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Novel Ways to See More: Using Dual-Polarisation Doppler Weather Radar Observations to Improve Our Understanding of Winter Weather

Dr. Ryan R. Neely III (SEE/NCAS), Prof. Alan Blyth (NCAS/SEE)

Contact email: r.neely@leeds.ac.uk

This PhD project will utilise the latest developments in dual-polarisation radar technology and in situ aircraft measurements to explore novel ways of observing winter storms as well as improving the representation of cold cloud microphysics in high resolution microphysical models.

Project Summary

Polarization is the phenomenon in which waves of electromagnetic radiation are restricted in direction of vibration. The reason polarization state is worth contemplating in relation to the weather is that two beams of radiation, otherwise identical, may interact differently with matter if their polarization states are different. Thus, observing the polarization of scattered light in the atmosphere provides a unique way to probe clouds, precipitation and aerosol.

In this project, you will have the opportunity to explore novel polarimetric observations of winter weather by making observations with state-of-the-art radars.  To accomplish this you will utilize the new NCAS mobile X-band dual-polarisation Doppler radar located at the University of Leeds, observations from instruments on board the NCAS Facility for Airborne Atmospheric Measurements (FAAM) and the recently upgraded UK weather radar network to investigate the wealth of unexplored geophysical information provided by polarimetric variables. From the observations that you help collect and analyse using advanced computing techniques you will improve our understanding of cloud microphysics and precipitation. You will also explore how these experimental techniques may be incorporated into improving operational observational and prediction systems so that your results may have a broader societal impact.

Figure 1: NCAS Mobile Doppler Dual-Polarisation Radar making observations at Burn airfield.

Background

Conventional (single-polarization) radars operate by transmitting pulses of electromagnetic (EM) radiation and "listening" for echoes returned from various atmospheric targets, including hydrometeors, biological (e.g., insects and birds), and inorganic (e.g., dust, chaff, and smoke) scatterers. The energy propagates through the atmosphere as an EM wave with the electric field vector oscillating in the horizontal plane parallel to the ground; therefore, these waves are said to be horizontally polarized. When a horizontally polarized wave illuminates a particle in the atmosphere, the particle behaves as a tiny antenna, emitting radiation in all directions, with the amplitude of this "scattered" energy related to the size, shape, and orientation of the target, as well as its physical composition (e.g., liquid or ice).

Consider a spherical hydrometeor that is small compared to the radar wavelength. When the particle is illuminated by a horizontally polarized radar wave, the particle behaves like a horizontal dipole antenna that becomes excited and scatters energy having horizontal polarization, whereas it behaves like a vertical dipole antenna and scatters energy with vertical polarization when excited by a vertically polarized radar wave. Dual-polarization radars exploit this fact by transmitting radiation with horizontal polarization and vertical polarization simultaneously (Fig. 2). By comparing the signals received from returns at each polarization, one gains further information about the size, shape, and orientation of targets within the radar sampling volume due to the differences in the scattering of these two waves of EM radiation.

Figure 2: Schematic of the simultaneous propagation of horizontally (blue) and vertically polarized (orange) EM waves. Adapted from Kumjian, (2013 a).

Meaningful information is obtained by comparing the amplitudes and phases of the signals returned at the H and V polarizations, providing a suite of new variables from which useful geophysical parameters may be derived.

Scientific aims of the project

Winter storms, especially those that include large accumulations of precipitation that lead to flooding, are one of the most significant major natural hazards in the extratropics and typically inflict substantial economic damage and even casualties. Examples of such events over the last 10 years that significantly impacted widespread areas of Europe include winter storms Kyrill (2007) and Xynthia (2010). Each of these storms caused damage that is estimated to have cost several billion Euros. Notably, in the winter of 2013/2014 a series of severe storms brought precipitation to the UK that caused large-scale flooding, power outages and many fatalities [Lewis et al., 2015].

The present lack of understanding in the microphysical processes that lead to the formation of ice and the development of precipitation in all wintertime storms creates significant uncertainty in numerical weather prediction (NWP) models and, subsequently, in hydrological models used for flood predictions and warnings. Thus, the overall scientific objective of this project is to quantitatively improve our understanding of the microphysical processes in extratropical winter storms.

The ability of dual-polarisation radars to be used to improve our understanding of cold cloud microphysics by initializing, evaluating and validating microphysical models is directly related to their ability to accurately characterize their targets (which must first be accurately validated by in situ observations). This information will be used in this project to implement and improve an existing hydrometeor classification algorithm (HCA) to determine the evolution of hydrometeors within wintertime storms. This will notably include talking questions such as:

  1. What is the structure of the melting layer and why does it vary?

  2. What is the evolution and role of ice generating cells in ice/mixed phase clouds?

  3. How can we better determine parameters (such as the drop size distribution) that allow for more accurate near-real time estimates of rain? This is especially a concern during times of flooding.

The overall objective of this project will be primarily accomplished by determining the temporal and spatial distribution of hydrometeors (i.e., snow, graupel, rain, mixed-phase particles) from observations taken with NCAS’s mobile dual-polarisation Doppler X-band radar and FAAM. Other observations to be examined will also include those obtained from the Met Office’s newly upgraded dual-polarisation Doppler C-band radar network and the other radars at the Chilbolton Observatory.

In addition to the primary analysis the observations described above, the observations will also be used to evaluate the cold cloud microphysical scheme of the Met Office’s high resolution NWP model (UKV) under the observed wintertime conditions. Due to the nature of remote sensing, there are many assumptions and large uncertainties involved with deriving information equivalent to model output comparisons from radar observations. Thus, in this project, the goal will be to reverse the typical process of creating information similar to NWP model output from observations by generating modelled dual-polarisation radar observations from the NWP model output provided by the supervisor’s partners in the Met Office. This will be accomplished by implementing a forward radar model, including detailed electromagnetic scattering calculations, which can be applied to NWP output (e.g. Pfeifer et al. [2008]). This will bridge the gap between polarimetric radar observations and NWP models by allowing for a comparison with as few assumptions as possible. By doing this, we will be able to more directly evaluate the capacity of the NWP model to realistically describe the processes involved in the formation and evolution of the hydrometeors.           

Project Objectives

This project will significantly contribute to the field of radar polarimetry, with specific applications to polarimetric classification of winter weather hydrometeors. In this project, you will work with world leading scientists at the University of Leeds and NCAS to:

  1. Become an expert in collecting and analysing dual-polarisation Doppler radar observations in conjunction with in situ observations of hydrometeors. This will be in part accomplished by having hands on experience with NCAS’s mobile dual-polarisation Doppler X-band radar and FAAM.
  2. Make use of the full range polarimetric radar observations (including multiple sources of radar observations and the full set of observable polarimetric parameters) to determine and quantify the evolution of hydrometer size, concentration, phase, shape, and orientation in wintertime clouds. This will be achieved by validating a hydrometeor classification scheme to analyse the observations.
  3. Use polarimetric radar observations to evaluate and improve the representation of cold cloud microphysics in a high resolution numerical prediction model. This objective will be accomplished by implementing and validating a radar forward operator that utilises the output the output of the Met Office’s UKV and evaluation the results to your observational analysis.

One unique aspect of this project is that you will be trained to go into the field and make observations with the new NCAS X-band radar and FAAM (i.e. you will have a direct hands on experience in deploying and operating a state-of-the-art instrument). This will notbly include working with all of the radars located at the Chilbolton Observatory and you will be included in any upcoming campaigns that the larger research group is involved with. This may include working in Greenland, Indonesia and the Arctic Ocean. The observations you make with both of these will provide the basis of your exploration in the use of polarimetric radar observations, though the total fieldwork component of the project may vary depending on the role of the radar in upcoming field campaigns and your interest.

Potential for high impact outcome

Polarimetry is currently becoming an operational feature of weather radar systems around the world. Such a choice has been motivated by the capability of polarimetric variables to distinguish different hydrometeor types and to improve the accuracy of quantitative precipitation estimation. These technologies have attained a degree of maturity such that major investments were undertaken by many national governments (notably the US, UK and Europe) for the implementation of polarimetric upgrades of weather radar networks. Yet, much work remains to fully incorporate all of the new information these observations provide into operational observation and forecast systems.

Thus, we anticipate the project generating several papers with at least one being suitable for submission to a high impact journal due to the relevancy to radar polarimetry for greatly improving operational radar meteorology and forecasting.

This work will also have the possibility of impacting society. The new observational abilities being investigated will eventually lead to better information being provided directly to forecasting systems from operational radar networks. This will directly result in more accurate predictions and as well as lead to improved model parameterizations.

Training

You will work directly under the supervision of Dr. Ryan R. Neely III and Dr. Alan Blyth within ICAS and the NCAS Weather directorate. You will become a member of the University of Leeds Radar Group and benefit from interactions with other staff and students within NCAS and SEE who have a range of interests and expertise.

This project will equip you with the necessary expertise to become a leader in the next generation of atmospheric scientists, ready to carry out your own programme of innovative scientific research. These skills will be developed by a mixture of hands on experience, attending external training courses, and taking part in the Leeds – York NERC doctoral training partnership programme. This includes access to a broad spectrum of training workshops put on by the Faculty that consist of a range of extensive training workshops that will help you manage your degree and prepare for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Specifically, this project will provide a high level of specialist scientific training in: (i) State-of-the-art analysis and application polarmetric Doppler radar observations; (ii) radiative transfer (with an emphasis on polarization and scattering processes); (iii) comparison of radar derived geophysical parameters to forecast models; (iv) planning and executing measurement campaigns.

You will also have the opportunity to present research results at national and international conferences, and you will benefit from established collaborations with world-leading scientists within NCAS, the Met Office, NCAR and NOAA.

Requirements

A good first degree (1 or good 2-1), or a good Masters degree in a physical or mathematical discipline, such as mathematics, physics, geophysics, engineering or meteorology is required. Experience in programming (e.g. Python, Matlab, IDL, R…) and fieldwork is of advantage.

Contact Information

Contact is strongly encourage before application so that we may discuss your interests and project specifics.  Help with the application process may also be provided. Enquires should be made by contacting Dr. Ryan Neely, Lecturer of Observational Atmospheric Science (R.Neely@leeds.ac.uk).  

References

  • Galletti, M. (2009), Fully polarimetric analysis of weather radar signatures, Ph.D. Thesis,  Technische Universität Chemnitz: Germany.

  • Kumjian, M. (2013), Principles and applications of dual-polarization weather radar. Part I: Description of the polarimetric radar variables, J. Operational Meteor., 1(19), 226–242, doi:10.15191/nwajom.2013.0119.

  • Kumjian, M. (2013), Principles and applications of dual-polarization weather radar. Part II: Warm- and cold-season applications, J. Operational Meteor., 1(20), 243–264, doi:10.15191/nwajom.2013.0120.

  • Pfeifer, M., et al. (2008), A polarimetric radar forward operator for model evaluation, Journal of Applied Meteorology and Climatology, 47.12, 3202-3220, doi:10.1175/2008JAMC1793.1.

  • Picca, J. C., Schultz, D. M. and Colle, B. A. (2014), The value of dual-polarization radar in diagnosing the complex microphysical evolution of an intense snowband, BAMS, doi:10.1175/BAMS-D-13-00258.1.

  • Putnam, B. J. et al. (2014), The analysis and prediction of microphysical states and polarimetric radar variables in a mesoscale convective system using double-moment microphysics, multinetwork radar data, and the ensemble Kalman filter, Monthly Weather Review 142, no. 1, 141-162, doi:10.1175/MWR-D-13-00042.1 .

  • Thompson, E. J., Rutledge, S. A., Dolan, B., Chandrasekar, V., & Cheong, B. L. (2014), A dual-polarization radar hydrometeor classification algorithm for winter precipitation. Journal of Atmospheric and Oceanic Technology, 31(7), 1457-1481.

    Further reading

  • The best reference for everything to do with polarimetric radar: of Bringi, V. N., and V. Chandrasekar, 2001: Polarimetric Doppler Weather Radar: Principles and Applications. Cambridge University Press, 636 pp.

  • Information about the NCAS X-band Dual-Polarisation Mobile Radar: https://www.ncas.ac.uk/index.php/en/the-facility-amf/mobile-instruments/radars/263-amf-main-category/amf-x-band-radar/1098-x-band-radar-overview

  • Information about the Met Office Dual-Polarisation Radar Upgrade: http://www.rmets.org/sites/default/files/abstracts/Mar/20032013-sugier.pdf

Related undergraduate subjects:

  • Electrical engineering
  • Engineering
  • Geology
  • Geophysics
  • Hydrology
  • Meteorology
  • Physics
  • Statistics