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Tropical Cyclone Observations
1. What is the Dvorak technique and how is it used?
The Dvorak technique is a methodology to get estimates of tropical cyclone intensity from satellite pictures. Vern Dvorak developed the scheme using a pattern recognition decision tree in the early 1970s (Dvorak 1975, 1984) .
Utilizing the current satellite picture of a tropical cyclone, one matches the image versus a number of possible pattern types: Curved band Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If infrared satellite imagery is available for Eye Patterns (generally the pattern seen for hurricanes, severe tropical cyclones and typhoons), then the scheme utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops. The larger the difference, the more intense the tropical cyclone is estimated to be. From this one gets a T-number and a Current Intensity (CI) Number. CI numbers have been calibrated against aircraft measurements of tropical cyclones in the Northwest Pacific and Atlantic basins. On average, the CI numbers correspond to the following intensities:
Current Intensity Numbers
One Minute Winds
Note that this estimation of both maximum winds and central pressure assumes that the winds and pressures are always consistent. However, since the winds are really determined by the pressure gradient, small tropical cyclones (like the Atlantic's Andrew in 1992, for example) can have stronger winds for a given central pressure than a larger tropical cyclone with the same central pressure. Thus caution is urged in not blindly forcing tropical cyclones to "fit" the above pressure- wind relationships. (The reason that lower pressures are given to the Northwest Pacific tropical cyclones in comparison to the higher pressures of the Atlantic basin tropical cyclones is because of the difference in the background climatology. The Northwest Pacific basin has a lower background sea level pressure field. Thus to sustain a given pressure gradient and thus the winds, the central pressure must accordingly be lower in this basin.)
The errors for using the above Dvorak technique in comparison to aircraft measurements taken in the Northwest Pacific average 10 mb with a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical cyclone estimates likely have similar errors. Thus an Atlantic hurricane that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb) could in reality have winds anywhere from 60 to 90 kt and pressures of 989 to 969 mb. These would be typical ranges to be expected; errors could be higher. In the absence of other observations, the Dvorak technique does a consistent estimate of the true intensity of the system.
While the Dvorak technique was calibrated for the Atlantic and Northwest Pacific basin with aircraft reconnaissance data as ground truth, the technique has also been quite useful in other basins that have limited observational platforms. At some point it would be preferable to re-derive the Dvorak technique to calibrate tropical cyclones with available data in the other basins.
Lastly, while the Dvorak technique is primarily designed to provide estimates of the current intensity of the storm, a 24 hour forecast of the intensity can be obtained also by extrapolating the trend of the CI number. Whether this methodology provides skillful forecasts is unknown.
2. Who are the "Hurricane Hunters" and what are they looking for?
Hurricane reconnaissance is carried out by two government agencies, the U.S. Air Force Reserves' 53rd Weather Reconnaissance Squadron and NOAA's Aircraft Operations Center.
The 53rd WRS is based at Keesler AFB in Mississippi and maintains a fleet of ten WC-130 planes. These cargo airframes have been modified to carry weather instruments to measure wind, pressure, temperature and dew point as well as drop instrumented sondes and make other observations.
AOC is presently based at MacDill AFB in Tampa, Florida and among its fleet of planes has two P-3 Orions, originally made as Navy sub hunters, but modified to include three radars as well as a suite of meteorological instruments and dropsonde capability. Starting in 1996 AOC added to its fleet a Gulfstream IV jet that is able to make hurricane observations from much higher altitudes (up to 45,000 feet). It is used primarily to drop sondes around the hurricane's environment to measure synoptic-scale parameters in the usually data-free oceanic areas.
The USAF planes are the workhorses of the hurricane hunting effort. They are often deployed to a forward base, such as Antigua, and carry out most of the reconnaissance of developing waves and depressions. Their mission in these situations is to look for signs of a closed circulation and any strengthening or organizing that the storm might be showing. This information is relayed by satellite to the hurricane specialists for evaluation.
The NOAA planes are more highly instrumented and are generally reserved for when developed hurricanes are threatening landfall, especially on U.S. territory. They are also used to conduct scientific research on storms.
The planes carry between six to fifteen people, both the flight crew and the meteorologists. Flight crews consist of a pilot, co-pilot, flight engineer, navigator, and electrical technicians. The weather crew might consist of a flight meteorologist, lead project scientist, cloud physicist, radar specialist, and dropsonde operators.
The primary purpose of reconnaissance is to track the center of circulation, the co-ordinates that identify the storm's location, and to measure the maximum winds. The crews are also evaluating the storm's size, structure, and development and this information is also relayed via radio and satellite link. Most of this data, which is critical in determining the hurricane's threat, cannot be obtained from satellite.
3. What is it like to fly into a hurricane?
One might not believe this, but most hurricane flights are fairly boring. They last 10 hours, there are clouds above you and clouds below - so all you see is gray, and you don't feel the winds swirling around the hurricane.
What does get interesting is flying through the hurricane's rainbands and the eyewall, which can get a bit turbulent. The eyewall is a donut-like ring of thunderstorms that surround the calm eye. The winds within the eyewall can reach as much as 200 mph [325 km/hr] at the flight level, but you can't feel these aboard the plane. What makes flying through the eyewall exhilarating and at times somewhat scary, are the turbulent updrafts and downdrafts. Those flying in the plane definitely feel these wind currents (and sometimes reach for the air-sickness bags). These vertical winds may reach up to 50 mph [80 km/hr] either up or down, but are actually much weaker in general than what one would encounter flying through a continental supercell thunderstorm.
Once the plane gets into the calm eye of a hurricane like Andrew or Gilbert, it is a place of powerful beauty: sunshine streams into the windows of the plane from a perfect circle of blue sky directly above the plane, surrounding the plane on all sides is the blackness of the eyewall's thunderstorms,
and directly below the plane peeking through the low clouds one can see the violent ocean with waves sometimes 60 feet high [20 m] crashing into one another. The partial vacuum of the hurricane's eye (where one tenth of the atmosphere is gone) is like nothing else on earth.
The USAFR 53rd Hurricane Hunters have a 'cyber flight' through a hurricane. To try it CLICK HERE.
5. What's it like to go through a hurricane on the ground ? What are the early warning signs of an approaching tropical cyclone?
Just as every person is an individual, every hurricane is different so every experience with a storm will be unique. The summary below is of a general sequence of events one might expect from a Category 2 hurricane approaching a coastal area. What you might experience could be vastly different.
- 96 hours before landfall
At first there aren't any apparent signs of a storm. The barometer is steady, winds are light and variable, and fair weather cumulus clouds dot the sky. The perceptive observer will note a swell on the ocean surface of about a meter (3 feet) in height with a wave coming ashore every ten seconds. These waves race out far ahead of a storm at sea, but could easily be masked by locally wind driven waves.
- 72 hours before landfall
Little has changed, except that the swell has increase to about 2 meters (6 feet) in height and the waves now come in every nine seconds. This means that the storm, still far over the horizon, is approaching.
- 48 hours before landfall
If anything, conditions have improved. The sky is now clear of clouds, the barometer is steady, and the wind is almost calm. The swell is now about 3 m (9 feet) and coming in every 8 seconds. A hurricane watch is issued, and areas with long evacuation times are given the order to begin evacuating.
- 36 hours before landfall
The first signs of the storm appear. The barometer is falling slightly, the wind is around 5 m/s (10 kts, 11 mph), and the ocean swell is about 4m (13 feet) in height and coming in 7 seconds apart. On the horizon a large mass of white cirrus clouds appear. As the veil of clouds approaches it covers more of the horizon.
- 30 hours before landfall
The sky is now covered by a high overcast. The barometer is falling at 0.1 millibar per hour (.003 inches of Hg/hr), and the winds pick up to about 10 m/s (20 kts, 23 mph). The ocean swell, coming in only 5 seconds apart, is beginning to be obscured by wind driven waves, and small whitecaps begin to appear on the ocean surface. A hurricane warning is issued and low lying areas and people living in mobile homes are ordered to evacuate.
- 24 hours before landfall
In addition to the overcast, small low clouds streak by overhead. The barometer is falling by .2 mb/hr (.006"Hg/hr), the wind picks up to 15 m/s (30 kts, 34 mph). The wind driven waves are covered in whitecaps and streaks of foam begin to ride over the surface. Evacuations should be completed and final preparations made by this time.
- 18 hours before landfall
The low clouds are thicker and bring driving rain squalls with gusty winds. The barometer is steadily falling at half a millibar per hour (.015 "Hg/hr), and the winds are whistling by at 20 m/s (40 kts, 46 mph). It is hard to stand against the wind.
- 12 hours before landfall
The rain squalls are more frequent and the winds don't diminish after they depart. The cloud ceiling is getting lower, and the barometer is falling at 1 mb/hr (.029 "Hg/hr). The wind is howling at hurricane force at 32 m/s (64 kts, 74 mph), and small, loose objects are flying through the air and branches are stripped from trees. The sea advances with every storm wave that crashes ashore and the surface is covered with white streaks and foam patches.
- 6 hours before landfall
The rain is constant now and the 40 m/s wind (80 kts, 92 mph) drives it horizontally. The barometer is falling 1.5 mb/hr (.044 "Hg/hr), and the storm surge has advanced above the high tide mark. It is impossible to stand upright outside without bracing yourself, and heavy objects like coconuts and plywood sheets become airborne missiles. The wave tops are cut off and make the sea surface a whitish mass of spray.
- 1 hour before landfall
It didn't seem possible, but the rain has become heavier, a torrential downpour. Low areas inland become flooded from the rain. The winds are roaring at 45 m/s (90 kts, 104 mph), and the barometer is free-falling at 2 mb/hr (.058 "Hg/hr). The sea is white with foam and streaks. The storm surge has covered coastal roads and 5 meter (16 foot) waves crash into buildings near the shore.
- The eye
Just as the storm reaches its peak, the winds begin to slacken, and the sky starts to brighten. The rain ends abruptly and the clouds break and blue sky is seen. However the barometer continues falling at 3 mb/hr (.09 "Hg/hr) and the storm surge reaches the furthest inland. Wild waves crash into anything in the grasp of the surge. Soon the winds fall to near calm, but the air is uncomfortably warm and humid. Looking up you can see huge walls of cloud on every side, brilliant white in the sunlight.
At this point, the barometer stops falling and in a moment begins to rise, soon as fast as it fell. The winds begin to pick up slightly and the clouds on the far side of the eyewall loom overhead.
- 1 hour after landfall
The sky darkens and the winds and rain return just a heavy as they were before the eye. The storm surge begins a slow retreat, but the monstrous waves continue to crash ashore. The barometer is now rising at 2 mb/hr (.058 "Hg/hr). The winds top out at 45 m/s (90 kts, 104 mph), and heavy items torn loose by the front side of the storm are thrown about and into sides of buildings that had been in the lee before the eye passed.
- 6 hours after landfall
The flooding rains continue, but the winds have diminished to 40 m/s (80 kts, 92 mph). The storm surge is retreating and pulling debris out to sea or stranding sea borne objects well inland. It is still impossible to go outside.
- 12 hours after landfall
The rain now comes in squalls and the winds begin to diminish after each squall passes. The cloud ceiling is rising, as is the barometer at 1 mb/hr (.029 "Hg/hr). The wind is still howling at near hurricane force at 30 m/s (60 kts, 69 mph), and the ocean is covered with streaks and foam patches. The sea level returns to the high tide mark.
- 24 hours after landfall
The low clouds break into smaller fragments and the high overcast is seen again. The barometer is rising by .2 mb/hr (.006"Hg/hr), the wind falls to 15 m/s (30 kts, 34 mph). The surge has fully retreated from land, but the ocean surface is still covered by small whitecaps and large waves.
- 36 hours after landfall
The overcast has broken and the large mass of white cirrus clouds disappears over the horizon. The sky is clear and the sun seems brilliant. The barometer is rising slightly, the wind are a steady 5 m/s (10 kts, 11 mph). All around are torn trees and battered buildings. The air stinks of dead vegetation and muck that was dredged by the storm from the bottom of the sea to cover the shore. The all clear is given.
6. Are there hurricanes on other planets ?
There are no other planets known to have warm water oceans from which true water cloud hurricanes could form. However, many astronomers and planetary meteorologists believe Jupiter exhibits such storms, in which ammonia takes the place of water. The principal candidate is the famous Great Red Spot, and the numerous whorls that surround it. The Spot exhibits an anticyclonic circulation at its top, just as tropical cyclones do at the top of the troposphere.
Jupiter's Great Red Spot (NASA/JPL)
7. How does the ocean respond to a hurricane ?
The ocean's primary direct response to a hurricane is cooling of the sea surface temperature (SST). How does this occur? When the strong winds of a hurricane move over the ocean they churn-up much cooler water from below. The net result is that the SST of the ocean after storm passage can be lowered by several degrees Celsius (up to 10° Fahrenheit).
Figure 1 shows SSTs ranging between 25- 27°C (77-81°F) several days after the passage of Hurricane Georges in 1998. As Figure 1 illustrates, Georges' post storm 'cold wake' along and to the right of the superimposed track is 3-5°C (6-9°F) cooler than the undisturbed SST to the west and south (i.e. red/orange regions are ~30°'C [86°'F]). The magnitude and distribution of the cooling pattern shown in this illustration is fairly typical for a post-storm SST analysis.
One important caveat to realize however is that most of the 3-5°C (6-9°F) ocean cooling shown in Figure 1 occurs well after the storm has moved away from the region (in this case several days after Georges made landfall). The amount of ocean cooling that occurs directly beneath the hurricane within the high wind region of the storm is a much more important question scientists would like to have answered. Why? Hurricanes get their energy from the warm ocean water beneath them. In order to get a more accurate estimate of just how much energy is being transferred from the sea to the storm, scientists need to know ocean temperature conditions directly beneath the hurricane. Unfortunately, with 150kph+ (100mph+) winds, 20m+ (60ft+) seas and heavy cloud cover being the norm in this region of the storm, direct (or even indirect) measurement of SST conditions within the storm's inner core environment are very rare.
Thankfully in this case "very rare" does not mean "once in a lifetime". Recently, scientists at the Hurricane Research Division were able to get a better idea of how much SST cooling occurs directly under a hurricane by looking at many storms over a 28 year period. By combining these rare events, HRD scientists put together a "composite average" of ocean cooling directly under the storm.
Figure 2 illustrates that, on average, cooling patterns are a lot less than the post storm 3-5°C (6-9°F) cold wake estimates shown in Figure 1. In most cases, the ocean temperature under a hurricane will range somewhere between 0.2 and 1.2°C (0.4 and 2.2°F) cooler that the surrounding ocean environment. Exactly how much depends on many factors including ocean structure beneath the storm (i.e. location), storm speed, time of year and to a lesser extent, storm intensity (Cione and Uhlhorn 2003).
While the estimates in Figure 2 represent a dramatic improvement when it comes to more accurately representing actual SST cooling patterns experienced under a hurricane, even small errors in inner core SST can result in significant miscalculations when it comes to accurately assessing how much energy is transferred from the warm ocean environment directly to the hurricane. With all other factors being equal, even an error of 0.5°C (1°F) can be the difference between a storm that rapidly intensifies to one that falls apart! With that much at stake, scientists at HRD and other government and academic institutions are working to improve our ability to accurately estimate, observe and predict under-the-storm upper ocean conditions. These efforts include statistical studies, modeling efforts and enhanced observational capabilities designed to help scientists better assess upper ocean thermal conditions under the storm. With such improvements, it is believed that future forecasts of tropical cyclone intensity change will be significantly improved.
Cione, J. J., and E. W. Uhlhorn, 2003: Sea Surface Temperature Variability in Hurricanes: Implications with Respect to Intensity Change. Monthly Weather Review, 131, 1783-1796.