NCAR Accelerated Scientific Discovery projects 2012

Prediction of North American air quality

Project lead: Gabriele Pfister, NESL/ACD
Yellowstone allocation: 6.25 million core-hours

We propose to perform high-resolution simulations with the nested regional climate model with chemistry (NRCM-Chem) to study possible changes in weather and air quality over North America between present-day and two future time periods: 2020-2030 and 2045-2055. These time horizons are chosen to be compatible and complementary with already performed NRCM simulations without inclusion of chemistry (NRCM-Met; Done et al., 2012). The performed simulations will provide insights into expected future changes related to air quality and will also be used for dynamical downscaling (of meteorology and air quality) of global climate simulations performed at NCAR in support of the 2013 IPCC AR5. Running the model with chemistry will require some different physics options than used in NRCM-Met, and comparison of NRCM-Met with simulations performed in the frame of this proposal will provide an estimate of how simulations of future weather and climate vary for different physics parameterizations. The NRCM-Chem simulations will span summertime conditions for 10 years each time horizon, in order to document the importance of interannual variability. The analysis will focus on regional air pollution events over the conterminous U.S. and their potential change in a future climate and economy. We will be using NRCM-Chem over the conterminous U.S. at a resolution of 12 x 12 km2 with inclusion of atmospheric chemistry to examine impacts on air quality and other regional climate processes.

Rotation and stratification at unit Burger number in turbulent flows

Project lead: Annick Pouquet, CISL/IMAGe
Yellowstone allocation: 11.2 million core-hours

We propose to study rotating stratified turbulence and contrast the behavior of such flows with and without helicity (velocity-vorticity correlations), performing two direct numerical simulations on grids of 20483 points at Rossby and Froude numbers comparable to atmospheric values, and at a Reynolds number of 5 x 104. The questions we wish to address in this proposal are: How and where is helicity created in such flows? What is the role of helicity on structures, nonlinear dynamics, and statistics in the context of the combined effects of rotation and stratification with comparable wave periods, when contrasted to a non-helical flow?

Turbulence modification in the spray-laden atmospheric marine boundary layer

Project lead: David Richter, NESL/MMM, ASP
Yellowstone allocation: 5.2 million core-hours

In the atmosphere, turbulent flows which contain a dispersed phase such as water droplets, dust, or aerosols are common, and these particle-laden flows are oftentimes key components in larger-scale atmospheric processes. Specifically, the current study is focused on the effect which sea spray, suspended by turbulence in the high-wind marine atmospheric boundary layer, has on both the turbulence itself as well as the transfer of momentum and heat to the ocean surface. For example, from measurements of the surface drag coefficient of tropical cyclones, evidence suggests that the drag saturates, or even decreases with wind speed, contrary to previous understanding. One potential explanation of this effect is that sea spray modifies the turbulence near the air-sea interface in a way that ultimately reduces the drag seen by the surface. To study this problem, a fundamental approach is taken where direct numerical simulation (DNS) coupled with Lagrangian particle-tracking is employed to focus generally on how a dispersed phase (such as sea spray) modifies turbulence. Systematic runs of turbulent Couette flow at multiple Reynolds numbers (Re = 8000; 32,000; 64,000), each with various particle sizes, time scales (Stokes numbers from 1 to 10), and concentrations (mass loadings up to 25%), are performed which aim to identify the critical mechanisms of turbulence modification, as well as determine the extent to which suspended particles can affect transport of heat and momentum.

Global cloud-permitting atmospheric simulations using MPAS

Project lead: William Skamarock, NESL/MMM
Yellowstone allocation: 6.5 million core-hours

The Model for Prediction Across Scales (MPAS) is comprised of geophysical fluid-flow solvers that use spherical centroidal Voronoi tesselations (SCVTs) to tile the globe. SCVTs (nominally hexagons) allow for both quasi-uniform tiling of the sphere (in an icosahedral-mesh configuration) as well as variable resolution tiling where the change in resolution is gradual and in which there are no hanging nodes (in contrast to models using traditional nesting approaches to enable refinement, or adaptive mesh refinement (AMR) techniques using cell division for refinement). We propose to test the global nonhydrostatic atmospheric solver (MPAS-ANH) by producing 10-day (medium-range) forecasts for two periods using uniform mesh spacings (mean cell-center spacings on the SCVT) of 60, 30, 15, 7.5, and 3 km.

Meso- to planetary-scale processes in a global ultra-high-resolution climate model

Project lead: R. Justin Small, NESL/CGD
Yellowstone allocation: 25.2 million core-hours

This is a collaborative multi-institution request responding to the NWSC science justification of using global high resolution to explore interactions between different scales, from mesoscale to planetary. The main computational objective is to perform and assess Community Earth System Model (CESM) simulations with 1/8-degree atmosphere and land models and 1/10-degree ocean and ice models. The climate science objectives are (1) to investigate the climate response to the coupling of ocean and atmosphere mesoscale features, (2) to assess the ability of a high-resolution and frequently coupled (two-hour) ocean and atmosphere simulation to represent near-inertial waves in the ocean, and (3) to investigate the role of small-scale ice features such as polynyas in the climate system.

Petascale simulation of physics and dynamics of turbulent clouds

Project lead: Andrzej Wyszogrodzki, RAL/NSAP
Yellowstone allocation: 6.25 million core-hours

The reliability of current weather and climate models at regional and global scales depends critically on the parameterizations of unresolved microphysical processes and small-scale cloud dynamics. The cloud droplet size distribution critically affects the amount of solar radiation reflected back to space, whereas removal of cloud water through precipitation affects the water cycle and bulk cloud properties such as the cloud fraction and cloud lifetime. This project devises numerical experiments to simulate, from first principles, cloud dynamical and microphysical processes from the scale of individual shallow cumulus clouds (i.e., 1 km scale) to the scale of cloud droplets (i.e., 10 microns), by integrating cloud-resolving large-eddy simulations (LES) of cloud dynamics and hybrid direct numerical simulation (HDNS) of cloud microphysics. Using the upcoming Yellowstone system, we will be able to close the scale gap between LES and HDNS. This capability represents a breakthrough in cloud physics research and will allow us to investigate several open issues including effects of entrainment and mixing at all scales on droplet size distribution, effects of turbulent fluctuations on droplet growth by diffusion and collision-coalescence, and precipitation formation. Resolving these open issues benefits weather and climate modeling from short-term storm-scale forecasts to longer-term climate change studies. The advanced simulation, optimization, and analysis tools developed here will impact many areas of atmospheric sciences and engineering involving turbulent multiphase flows and computational sciences.