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 two general types, statistical and dynamical, and two primary types of forecasts, track and intensity.
Track models
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Some of the track models used by the National Hurricane Center follows:[1][2][3]
- 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.[3]
- NHC90 and NHC98
- 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.
- VICBAR
- 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. .[4][5] 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. Not a large amount is known about this model as the model is owned by a private company which charges for access to the data.
Some models do not produce output quickly enough to be used for the forecast cycle immediately after the model starts running. Most of the above track models (except CLIPER) require data from global weather models, 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.[2][3]
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 always totally accurate. Thus, the 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 300 nautical miles in 1, 2, and 3 days, respectively - the source of the 1-2-3 rule - although the errors have been decreasing.
Intensity models
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Some of the intensity models used by the National Hurricane Center follows:[1][3]
- 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.[3]
- SHIPS (Statistical Hurricane Intensity Prediction Scheme) is a statistical model that uses climatological, persistence and synoptic predictors.
- 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 is the same model used in track forecasting.
- HWRF also provides intensity forecasts along with its track.
- RI Scheme (Rapid Intensification) uses output from SHIPS to determine the probability of rapid intensification.
- FSSE provides inensity 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 not perfect. They increase in accuracy as the future time of the forecast approaches the present. That is, a model is 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.
See also
References
- ^ a b NHC Tropical Cyclone FAQ Subject F2
- ^ a b Summary of the NHC/TPC Tropical Cyclone Track and Intensity Guidance Models
- ^ a b c d e 2005 NHC Forecast Verification Report
- ^ http://www.noaanews.noaa.gov/stories2007/s2885.htm
- ^ WRF Program Coordinator. Monthly Report of the WRF Program Coordinator. Retrieved on 2007-04-10.