A New Dynamical Core of the Global Environmental Multiscale (GEM) Model with a Height-Based Terrain-Following Vertical Coordinate

Syed Zahid Husain,Abdessamad Qaddouri,Claude Girard,André Plante

doi:10.1175/mwr-d-18-0438.1

Syed Zahid Husain, Abdessamad Qaddouri + Show 2 more

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https://doi.org/10.1175/mwr-d-18-0438.1

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Abstract

Abstract A new dynamical core of Environment and Climate Change Canada’s Global Environmental Multiscale (GEM) atmospheric model is presented. Unlike the existing log-hydrostatic-pressure-type terrain-following vertical coordinate, the proposed core adopts a height-based approach. The move to a height-based vertical coordinate is motivated by its potential for improving model stability over steep terrain, which is expected to become more prevalent with the increasing demand for very high-resolution forecasting systems. A dynamical core with height-based vertical coordinate generally requires an iterative solution approach. In addition to a three-dimensional iterative solver, a simplified approach has been devised allowing the use of a direct solver for the new dynamical core that separates a three-dimensional elliptic boundary value problem into a set of two-dimensional independent Helmholtz problems. The issue of dynamics–physics coupling has also been studied, and incorporating the physics tendencies within the discretized dynamical equations is found to be the most acceptable approach for the height-based vertical coordinate. The new dynamical core is evaluated using numerical experiments that include two-dimensional nonhydrostatic theoretical cases as well as 25-km resolution global forecasts. For a wide range of horizontal grid resolutions—from a few meters to up to 25 km—the results from the direct solution approach are found to be equivalent to the iterative approach for the new dynamical core. Furthermore, results from the different numerical experiments confirm that the new height-based dynamical core is equivalent to the existing pressure-based core in terms of solution accuracy.

Highlights

The dynamical core of the Global Environmental Multiscale (GEM) model, used operationally by Environment and Climate Change Canada (ECCC) for numerical weather prediction (NWP), employs a log-hydrostaticpressure-type terrain-following vertical coordinate
The primary objective of the present study is to demonstrate that, for the model configurations where orography-induced numerical instability is not relevant—that is, for horizontal grid resolutions within the hydrostatic regime— the new dynamical core developed at ECCC with heightbased terrain-following coordinate (TFC) makes predictions that are equivalent to those from the existing model
The operational GEM-P dynamical core, which is based on a log-hydrostaticpressure-type vertical coordinate, suffers from strong numerical instability when subjected to steep orography which becomes more prevalent with very highresolution NWP simulations

Summary

Introduction

The dynamical core of the Global Environmental Multiscale (GEM) model, used operationally by Environment and Climate Change Canada (ECCC) for numerical weather prediction (NWP), employs a log-hydrostaticpressure-type terrain-following vertical coordinate. The existing pressure-type coordinate system permits the use of a direct solver for the discretized EBV problem to resolve the dynamical component of the flow. The direct solver starts by separating the EBV problem vertically in terms of the vertical eigenvectors of the part of the coefficient matrix that only includes the discretized difference and average operators in the vertical direction (Qaddouri and Lee 2010). The tridiagonal problems are solved using LU decomposition without pivoting. Such an approach is computationally more efficient than most iterative methods, for the spatial resolutions of the current operational NWP systems at ECCC

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