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| Titel |
Extreme Rainfall Intensities and Long-term Rainfall Risk from Tropical Cyclones |
| VerfasserIn |
A. Langousis, D. Veneziano |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024933
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| Zusammenfassung |
| We develop a methodology to estimate the rate of extreme rainfalls at coastal sites due to
tropical cyclones (TCs). A basic component of the methodology is the probability distribution
of ID,max, the maximum rainfall intensity at the site over a period D during the passage
of a TC with given characteristics θ. The long-term rainfall risk is obtained by
combining the conditional distribution of (ID,max|θ) with a recurrence model for
θ.
The lack of extensive TC rainfall records and the many parameters needed to characterize
the motion, size and intensity of tropical cyclones make it difficult to estimate the distribution
of (ID,max|θ) directly from data. Hence, we have resorted to a combination of physical
modeling to obtain the mean rainfall field for a TC with given characteristics θ, and
statistical analysis to include storm-to-storm variability, as well as intra-storm rainfall
fluctuations due to rainbands and local convection. The vector θ includes the maximum
tangential wind velocity V max, the radius of maximum winds Rmax and the translation
speed V t of the storm, in addition to the distance y of the coastal site from the TC
center.
The physical model of TC rainfall uses an extension of Smith’s (1968) boundary layer
(BL) formulation and simple moist air thermodynamics to calculate the vertical outflow of
water vapor from the top of the TC boundary layer, which is assumed to be all converted into
rainfall. However, the calculated rainfall field is not simply proportional to the vertical flux of
moisture. This is because (1) the trajectory of moisted air parcels has an outward slant
depending on distance from the TC center and (2) the ascending air parcels and descending
rain drops are advected into a helical motion by the cyclonic circulation; therefore a parcel of
air that leaves the TC boundary layer contributes rainfall to a range of azimuthal
locations.
The statistical component of the model characterizes the distribution of (ID,max|θ) by
comparing the physical model results with precipitation radar (PR) data from the TRMM
mission. Taylor’s hypothesis is used to convert spatial rainfall intensity fluctuations to
temporal fluctuations at a given location A.
To illustrate the use of the model for long-term rainfall risk analysis, we formulate a
recurrence model for tropical cyclones in the Gulf of Mexico that make landfall between
longitudes 85o-95oW and compare the intensity-duration-frequency (IDF) curves for New
Orleans obtained by the present model with similar curves in the literature based on
continuous rainfall records. The latter include all types of rainstorms. We find that for return
periods of 100 years or more and long averaging durations (D around 12-24 hours), tropical
cyclones dominate over other rainfall event types, whereas the reverse is true for shorter
return periods or shorter averaging durations.
We also determine how the most likely TC scenario varies with the averaging duration D
and the return period T . We do so by plotting the modal values of V max, Rmax, and V t
conditioned on exceeding the T -yr rainfall intensity for duration D. The mode of V t
decreases as T increases, because more intense rainfalls are generally produced by
slower-moving systems. The mode of Rmax decreases when D or T increase, whereas
the opposite is true for V max. For the distance y from the TC center, the modal
value is always close to Rmax, the location where maximum rainfall intensities
tend to occur. These modal values can be used to define T -year scenario events. |
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