Methodology
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Methodology

The web presentation of this section is still a work in progress.  Here's a brief summary of the ensemble system design.

  • Initial conditions and boundary conditions are provided by the NCEP Global Forecasting System (GFS) ensemble fields, obtained at 2º by 2º resolution. By the use of a breeding method, five rapidly growing modes are bred from a single control. These modes are then added and subtracted from the control to obtain a total of 10 initial conditions and subsequent model runs.  The  10 GFS ensemble members, plus one control, give a total of 11 different initial background fields to use for initializing the ensemble.  The GFS ensemble also provide time-dependent  boundary conditions for the kilo-ensemble.  Thus, uncertainties associated with the evolution of the storm environment are taken into account. By means of the boundary conditions, information enters from outside the 6000 km square domain to help improve the forecast at later time periods. .  
  • Due to uncertainties associated with the storm steering layer, four different deep layer means (shallow, mid, deep, and entire depth to 100 mb) are used to calculate the wind fields used to initialize the barotropic model.
  • Due to uncertainties in the decomposition of the tropical atmosphere, three values are used for gamma in the modified vorticity equation.
  • A synthetic vortex is inserted at location of operational storm position estimate with associated storm motion vector. 
  • Uncertainties relating to vortex size/strength and storm motion vector are simulated by varying the parameters in the synthetic vortex.
  • The perturbations from these five classes are cross-multiplied to obtain 1980 initial conditions:

                11 environmental evolutions

                4 deep layer means

                3 gammas

                3 sizes/strengths

           x    5 storm motion vector perturbations

            -----------------------------------------------

                1980 ensemble members
  • The MUltigriD BARotropic (MUDBAR) tropical cyclone track model is used to calculate track of each ensemble member.
  • The resulting tracks from the 1980 ensembles are then processed and displayed for the forecast user.

 

On to real-time testing . . .