Tropical cyclone rainfall forecasting
One of the most significant threats from tropical cyclones is heavy rainfall. Between 1970-2004, inland flooding from tropical cyclones caused a majority of the fatalities in the United States.[1] This statistic changed in 2005, when Hurricane Katrina's impact alone shifted the most deadly aspect of tropical cyclones back to storm surge, which has historically been the most deadly aspect of strong tropical cyclones.[2] During the 2005 season, flooding related to Hurricane Stan's broad circulation lead to 1662-2000 deaths.
While flooding is common to tropical cyclones near a landmass, there are a few factors which lead to excessive rainfall from tropical cyclones. Slow motion, as was seen during Hurricane Danny (1997) and Hurricane Wilma, can lead to high amounts. The presence of topography near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan acts to magnify amounts due to upslope flow into the mountains. Strong upper level forcing from a trough moving through the Westerlies, as was the case during Hurricane Floyd, can lead to high amounts even from systems moving at an average forward motion. A combination of two of these factors could be especially crippling, as was seen during Hurricane Mitch in Central America.[3]
Rainfall distribution around a tropical cyclone
General distribution
| Rainfall Rate per day within radius of the center (Riehl) | |||
|---|---|---|---|
| Radius (mi) | Radius (km) | Amount (in) | Amount (mm) |
| 35 | 56 | 33.98 | 863 |
| 70 | 112 | 13.27 | 337 |
| 140 | 224 | 4.25 | 108 |
| 280 | 448 | 1.18 | 30 |
Isaac Cline underwent the first investigations into rainfall distribution around tropical cyclones in the early 1900s. He found that a larger proportion of rainfall typically falls in advance of the center (or eye) than after the center's passage, with the highest percentage falling in the right front quadrant. Father Viñes of Cuba found that some tropical cyclones can have their highest rainfall rates in the rear quadrant within a training (non-moving) inflow band (Tannehill 1942). Rainfall is found to be strongest in their inner core, with a degree of the center, with lesser amounts farther away from the center (Riehl 1954). Most of the rainfall in hurricanes is concentrated within its radius of gale-force winds.[4] The chart to the right was developed by Riehl in 1954 using meteorological equations which assume a gale radius of about 140 statute miles, a fairly symmetric cyclone, and does not consider topographic effects or vertical wind shear. As seen in the statistics from Taiwan/Taipei below, local amounts can exceed this chart by a factor of two due to topography. Wind shear tends to lessen the amounts below what is shown on the table.
Storm Size
Larger tropical cyclones have larger rain shields, which can lead to higher rainfall amounts farther from the cyclone's center.[4] This is generally due to the longer time frame rainfall falls at any one spot in a larger system, as long as forward motion is constant. However, some of the difference seen between larger and small storms could be the increased sampling of rainfall within a larger tropical cyclone when compared to that of a compact cyclone; a statistical problem.
Slow/Looping motion on rainfall magnitude
Storms which have moved slowly, or loop, over a succession of days lead to the highest rainfall amounts for several countries. Riehl calculated that 33.97 inches/863 mm of rainfall per day can be expected within one-half degree, or 35 miles/56 km, of the center of a mature tropical cyclone. Many tropical cyclones progress at a forward motion of 10 knots, which would limit the duration of this excessive rainfall to around one-quarter of a day, which would yield about 8.50 inches/216 mm of rainfall. This would be true over water, within 100 miles/160 km of the coastline,[5] and outside topographic features. As a cyclone moves farther inland and is cut off from its supply of warmth and moisture (the ocean), these amounts decrease quickly.
Vertical Wind Shear
Vertical wind shear forces the rainfall pattern around a tropical cyclone to become highly asymmetric, with most of the precipitation falling downwind of the shear, or downshear. In other words, southwesterly shear forces the bulk of the rainfall northeast of the center. If the wind shear is strong enough, the bulk of the rainfall will move away from the center leading to what is known as an exposed circulation center. When this occurs, the potential magnitude of rainfall with the tropical cyclone will be significantly reduced.
Interaction with frontal boundaries/upper level troughs
As a tropical cyclone interacts with an upper level trough, and related surface front, a distinct northern area of precipitation is seen along the front ahead of the axis of the upper level trough. This type of interaction can lead to the appearance of the heaviest rainfall falling along and to the left of the tropical cyclone track the precipitation streaking hundreds of miles/kilometers downwind of the tropical cyclone.[6]
- ^ Ed Rappaport. "Inland Flooding". National Oceanic & Atmospheric Administration. Retrieved 2006-06-24.
- ^ Eric S. Blake. "The Deadliest, Costliest, and Most Intense United States Tropical Cyclones From 1851 to 2004". National Oceanic & Atmospheric Administration. Retrieved 2006-06-24.
{{cite web}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ "Are You Ready?". Federal Emergency Management Agency. 2006-04-05. Retrieved 2006-06-24.
- ^ a b Corene J. Matyas. Relating Tropical Cyclone Rainfall Patterns to Storm Size. Retrieved on 2007-02-14.
- ^ Russell Pfost. Tropical Cyclone Quantitative Precipitation Forecasting. Retrieved on 2007-02-25.
- ^ Norman. W. Junker. Hurricanes and extreme rainfall. Retrieved on 2006-02-13.