The Tangent-Linear and Adjoint Models of the NEPTUNE Dynamical Core

An approximate tangent-linear model and its adjoint have been developed for the dynamical core of the NEPTUNE model (Navy Environmental Prediction sysTem Utilizing a Nonhydrostatic Engine). NEPTUNE is a spectral element solver for the compressible, nonhydrostatic, deep atmosphere equations of motion on a cubed sphere grid. The tangent-linear model (TLM) and adjoint model (ADJ) were developed to enable 4D-Var-based forecast initialization systems for NEPTUNE by interfacing with driver software, such as JEDI (Joint Effort for Data assimilation Integration), but they are also useful tools for studies of linear instability and forecast sensitivity. Jablonowski and Williamson's baroclinic wave test is used to illustrate the accuracy and characteristics of the TLM and ADJ for sensitivity studies and variational assimilation.