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Robert X. Black and Katherine J. Evans

Abstract

The statistics, horizontal structure, and linear barotropic dynamics of anomalous weather regimes are evaluated in a 15-winter integration of the NCAR Community Climate Model (CCM2). Statistical and ensemble analyses of simulated regimes are contrasted with parallel analyses derived from NCEP–NCAR reanalyses. The CCM2 replicates much of the structure of observed frequency distributions for anomalous weather regimes over the North Pacific and North Atlantic regions. The main differences are a northward shift and longitudinal broadening of the North Pacific frequency maximum and a weakening and southward shift of the North Atlantic maximum.

Ensemble analyses reveal that simulated North Pacific regimes attain a more isotropic horizontal anomaly structure than observed cases, which are zonally elongated. The E-vector diagnoses indicate that North Pacific cases in the CCM2 are also associated with much weaker local barotropic energy conversions from the climatological-mean flow. This is partly due to the relatively weak climatological-mean diffluence simulated by the CCM2 in the jet exit region over the eastern North Pacific. The model’s North Atlantic regimes have horizontal anomaly patterns quite similar to observed cases, except for a southwestward shift relative to observations. Both simulated and observed North Atlantic cases exhibit robust local barotropic interactions with the climatological-mean flow, with the strongest conversions shifted southwestward in the model.

The results suggest a larger role for mechanisms besides barotropic instability in maintaining anomalous weather regimes over the North Pacific in the CCM2. The model’s North Atlantic events occur southwest of observed cases apparently in order to more efficiently utilize the available “barotropic energy reservoir” in the model climatology. The authors conclude that for GCMs to properly represent important dynamical characteristics of anomalous weather regimes, it is paramount that the model accurately depict the climatological-mean stationary wave field.

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Katherine J. Evans and Robert X. Black

Abstract

Piecewise tendency diagnosis (PTD) is extended and employed to study the dynamics of weather regime transitions. Originally developed for adiabatic and inviscid quasigeostrophic flow on a beta plane, PTD partitions local geopotential tendencies into a linear combination of dynamically meaningful source terms within a potential vorticity (PV) framework. Here PTD is amended to account for spherical geometry, diabatic heating, and ageostrophic processes, and is then used to identify the primary mechanisms responsible for Northern Hemisphere weather regime transitions.

Height tendency patterns obtained by summing the contributions of individual PTD forcing terms correspond very well to actual height tendencies. Composite PTD analyses reveal that linear PV advections provide the largest dynamical forcing for the weather regime development over the North Pacific. Specifically, linear baroclinic growth provides the primary forcing while barotropic deformation of PV anomalies provides a secondary contribution. North Atlantic anticyclonic anomalies develop from the combined effects of barotropic deformation, baroclinic growth, and nonlinear eddy feedback. The Atlantic cyclonic cases develop primarily from barotropic deformation and nonlinear eddy feedback. All four weather regime types decay primarily due to enhanced wave energy propagation away from the primary circulation anomaly. In some cases, regime decay is aided by decreasing positive contributions from barotropic deformation as the circulation anomaly attains a deformed horizontal shape. The current results 1) provide quantitative measures of the primary mechanisms responsible for weather regime transition and 2) demonstrate the utility of the extended PTD as a concise diagnostic paradigm for studying large-scale dynamical processes in the midlatitude troposphere.

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Katherine J. Evans, Mark A. Taylor, and John B. Drake

Abstract

A fully implicit (FI) time integration method has been implemented into a spectral finite-element shallow-water equation model on a sphere, and it is compared to existing fully explicit leapfrog and semi-implicit methods for a suite of test cases. This experiment is designed to determine the time step sizes that minimize simulation time while maintaining sufficient accuracy for these problems. For test cases without an analytical solution from which to compare, it is demonstrated that time step sizes 30–60 times larger than the gravity wave stability limits and 6–20 times larger than the advective-scale stability limits are possible using the FI method without a loss in accuracy, depending on the problem being solved. For a steady-state test case, the FI method produces error within machine accuracy limits as with existing methods, but using an arbitrarily large time step size.

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Salil Mahajan, Katherine J. Evans, John E. Truesdale, James J. Hack, and Jean-François Lamarque

Abstract

A new high-resolution global tropospheric aerosol dataset with monthly resolution is generated using version 4 of the Community Atmosphere Model (CAM4) coupled to a bulk aerosol model and forced with recent estimates of surface emissions for the period 1961–2000 to identify tropospheric aerosol-induced interannual climate variations. The surface emissions dataset is constructed from phase 5 of the Coupled Model Intercomparison Project (CMIP5) decadal-resolution surface emissions dataset to include reanalysis of tropospheric chemical composition [40-yr Reanalysis of Tropospheric Chemical Composition (RETRO)] wildfire monthly emissions data. A four-member ensemble run is conducted using the spectral configuration of CAM4, forced with the new tropospheric aerosol dataset and prescribed with observed sea surface temperature, sea ice, and greenhouse gases. CAM4 only simulates the direct and semidirect effects of aerosols on the climate. The simulations reveal that variations in tropospheric aerosol levels can induce significant regional climate variability on the interannual time scales. Regression analyses over tropical Atlantic and Africa suggest that increasing dust aerosols can cool the North African landmass and shift convection southward from West Africa into the Gulf of Guinea in the spring season. Further, it is found that carbonaceous aerosols emanating from the southwestern African savannas can significantly cool the region and increase the marine stratocumulus cloud cover over the southeast tropical Atlantic Ocean by aerosol-induced diabatic heating of the free troposphere above the low clouds. Experiments conducted with CAM4 coupled to a slab ocean model suggest that present-day aerosols can cool the tropical North Atlantic and shift the intertropical convergence zone southward and can reduce the ocean mixed layer temperature beneath the increased marine stratocumulus clouds in the southeastern tropical Atlantic.

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Salil Mahajan, Katherine J. Evans, James J. Hack, and John E. Truesdale

Abstract

The impacts of absorbing aerosols on global climate are not completely understood. This paper presents the results of idealized experiments conducted with the Community Atmosphere Model, version 4 (CAM4), coupled to a slab ocean model (CAM4–SOM) to simulate the climate response to increases in tropospheric black carbon aerosols (BC) by direct and semidirect effects. CAM4-SOM was forced with 0, 1×, 2×, 5×, and 10× an estimate of the present day concentration of BC while maintaining the estimated present day global spatial and vertical distribution. The top-of-atmosphere (TOA) radiative forcing of BC in these experiments is positive (warming) and increases linearly as the BC burden increases. The total semidirect effect for the 1 × BC experiment is positive but becomes increasingly negative for higher BC concentrations. The global-average surface temperature response is found to be a linear function of the TOA radiative forcing. The climate sensitivity to BC from these experiments is estimated to be 0.42 K W−1 m2 when the semidirect effects are accounted for and 0.22 K W−1 m2 with only the direct effects considered. Global-average precipitation decreases linearly as BC increases, with a precipitation sensitivity to atmospheric absorption of 0.4% W−1 m2. The hemispheric asymmetry of BC also causes an increase in southward cross-equatorial heat transport and a resulting northward shift of the intertropical convergence zone in the simulations at a rate of 4° PW−1. Global-average mid- and high-level clouds decrease, whereas the low-level clouds increase linearly with BC. The increase in marine stratocumulus cloud fraction over the southern tropical Atlantic is caused by increased BC-induced diabatic heating of the free troposphere.

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Rick Archibald, Katherine J. Evans, John Drake, and James B. White III

Abstract

In this paper a new approach is presented to increase the time-step size for an explicit discontinuous Galerkin numerical method. The attributes of this approach are demonstrated on standard tests for the shallow-water equations on the sphere. The addition of multiwavelets to the discontinuous Galerkin method, which has the benefit of being scalable, flexible, and conservative, provides a hierarchical scale structure that can be exploited to improve computational efficiency in both the spatial and temporal dimensions. This paper explains how combining a multiwavelet discontinuous Galerkin method with exact-linear-part time evolution schemes, which can remain stable for implicit-sized time steps, can help increase the time-step size for shallow-water equations on the sphere.

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Jun Jia, Judith C. Hill, Katherine J. Evans, George I. Fann, and Mark A. Taylor

Abstract

Although significant gains have been made in achieving high-order spatial accuracy in global climate modeling, less attention has been given to the impact imposed by low-order temporal discretizations. For long-time simulations, the error accumulation can be significant, indicating a need for higher-order temporal accuracy. A spectral deferred correction (SDC) method is demonstrated of even order, with second- to eighth-order accuracy and A-stability for the temporal discretization of the shallow water equations within the spectral-element High-Order Methods Modeling Environment (HOMME). Because this method is stable and of high order, larger time-step sizes can be taken while still yielding accurate long-time simulations. The spectral deferred correction method has been tested on a suite of popular benchmark problems for the shallow water equations, and when compared to the explicit leapfrog, five-stage Runge–Kutta, and fully implicit (FI) second-order backward differentiation formula (BDF2) time-integration methods, it achieves higher accuracy for the same or larger time-step sizes. One of the benchmark problems, the linear advection of a Gaussian bell height anomaly, is extended to run for longer time periods to mimic climate-length simulations, and the leapfrog integration method exhibited visible degradation for climate length simulations whereas the second-order and higher methods did not. When integrated with higher-order SDC methods, a suite of shallow water test problems is able to replicate the test with better accuracy.

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