Written by Erika Duran and Emily Berndt
In our previous blog post, we highlighted the value of using Red-Green-Blue (RGB) composite imagery from the NASA Global Precipitation Measurement (GPM) constellation to analyze the evolution of storm structure in Hurricane Dorian. Several structural changes were visible in the 37 GHz and 89 GHz passive microwave RGB imagery from September 1-3, 2019, as Dorian approached and made landfall in the Bahamas, and then remained nearly stationary over Grand Bahama Island for nearly two days.
The above animation of the 37 GHz passive microwave RGB demonstrates an eyewall replacement cycle that occurred from September 1-2, as well as the erosion of the western eyewall from September 2-3. A secondary eyewall is visible at 20:46 UTC on September 1 (below, Fig 2c), followed by a wider, single eyewall and a much wider eye at 10:36 UTC on September 2 (Fig 2d), as compare to the previous day (Fig 2a). The erosion of the western eyewall is visible in Fig 2f, likely a result from the storm’s interaction with land.
An animation of the 89 GHz Passive Microwave RGB (Fig 3) provides a slightly different perspective of these same events, highlighting changes in Dorian’s deep convection.
While the eyewall replacement cycle is much more clear in the 37 GHz RGB imagery, structural changes associated with this cycle are still evident using the 89 GHz RGB, showing the presence of a “moat,” or a region of low-echo reflectivity and subsidence between the primary and secondary eyewalls (e.g., Sitkowski et al. 2011, Kuo et al. 2009). The moat is seen as the darker red colors surrounding the eye in Fig 4c, which is located between the primary and secondary eyewalls indicated in the 37 GHz imagery. Additionally, the erosion of the western eyewall is much more clear using the 89 GHz RGB, showing the asymmetry in Dorian’s deep convection on the left side of the eye (Fig 4f).
Sitkowski, M., J.P. Kossin, and C.M. Rozoff, 2011: Intensity and Structure Changes during Hurricane Eyewall Replacement Cycles.Mon. Wea. Rev.,139, 3829–3847,https://doi.org/10.1175/MWR-D-11-00034.1
Kuo, H., C. Chang, Y. Yang, and H. Jiang, 2009: Western North Pacific Typhoons with Concentric Eyewalls.Mon. Wea. Rev.,137, 3758–3770,https://doi.org/10.1175/2009MWR2850.1