Study of daily cloud pulses in tropical cyclones around the world released online in the Journal of Geophysical Research – Hurricane Research Division

Tropical cyclone (TC) diurnal pulses have been well documented in the North Atlantic, but their characteristics in other regions have not been thoroughly investigated. This study details the global and regional variability of diurnal pulses from 2001–2018. Further research is needed to examine the structure of diurnal pulses and to improve understanding of the cause of diurnal pulses, their development, and their impact on TC structure and intensity.

Figure 1. The diurnal pulse in Hurricane Sam on 25 September 2021. Panels A and B depict cloud-top temperature at midnight and 6 am local time (LT), respectively. Panel C is the 24-h conceptual clock showing that the diurnal pulse’s expected location at 6 am LT is around 200 km away from the TC center. Panel D depicts the difference between panels B and A. Note the difference field clearly shows that an area of cold cloud tops in panel B compared to panel A (i.e., warm colors in panel D) is located around the 200 km radius (i.e., the white circle) from the center and that the location of these colder cloud tops matches the expected location of the diurnal pulse found in panel C.

TCs routinely have clouds that move outward away from their centers at the same time each day. This feature is called a diurnal pulse and is most clearly seen in the difference between cloud-top temperature measurements taken by infrared sensors on satellites six hours apart (Fig. 1a-b; Fig. 1d). Since the pulse is predictable, a 24-h conceptual clock to identify where the pulse should be located was created by earlier researchers (Fig. 1c). 

Previous research found that this diurnal pulse nearly always exists in TCs in the North Atlantic. This study documents worldwide diurnal pulses that last at least 6 h and travel at least 300 km.We identified days between 2001 and 2018 when TCs had diurnal pulses and when they did not. For those days that had diurnal pulses, the initial and ending time/location, duration, speed that the clouds move away from the center, and strength of the diurnal pulse were documented. TCs with and without diurnal pulses were analyzed to look for differences in properties such as the amount of moisture in the environment and the sea surface temperature. We also looked at regional differences (Fig. 2). 

Figure 2. The global distribution of diurnal-pulse frequency. Regions are indicated by black boxes. The regions are the Northwest Pacific (NWP), the Northeast Pacific (NEP), the North Atlantic (NA), the South Indian Ocean (SI), the South Pacific (SP), and the North Indian Ocean (NI).

Though this study documents diurnal pulses globally, further research is needed using high-resolution aircraft observations and advanced computer models to examine their structure overall in time and space and to improve the understanding of what causes them, their development, and their impact on TC structure and intensity.

■ Important Conclusions:

• Diurnal pulses occur on 52% of TC days globally. They are most frequent over the Northwest Pacific, occurring on 60% of all TC days in that region. The Northwest Pacific also has the largest fraction of diurnal pulses worldwide compared to other basins (34%).
• The middle value of the length of time a diurnal pulse exists is between 12 and 15 h. Their length of time did not vary by region except that TCs in the Northwest Pacific had diurnal pulses that lasted slightly longer than elsewhere. 
• The middle value of the distance an diurnal pulse moves outward from the TC center is between 500 and 600 km.  Their distance traveled was greater in the Northwest Pacific and North Atlantic regions than elsewhere.
• Diurnal pulses travel outward at 5–10 m/s, and the ones that last the longest move the slowest. Diurnal pulses in the Northwest Pacific moved outward the fastest, whereas those in the Northeast Pacific traveled the slowest.
• The longer lasting the diurnal pulse, the stronger (greater difference in cloud-top temperature  in time) the diurnal pulse.
• Diurnal pulses were most likely to develop in the early morning hours in the TC core.
• Strong inner-core convection and high sea surface temperature below the TC, ample moisture, and low shear (difference between the wind at the top and the bottom of the TC) are favorable for diurnal pulses to occur.
• About 80% of the diurnal pulses identified globally followed the 24-h conceptual clock in Fig. 1c. These clock-following diurnal pulses occurred most often in the Northwest Pacific (85%) and least often in the North Atlantic (69%).

For more information, contact aoml.communications@noaa.gov. The study can be found at https://doi.org/10.1029/2022JD037660.