Exploring Landfalling Tropical Cyclones with Weather Radar

What we know - What we do not yet know - What we would like to know!

Motivation

When we think about various weather phenomena, our answer might be correlated to geography! For Oklahomans, this may mean thinking of tornadoes or critical fire conditions. However, in other parts of the United States - especially along coastlines - the answer may change to hurricanes, which just so happen to be among the most costly weather phenomena each year!

According to the National Centers for Environmental Information (NCEI), a study completed in 2023 showed that flooding causes a majority of property damage and loss of life during the landfall of Tropical Cyclones (TC's). This might seem contradictory since the first thing one might think of when hearing of a tropical storm/hurricane/typhoon are high winds, but TC's host a plethora of associated impacts that threaten both life and property.

NCEI overview of billion dollar disasters, NOAA, 2023

These impacts include but are not limited to:

  • Strong Winds
  • Flooding
    • Storm Surge
    • Localized Areas of Maximum Rainfall
  • Heavy Rain
  • Erosion
  • Tornadoes

To aid in studying these complex storm systems, researchers utilize radar observations. Whether it be from the WSR-88D national network or mobile radar platform deployments, the data obtained gives critical insight to various processes within TC's. While radar has been very effective at studying maximum winds during tropical cyclone landfall, there is still limited understanding regarding rainfall processes, outer-band supercells and tornadoes, and how electrification may be related to TC intensification.


Tropical Cyclones

Let's dive right into some meteorological aspects of TC's! The abridged definition best summarized by Wang, 2012; Montgomery and Farrell, 1993 states that Tropical Cyclones are warm-cored, intense low-pressure systems that produce damaging winds and heavy amounts of rainfall. But what exactly promotes TC formation and further development?

Great question! Starting with Warm Sea-Surface Temperatures (SSTs), this increases evaporation over the ocean. If weak wind shear is present, then this aids in the ability for warm-cored convection to occur and maintain itself, but there must also be some type of disturbance that triggers said convection. Once updrafts and convection lowers the surface pressure, rotation is created and influenced by the Coriolis Force. Finally, intensification can ensue with perpetuation of the aforementioned effects which further lowers pressure and thus increases surface winds.


The Role of Radar

Reverting back to the importance of radar (Radio Detection and Ranging) in studying Tropical Cyclones, this tool allows for researchers to examine both dynamical and microphysical processes inside ongoing storms. Compared to the early years of weather radar development, decades of advancements in technology has led to the implementation of dual-polarization capabilities. This allows for pulses to be transmitted and received both horizontally and vertically oriented which better depicts the size, shape, composition, and motion of hydrometeors.

While this instrument is wonderful in analyzing rather complex processes, problems such as signal degradation, coverage gaps, and slow update times do unfortunately exist. There have been efforts via mobile radar deployments to gather high-resolution data at faster scanning speeds, however, the associated impacts of TC's pose great dangers to operators. Despite this, there have been some success in deployments of mobile radar and other sensors to gather mesoscale and microphysical observations.


Structure of TC's

Understanding the structure of TC's and where maximum winds occur is important for forecasting and post-analysis to understand where and why areas were damaged. By analyzing areas impacted by strong winds during landfall using radar, researchers can help differentiate water damage from wind damage.

GOES 16 GeoColor Satellite Image of Hurricane Harvey at 22:32 UTC (5:32 CDT) on August 25, 2017.

Wind Field Analysis

  • Alford et al. (2019) used the SMART mobile radar platform and WSR-88D to perform a dual-doppler analysis of Hurricane Harvey as it made landfall.
  • This analysis revealed mesovortices in Harvey’s eyewall, which were associated with the strongest winds.
  • Because radar beams point upward, it is impossible to directly measure winds at the surface with radar. To address this issue, this study applied a logarithmic profile which accounts for surface drag to accurately measure winds at the surface.

Boundary Layer Analysis

RADARSAT-1 synthetic aperture radar (ScanSAR Wide B) image of Typhoon Fengshen (Canadian Space Agency)

Furthermore, radar can also provide detailed information about boundary layer structure and how it impacts numerical modeling and forecasting of TC's.

  • Morrison et al. (2005) used WSR-88D data to explore hurricane boundary layer (HBL) structure.
  • Numerous HBL rolls were found in significant areas of TC's studied.
  • Found forecast models are underrepresenting fluxes between horizontal and vertical winds.
  • This study highlights how big of a role the HBL plays in tropical cyclone development and that forecast models are not accurately depicting its structure.

Sparsely Studied Topics

With these characteristics of Tropical Cyclones and their associated impacts in mind, there is a necessity for further research regarding tornadoes, rainfall, and electrification processes. There are new science questions around the identification of tornadic precursors in landfalling TC tornadoes, the microphysics behind rainband development, and the role of lightning flashes in storm intensity. The answers, or at least knowledge gained, is expected to be helpful for both forecasting and nowcasting efforts.

Tornadoes

The 0.5° storm-relative velocity image from KRAX at 1635 UTC 29 Aug 2004 in Tropical Storm Gaston, from Schneider and Sharp (2007) depicting an ongoing TC tornado.

More often than not, landfalling TC's bring tornadoes in the form of embedded supercells along the spiral rainbands. What makes these tornadoes so unique is the fact they are usually much shallower and smaller than traditional supercells. This is due to the saturated nature of the environment and the amount of strong wind shear present. However, they are much more difficult to detect using radar as the returned signal back is quite weak. In operational settings, this is less than ideal when needing to warn the public. A testimony to this is the image provided here... which is actually of an ongoing tornado from a TC! Studies such as Schneider & Sharp, 2007, have found identifiable precursors that could potentially be useful in forecasting, but further research is necessary to determine if they vary on a case by case basis.

Rainfall

Flooding on Interstate 10 after Hurricane Harvey. (Scott Olson, Getty Images)

To gain more insight on patterns regarding rainfall in Tropical Cyclones, the microphysical aspects are being examined using radar. Given that rainband development can result in localized areas away from the coastline being impacted by high rainfall totals for prolonged amounts of time, it is important to investigate potential causes! In recent studies from Zheng et. al., in 2021, an analysis of polarimetric data found a correlation between degrees of rainband development and graupel content. This study imposed further science questions of the role of ice particles melting near the freezing level in the atmosphere and how that impacts rainfall processes.

Electrification

NASA SPoRT GLM Average Flash Area lapse at 04:33 UTC on September 28th during the intensification of Hurricane Ian (2022). Plot produced by Kiahna Mollette, University of Alabama Huntsville.

There have been previous studies noting the correlation between strength of lightning flashes and Tropical Cyclone intensity. While a wonderful example of this would be Hurricane Ian in 2022 (shown in the image), as it was quite the electrical spectacle well sampled by researchers, gathering good data while also maintaining safety is not always sunshine and rainbows. In lieu of both mesoscale and microphysical observations, some researchers have turned to "Model Land" to help bridge the gap in knowledge. In Fierro et. al., 2015, high-resolution simulations of various case studies led to more questions of the role of intracloud flashes, cloud-to-ground flashes, and deep convective cells in intensification.


Future Research

Since the entire point of all this reading, writing, and information gathering was to narrow down our sights towards an avenue of Senior Capstone research, we decided to reach out to researchers who currently study Tropical Cyclones. After some fun discussions, we formulated a few science questions together:

  • How can we use radar to investigate associated processes of landfalling TC’s including rainfall and electrification?
  • What variables have not yet been sufficiently analyzed?
  • Is there a link between rapid intensification and electrification processes of landfalling cyclones?
  • Does the diurnal cycle have any impacts on maximum areas of rainfall?

We will investigate one of these questions in our Capstone project based on which area we feel we can provide the best analysis. So, stay tuned during the Fall 2023 semester to see which direction we decide to go in!


Resources

Tory, K. J., and W. M. Frank, 2010: Tropical cyclone formation. Global Perspectives on Tropical Cyclones, 2nd ed. J. C. L. Chan and J. D. Kepert, Eds., World Scientific Series on Asia-Pacific Weather and Climate, Vol. 4, World Scientific, 55–91.

Alford, A. A., Biggerstaff, M. I., Carrie, G. D., Schroeder, J. L., Hirth, B. D., & Waugh, S. M. (2019). Near-Surface Maximum Winds During the Landfall of Hurricane Harvey. Geophysical Research Letters, 46(2), 973–982.  https://doi.org/10.1029/2018GL080013 

Fierro, A. O., Mansell, E. R., Ziegler, C. L., & MacGorman, D. R. (2015). Explicitly Simulated Electrification and Lightning within a Tropical Cyclone Based on the Environment of Hurricane Isaac (2012). Journal of the Atmospheric Sciences, 72(11), 4167–4193.  https://doi.org/10.1175/JAS-D-14-0374.1 

Montgomery, M. T., & Farrell, B. F. (1993). Tropical Cyclone Formation. Journal of the Atmospheric Sciences, 50(2), 285–310.  https://doi.org/10.1175/1520-0469(1993)0502.0.CO;2 

Morrison, I., Businger, S., Marks, F., Dodge, P., & Businger, J. A. (2005). An Observational Case for the Prevalence of Roll Vortices in the Hurricane Boundary Layer. Journal of the Atmospheric Sciences, 62(8), 2662–2673.  https://doi.org/10.1175/JAS3508.1 

Schneider, D., & Sharp, S. (2007). Radar Signatures of Tropical Cyclone Tornadoes in Central North Carolina. Weather and Forecasting, 22(2), 278–286.  https://doi.org/10.1175/WAF992.1 

Wang, Y. (2012). Recent Research Progress on Tropical Cyclone Structure and Intensity. Tropical Cyclone Research and Review, 1(2), 254–275.  https://doi.org/10.6057/2012TCRR02.05 

Zheng, H., Zhang, Y., Zhang, L., Lei, H., & Wu, Z. (2021). Precipitation Microphysical Processes in the Inner Rainband of Tropical Cyclone Kajiki (2019) over the South China Sea Revealed by Polarimetric Radar. Advances in Atmospheric Sciences, 38(1), 65–80.  https://doi.org/10.1007/s00376-020-0179-3 

In Gratitude

Thank you for taking the time to read about our semester's work. We hope you learned something or at least enjoyed the pretty pictures along the way!

We would also like to extend a thank you to Dr. Kathy Pegion, Dr. Addison Alford, & Dr. James Ruppert for providing additional insight and resources during the duration of our Literature Review and Presentation process for METR-3334: Principles of Research & Communication. Double shout-out to Dr. Pegion for being an amazing professor of our class!

@robbyfrostwx

@savsouthwardwx

NCEI overview of billion dollar disasters, NOAA, 2023

GOES 16 GeoColor Satellite Image of Hurricane Harvey at 22:32 UTC (5:32 CDT) on August 25, 2017.

RADARSAT-1 synthetic aperture radar (ScanSAR Wide B) image of Typhoon Fengshen (Canadian Space Agency)

The 0.5° storm-relative velocity image from KRAX at 1635 UTC 29 Aug 2004 in Tropical Storm Gaston, from Schneider and Sharp (2007) depicting an ongoing TC tornado.

Flooding on Interstate 10 after Hurricane Harvey. (Scott Olson, Getty Images)

NASA SPoRT GLM Average Flash Area lapse at 04:33 UTC on September 28th during the intensification of Hurricane Ian (2022). Plot produced by Kiahna Mollette, University of Alabama Huntsville.