William C. Skamarock

Development of a next-generation regional weather research and forecast model.

The Weather Research and Forecast (WRF) project is a multi-institutional effort to develop an advanced mesoscale forecast and data assimilation system that is accurate, efficient, and scalable across a range of scales and over a host of computer platforms. The first release, WRF 1.0, was November 30, 2000, with operational deployment targeted for the 2004-05 time frame. This paper provides an overview of the project and current status of the WRF development effort in the areas of numerics and physics, software and data architecture, and single-source parallelism and performance portability.

Preconditioned Conjugate-Residual Solvers for Helmholtz Equations in Nonhydrostatic Models

Numerical integration of the compressible nonhydrostatic equations using semi-implicit techniques is complicated by the need to solve a Helmholtz equation at each time step. The authors present an accurate and efficient technique for solving the Helmholtz equation using a conjugate-residual (CR) algorithm that is accelerated by ADI preconditioners. These preconditioned CR solvers possess four distinct advantages over most other solvers that have been used with the Helmholtz equations that arise in compressible nonhydrostatic semi-implicit atmospheric models: the preconditioned CR methods 1) can solve Helmholtz equations containing variable coefficients, alleviating the need to prescribe a reference state in order to simplify the elliptic problem; 2) transparently include the cross-derivative terms arising from terrain transformations; 3) are efficient and accurate for nonhydrostatic models used across a broad range of scales, from cloud scales to synoptic-global scales; and 4) are easy to formulate and program. These features of the CR solver allow semi-implicit formulations that are unconstrained by the form of the Helmholtz equations, and the authors propose a formulation that is more consistent than those most often used in that it includes implicit treatment of all terms associated with the pressure gradients and divergence. This formulation is stable for nonhydrostatic-scale simulations involving steep terrain, whereas the more common semi-implicit formulation is not. The ADI preconditioners are presented for use in simulations of both hydrostatic and nonhydrostatic scale flows. These simulations demonstrate the efficiency and accuracy of the preconditioned CR method and the overall stability of the model formulation. The simulations also suggest a general convergence criteria for the iterative algorithm in terms of the solution divergence.

Models of coastally trapped disturbances

During the spring and summer, the climatological northerly flow along the U.S. west coast is occasionally interrupted by transitions to southerly flow that have a limited offshore scale and appear to be manifestations of marine-layer flow that is rotationally trapped by the coastal mountains. Existing climatological and observational studies suggest that a synoptic-scale offshore flow initiates these coastally trapped disturbances (CTDs). Using idealized simulations produced with a 3D nonhydrostatic model, the authors find that an imposed offshore flow will produce CTDs in idealized coastal environments. The imposed flow first weakens the prevailing northerly flow in the marine layer and lowers the pressure at the coast. The marine-layer flow around this low pressure evolves toward geostrophic balance, but is retarded as it encounters the coastal mountains to the south of the low and subsequently deepens the marine layer in this region. The elevated marine layer then begins progressing northward as a Kelvin wave and later may steepen into a bore or gravity current, this progression being the CTD. Many observed features accompanying CTDs are found in the numerical simulations, including the formation of a mesoscale pressure trough offshore and deep southerlies in the CTD at the coast. Stability in the atmosphere above the marine layer can give rise to topographically trapped Rossby waves and stronger CTD winds. In these stable conditions, propagation of wave energy away from the disturbance does not preclude strong, quasi-steady, propagating CTDs.

Catalina Eddies and Coastally Trapped Disturbances

Abstract Observations show that coastally trapped disturbances (CTDs) often accompany Catalina eddies that appear in the southern California bight. In a previous modeling study of CTDs using simple environments and forcings, simulations of CTDs also evolved a mesoscale eddy that played a critical role in CTD formation and evolution. In this study the simple environments and forcings are extended to model the southern California bight, and simulations produce both Catalina eddies and CTDs. The simulated Catalina eddies and the mesoscale eddies in the previous CTD studies are dynamically equivalent. The primary mechanism for eddy formation in the simulations is lee troughing, and eddies and CTDs are produced provided that 1) there is sufficient stratification in the environment; 2) the offshore flow is of sufficient strength, breadth, and duration; and 3) there is sufficient terrain to produce significant lee troughing. The simulations show that CTDs can propagate out of the bight region when synoptic winds...

Idealized global nonhydrostatic atmospheric test cases on a reduced‐radius sphere

Idealized simulations on a reduced-radius sphere can provide a useful vehicle for evaluating the behavior of nonhydrostatic processes in nonhydrostatic global atmospheric dynamical cores provided the simulated cases exhibit good agreement with corresponding flows in a Cartesian geometry, and for which there are known solutions. Idealized test cases on a reduced-radius sphere are presented here that focus on both dry and moist dynamics. The dry dynamics cases are variations of mountain-wave simulations designed for the Dynamical Core Model Intercomparison Project (DCMIP), and permit quantitative comparisons with linear analytic mountain-wave solutions in a Cartesian geometry. To evaluate moist dynamics, an idealized supercell thunderstorm is simulated that has strong correspondence to results obtained on a flat plane, and which can be numerically converged by specifying a constant physical diffusion. A simple Kessler-type routine for cloud microphysics is provided that can be readily implemented in atmospheric simulation models. Results for these test cases are evaluated for simulations with the Model for Prediction across scales (MPAS). They confirm close agreement with corresponding simulations in a Cartestian geometry; the mountain-wave results agree well with analytic mountain-wave solutions, and the simulated supercells are consistent with other idealized supercell simulation studies and exhibit convergent behavior.