Tropical Cyclone Diurnal Cycle in Lightning


Background:


Dunion et al. (2014) recently found a diurnal cycle evident in infrared satellite images when applying a 6-h differencing technique. By taking the differences in brightness temperature between the two infrared images, a "cool" ring was found to emanate from the inner core and propagate radially outward (see figure below from Hurricane Felix (2007); images courtesy of Jason Dunion). The timing of these pulses follow closely to the local standard time (LST), beginning near the core just after sunset each day and propagating several hundred kilometers away by the following afternoon (see clock below; developed by Jason Dunion). Dunion et al. (2014) suggest that these pulses may have an impact on the tropical cyclone structure and intensity.




Data and Methods:


We chose to analyze the lightning in the 56 major hurricanes from 2005-2012 to see if a diurnal signal was present. The NHC best tracks of the tropical cyclones chosen in the North Atlantic (NATL) and East Pacific (EPAC) basins are shown in the figure below. The lightning used in this study is from the World Wide Lightning Location Network (WWLLN). To diagnose the diurnal cycle, the lightning strikes were transformed into LST based on the tropical cyclone center at that time.



Results:


When compositing all the tropical cyclones in the North Atlantic basin and all the tropical cyclones in the East Pacific basin, a diurnal signal emerges, as indicated by the dashed purple lines in the figures below. Previous studies have identified that most lightning in tropical cyclones is concentrated in two regions: near the eyewall in the inner core and in the outer rainbands. A relative minimum occurs between the two regions, in the inner rainband region. This is reflected well in the composite of lightning strikes in these major hurricanes. A maximum in lightning activity can be seen within 100 km, or in the inner core, and in around 300 km, or in the outer rainbands. The diurnal signal, moving radially outward throughout the day, is an apparent feature embedded within the well-studied usual lightning maxima regions.


While these results are promising, the composite of several tropical cyclone lightning strike counts can mask what is going on in individual tropical cyclones. The lightning patterns in individual tropical cyclones led to interesting results, that perhaps draw more questions than answers. Shown below are a few examples. Hurricane Hilary (2011) exhibiting a very nice lightning pattern that moved radially outward throughout the day, consistent with the ideas presented by Jason Dunion (compare maximum lightning location with the diurnal clock above). While several tropical cyclones had these clear diurnal pulsing occurring, not all did. Hurricane Jova (2011) and Hurricane Julia (2010) are two such examples. Jova had a lot of inner core lightning, but it never managed to propagate away from the inner core. Julia (2010) had very little lightning at all.



Future Work:


This research is in the early stages. Given what was presented above, the following questions have arisen:

  • Why don't all tropical cyclones have a diurnal signal in the lightning when the infrared imagery suggests a diurnal cycle is occurring?
  • What causes low lightning activity in certain tropical cyclones?
  • If a tropical cyclone can strengthen into a major hurricane and have very few lightning strikes, how important of a signal is lightning for tropical cyclone intensity changes?

The following questions could be answered by the make-up of the deep convection present in a tropical cyclone, or perhaps the lack of lightning activity is a result of detection errors. Future work will aim to work through these questions.



For more information on this research, please refer to the following: