The National Meteorological Center's Quasi-Lagrangian Model for Hurricane Prediction

Abstract

A description is presented of the National Meteorological Center's Quasi-Lagrangian Model (QLM), which is used for operational hurricane prediction. The model uses the primitive equations with high horizontal and vertical resolution, and includes parameterizations of principal physical and dynamical processes that affect the motion and development of a hurricane. Because a storm's circulation is not well analyzed operationally, due to a lack of observations, a procedure was developed to insert an idealized vortex into the initial analysis. The geopotential height and wind fields in the vortex satisfy the gradient wind relation with variable Coriolis parameter, and its structure depends on the size and intensity of the observed storm. The primary purpose of the QLM is to provide numerical guidance for forecasting hurricane track. Considerable improvement in the track and the structure of a storm is obtained through the use of the idealized vortex. Further improvement in forecasts is attained with the use of a secondary circulation (a dipole). Based on the current storm motion, the dipole imposes a steering current on the vortex. Several 72-h track forecasts from the QLM and the most accurate statistical-dynamical track prediction model (NHC83) in use over the Atlantic Ocean area are compared. Results show that the QLM forecasts of landfall compare well with the NHC83, but over the open oceans, where observations are sparse, the NHC83 performs better than the QLM.