Project Scientist I, National Center for Atmospheric Research


Extreme weather and climate

North Atlantic TC genesis as function of resolutionAn attractive highlight of variable-resolution techniques is the ability to directly resolve extreme phenomena in climate assessments. This allows for direct projection of future climate extremes without the need for downscaling that may introduce biases by requiring a non-unified modeling framework. For example, we have shown that variable-resolution models greatly improve the simulated representation of tropical cyclones, orographically-influenced snowfall, the propagation of mesoscale convective systems, and the structure of winter storms in the eastern United States, while expending only a fraction of the computing resources required by a globally-uniform simulation.

The concepts discussed here are also pertinent to other geophysical models. For example, with a new variable-resolution options of the Community Land Model (CLM), we can investigate land-atmosphere interactions, such as convective triggering or snow-on-canopy albedo feedbacks over small spatial scales.

(Pictured: 20-year North Atlantic tropical cyclone genesis locations (color-coded by maximum lifetime intensity) for the default 1° version of the Community Earth System Model (a.), for a regionally-refined, 0.25° version of the same model (b.), and observations (c.). Note that the variable-resolution model produces not only a more realistic number of cyclogenesis events but a much more realistic distribution of storm intensity when compared to the low-resolution model. From Zarzycki and Jablonowski, 2014, JAMES.)

Objective detection of weather extremes in climate data

Schematic of objective detection algorithm for finding tropical cyclones in 3-dimensional climate dataOne area contributing to uncertainty in understanding the role of atmospheric extremes within the climate system (and their simulated future changes) is the multitude of automated detection algorithms used to find and track storms in model data. For example, the combination of the under-resolution of tropical cyclones in traditional climate models in addition to the relatively low number of storms per year (approximately 90 globally) makes quantifying the number of cyclones in a given model simulation difficult. Storm counts are sensitive to choices in threshold parameters such as surface pressure minimum, warm core anomaly, and wind speed. Ongoing research seeks to understand the uncertainty in these calculations and provide more unified algorithms for the climate community. This framework is not only tropical cyclone specific, but is extendable to the detection of wintertime storms, mesoscale convective systems, atmospheric rivers, heat waves, droughts, and other climate extremes. Current detection methods are also handicapped by 'big data.' As grid spacings grow finer and models increase in complexity, the burden of processing output grows larger due to computational limitations exposed by colossal quantities of information. Some of my work aims to eliminate this bottleneck.

(Pictured: Simplified schematic of a generalized tropical cyclone detection algorithm. The software first searches for a suitable sea level pressure minimum and then uses a geopotential thickness criterion to verify that the cyclone as a warm core. Candidate points are then stitched together in time to form trajectories.)

Tropical cyclone dynamics

Radial and tangential wind profiles for tropical cyclones using two different physical parameterization suitesAs the resolutions used for global modeling continue to increase, the ability of these models to discretely resolve tropical cyclones correspondingly improves. At horizontal resolutions of 0.25° and finer, features such as calm eyes, spiral rainbands, and tilted eyewalls are noticeable in simulated storms. This provides new insight into how tropical cyclones not only are influenced by the global climate system but also how they impact the transport of moisture, heat, and momentum from the tropics. Additionally, we are now capable of investigating potential impacts of different climates on aspects of storms such as surface wind fields, rainfall, and storm motion. Given that observational (and limited area modeling) studies have shown that certain features in tropical cyclone cores occur at spatial scales on the order of a single kilometer, the dynamical response of cyclones in climate models serve as interesting test beds for understanding the relative role of resolved versus parameterized processes driving their genesis, maintenance, and lysis.

(Pictured: Differences in radial (top) and tangential (bottom) wind flow for identically-initialized tropical cyclones using two different sub-grid parameterizations of the planetary boundary layer, macrophysics, and shallow convection.)

Next-generation general circulation models

An example of a variable-resolution global grid with enhanced resolution over the United StatesSimulating the global atmosphere at high spatial resolution is computationally burdensome and inhibits the use of these models for either fine-scale or long-term analysis of weather and climate. To alleviate these issues, limited area models (or regional climate models) have become popular, although they suffer from issues such as lack of conservation properties, mathematically (or physically) inconsistent lateral boundary conditions, and additional biases "adopted" from coarser driving models. Variable-resolution models can serve to bridge this gap, providing spatial "targeting" of computing resources to a specific region or feature of interest while maintaining a unified modeling framework. Variable-resolution models are central to both the strategic plans of the Department of Energy and National Science Foundation and are a required capability of the next generation global forecast system for the National Centers for Environmental Prediction.

(Pictured: a grid from the Community Earth System Model (CESM) with regional refinement over the eastern two-thirds of the continental United States.)

Scale-aware physical parameterizations

Cloud fraction for various aquaplanet simulations using different horizontal resolutionsMulti-resolution models will only be as effective as the subgrid physical parameterizations used to approximate phenomena such as clouds, turbulence, radiation, and precipitation. Legacy parameterizations have been shown to require significant empirical tuning if climate statistics within the same model are to match at different resolutions. In some cases, parameterizations are formulated for such a specific range of resolutions that a complete overhaul is required as model grid spacing decreases. In addition, as grid spacings in multi-resolution simulations push below 10km, modelers enter a 'gray zone,' where features such as convection become partially (or even fully) resolved and may require less (or no) subgrid parameterization.

(Pictured: An example of physical parameterizations which suffer from scale sensitivity. The left panels show multi-year mean cloud fraction in aquaplanet simulations at low resolution (top), high resolution (bottom) and with a variable-resolution grid (middle). Note the strong differences due purely to changes in grid spacing (highlighed by difference panels on the right). From Zarzycki et al., 2014, J. Clim.)

Aerosols and climate forcing

Schematic of black carbon forcing above highly reflective clouds The impact of aerosols (short-lived forcers) on climate impacts remains poorly constrained. For example, absorbing aerosols (such as black carbon) can have dramatically different heating impacts based on their spatial location in the atmosphere. Black carbon above highly reflective (high albedo) surfaces (such as snow and ice) can have nearly double the amount of forcing (per unit mass) due to incident shortwave radiation not only coming from above, but also below. This same behavior holds true for aerosols that can be lofted above highly reflective clouds. Additional work in terms of both modeling and observations are needed to reduce the uncertainty in how aerosols are emitted, where they move in the atmosphere, and how they are removed.

(Pictured: A schematic demonstrating how aerosols that absorb shortwave radiation (such as black carbon) can have a significantly larger direct radiative forcing if lofted above reflective clouds. Based on Zarzycki and Bond, 2010, GRL.)