Structure
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Eye and center
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At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear "eye". Weather in the eye is normally calm and free of convective clouds, although the sea may be extremely violent.[1] The eye is normally circular and is typically 30–65 km (19–40 mi) in diameter, though eyes as small as 3 km (1.9 mi) and as large as 370 km (230 mi) have been observed.[2][3]
The cloudy outer edge of the eye is called the "eyewall". The eyewall typically expands outward with height, resembling an arena football stadium; this phenomenon is sometimes referred to as the "stadium effect".[3] The eyewall is where the greatest wind speeds are found, air rises most rapidly, clouds reach their highest altitude, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[1]
In a weaker storm, the eye may be obscured by the central dense overcast, which is the upper-level cirrus shield that is associated with a concentrated area of strong thunderstorm activity near the center of a tropical cyclone.[4]
The eyewall may vary over time in the form of eyewall replacement cycles, particularly in intense tropical cyclones. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward, which is believed to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the tropical cyclone weakens temporarily. The outer eyewall eventually replaces the primary one at the end of the cycle, at which time the storm may return to its original intensity.[5]
Rainbands
editClouds
editAltitude
editSize
editThere are a variety of metrics commonly used to measure storm size. The most common metrics include the radius of maximum wind, the radius of 34-knot (17 m/s; 63 km/h; 39 mph) wind (i.e. gale force), the radius of outermost closed isobar (ROCI), and the radius of vanishing wind.[6][7] An additional metric is the radius at which the cyclone's relative vorticity field decreases to 1×10−5 s−1.[3]
| Size descriptions of tropical cyclones | |
|---|---|
| ROCI (Diameter) | Type |
| Less than 2 degrees latitude | Very small/minor |
| 2 to 3 degrees of latitude | Small |
| 3 to 6 degrees of latitude | Medium/Average/Normal |
| 6 to 8 degrees of latitude | Large |
| Over 8 degrees of latitude | Very large[8] |
On Earth, tropical cyclones span a large range of sizes, from 100–2,000 km (62–1,243 mi) as measured by the radius of vanishing wind. They are largest on average in the northwest Pacific Ocean basin and smallest in the northeastern Pacific Ocean basin.[9] If the radius of outermost closed isobar is less than two degrees of latitude (222 km (138 mi)), then the cyclone is "very small" or a "midget". A radius of 3–6 latitude degrees (333–670 km (207–416 mi)) is considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees (888 km (552 mi)).[8] Observations indicate that size is only weakly correlated to variables such as storm intensity (i.e. maximum wind speed), radius of maximum wind, latitude, and maximum potential intensity.[7][9] Typhoon Tip is the largest cyclone on record, with tropical storm-force winds 2,170 km (1,350 mi) in diameter. The smallest storm on record is Tropical Storm Marco (2008), which generated tropical storm-force winds only 37 km (23 mi) in diameter.[10]
- ^ a b National Weather Service (October 19, 2005). "Tropical Cyclone Structure". JetStream – An Online School for Weather. National Oceanic & Atmospheric Administration. Archived from the original on December 7, 2013. Retrieved May 7, 2009.
- ^ Pasch, Richard J.; Eric S. Blake; Hugh D. Cobb III; David P. Roberts (September 28, 2006). "Tropical Cyclone Report: Hurricane Wilma: 15–25 October 2005" (PDF). National Hurricane Center. Retrieved December 14, 2006.
- ^ a b c Annamalai, H.; Slingo, J.M.; Sperber, K.R.; Hodges, K. (1999). "The Mean Evolution and Variability of the Asian Summer Monsoon: Comparison of ECMWF and NCEP–NCAR Reanalyses". Monthly Weather Review. 127 (6): 1157–1186. Bibcode:1999MWRv..127.1157A. doi:10.1175/1520-0493(1999)127<1157:TMEAVO>2.0.CO;2. Archived from the original on August 1, 2020. Retrieved December 12, 2019.
- ^ American Meteorological Society. "AMS Glossary: C". Glossary of Meteorology. Allen Press. Archived from the original on January 26, 2011. Retrieved December 14, 2006.
- ^ Atlantic Oceanographic and Hurricane Research Division. "Frequently Asked Questions: What are "concentric eyewall cycles" (or "eyewall replacement cycles") and why do they cause a hurricane's maximum winds to weaken?". National Oceanic and Atmospheric Administration. Archived from the original on December 6, 2006. Retrieved December 14, 2006.
- ^ "Global Guide to Tropical Cyclone Forecasting: chapter 2: Tropical Cyclone Structure". Bureau of Meteorology. May 7, 2009. Archived from the original on June 1, 2011. Retrieved May 6, 2009.
- ^ a b Chavas, D.R.; Emanuel, K.A. (2010). "A QuikSCAT climatology of tropical cyclone size". Geophysical Research Letters. 37 (18): n/a. Bibcode:2010GeoRL..3718816C. doi:10.1029/2010GL044558. hdl:1721.1/64407. S2CID 16166641.
- ^ a b "Q: What is the average size of a tropical cyclone?". Joint Typhoon Warning Center. 2009. Archived from the original on October 4, 2013. Retrieved May 7, 2009.
- ^ a b Merrill, Robert T (1984). "A comparison of Large and Small Tropical cyclones". Monthly Weather Review. 112 (7): 1408–1418. Bibcode:1984MWRv..112.1408M. doi:10.1175/1520-0493(1984)112<1408:ACOLAS>2.0.CO;2. hdl:10217/200. S2CID 123276607. Archived from the original on May 23, 2022. Retrieved December 12, 2019.
- ^ Dorst, Neal; Hurricane Research Division (May 29, 2009). "Frequently Asked Questions: Subject: E5) Which are the largest and smallest tropical cyclones on record?". National Oceanic and Atmospheric Administration's Atlantic Oceanographic and Meteorological Laboratory. Archived from the original on December 22, 2008. Retrieved June 12, 2013.