Showing papers by "Isaac M. Held published in 2011"

The dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component AM3 of the GFDL global coupled model CM3

Abstract: The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for the atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol–cloud interactions, chemistry–climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical system component of earth system models and models for decadal prediction in the near-term future—for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud droplet activation by aerosols, subgrid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emiss...

Statistical–Dynamical Predictions of Seasonal North Atlantic Hurricane Activity

Abstract: Skillfullypredicting North Atlantichurricane activitymonths in advance is of potential societal significance and a useful test of our understanding of the factors controlling hurricane activity. In this paper, a statistical‐ dynamical hurricane forecasting system, based on a statistical hurricane model, with explicit uncertainty estimates,andbuiltfromasuiteofhigh-resolutionglobalatmosphericdynamicalmodelintegrationsspanning a broad range of climate states is described. The statistical model uses two climate predictors: the sea surface temperature (SST) in the tropical North Atlantic and SST averaged over the global tropics. The choice of predictorsis motivatedby physicalconsiderations, aswell astheresultsofhigh-resolutionhurricanemodeling and statistical modeling of the observed record. The statistical hurricane model is applied to a suite of initialized dynamical global climate model forecasts of SST to predict North Atlantic hurricane frequency, which peaks during the August‐October season, from different starting dates. Retrospective forecasts of the 1982‐2009 period indicate that skillful predictions can be made from as early as November of the previous year; that is, skillful forecasts for the coming North Atlantic hurricane season could be made as the current one is closing. Based on forecasts initialized between November 2009 and March 2010, the model system predictsthattheupcoming2010NorthAtlantichurricaneseasonwilllikelybemoreactivethanthe1982‐2009 climatology, with the forecasts initialized in March 2010 predicting an expected hurricane count of eight and a 50% probability of counts between six (the 1966‐2009 median) and nine.

The Response of Tropical Cyclone Statistics to an Increase in CO2 with Fixed Sea Surface Temperatures

Abstract: The effects on tropical cyclone statistics of doubling CO2, with fixed sea surface temperatures (SSTs), are compared to the effects of a 2-K increase in SST, with fixed CO2, using a 50-km resolution global atmospheric model. Confirming earlier results of Yoshimura and Sugi, a significant fraction of the reduction in globally averaged tropical storm frequency seen in simulations in which both SST and CO2 are increased can be thought of as the effect of the CO2 increase with fixed SSTs. Globally, the model produces a decrease in tropical cyclone frequency of about 10% due to doubling of CO2 and an additional 10% for a 2-K increase in SST, resulting in roughly a 20% reduction when both effects are present. The relative contribution of the CO2 effect to the total reduction is larger in the Northern than in the Southern Hemisphere. The average intensity of storms increases in the model with increasing SST, but intensity remains roughly unchanged, or decreases slightly, with the increase in CO2 alone. A...

Climate Variability and Radiocarbon in the CM2Mc Earth System Model

Abstract: The distribution of radiocarbon (14C) in the ocean and atmosphere has fluctuated on time scales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute 14C within the earth system on interannual to centennial time scales. The model, the Geophysical Fluid Dynamics Laboratory Climate Model version 2 (GFDL CM2) with Modular Ocean Model version 4p1(MOM4p1) at coarse-resolution (CM2Mc), is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory’s CM2M model, uses no flux adjustments, and is run here with a simple prognostic ocean biogeochemistry model including 14C. The atmospheric 14C and radiative boundary conditions are held constant so that the oceanic distribution of 14C is only a function of internal climate variab...

The impacts of changing transport and precipitation on pollutant distributions in a future climate

Abstract: [1] Air pollution (ozone and particulate matter in surface air) is strongly linked to synoptic weather and thus is likely sensitive to climate change. In order to isolate the responses of air pollutant transport and wet removal to a warming climate, we examine a simple carbon monoxide–like (CO) tracer (COt) and a soluble version (SAt), both with the 2001 CO emissions, in simulations with the Geophysical Fluid Dynamics Laboratory chemistry-climate model (AM3) for present (1981–2000) and future (2081–2100) climates. In 2081–2100, projected reductions in lower-tropospheric ventilation and wet deposition exacerbate surface air pollution as evidenced by higher surface COt and SAt concentrations. However, the average horizontal general circulation patterns in 2081–2100 are similar to 1981–2000, so the spatial distribution of COt changes little. Precipitation is an important factor controlling soluble pollutant wet removal, but the total global precipitation change alone does not necessarily indicate the sign of the soluble pollutant response to climate change. Over certain latitudinal bands, however, the annual wet deposition change can be explained mainly by the simulated changes in large-scale (LS) precipitation. In regions such as North America, differences in the seasonality of LS precipitation and tracer burdens contribute to an apparent inconsistency of changes in annual wet deposition versus annual precipitation. As a step toward an ultimate goal of developing a simple index that can be applied to infer changes in soluble pollutants directly from changes in precipitation fields as projected by physical climate models, we explore here a “Diagnosed Precipitation Impact” (DPI) index. This index captures the sign and magnitude (within 50%) of the relative annual mean changes in the global wet deposition of the soluble pollutant. DPI can only be usefully applied in climate models in which LS precipitation dominates wet deposition and horizontal transport patterns change little as climate warms. Our findings support the need for tighter emission regulations, for both soluble and insoluble pollutants, to obtain a desired level of air quality as climate warms.