800 km. A number of these features closely resemble PMC imaging by the Cloud Imaging and Particle Size (CIPS) instrument aboard the Aeronomy of Ice in the Mesosphere (AIM) satellite. Specifically, initial CGCAM‐PMC results closely approximate various CIPS images of large voids surrounded by smaller void(s) for which dynamical explanations have not been offered to date. In these cases, the GW and instabilities dynamics of the initial GW packet are responsible for formation of the large void. The smaller void(s) at the trailing edge of a large void is (are) linked to the lower‐ or higher‐altitude SGW generation and primary mean‐flow forcing. We expect an important benefit of such modeling to be the ability to infer local forcing of the mesosphere and lower thermosphere (MLT) over significant depths when CGCAM‐PMC modeling is able to reasonably replicate PMC responses.

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.