G. Mioche

Using in-situ airborne measurements to evaluate three cloud phase products derived from CALIPSO

We compare the cloud detection and cloud phase determination of three independent climatologies based on Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) to airborne in situ measurements. Our analysis of the cloud detection shows that the differences between the satellite and in situ measurements mainly arise from three factors. First, averaging CALIPSO Level l data along track before cloud detection increases the estimate of high- and low-level cloud fractions. Second, the vertical averaging of Level 1 data before cloud detection tends to artificially increase the cloud vertical extent. Third, the differences in classification of fully attenuated pixels among the CALIPSO climatologies lead to differences in the low-level Arctic cloud fractions. In another section, we compare the cloudy pixels detected by colocated in situ and satellite observations to study the cloud phase determination. At midlatitudes, retrievals of homogeneous high ice clouds by CALIPSO data sets are very robust (more than 94.6% of agreement with in situ). In the Arctic, where the cloud phase vertical variability is larger within a 480 m pixel, all climatologies show disagreements with the in situ measurements and CALIPSO-General Circulation Models-Oriented Cloud Product (GOCCP) report significant undefined-phase clouds, which likely correspond to mixed-phase clouds. In all CALIPSO products, the phase determination is dominated by the cloud top phase. Finally, we use global statistics to demonstrate that main differences between the CALIPSO cloud phase products stem from the cloud detection (horizontal averaging, fully attenuated pixels) rather than the cloud phase determination procedures.

Influence of ice crystal shape on retrieval of cirrus optical thickness and effective radius: A case study

Airborne measurements of spectral upwelling radiances (350A¢Â�Â�2200 nm) reflected by cirrus using the Spectral Modular Airborne Radiation measurement sysTem (SMART)-Albedometer were made over land and water surfaces. Based on these data, cloud optical thickness tau and effective radius Reff of the observed cirrus were retrieved. By using different crystal shape assumptions (hexagonal plates, solid and hollow columns, rough aggregates, planar and spatial rosettes, ice spheres, and a mixture of particle habits) in the retrieval, the influence of crystal shape on the retrieved tau and Reff was evaluated. With relative differences of up to 70%, the influence of particle habit on t is larger than on Reff (up to 20% differences). Retrieved tau values agreed with values derived from concurrent lidar measurements within the measurement uncertainties.

Quasi-Spherical Ice in Convective Clouds

AbstractHomogeneous freezing of supercooled droplets occurs in convective systems in low and midlatitudes. This droplet-freezing process leads to the formation of a large amount of small ice particles, so-called frozen droplets, that are transported to the upper parts of anvil outflows, where they can influence the cloud radiative properties. However, the detailed microphysics and, thus, the scattering properties of these small ice particles are highly uncertain. Here, the link between the microphysical and optical properties of frozen droplets is investigated in cloud chamber experiments, where the frozen droplets were formed, grown, and sublimated under controlled conditions. It was found that frozen droplets developed a high degree of small-scale complexity after their initial formation and subsequent growth. During sublimation, the small-scale complexity disappeared, releasing a smooth and near-spherical ice particle. Angular light scattering and depolarization measurements confirmed that these sublim...

Spaceborne Remote Sensing and Airborne In Situ Observations of Arctic Mixed-Phase Clouds

Our predictive capability of Arctic climate change is severely hampered by a lack of understanding about key processes, notably related to mixed-phase clouds (MPCs). These cloud-related processes are suspected to play a major role in the Arctic energy budget due to their persistence and peculiar microphysical properties. This chapter focuses on the characterization of MPC properties in the Arctic region from in situ and satellite observations. The frequency of MPC occurrence over the Arctic region is determined from the CALIPSO and CloudSat satellite active remote sensing observations. Results highlight that the spatial, vertical and seasonal variability of MPC can be in part linked to the influence of the North Atlantic Ocean and the melting of sea ice. The microphysical and optical properties of the ice crystals and liquid droplets within MPC and the associated formation and growth processes responsible for the cloud life cycle are evaluated based on in situ airborne observations. Finally, the coupling of in situ MPC airborne measurements with the satellite active remote sensing is presented through a validation study to evaluate remote sensing retrieval algorithms and products such as cloud detection or cloud phase.