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# Tropical cyclone forecast model

[
National Hurricane Center (NHC)]

A tropical cyclone forecast model is a computer program that uses meteorological data to forecast the motion and intensity of tropical cyclones. Such models utilize powerful supercomputers with sophisticated mathematical modeling software and meteorological data to calculate paths and intensities. There are three types of models: statistical, dynamical, or combined statistical-dynamic, and two primary types of forecasts, track and intensity.

Track models

Some of the track models used by the National Hurricane Center (NHC) are as follows: [http://www.aoml.noaa.gov/hrd/tcfaq/F2.html NHC Tropical Cyclone FAQ Subject F2] ] [http://www.nhc.noaa.gov/modelsummary.shtml Summary of the NHC/TPC Tropical Cyclone Track and Intensity Guidance Models] ] [http://www.nhc.noaa.gov/verification/pdfs/Verification_2005.pdf 2005 NHC Forecast Verification Report] ]
*CLIPER (CLImatology and PERsistence) is a 3-day statistical model (CLIPER5 is a 5-day version of the same model). It uses the current path of a tropical cyclone and an average of historical paths of similar cyclones to come up with a track, and is considered a "no-skill" model. Curiously, until the late 1980s, this was actually the most accurate model. Forecast skill is determined by comparing forecasts against this model.
*NHC98 (after NHC90) is the sixth in a series of models that are combination statistical and dynamical models; they use the output from CLIPER (above), in combination with vertically averaged winds through the atmosphere and upper-atmosphere air pressures from the AVN (Aviation) run of the MRF (Medium Range Forecast) model as predictors. In NHC98, storms are stratified based on their latitude and their recent motion, with different equations used for westward or eastward-moving storms.
*BAM (Beta and Advection) uses vertically-averaged winds and computes trajectories provided by the GFS model to provide track forecasts. There are three forms: BAMD (BAM Deep, 850-200 hPa), BAMM (BAM Medium, 850-400 hPa) and BAMS (BAM Shallow, 850-700 hPa) that use different altitudes of wind. For a weak hurricane without a well-developed eye wall extending deep into the atmosphere (or for a tropical storm), the shallow-version model may work well, because weak storms tend to be steered by low-level winds. As the storm grows stronger and the eye wall gets deeper, the deeper versions become more accurate, as these types of storms are steered more by the winds in the upper-level. If the forecast from the three versions is similar, then the forecaster can conclude that the storm may go as predicted, but if the versions vary by a great deal, then the forecaster has less confidence in the track predicted. Large differences between model predictions can also indicate wind shear in the atmosphere, which could affect the intensity forecast as well. "Hurricane Models Information" "Hurricane Alley", 2008, webpage: [http://www.hurricanealley.net/hurmdls.htm HurricaneAlley-models] .]
*VICBAR (VIC ooyama BARotropic [cite journal | doi = 10.1175/1520-0493(1992)120<1628:ANSMFH>2.0.CO;2 | year = 1992 | volume = 120 | pages = 1628 | title = A Nested Spectral Model for Hurricane Track Forecasting | author = Demaria, Mark | journal = Monthly Weather Review] )
*LBAR (Limited area sine transform BARotropic) is a two-dimensional track prediction model.
*NHCP Aviation primarily forecasts wind direction and speed and provides input for other models.
*GFDL (Geophysical Fluid Dynamics Laboratory) is a limited-area baroclinic model developed specifically for hurricane prediction. It is run four times a day (six hours apart).
*UKMET (United Kingdom Meteorological Office)
*NOGAPS (United States Navy Global Atmospheric Prediction System)
*A98E is a statistical-dynamical prediction model using geopotential heights from the GFS forecast to modify the CLIPER forecast. It incorporates portions of the NHC90 and NHC98 Atlantic forecast models.
*GFS (National Weather Service Global Forecast System) is the main meteorological model for the United States. The model is run four times a day (six hours apart). It is a merger and expansion of the AVN and MRF models and is extended out to 384 hours.
*HWRF Hurricane-WRF is a dynamical nested model that uses the NAM as a source of its background data. HWRF is intended to eventually replace the GFDL as the main nested model for hurricane prediction, but also aides in the forecasting of track, intensity, and rainfall from tropical cyclones. . [ [http://www.noaanews.noaa.gov/stories2007/s2885.htm NOAA News Online (Story 2885) ] ] [WRF Program Coordinator. [http://wrf-model.org/development/wexob/meetings/Monthly-PC-Report-31Oct04.pdf Monthly Report of the WRF Program Coordinator.] Retrieved on 2007-04-10.] The HWRF was introduced at the beginning of the 2007 hurricane season.
*FSSE Florida State Superensemble is a model first used operationally in the 2005 hurricane season and has shown to verify well in tropical cyclone track forecasting. Little is publicly known about this model because the model is owned by a private company which charges for access to the data.

Timeliness

Some models do not produce output quickly enough to be used for the forecast cycle immediately after the model starts running (e.g. HWRF, GFDL, FSSE.) Most of the above track models (except CLIPER) require data from global weather models, such as the GFS, which produce output about four hours after synoptic time. However, for half of their forecasts, the NHC issues forecasts only three hours after that time, so some "early" models - NHC90, BAM, and LBAR - are run using a 12-hour-old forecast for the current time. "Late" models, such as the GFS and GFDL, finish after the advisory has already been issued. These models are interpolated to the current storm position for use in the following forecast cycle - for example, GFDI.

No model is ever perfectly accurate because it is impossible to learn exactly everything about the atmosphere in a timely enough manner, and atmospheric measurements that are taken are not completely accurate. The use of the ensemble method of forecasting, whether it be a multi-model ensemble, or numerous ensemble members based on the global model, helps define the uncertainty and further limit errors. The forecast models are used as a tool that an experienced forecaster will use to assemble an official track forecast.

Like all weather forecasts, track forecasts are more accurate in the short term than in the long term. Average errors for the United States National Hurricane Center are around 100, 200, and convert|300|nmi|km in 1, 2, and 3 days, respectively - the source of the 1-2-3 rule - although the errors have been decreasing.

Intensity models

Some of the intensity models used by the National Hurricane Center follows:

*SHIFOR (Statistical Hurricane Intensity FORecast) and SHIFOR5 uses the average of past storms with similar behaviour as well as an extrapolation of recent behaviour by a storm to arrive at a forecast, similar to CLIPER and CLIPER5 above. Until recently, this was the most consistently accurate.
*SHIPS (Statistical Hurricane Intensity Prediction Scheme) is a statistical model that uses climatological, persistence and synoptic predictors.
***RI Scheme (Rapid Intensification) uses output from SHIPS to determine the probability of rapid intensification.
*DSHP (Decay SHIP) is identical to the SHIPS model; however, if the cyclone is forecast to cross land, the intensity will be reduced accordingly.
*GFDL and HWRF provide intensity forecasts along with their track forecasts.
*FSSE provides intensity output in relation to its storm forecast, though varies from other models in taking into account the National Hurricane Center's forecasts in addition to standard data sources.

As with track models, intensity models are never perfect. They decrease in accuracy the farther you get from the present. In other words, a forecast model is normally more accurate in the short term than in the long term. Intensity models are considered to be inferior to track models in that it is much more difficult to forecast intensity changes of a tropical cyclone than it is to forecast its course. Also, errors in the potential course of a tropical cyclone, especially regarding landfall timing, can compound the intensity error.

ee also

*Numerical weather prediction
*Tropical cyclone
*Tropical cyclone forecasting
*Tropical cyclone rainfall forecasting
*Weather forecasting

References

* [http://www.nrlmry.navy.mil/~chu/chap5/se000.htm Tropical Cyclone Forecasters Reference Guide, Chapter 5]
* [http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/ Model Analyses and Forecasts from NCEP]
* [http://www.nhc.noaa.gov/aboutmodels.shtml National Hurricane Center Forecast Model Background and Information]

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