Footprinting Global Tropical Cyclones - Willis … Autumn Seminar – Nov 1st 2017 Footprinting...

WRN Autumn Seminar – Nov 1st 2017

Footprinting Global Tropical Cyclones

James Done, Greg Holland and Ming Ge: NCAR, Willis Research Network

Ioana Dima-West, Geoffrey Saville and Sam Phibbs: Willis Towers Watson

Yuqing Wang: U. Hawaii


The 2017 Hurricane Season

Named storms: 16 (12.1) Hurricanes: 10 (6.4) Major hurricanes: 6 (2.7)

Easterly Wave Developments

Harvey

Irma

Maria


Records Smashed

September: (Phil Klotzbach, CSU)

Harvey: 1.54m of rain Irma: 185mph for 37 hours. Three cat 4+ U.S. landfalls.

Named-Storm Days 53.25 52.25 (2004)

Hurricane Days 40.25 34.50 (2006)

Major Hurricane Days 18 17.25 (1961)

ACE 175 155 (1928)

Phil Klotzbach (CSU)


A Hurricane Drought? Harvey was the first major hurricane U.S. landfall since Wilma (2005).

Why the drought?

- physical?

- luck?

- definitional?

Emphasis on count of storms above wind speed threshold is misplaced.

2006-2016 storms


Warm Oceans



Warm Oceans


Promotes Rapid Intensification: Harvey and Jose: Cat 1 to Cat 4 in 24 hours Maria: Cat 1 to Cat 5 in 15 hours


Light Winds



Slow-Moving Harvey

• Rapid coastal strengthening to category 4.

• Track guidance fell away.

• Perfect set-up for record rains over Houston.


Intense Irma

• 10-day warning of a strong hurricane in the U.S. Southeast.

• Repeated eye-wall cycles kept winds below 200mph.

• Uncertain right turn. Best possible route to minimize damage.


Pinhole Maria

• Pinhole eye, 10 miles across.

• Eye-wall cycle prevented a category 5 landfall on PR.

• Second strongest behind San Felipe II (1928).


Fast-Moving Nate

• Crossed Gulf of Mexico in 24 hours.

• Warm waters, loop current.

• Landfalling depression or Category 3?

WRN Autumn Seminar – Nov 1st 2017 12

200 Historical Global Footprints


Current Footprinting Approaches

Parametric radial wind profiles:

- fast, simple terrain, landuse, inland decay factors.





Spatial analysis of observations:

- asymmetries, but few storms, globally inconsistent.







Geostatistical spatial modeling:

- fast, only applied to European windstorm so far.







Numerical modeling:

- many physical processes, but slow, track error.

Geostatistical spatial modeling:

- fast, only applied to European windstorm so far.


Topographic Speed-Up

A factor proportional to terrain height and shape. Can reach 2.

(Miller et al. 2012)


Channel Effects

Winds parallel to mountains.

Winds between mountains.


Terrain drives damage

Damaged roof locations in hurricane Fabian (2003).

Miller et al. (2012)


Footprinting Approach

Fast, topographic and roughness effects, no track error, but some missing processes (e.g., strong thermal effects).

Kepert and Wang (2001) numerical boundary layer model.

Historical track data

(EBTrACS and JTWC)

Holland et al. (2010)

parametric pressure profile


Diagnoses boundary-layer flow using dry equations of motion for a given pressure field.

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The KW01 Model

• High-order turbulence scheme - prognostic TKE, turbulence dissipation. - diagnostic length scale (<80m).

• Ignores strong thermal effects. • Rapidly achieves steady state.


1. Allow storms to move:

- add environment pressure gradient to TC forcing,

- add storm translation velocity to horizontal advection.

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Mods to KW01





2. Allow storms to change intensity and size:

- update pressure gradient and allow winds to respond,

- force gradient winds at model top.

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Mods to KW01





2. Allow storms to change intensity and size:

- update pressure gradient and allow winds to respond,

- force gradient winds at model top.

3. Include some topographic effects:

- included in model equations.

4. Include variable surface roughness effects:

- drag coefficient = f(terrain height and landuse).

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Mods to KW01


Effects of Complex Topography

Hurricane Maria (2017)

ms-1


Hurricane

Wilma (2005)

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Variable Surface Roughness

ms-1


Eye-Wall Asymmetries?

Hurricane Patricia (2015)


Good comparison with HWIND

• Similar asymmetry. • Similar wind speed reduction at the coast.

Hurricane Wilma (2005) WRN HWIND

ms-1


Surface Station Comparisons

Hurricane Maria (2017)

ms-1


Hurricane Harvey

ms-1 ms-1

ms-1


Hurricane Harvey


Hurricane Irma

ms-1 ms-1


Reconstructing older events

Typhoon Ellen (1983): Data: Vmax, lat, lon: IBTrACS. Rmax: Hong Kong radar. Constant 35km. R34: Hong Kong barogram. Set to 150km at landfall, then decrease following wind decay


Ellen’s Footprint at 78m

Unique view of risk to high rise structures.

ms-1


Application to Hazard Risk

10% annual wind speed (ms-1)

• Combining with TC risk modeling to produce new views of global wind risk.

• Particularly valuable in regions of complex terrain.


• A globally-consistent topography-aware historical footprint dataset.

• New views of historical events. • Dataset version 1.0.

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Footprinting Global Tropical Cyclones - Willis … Autumn Seminar – Nov 1st 2017 Footprinting...

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