# Presentation: 133580

## HURRICANES!...An overview of tropical cyclones

*HURRICANES!**...An overview of tropical cyclones*

**Tim Marchok**
**NOAA/Geophysical Fluid Dynamics Laboratory**
**timothy.marchok@noaa.gov**

- Princeton University

- Quest Science Workshop

- July, 2007

## Outline

- What hurricanes are and where they form
- How hurricanes work
- How hurricanes cause damage
- How hurricanes differ from tornadoes
- How we forecast hurricanes
- Hurricanes and climate
- Hurricanes that have affected NJ and PA

## “Hurricane” is a specific regional name. In general, these storms are known as “tropical cyclones”:
Tropical depression:	 winds < 39 mph
Tropical storm:	   winds < 74 mph
Hurricane:	   winds > 74 mph

- A hurricane is an intense cyclonic storm (i.e., a cyclone) that begins as an area of low pressure  over the tropical oceans.

- “Hurricane” is a specific regional name. In general, these storms are known as “tropical cyclones”:
- Tropical depression:	 winds < 39 mph
- Tropical storm:	   winds < 74 mph
- Hurricane:	   winds _>_ 74 mph

- ( gets named! )

## - Atlantic and Eastern Pacific Oceans (hurricanes)- Western North Pacific Ocean (typhoons)- South Pacific and Indian Oceans (cyclones)

- Where do tropical cyclones form?

- (10)

- (26)

- (14)

- (15)

- (5)

- (9)

- (nn)  Number of storms per year

## Outline

- What hurricanes are and where they form
**How hurricanes work**
- How hurricanes cause damage
- How hurricanes differ from tornadoes
- How we forecast hurricanes
- Hurricanes and climate
- Hurricanes that have affected NJ and PA

## Why are the winds so strong in a hurricane?

- Winds always move from areas of higher pressure to areas of lower pressure.  The bigger the difference between the high and low pressure, the stronger the winds.

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- The winds from Hurricane Fabian battering Bermuda on September 5, 2003.

## What causes the winds to rotate around a hurricane?

- Hurricane Isabel making landfall on September 18, 2003

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- The Coriolis Effect:  A change in the direction of moving objects due to those objects moving on a rotating, spherical planet.

## If the Coriolis effect causes objects to move to the right of the direction of motion in the Northern Hemisphere, then why is the circulation counter-clockwise around a low-pressure center?

- Keep in mind that there is a mixture of forces that determines the path a particle of air takes, with the Coriolis effect being just one force.  Previously, we’ve seen how air moves from high to low pressure.  The force that causes this is called the “pressure gradient force” (PGF).  At large distances from a low (e.g., at point A in the diagram on the right), there is almost a balance between the Coriolis effect and the PGF.  As a parcel of air gets closer to the low pressure center, often the pressure gradient intensifies (analogous to a terrain gradient increasing when on a mountainside and getting ready to drop rapidly into a canyon), accelerating the parcel (this is felt in the increase of winds as you near a hurricane’s center).  Close to a strong low, the influence of the Coriolis effect becomes unimportant compared to the PGF, and the parcel moves more in a direction towards the low.  Very close to an intense hurricane’s center (e.g., at point B in the diagram on the right), as the air rotates rapidly around the center, there is nearly a balance between the PGF and an outward force due to centripetal acceleration.

- A

- B

## How is a lawn mower like a hurricane?

- Gas provides the fuel for the mower’s engine

- The engine uses the fuel to perform work  and move the grass-cutting blade

- The starter rope and spark plug are the “triggers” for making the mower start

- Exhaust fumes are expelled out away from the engine

## The hurricane heat engine: how it works

**3.** Having lost most of its heat and energy, the air is now expelled outward away from the center of the storm in the cooler hurricane outflow at high levels.

- In mature hurricanes, an eye forms in the center.  Here, the air is descending and the region is generally cloud-free.

**2.** The warm, moist air rises in thunderstorms surrounding the eye, supplying the fuel that helps strengthen the** **storm and intensify the winds.

**1.** As air rushes into the hurricane’s low pressure center, the air picks up heat and moisture (fuel) from the warm ocean surface.

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## Example: Slicing through a Katrina GFDL forecast

- This is an x-y view of the forecasted winds at 35 meters above the surface 66 hours into a forecast of Katrina

- We can take a slice through the core of the storm to see the structure of the storm through the depth of the troposphere

- The shaded contours show the strength of the horizontal winds.  The wind barbs show the circulation through the storm

- Here, the shaded contours show the temperature deviation inside the storm and indicate the hurricane’s  warm core structure.

## ...show movie “katrina-visir.avi” to illustrate rotation of storm as well as the blow-off of high-level cirrus clouds....

## ...but where’s the trigger?

- Hurricanes don’t just form out of thin air.  They typically grow from organized clusters of smaller thunderstorms that move across the oceans.

## Tropical Cyclogenesis (TC genesis)

- “TC genesis”:  Transition from a tropical disturbance to a tropical depression, that is,  the initial formation of a rotational circulation with a scale of a few hundred km.

- Montage of the lifecycle of Hurricane Frances (2004)

- Image courtesy of CIMMS / Univ of Wisconsin

## TC Genesis:  geographic origins of disturbances

- About 85% of all TCs form within 20o latitude of the equator.  The vast majority originate in or just poleward of a region of preferred convection (thunderstorm activity) known as the Inter-Tropical Convergence Zone (ITCZ).

**Atlantic Ocean:** Majority of TCs originate from  “African Easterly Waves”, disturbances that originate as intense thunderstorm complexes over Africa and move westward off the coast, just to the north of the ITCZ.

- A smaller fraction of Atlantic TCs originate in old frontal regions from the remnants of mid-latitude circulations off the Eastern Seaboard or in the Gulf of Mexico.

**The “right mixture of fuel”:** Sea surface temperatures must be at least 26.5oC (~80oF)  for a TC _**to form **_(once formed, they can survive over cooler waters).

## ...show movie “storms_2004_wv_mini.mov” to illustrate how storms develop from clusters of smaller thunderstorms, and to illustrate how some clusters do in fact organize and strengthen into tropical cyclones while others never get their act together....

## Summer-Autumn variations in Atlantic TC genesis

- Climatological SSTs: June

- Climatological SSTs: September

- TCs beginning June 11-20 (1886-1992)

- TCs beginning Sept 11-20 (1886-1992)

## Thick Mixed Layer and Large Oceanic Heat Content of the Loop Current Limited the Ocean Cooling from Rita and Katrina.This contributed to the explosive intensification of the storms in the Gulf of Mexico

## Outline

- What hurricanes are and where they form
- How hurricanes work
**How hurricanes cause damage**
- How hurricanes differ from tornadoes
- How we forecast hurricanes
- Hurricanes and climate
- Hurricanes that have affected NJ and PA

## The triple threat of damage from a hurricane:

**Wind Damage**:  Wind gusts can approach 200 mph in the strongest hurricanes.
**Storm Surge**:  The winds of a hurricane pile up ocean water that can inundate coastlines with water levels up to 30 feet higher than normal.
**Inland Flooding**:  Today, this is the greatest risk to life from a hurricane.  Hurricanes and tropical storms, especially slow-moving ones, can drop huge amounts of rain in a very short time, even inland far from where the storm made landfall.  Over the past 30 years, 9 out of every 10 fatalities in tropical cyclones have been due to drowning from inland flooding.

## Wind Damage examples

- A 1x4 board of wood was pierced through a palm tree by Hurricane Andrew’s winds in August, 1992.

- A mobile home park near Miami, FL was completely destroyed by Hurricane Andrew in August, 1992.

- An historic home along the Gulf Coast before and after the passage of Category 5 Hurricane Camille in August, 1969.

## The Saffir-Simpson Scale: A method for categorizing hurricanes based on wind speed

|  | Category 1 | Category 2 | Category 3 | Category 4 | Category 5 |
| --- | --- | --- | --- | --- | --- |
| Wind
speed (mph) | 74-95 | 96-110 | 111-130 | 131-155 | >155 |
| Storm 
surge (ft) | 4-5 | 6-8 | 9-12 | 13-18 | >18 |
| Damage 
description | Minimal | Moderate | Extensive | Extreme | Catastrophic |

## Storm Surge examples

- Storm surge from Hurricane Carol completely inundates a yacht club in August, 1954.

- Apartment building in Pass Christian, Mississippi before and after the passage of Category 5 Hurricane Camille in August, 1969.  Thirty-two people ignored warnings and stayed for a “hurricane party”.  Only two survived.

- Hurricane Katrina’s 30-foot storm surge obliterated every home within several blocks of the shoreline in Pass Christian and Waveland, Mississippi.

## How are hurricanes different from tornadoes?

|  | Hurricanes | Tornadoes |
| --- | --- | --- |
| Where they form | Hurricanes form over warm water in the tropical oceans and develop best when far from the jet stream. | Tornadoes form over land and form within storms that are often very close to the jet stream |
| How big they are | Up to several hundred miles wide | No more than 1⁄4 mile wide |
| How long they last | Average of 9 days (up to 3 weeks) | No more than an hour |
| How strong the winds are | Less than 200 mph | Up to 300 mph |
| Occurrences per year | An average of 10 tropical storms in the Atlantic Ocean | In the United States, 800-1000 |
| Advance warning from
forecasters | Several days | No more than 15-30 minutes |

## Outline

- What hurricanes are and where they form
- How hurricanes work
- How hurricanes cause damage
- How hurricanes differ from tornadoes
**How we forecast hurricanes**
- Hurricanes and climate
- Hurricanes that have affected NJ and PA

## Forecasting a hurricane’s track and intensity

- View from a Hurricane Hunter aircraft inside the eye of Hurricane Isabel while a category 5 storm.

_**Aircraft:**_** **Commercial aircraft and “Hurricane Hunters”

- _Forecasting a hurricane’s track and intensity_

- 1. Observations: Measuring the atmosphere

- Before we can predict what a storm will do in the future, we need to know what the storm and the atmosphere look like right now.

_**Radiosondes:**_ These instruments are attached to weather balloons

_**Satellites**_

_**Surface**_

## On board a recon flight into Hurricane Rita....

## On board a recon flight into Hurricane Rita....

## On board a recon flight into Hurricane Rita....

## Forecasting a hurricane’s track and intensity

- 2. All observations of the atmosphere are fed into a 3-D analysis that depicts the current state of the atmosphere

- 3. A supercomputer is needed to _quickly_ solve the complex equations that describe the motions of the atmosphere using the analysis as its initial condition.

- A graphical snapshot from a GFDL Hurricane Model forecast for Hurricane Floyd, September, 1999

- _Triply-nested grid_:  Computations are performed for ~1.8 billion data values every 60 seconds.

## ...show “katrina_gfdl_movie.mpg” as an example of a 3-D visualization of a hurricane computer model forecast....

## Forecasting a hurricane’s track and intensity

- 4. Making heads or tails of all the model forecast data

- Sometimes the models agree with each other...

- This plot shows the computer model track forecasts for Hurricane Katrina issued almost 3 days before she made landfall.  The fact that the models agreed so well with one another made it relatively easy for the National Hurricane Center to forecast her landfall location.

- In this case, the forecast at this particular time for Hurricane Kate became a difficult one for the National Hurricane Center because the different computer model forecast tracks varied so wildly.

- ...and sometimes they don’t!

## Forecasting a hurricane’s track and intensity

- 5. Consensus forecasts: A mean of several skillful forecasts normally outperforms any of the individual forecasts...

- Forecasters at the National Hurricane Center rely on forecasts from a suite of models, which can often provide dissimilar solutions.
- For this case of Hurricane Dennis, should the forecasters trust the model that predicts the storm will make landfall near New Orleans, or the one that predicts the storm will make landfall on the Florida Panhandle?

- A consensus mean forecast (here, “GUNA”) is computed by averaging together the tracks from some of the more skillful models.  This helps to smooth out the biases of some of the individual forecasts.

- Forecasters often rely heavily on these consensus forecasts.  Note how close the National Hurricane Center’s Official forecast (“OFCL”) is to the “GUNA” consensus forecast here.

- For this case, the consensus forecast was very accurate, leading to very small track forecast errors at landfall near Pensacola, Florida (the eventual observed positions of Dennis are shown by the black hurricane symbols)

## How accurate are track forecasts in the Atlantic?

- _Forecasting a hurricane’s track and intensity_

- Track errors have been cut in half over the last 15 years...

- Source:   James Franklin, National Hurricane Center

## Forecasting a hurricane’s track and intensity

- Improvements in intensity forecasting lag far behind those of track forecasting...  but why?

- 1. Until recently, the vast majority of R&D efforts were focused primarily on track forecasting.  The driving philosophy:  It is far more important to know _**where**_ a storm is going to hit than exactly how intense it will be.

- 2. _**Track**_ forecasting is primarily determined by accurate simulations of the large-scale atmospheric flow, which most models handle very well.  Factors that determine _**intensity**_ forecasting often occur on very small spatial scales that current models have difficulty depicting.

- 3. While it has long been known that the temperature of the sea surface affects a storm’s intensity, there are numerous physical processes involved in the exchange of energy between the sea surface and the atmosphere that are not well understood.  These processes affect storm intensity and have only recently begun to get widespread attention from the research community.

## Outline

- What hurricanes are and where they form
- How hurricanes work
- How hurricanes cause damage
- How hurricanes differ from tornadoes
- How we forecast hurricanes
**Hurricanes and climate**
- Hurricanes that have affected NJ and PA

## Hurricanes and climate

- 1. What is El Nino and does it have any effect on hurricanes?

- El Nino and the accompanying Southern Oscillation, referred to collectively as “ENSO”, occur in the tropical regions of the Pacific Ocean.  During an El Nino event, the trade winds weaken and sea surface layers become anomalously warm in the central and eastern Pacific Ocean.

- A fairly strong relationship exists between El Nino (“ENSO warm events”) and the suppression of hurricane activity in the Atlantic Ocean.

- El Nino
- “ENSO warm event”

- La Nina
- “ENSO cold event”

**The Walker Circulation**

**Normal Pacific pattern**

**ENSO warm event**

**ENSO cold event**

## Hurricanes and climate: Global Warming

- 2. Is global warming responsible for the very active Atlantic  hurricane seasons we’ve seen over the last 10 years?

- Other research has shown the existence of natural swings in Atlantic hurricane activity that can last for decades.  These swings are tied to changes in ocean circulations, affecting not only ocean temperatures but also the atmosphere above.  The 1930’s to 1960’s were very active, followed by a relatively  inactive period from 1970-1994.  It appears that in 1995 we entered into another active period.

- Recent research has indicated that while the number of hurricanes has **not** increased over the last 50 years, the percentage of storms that reach the more intense categories has increased.   The authors point to global warming as a likely explanation.

- This is currently the hottest topic in hurricane research.  There are 2 opposing views

**YES!**

**NO!**

## Hurricanes and climate: The AMO argument

- This figure shows the variation of Sea Surface Temperature (SST) anomalies in the North Atlantic Ocean associated with alternating cycles of the Atlantic Multidecadal Oscillation (AMO)

- This figure shows the difference in East Coast major (Cat 3+) hurricane landfalls between seasons with colder and warmer values of the AMO

- (Courtesy Stan Goldenberg)

## Hurricanes and climate: Global Warming argument

- 1. Kerry Emanuel (MIT) uses a simple metric – the Power Dissipation Index (PDI), a function of  (**V**max)3 – to describe hurricane intensity & activity over the last ~130 years

- 2.  The PDI in the Atlantic co-varies with SST very well, leading Emanuel to conclude that increases in SST, which he attributes to anthropogenic (man-induced) warming, have led to the increases in PDI in the Atlantic, and that the AMO is insignificant in modifying Atlantic SSTs.

- (Courtesy Kerry Emanuel)

## Increase of SSTs and the Number of Intense TCs 

## Hurricanes and climate: Be careful of the data!

**Simple example of data inconsistencies inflating a trend.**

- GMS-1 launched 14 July 1977

- Prior to the regular use of satellites in the Indian Ocean, detection of tropical cyclones was extremely poor.

- Courtesy Jim Kossin, Univ of Wisc

## Web links

- National Hurricane Center:   _[http://www.nhc.noaa.gov/](http://www.nhc.noaa.gov/)_
- Hurricane FAQ page (an excellent resource for any questions related to hurricanes):  _[http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html](http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html)_
- Tornado FAQ page (another excellent resource): _[http://www.spc.noaa.gov/faq/tornado/](http://www.spc.noaa.gov/faq/tornado/)_
- Philadelphia / Mount Holly National Weather Service office:  _[http://www.erh.noaa.gov/er/phi/](http://www.erh.noaa.gov/er/phi/)_
- The Weather Underground (There is a link on this page to “Computer Models”, which will show you the latest track forecasts from various models): _[http://www.wunderground.com/tropical/](http://www.wunderground.com/tropical/)_
- Unisys tropical weather site (good info on both current and past hurricanes):  _[http://weather.unisys.com/hurricane/](http://weather.unisys.com/hurricane/)_
- University of Wisconsin archive of 2005 season images and movies:  _[http://cimss.ssec.wisc.edu/tropic/archive/atlantic.html](http://cimss.ssec.wisc.edu/tropic/archive/atlantic.html)_

## The end.

## Outline

- What hurricanes are and where they form
- How hurricanes work
- How hurricanes cause damage
- How hurricanes differ from tornadoes
- How we forecast hurricanes
- Hurricanes and climate
**Hurricanes that have affected NJ and PA**** **

## Hurricanes Carol and Hazel

- August & October, 1954

- Hurricane Carol caused $460 million in damage and was responsible for 60 deaths on the U.S. East Coast

- Hurricane Hazel produced the strongest winds ever officially recorded in Philadelphia (94 mph)

- Storm surge from Hurricane Carol pounds the New England Coast

## Hurricanes Connie and Diane

- August, 1955

- Floods from the storms affected Yardley, washing out the Yardley-Trenton Bridge.  Fifty other bridges along the Delaware River were either damaged or destroyed.

- Flooding of the Delaware River at the Washington Crossing Bridge, August, 1955.

- The 2 storms hit within a week of each other, causing record rainfall and flooding, killing 180 people (90 along the Delaware River) and causing $680 million in damage on the U.S. East Coast.

## Hurricane Agnes

- June, 1972

- The storm stalled over northern Pennsylvania as a weakened tropical depression, leading to record rainfall amounts and disastrous flooding.

- Costliest natural disaster in Pennsylvania history ($2.1 billion in damage)

- 48 deaths in Pennsylvania from the flooding (122 total in the United States)

## Hurricane Floyd

- September, 1999

- Hurricane Floyd dumped 8-13 inches of rain in parts of the Philadelphia suburbs

- Eight people in southeastern Pennsylvania drowned in the flooding from Hurricane Floyd.

- In total, Floyd caused more than $4 billion in damage in the U.S.

- Satellite image of Hurricane Floyd making landfall in North Carolina, September 16, 1999

- Flooding of the Raritan River near Flemington, NJ, in the aftermath of Hurricane Floyd.

## Washington Crossing Bridge

- August, 1955

- September 19, 2004

- Photo copyright: Tim Marchok, 2004