John E. Truesdale

Simulation of the Global Hydrological Cycle in the CCSM Community Atmosphere Model Version 3 (CAM3): Mean Features

The seasonal and annual climatological behavior of selected components of the hydrological cycle are presented from coupled and uncoupled configurations of the atmospheric component of the Community Climate System Model (CCSM) Community Atmosphere Model version 3 (CAM3). The formulations of processes that play a role in the hydrological cycle are significantly more complex when compared with earlier versions of the atmospheric model. Major features of the simulated hydrological cycle are compared against available observational data, and the strengths and weaknesses are discussed in the context of specified sea surface temperature and fully coupled model simulations. The magnitude of the CAM3 hydrological cycle is weaker than in earlier versions of the model, and is more consistent with observational estimates. Major features of the exchange of water with the surface, and the vertically integrated storage of water in the atmosphere, are generally well captured on seasonal and longer time scales. The water cycle response to ENSO events is also very realistic. The simulation, however, continues to exhibit a number of long-standing biases, such as a tendency to produce double ITCZ-like structures in the deep Tropics, and to overestimate precipitation rates poleward of the extratropical storm tracks. The lower-tropospheric dry bias, associated with the parameterized treatment of convection, also remains a simulation deficiency. Several of these biases are exacerbated when the atmosphere is coupled to fully interactive surface models, although the larger-scale behavior of the hydrological cycle remains nearly identical to simulations with prescribed distributions of sea surface temperature and sea ice.

Exploratory High-Resolution Climate Simulations using the Community Atmosphere Model (CAM)

AbstractExtended, high-resolution (0.23° latitude × 0.31° longitude) simulations with Community Atmosphere Model versions 4 and 5 (CAM4 and CAM5) are examined and compared with results from climate simulations conducted at a more typical resolution of 0.9° latitude × 1.25° longitude. Overall, the simulated climate of the high-resolution experiments is not dramatically better than that of their low-resolution counterparts. Improvements appear primarily where topographic effects may be playing a role, including a substantially improved summertime Indian monsoon simulation in CAM4 at high resolution. Significant sensitivity to resolution is found in simulated precipitation over the southeast United States during winter. Some aspects of the simulated seasonal mean precipitation deteriorate notably at high resolution. Prominent among these is an exacerbated Pacific “double ITCZ” bias in both models. Nevertheless, while large-scale seasonal means are not dramatically better at high resolution, realistic tropica...

CCSM-CAM3 climate simulation sensitivity to changes in horizontal resolution

Abstract The latest version of the Community Climate System Model (CCSM) Community Atmosphere Model version 3 (CAM3) has been released to allow for numerical integration at a variety of horizontal resolutions. One goal of the CAM3 design was to provide comparable large-scale simulation fidelity over a range of horizontal resolutions through modifications to adjustable coefficients in the parameterized treatment of clouds and precipitation. Coefficients are modified to provide similar cloud radiative forcing characteristics for each resolution. Simulations with the CAM3 show robust systematic improvements with higher horizontal resolution for a variety of features, most notably associated with the large-scale dynamical circulation. This paper will focus on simulation differences between the two principal configurations of the CAM3, which differ by a factor of 2 in their horizontal resolution.

Implementation of new diffusion/filtering operators in the CAM-FV dynamical core

Two new filtering/diffusion operators have been implemented in the Community Atmosphere Model finite-volume dynamical core (CAM-FV). First, a fourth-order divergence damping operator has been added to optionally replace the second-order version that has traditionally been used. This provides more scale selective dissipation of divergent modes that can generate grid-scale noise in CAM-FV if not damped properly. For example, data assimilation runs using CAM-FV DART (Data Assimilation Research Testbed) have revealed potential noise problems at the grid scale that can be alleviated significantly using higher-order divergence damping. Second, a ‘Laplacian’-type damping operator has been implemented to increase the explicit momentum dissipation in the top-of-atmosphere sponge layers. This helps control the excessive polar night jets that have been observed in ultra-high-resolution CAM-FV simulations. Results from stand-alone CAM-FV and CAM-FV DART are presented in this paper along with details on the implementation of the new operators.

Impact of the dynamical core on the direct simulation of tropical cyclones in a high-resolution global model

This paper examines the impact of the dynamical core on the simulation of tropical cyclone (TC) frequency, distribution, and intensity. The dynamical core, the central fluid flow component of any general circulation model (GCM), is often overlooked in the analysis of a models ability to simulate TCs compared to the impact of more commonly documented components (e.g., physical parameterizations). The Community Atmosphere Model version 5 is configured with multiple dynamics packages. This analysis demonstrates that the dynamical core has a significant impact on storm intensity and frequency, even in the presence of similar large-scale environments. In particular, the spectral element core produces stronger TCs and more hurricanes than the finite-volume core using very similar parameterization packages despite the latter having a slightly more favorable TC environment. The results suggest that more detailed investigations into the impact of the GCM dynamical core on TC climatology are needed to fully understand these uncertainties.

Computational design of the NCAR community climate model

Abstract An overview of the computational design for the latest version of the NCAR Atmospheric General Circulation Model, designated CCM2, is presented. Parallel implementation details are driven by two major algorithmic classes of computation that require different patterns of data communication, the spectral transform method and the semi-Lagrangian advection technique. The organization and performance characteristics of a shared-memory parallel implementation, and an analogous distributed-memory message-passing parallel implementation are described. The advantages and limitations of this coarse-grained partitioning are discussed in the context of global climate modeling research.

Linearity of Climate Response to Increases in Black Carbon Aerosols

AbstractThe 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...

Interannual Tropospheric Aerosol Variability in the Late Twentieth Century and Its Impact on Tropical Atlantic and West African Climate by Direct and Semidirect Effects

AbstractA 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 lev...

Projections of future tropical cyclone damage with a high-resolution global climate model

High-resolution climate model simulations and a tropical cyclone damage model are used to simulate the economic damage due to tropical cyclones. The damage model produces reasonable damage estimates compared to observations. The climate model produces realistically intense tropical cyclones over a historical simulation, with significant basin scale correlation of the inter-annual variability of cyclone numbers to observed storm numbers. However, the climate model produces too many moderate tropical cyclones, particularly in the N. Pacific. Annual mean cyclone damage with simulated storms is similar to estimates with the damage model and observed storms, and with actual economic losses. Ensembles of future simulations with different mitigation scenarios and different sea surface temperatures (SSTs), as well as societal changes, are used to assess future projections of cyclone damage. Damage estimates are highly dependent on the internal variability of the coupled system. Using different ensemble members or different SSTs affects damage results by ±40 %. Experiments indicate that despite decreases in storm numbers in the future, strong landfalling storms increase in E. Asia, increasing global storm damage by ∼50 % in 2070 over 2015. Little significant benefit is seen from mitigation, but only one ensemble is available. Projected increases in vulnerable assets increase damage from simulated storms by more than threefold (∼300 %, assuming no adaptation) indicating future growth will swamp potential changes in tropical cyclones.