A Database of Microwave Single Scattering Properties of Ice Hydrometeors
Licentiate thesis, 2018
Microwave remote sensing by satellites is important for global observations of ice hydrometeors. Interpretation of the measurements requires sufficiently accurate knowledge of hydrometeors’ interaction with photons, i.e. article scattering and absorption. This presents a challenge for several reasons. Liquid hydrometeors can typically be modelled by spheroids, while the shapes of ice hydrometeors are known to be significantly more complex and variable. Also, the shapes can from a remote sensing perspective generally not be known exactly, as they vary from case to case. Finally, calculating the light scattering properties is challenging and computationally costly.
This thesis presents work related to recent efforts in improving the representation of light scattering by ice hydrometeors. A new single scattering database is presented, which includes 34 frequencies in between 1 and 874 GHz, and supports both passive and active microwave applications. A total of 34 different particle models were included, ranging from pristine crystals to aggregates. Complete random orientation is assumed throughout, slightly limiting its usefulness with respect to polarimetric measurements. Most aggregates were generated through simulation of aggregation, by letting particles collide randomly. The database can be considered the most extensive of this type to date, and future versions are intended to include oriented and melting particles. The general intention is to aid existing and future satellite retrievals, and satellite data assimilation into weather prediction models, all requiring accurate modelling of measured radiances. Special attention has been given to the upcoming Ice Cloud Imager (ICI), part of Europe’s next generation of weather satellites.
Using the aggregation simulation tools developed for the database, a more dedicated case study was performed, which looked at the impact of different aggregate shape parameters on the resulting scattering properties. Both the amount and aspect ratio of the aggregate constituent crystals was found to have a high impact on both extinction (183, 325 and 664 GHz) and back-scattering (13, 36 and 94 GHz). Effective density and aerodynamic area had a high impact as well. Calculated radar triple frequency signatures were seen to clearly depend on the particle shape, consistent with previous studies. Overall, the results indicate that the particle shape should be considered in both passive and active applications above 13 GHz, and future database development will consider this. A potential application is also retrieval of ice particle shape through remote sensing.