Defence in the field of Space Science and Technology, M. Sc. (Tech.) Annakaisa von Lerber
The title of thesis is “Challenges in measuring winter precipitation: Advances in combining microwave remote sensing and surface observations”
In mountainous regions and at high latitudes societies are prepared for the challenges caused by snow storms. Snowfall has an impact on transportation services, both in aviation and road maintenance, and the knowledge of the expected accumulation and location of snowfall helps authorities to mitigate the possible damages. Snow particles have an irregular structure, and their properties evolve continuously due to microphysical processes that take place aloft. Hence the quantitative precipitation estimation with remote sensing instruments, such as radars and radiometers, is challenging.
In this thesis, the microphysical properties of snowfall are studied with ground-based measurements, and the changes in prevailing snow particle characteristics are linked to remote sensing observations. For example, it is shown that heavily rimed snow particles and open-structured low-density snowflakes have different signatures with triple-frequency radar observations. This can be applied to snowfall estimation with space-based radars.
As a part of this work, two methods are implemented to retrieve mass estimates for an ensemble of snow particles and the changes in the retrieved mass-dimensional relations are shown to correspond to microphysical growth processes. The dependence of weather radar observations on the properties of snow is investigated and parametrized. The results improve the accuracy of the radar-based snowfall estimation. Furthermore, the created data set is utilized to validate space-borne snowfall measurements.
The attenuation caused by melting snow particles is shown to be significant for weather radars, and the expected attenuation is parametrized according to properties of snow at the top of the melting layer.
Opponent: Professor Herman Russchenberg, Delft University of Technology, Netherlands.
Supervisor: Professor Tuija Pulkkinen, Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering.