Lecturer: Gerrit Holl (LTU)
Date: 2013-11-15 10:00
Place: Aulan

Remote sensing of ice clouds: synergistic measurements and radiative transfer simulations

Gerrit Holl
Department of Computer Science, Electrical and Space Engineering
Division of Space Technology
Luleå University of Technology

PhD Thesis Defence.
Faculty opponent: Jay Mace

This thesis primarily considers the spaceborne remote sensing of ice clouds and frozen precipitation. Ice clouds are important for hydrology and for the Earth’s radiation budget, but many properties are difficult to measure, in particular using spaceborne instruments.
A better quantification of ice clouds is needed to improve global climate models. This thesis presents steps toward such an improvement.

The first part of the thesis introduces topics related to the research presented in the second part, but presents no new scientific results.
It gives a brief introduction to the history of atmospheric remote sensing and describes how the different parts of the electromagnetic spectrum can be used actively or passively. Then, it describes why ice clouds are important and what microphysical, optical, and
macrophysical properties are used to describe atmospheric ice. Next, it briefly introduces the relevant topics in atmospheric radiative transfer. The first part concludes with a description of various approaches to retrievals, with a particular focus on those applied in
this thesis.

The second part of the thesis describes new results. The bulk of the new results is described in five peer-reviewed publications, that are appended verbatim.

A major part of the work builds on the development of a toolkit to easily find co-incident measurements, or collocations, between any pair of satellite sensors. Four appended articles rely on this toolkit.

The first appended article uses the toolkit to obtain collocations between passive microwave and infrared on operational meteorological satellites with the Cloud Profiling Radar on CloudSat. It presents three examples. Firstly, from the collocated dataset and a dataset of
synthetic profiles, the article compares the statistical relations between an official CloudSat Ice Water Path (IWP) product and microwave radiances. Secondly, it shows a point-by-point comparison between the same CloudSat IWP product, and an IWP product based on
passive microwave. A more sophisticated set of systematic comparisons, including more satellites and sensors, is presented in a dedicated paper. Finally, the first paper provides a first preview of how the collocations can be used to train a new IWP retrieval from passive operational measurements. This too is the topic of a dedicated paper, where solar, terrestrial infrared, and microwave radiances are combined to obtain an improved IWP product from passive operational
sensors, by training with an active combined radar-lidar product from CloudSat-CALIPSO.

The second appended article also relies on the collocations toolkit.
Here, collocations between different copies of identical or very similar microwave sounders are used to assess how the inter-satellite bias depends on radiance and latitude.

The remaining two studies described in the thesis do not use existing measurements, but are based on radiative transfer modelling. One attached paper verifies that optimised frequency grids obtained in clear-sky simulations for terrestrial infrared instrument studies, can be applied directly for cloudy simulations. This result is relevant for future studies. Finally, the thesis includes a short study with retrieval simulations for a new sub-millimetre instrument concept.

Created 2013-10-11 07:50:22 by Mats Holmström
Last changed 2013-10-11 07:50:22 by Mats Holmström