Since its initial release in 2000, the Weather Research and Forecasting (WRF) Model has become one of the world’s most widely used numerical weather prediction models. Designed to serve both research and operational needs, it has grown to offer a spectrum of options and capabilities for a wide range of applications. In addition, it underlies a number of tailored systems that address Earth system modeling beyond weather. While the WRF Model has a centralized support effort, it has become a truly community model, driven by the developments and contributions of an active worldwide user base. The WRF Model sees significant use for operational forecasting, and its research implementations are pushing the boundaries of finescale atmospheric simulation. Future model directions include developments in physics, exploiting emerging compute technologies, and ever-innovative applications. From its contributions to research, forecasting, educational, and commercial efforts worldwide, the WRF Model has made a significant mark on numerical weather prediction and atmospheric science.

The formulation of a fully compressible nonhydrostatic atmospheric model called the Model for Prediction Across Scales-Atmosphere (MPAS-A) is described. The solver is discretized using centroidal Voronoi meshes and a C-grid staggering of the prognostic variables, and it incorporates a split-explicit time-integration technique used in many existing nonhydrostatic meso-and cloud-scale models. MPAS can be applied to the globe, over limited areas of the globe, and on Cartesian planes. The Voronoi meshes are unstructured grids that permit variable horizontal resolution. These meshes allow for applications beyond uniform-resolution NWP and climate prediction, in particular allowing embedded high-resolution regions to be used for regional NWP and regional climate applications. The rationales for aspects of this formulation are discussed, and results from tests for nonhydrostatic flows on Cartesian planes and for large-scale flow on the sphere are presented. The results indicate that the solver is as accurate as existing nonhydrostatic solvers for nonhydrostatic-scale flows, and has accuracy comparable to existing global models using icosahedral (hexagonal) meshes for large-scale flows in idealized tests. Preliminary full-physics forecast results indicate that the solver formulation is robust and that the variable-resolution-mesh solutions are well resolved and exhibit no obvious problems in the mesh-transition zones.

[1] High-resolution Antarctic Mesoscale Prediction System archive data were used to investigate high-precipitation events at the deep ice core drilling site Kohnen Station, Dronning Maud Land, Antarctica, during the period [2001][2002][2003][2004][2005][2006]. The precipitation is found to be highly episodic, with, on average, approximately eight high-precipitation events per year that can bring more than half of the total annual accumulation. The duration of the events varies between 1 day and about 1 week. On most days in the remaining time of the year, however, daily precipitation sums are about one order of magnitude smaller than that for the high-precipitation events. Synoptic weather patterns causing these events were directly connected to frontal systems of cyclones in only 20% of the 51 investigated cases. The majority of the events occurred in connection with (blocking) anticyclones and correspondingly amplified Rossby waves, which lead to advection of warm, moist air from relatively low latitudes. Possible changes in the seasonality and frequency of these events in a different climate can lead to a bias in ice core properties and might also strongly influence the mass balance of the Antarctic continent and thus global sea level change.

The ability to solve the global shallow-water equations with a conforming, variable-resolution mesh is evaluated using standard shallow-water test cases. While our long-term motivation is the creation of a global climate modeling framework capable of resolving different spatial and temporal scales in different regions, we begin with an analysis of the shallow-water system in order to better understand the strengths and weaknesses of our approach. The multiresolution meshes are spherical centroidal Voronoi tessellations where a single, user-supplied density function determines the region(s) of fine-and coarse-mesh resolution. We explore the shallow-water system with a suite of meshes ranging from quasi-uniform resolution meshes, where grid spacing is globally uniform, to highly-variable resolution meshes, where grid spacing varies by a factor of 16 between the fine and coarse regions. We find that potential vorticity is conserved to within machine precision and total available energy is conserved to within time-truncation error. This finding holds for the full suite of meshes, ranging from quasi-uniform resolution and highly-variable resolution meshes. Using shallow-water test cases 2 and 5, we find that solution error is controlled primarily by the grid resolution in the coarsest part of the model domain. This finding is consistent with results obtained by others.When these variable resolution meshes are used for the simulation of an unstable zonal jet, we find that the core features of the growing instability are largely unchanged as the variation in mesh resolution increases. The main differences between the simulations occur outside the region of mesh refinement and these differences are attributed to the additional truncation error that accompanies increases in grid spacing. Overall, the results demonstrate support for this approach as a path toward multi-resolution climate system modeling.1

[1] The precipitation regime of Dronning Maud Land (DML), Antarctica, was studied using Antarctic Mesoscale Prediction System (AMPS) archive data. Precipitation is the most important component of the mass balance of the Antarctic ice sheet. Precipitation studies of DML are particularly important because two deep ice core drilling sites, Kohnen Station and Dome Fuji, are located in this region. For the correct interpretation of the ice core properties a thorough understanding of the precipitation regime is necessary. We used the high-resolution AMPS archive data for the years 2001-2006 to investigate the temporal and spatial distribution of precipitation. The results were compared to a recently published mass balance map derived from glaciological data of western DML. The mass balance map and the AMPS mean annual precipitation field show fairly similar patterns, which are mostly related to topography and prevailing wind systems. Precipitation is found to generally decrease from the coast to the inland plateau. Along the escarpment between the low-altitude coastal areas and the interior plateau, local minima and maxima in precipitation correspond to the leeward and windward sides of topographical ridges. Interannual variability of monthly sums of precipitation is fairly high owing to the influence of cyclone activity on precipitation, which affects not only the coastal regions, but also the interior of the continent more than previously thought.