Ice clouds in satellite observations and climate models

Abstract

This thesis concerns the microphysical properties of clouds made up of ice particles, called ice clouds. Ice clouds are strong modulators of the outgoing longwave radiation and incoming shortwave radiation, yet our knowledge on several key ice cloud properties, which govern the magnitude and sign of the net contribution to the Earth’s atmospheric radiation budget, is inadequate. For instance, currently climate models are far from consensus on the magnitude and spatial distribution of ice water path (IWP), a vital radiative property of ice clouds, and the main property of concern in this thesis. The large spread amongst the models in terms of IWP is mostly due to the lack of constraints from observations on ice cloud properties. The lacking constraints reflect the major difficulties faced in observing global ice cloud properties.In-situ measurements provide useful sources of information on ice clouds, but are far from adequate due to the sparseness of measurements. Cloud ice observations from satellites provides a global view and is the most useful source of information. However, measurements from satellites also carry large uncertainties and are notoriously difficult to use for model evaluation, due to a mismatch on how IWP is defined in the models compared to what is actually observed. Not one satellite instrument can measure ice particle information from the entire ice cloud column, as desired from the model point of view. Satellite observations of IWP depend for the most part on the wavelength spectrum the instrument measures in, hence the instruments measure related, but different information on clouds.A study addressing the satellite observed and modeled IWP is presented in the first appended article: Eliasson et al. [2011]. Large differences between climate models are observations, especially in areas with frequent deep convection, were reported and discussed. The second appended article is a first evaluation study of cloud parameters, such as IWP, in the EC-Earth climate model using satellite A-Train observations. The model captures large-scale features for the most part but has problems related to ice water content and cloud fraction. This is strongly linked to the treatment of precipitation.The thesis contains introductory chapters on ice clouds; their formation, radiative importance, and representation in climate models. This is followed by a more in depth chapter on the observational data. The different satellite techniques are then discussed following a radiation physics and radiative transfer background section.Godkänd; 2011; 20110824 (ysko); LICENTIATSEMINARIUM Ämnesområde: Rymdteknik/Space Engineering Examinator: Professor Stefan Buehler, Institutionen för system- och rymdteknik, Luleå tekniska universitet Diskutant: Dr. sc. ETH Helmholtz Young Investigators Group Leader Corinna Hoose, Karlsruhe Institute of Technology, Germany Tid: Måndag den 24 oktober 2011 kl 10.00 Plats: Kiruna Rymdcampus, Luleå tekniska universite