Impact of the microstructure of precipitation and hydrometeors on multi-frequency radar observations

Jussi Leinonen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Continuous observations are needed to monitor and predict the state of the changing Earth system. These observations must be global, and therefore areas with poor or no infrastructure also have to be covered by them. Remote sensing systems, especially those based on satellites, are a practically achievable way to make measurements also in such remote areas. The hydrological cycle is a critical part of the atmosphere-ocean system. It is monitored remotely by many satellites, but the need for new technologies to improve the accuracy of the measurements is widely recognized. Precipitation and cloud radars appear to be promising tools, but have so far been operated in only two satellites. Typically, space-based radars use shorter wavelengths than most ground-based weather radars. This complicates the problem of modeling the radar scattering, whose nature depends on the size of the targets relative to the wavelength. Understanding the radar scattering at short wavelengths is particularly important for multi frequency radars, which are used to infer additional information about their targets from the difference of signals of different frequencies, and thus originating from different scattering processes. These radars require that one of the wavelengths be of the order of the typical target hydrometeor size or shorter. The Arctic and the Antarctic, which are particularly significant among Earth's remote areas because of their sensitivity to climate change, present specific challenges and opportunities for spaceborne radars. Compared to regions closer to the equator, the typically light precipitation rate, small size of precipitating particles and common occurrence of snowfall in these areas require radars to have higher sensitivity. Because of the small hydrometeor size, respectively shorter wavelengths are needed there to use a multi-frequency system. On the other hand, these factors also mean that signal attenuation by the hydrometeors is usually fairly weak. This increases the suitability of short-wavelength radars, whose signal is attenuated more strongly in the atmosphere than those with longer wavelengths. This thesis is also concerned with the complex shapes of snowflakes, which make the interpretation of the scattered signals more difficult. It has previously been a common practice in radar scattering computations to simplify the particle structure to an equivalent analytical model, but it turns out that such models are often not consistently usable at high frequencies, above roughly 30–90 GHz depending on the snowflake size. Instead, the shape model should describe also the microstructure of the snowflakes. An autocorrelation-based particle model is suggested herein as an alternative that can adequately account for that structure and yet remain simple enough to be suitable for the interpretation of radar observations.
Translated title of the contributionSateen ja hydrometeorien rakenteen vaikutus monitaajuustutkahavaintoihin
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Nieminen, Risto, Supervising Professor
Publisher
Print ISBNs978-951-697-777-8
Electronic ISBNs978-951-697-778-5
Publication statusPublished - 2013
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • radar
  • snowflake
  • precipitation microphysics
  • remote sensing

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