We can't do an series on
National Hurricane Preparedness Week without addressing the science (some say art) of Hurricane forecasting / prediction.
Hurricane forecasting is a specialized skill which involves analyzing highly complex atmospheric conditions. Many parameters at all layers of the atmosphere affect hurricane movement and intensity. Determining the dominant parameters is the most difficult factor in tropical cyclone forecasting.
Forecasts today can get hurricane tracks wrong by hundreds of miles and wind speeds by tens of miles per hour. As a result, Majumdar says, "people often return after an evacuation to find nothing really happened." The solution, he says, is to improve forecasting through better science. "That's the only way to get people to trust the warnings."
The stakes have never been higher. Population is burgeoning along vulnerable coasts in the U.S., Asia, and the Caribbean. In the southeastern U.S., for example, coastal populations grew more than 50 percent from 1980 to 2003. The North Atlantic hurricane nursery, responding to a natural climate cycle, is experiencing [In 2006] a baby boom that isn't expected to end for a decade or more. And behind it all lurks the grim possibility that global warming is making these storms stronger.
NHC Senior Hurricane Specialist Dr. Michael Brennan describes the collaborative process in forecasting the track and intensity of a tropical cyclone:
Tropical cyclones form from relatively common tropical weather systems, referred to as cloud clusters. These groups of loosely organized, deep cumulus clouds occur in a variety of tropical weather situations, but in the Atlantic the most common pattern for storm genesis has historically been intensification of tropical waves that regularly move off the west coast of Africa during the Atlantic hurricane season. Most cloud clusters and tropical waves, however, do not evolve into tropical cyclones. In this sense, the hurricane is a rare phenomenon.
The initiation of a vortex with winds of moderate strength (cyclogenesis) can occur very rapidly, often in less than a day. The climatology of Atlantic tropical cyclogenesis suggests that formation is favored when a strong convective disturbance occurs in a region where the air is already "spinning" in a cyclonic (counterclockwise) direction. Other favorable factors, such as weak vertical wind shear, low-level inflow, and high-level outflow, have also been identified. Interactions between incipient disturbances and upper-tropospheric systems often contribute to cyclone development as well. Genesis almost always occurs over warm tropical waters. The dynamics of the initial stage of the tropical cyclone's life cycle is not well understood due to the lack of observations in the regions of storm genesis and the complexity of the interactions between the many scales of motion involved in formation.
Intensification of the weak circulation into a hurricane can be thought of as the evolution of a vortex in which the dominant forces are in approximate balance. The balance of forces near the sea surface is altered by the friction, causing moisture-rich air to move toward the storm center. Clouds near the center are organized into spiral-rainband structures by a complex, poorly understood interaction between the physics of the clouds, the strong rotation in the vortex, and the atmospheric conditions in the environment of the storm. The strengthening winds extract ever larger amounts of water vapor from the warm ocean. As this water vapor rises near the center, it cools and condenses; the latent heat thus released creates a warm central core, and air is drawn toward the center, contracting the vortex and further spinning up the winds. The reasons that some disturbances intensify to hurricane, while others do not, are not well understood. Also unknown are the reasons that some hurricanes become severe while others do not.
Although the small-scale details of the storm may change continuously, and sometimes rapidly, the tropical cyclone, as a whole, is a stable system that may persist for many days over the warm tropical ocean. During this time, a tropical cyclone moves in the general direction of the broad-scale wind patterns in which it is embedded. Tropical cyclones dissipate rapidly after landfall, due primarily to the loss of the surface moisture source. The vortex may retain some organization, particularly in the middle troposphere, for several days after landfall. Storms that move poleward over cold waters tend to weaken at a slower rate than those storms that move over land. In either case, the circulation center frequently interacts or combines with a midlatitude weather system and, in the process, loses its warm core structure. The transformed system can still produce substantial rainfall.
Hurricane Forecast Computer Models
By Dr. Jeff Masters, Director of Meteorology from
Wunderground
The behavior of the atmosphere is governed by physical laws which can be expressed as mathematical equations. These equations represent how atmospheric quantities such as temperature, wind speed and direction, humidity, etc., will change from their initial current values (at the present time). If we can solve these equations, we will have a forecast. We can do this by sub-dividing the atmosphere into a 3-D grid of points and solving these equations at each point.
These models have three main sources of error:
1) Initialization: We have an imperfect description of what the atmosphere is doing right now, due to lack of data (particularly over the oceans). When the model starts, is has an incorrect picture of the initial state of the atmosphere, so will always generate a forecast that is imperfect.
2) Resolution: Models are run on 3-D grids that cover the entire globe. Each grid point represents of piece of atmosphere perhaps 40 km on a side. Thus, processes smaller than that (such as thunderstorms) are not handled well, and must be "parameterized". This means we make up parameters (fudge factors) that do a good job giving the right forecast most of the time. Obviously, the fudge factors aren't going to work for all situations.
3) Basic understanding: Our basic understanding of the physics governing the atmosphere is imperfect, so the equations we're using aren't quite right.
Types of hurricane forecasting models
The best hurricane forecasting models we have are "global" models that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called "dynamical" models. The four best hurricane forecast models—ECMWF, GFDL, GFS, and UKMET—are all global dynamical models. These models take several hours to run on the world's most advanced supercomputers.
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There are also dynamical models that cover just a portion of the globe. These are less useful, unless the hurricane happens to start out inside the domain the model covers and stay there. Hurricanes moving from outside the model domain into the model domain are not well handled. An example of this kind of model is the NAM model covering North America and the surrounding waters, run by the National Weather Service (NWS).
Another type of hurricane model is a statistical model. These models do not try to solve mathematical equations on a grid. The advantage of these statistical models is that they are fast to run and can provide output in a few minutes. There are also hybrid statistical/dynamical models, and simple trajectory models.
A full list of all of the tropical cyclone track and intensity models can be
found on the National Hurricane Center's website.
A summary of the top six models:
ECMWF: The
European Center for Medium-Range Weather Forecasting (ECMWF) model is the premier global model in the world for medium range weather forecasting in the mid-latitudes. In 2006, the ECMWF made improvements that starting producing very accurate hurricanes forecasts.
GFS: The Global Forecast System model run by the NWS. Excellent graphics are available on the web from the
National Center for Environmental Prediction. Wunderground.com also has GFS plots. I like the
Tropical Atlantic imagery. If you select "Shear" from the "level" menu, then click on "Add a Map", you'll get plots of the wind shear that I talk so much about.
GFDL: The NWS/Geophysical Fluid Dynamics Laboratory model. The GFDL and HWRF models are the only models that provide specific intensity forecasts of hurricanes. Wunderground.com makes these graphics available on
Wundermap. More detailed GFDL graphics are available at
NOAA/NCEP. See the "GHM" model under the heading, Hurricane Graphics.
UKMET: The United Kingdom Met Office model. Data from this model is restricted from being redistributed according to international agreement, and graphics from the UKMET are difficult to find on the web. Only paying subscribers are supposed to have access to the data.
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HWRF: The NWS/Hurricane Weather Research Model. HWRF is a non-hydrostatic a coupled ocean-atmosphere model, will utilize highly advanced physics of the atmosphere, ocean and waves in one prediction system, providing unparalleled understanding of the science of tropical cyclone evolution. Its output gives meteorologists an analysis of the hurricane in three-dimensions from real-time airborne Doppler radar. It will make use of a wide variety of observations from satellites, data buoys, and hurricane hunter aircraft. No other hurricane model accesses this wide of a range of meteorological information. The GFDL and HWRF models are the only models that provide specific intensity forecasts of hurricanes. Detailed HWRF graphics are available at
NOAA/NCEP. See the "HWRF" model under the heading, Hurricane Graphics.
NOGAPS: The U.S. Navy's Navy Operational Global Prediction Center System. Graphics are available at the
Navy web site. This model has been performing poorly in recent years compared to the other global models, so it has been removed from the consensus models that the National hurricane Center uses as of 2011.
One other model worth looking at, but not as good as the other six is the
Canadian GEM model.
Non-global models
The BAMM model (Beta and advection model, medium layer) is included on
Wundermap. The BAMM is a simple trajectory model that is very fast to run, and did the best of any individual model at 3-5 day track forecasts in 2005. Since this model is always available, we have included it along with the "big four". In general, one should not trust the BAMM model for the 1-2 day time period when output from "the big four" are available. "The big four" are generally not available for tropical disturbances, and for these situations we post plots of a number of other non-global models such as the LBAR, A98E, etc. All of these models are described in detail on
NHC's web site.
Model performance
So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF. This model out-performed the official NHC forecast in 2010 for 3-day and 4-day forecasts, and in 2009 for 4-day and 5-day forecasts. You can view ECMWF forecasts on our
Wundermap with the model layer turned on. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 3, over the past two years, the GFS and GFDL model have been the next best models, with the UKMET model not far behind. Last year, the NOGAPS model did very poorly, forcing NHC to come up with some new consensus models this year, the TCOA and TVCA, that do not include the NOGAPS model. For those interested in learning more about the models, NOAA has a great
training video (updated for 2011).
Figure 1. Forecast performance in 2010, compared to a simple "Climatological and Persistence' (CLIPER) model. OFCL=Official NHC forecast. The best global dynamical models are the ECMWF, the GFS, and the GFDL. Image credit:
National Hurricane Center.
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Categories: Disaster & Survival Preparedness
How are Tropical Storms different from Hurricanes?
Mainly - Wind speed. Both are storms of tropical origin, fueled by warm sea surface temperatures. Tropical storms are hurricanes that have not yet matured. Storms with wind speeds of 39 to 73 mph are called a tropical storm; those with wind speeds of 74 mph or greater are called hurricanes.
Really, they are both tropical cyclones. A tropical cyclone with wind speeds less than 39 mph is a tropical depression. A tropical depression lacks the familiar shape and eye of the more severe tropical cyclones
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A tropical storm is a tropical cyclone has wind speeds between 39 mph and 73 mph. While it has the familiar spiral shape it does not have the well defined eye of a hurricane.
A hurricane is a tropical cyclone with wind speeds in excess of 74 mph and has a defined eye.
Glossary of National Hurricane Center Terms Continue reading →