Dias Neto, Jose ORCID: 0000-0002-8488-8486 (2021). Investigating aggregation in ice and snow clouds using novel combination of triple-frequency cloud radars and radar Doppler spectra. PhD thesis, Universität zu Köln.

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Clouds are essential component of the hydrological cycle for transporting of water and distributing precipitation at different parts of the planet. On a global scale, around 63% of the precipitation originates via ice phase. Different ice microphysical processes can lead to growth (e.g. deposition, aggregation, riming) or reduction (e.g. sublimation, breakup) of ice particle sizes. Aggregation, in particular, rapidly increases ice particles sizes and continuously changes the particle size distribution. However, aggregation and other ice microphysical processes are not fully understood. In order to improve the current knowledge about aggregation and the other microphysical processes, microwave radars are used to observe clouds due to their capability of retrieving information through the different parts of the clouds. Additionally, if Doppler radars operating at different frequencies (multi-frequency setup) are used to observe the same region of clouds, the multi-frequency observations can be used to retrieve information of particles sizes and velocities. This thesis uses multi-frequency Doppler observations (6 months) to investigate scenarios that intensify aggregation and the impact of increasing aggregate sizes on raindrop sizes. To this end, a multi-frequency data processing framework is introduced to minimize the effect of attenuation (e.g. atmospheric gases, snow, wet radome) and radar miscalibration; it also assigns a set of quality flags to the different correction steps. The statistical analysis from the observations of the ice part of the clouds indicates that aggregation is intensified in two temperature regions. The first region is between -20 and -10 °C and coincides with the dendritic growth zone (DGZ). Dendritic crystals can favour aggregation due to their branched structure. In addition to the growth of dendrites, the statistical results suggest that an intensification of aggregation in this temperature region correlates with an intensification of updrafts (up to 0.3 m/s). The statistics also show that approximately 25% of the cases where aggregation intensifies an additional mode of small and slow falling particles are present. The temperature region between -10 and 0 °C coincides with the region where the stickiness of ice surfaces increases due to the effect of a quasi liquid layer on the ice surface. Due to this increased stickiness, aggregation intensifies towards the 0 °C isotherm. The statistical results indicate that the growth of large aggregates in the DGZ favour, but it is not sufficient to guarantee the presence of even larger aggregates close to 0 °C. The results also indicate that an increase in aggregate sizes close to the 0 °C correlates with an increase in raindrop sizes. The multi-frequency processing framework and the other filtering processes introduced in this thesis can be used as the foundations for future multi-frequency experiments. The highly qualified multi-frequency dataset and the statistical results from this thesis can be used to evaluate the ice microphysical processes implemented in numerical models.

Item Type: Thesis (PhD thesis)
Translated title:
CreatorsEmailORCIDORCID Put Code
Dias Neto, Josejdiasn@gmail.comorcid.org/0000-0002-8488-8486UNSPECIFIED
URN: urn:nbn:de:hbz:38-534056
Date: 1 March 2021
Place of Publication: Cologne
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects: Natural sciences and mathematics
Earth sciences
Uncontrolled Keywords:
radar remote sensing, cloud microphysics, snow, ice crystals, radar multi-frequencyEnglish
Date of oral exam: 1 March 2021
NameAcademic Title
Kneifel, StefanDr
Neggers, RoelProf Dr
Funders: KN 1112/2-1
Refereed: Yes
URI: http://kups.ub.uni-koeln.de/id/eprint/53405


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