CGD's Dr. Phil Rasch

Abstract

We apply our Snow, Ice, and Aerosol Radiative (SNICAR) model, coupled to a general circulation model with prognostic carbon aerosol transport, to improve understanding of climate forcing and response from black carbon (BC) in snow. Building on two previous studies, we account for interannually varying biomass burning BC emissions, snow aging, and aerosol scavenging by snow meltwater. We assess uncertainty in forcing estimates from these factors, as well as BC optical properties and snow cover fraction. BC emissions are the largest source of uncertainty, followed by snow aging. The rate of snow aging determines snowpack effective radius (r_e), which directly controls snow reflectance and the magnitude of albedo change caused by BC. For a reasonable r_e range, reflectance reduction from BC varies threefold. Inefficient meltwater scavenging keeps hydrophobic impurities near the surface during melt and enhances forcing. Applying biomass burning BC emission inventories for a strong (1998) and weak (2001) boreal fire year, we estimate global annual mean BC/snow surface radiative forcing from all sources (fossil fuel, biofuel, and biomass burning) of +0.054 (0.007-0.13) and +0.049 (0.007-0.12) W m^-2, respectively. Snow forcing from only fossil fuel + biofuel sources is +0.043 W m^-2 (forcing from only fossil fuels is +0.033 W m^-2), suggesting that the anthropogenic contribution to total forcing is at least 80%. The 1998 global land and sea-ice snowpack absorbed 0.60 and 0.23 W m^-2, respectively, because of direct BC/snow forcing. The forcing is maximum coincidentally with snowmelt onset, triggering strong snow-albedo feedback in local springtime. Consequently, the "efficacy" of BC/snow forcing is more than three times greater than forcing by CO₂. The 1998 and 2001 land snowmelt rates north of 50°N are 28% and 19% greater in the month preceding maximum melt of control simulations without BC in snow. With climate feedbacks, global annual mean 2-meter air temperature warms 0.15 and 0.10°C, when BC is included in snow, whereas annual arctic warming is 1.61 and 0.50°C. Stronger high-latitude climate response in 1998 than 2001 is at least partially caused by boreal fires, which account for nearly all of the 35% biomass burning contribution to 1998 arctic forcing. Efficacy was anomalously large in this experiment, however, and more research is required to elucidate the role of boreal fires, which we suggest have maximum arctic BC/snow forcing potential during April-June. Model BC concentrations in snow agree reasonably well (r = 0.78) with a set of 23 observations from various locations, spanning nearly 4 orders of magnitude. We predict concentrations in excess of 1000 ng g^-1 for snow in northeast China, enough to lower snow albedo by more than 0.13. The greatest instantaneous forcing is over the Tibetan Plateau, exceeding 20 W m^-2 in some places during spring. These results indicate that snow darkening is an important component of carbon aerosol climate forcing.

Figure caption: Zonal mean surface forcing from BC in snow as a function of montha nd latitutde for (top) 1998 and (middle) 2001 central estimates, and (bottom) 1998 biomass burning only. The biomass burning forcing contributions is estimated as the difference between 1998 central (FF + BF + BB) and FF + BF only forcing.

Support: NSF/NCAR SGER ATM-0503148 and NASA Earth System Science Fellowship NNG05GP30H. Computations are supported by Earth System Modeling Facility NSF ATM-0321380. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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G. Bala, R. B. Rood, A. Mirin, J. McClean, Krishna Achutarao, D. Bader, P. Gleckler, R. Neale, and P. Rasch, 2007:Evaluation of a CCSM3 Simulation with a Finite Volume Dynamical Core for the Atmosphere at 1o lat x 1.25o lon resolution. J. Climate, In press.

Abstract

We present a simulation of the present-day climate by the Community Climate System Model version 3 (CCSM3) that uses a Finite Volume (FV) numerical method for solving the equations governing the atmospheric dynamics. The simulation is compared to observations and to the well-documented simulation by the standard CCSM3 which uses the Eulerian spectral method for the atmospheric dynamics. The atmospheric component in our simulation uses a 1° lat x1.25° lon -grid which is a slightly finer resolution than the T85-grid used in the spectral transform. As in the T85 simulation, the ocean and ice models use a nominal 1-degree grid. Although the physical parameterizations are the same and the resolution is comparable to the standard model, substantial testing and slight retuning were required to obtain an acceptable control simulation. There are significant improvements in the simulation of the surface wind stress and sea surface temperature. Improvements are also seen in the simulations of the total variance in the tropical Pacific, the spatial pattern of ice thickness distribution in the Arctic, and the vertically integrated ocean circulation in the Antarctic Circumpolar Current. Our results demonstrate that the FV version of the CCSM coupled model is a state-of-the-art climate model whose simulation capabilities are in the class of those used for IPCC assessments. The simulated climate is very similar to that of the T85 version in terms of its biases, and more like the T85 model than the other IPCC models.

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Scott C. Doney, Natalie Mahowald, Ivan Lima, Richard A. Feely, Fred T. Mackenzie, Jean-Francois Lamarque, and Phil J. Rasch. Impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system, PNAS, pp 14580-14585, September 11, 2007, vol. 104, no. 37, doi 10.1073 pnas.0702218104.

Abstract

Fossil fuel combustion and agriculture result in atmospheric deposition of 0.8 Tmol/yr reactive sulfur and 2.7 Tmol/yr nitrogen to the coastal and open ocean near major source regions in North America, Europe, and South and East Asia. Atmospheric inputs of dissociation products of strong acids (HNO₃ and H2SO₄) and bases (NH₃) alter surface seawater alkalinity, pH, and inorganic carbon storage. We quantify the biogeochemical impacts by using atmosphere and ocean models. The direct acid/base flux to the ocean is predominately acidic (reducing total alkalinity) in the temperate Northern Hemisphere and alkaline in the tropics because of ammonia inputs. However, because most of the excess ammonia is nitrified to nitrate (NO₃^-) in the upper ocean, the effective net atmospheric input is acidic almost everywhere. The decrease in surface alkalinity drives a net air-sea efflux of CO₂, reducing surface dissolved inorganic carbon (DIC); the alkalinity and DIC changes mostly offset each other, and the decline in surface pH is small. Additional impacts arise from nitrogen fertilization, leading to elevated primary production and biological DIC drawdown that reverses in some places the sign of the surface pH and air-sea CO₂ flux perturbations. On a global scale, the alterations in surface water chemistry from anthropogenic nitrogen and sulfur deposition are a few percent of the acidification and DIC increases due to the oceanic uptake of anthropogenic CO₂. However, the impacts are more substantial in coastal waters, where the ecosystem responses to ocean acidification could have the most severe implications for mankind.

Figure caption: Perturbation maps of simulated surface water pH, DIC, and total alkalinity trends and air-sea CO₂ flux due to anthropogenic atmospheric nitrogen and sulfur deposition.

Support: NSF/NCAR SGER ATM-0503148 and NASA Earth System Science Fellowship NNG05GP30H. Computations are supported by Earth System Modeling Facility NSF ATM-0321380. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research, under sponsorship of the National Science Foundation.

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J. H. Richter, and P. J. Rasch: 2007, Effects of convective momentum transport on the atmospheric circulation in the Community Atmosphere Model, version 3 (CAM3). J. Climate, in press.

Abstract

Transport of momentum by convection is an important process affecting global circulation. Due to the lack of global observations, the quantification of the impact of this process on the tropospheric climate is difficult. Here, we present an implementation of two convective momentum transport parameterizations, Schneider and Lindzen (1976) and Gregory et al. (1997), in the Community Atmosphere Model, version 3 (CAM3), and we examine in detail how they affects global climate. An analysis of the tropospheric zonal momentum budget reveals that convective momentum transport affects tropospheric climate mainly through changes to the Coriolis torque. These changes result in improvement of the representation of the Hadley circulation: in DJF, the upward branch of the circulation is weakened in the Northern hemisphere and strengthened in the Southern hemisphere; the lower Northerly branch is weakened. In JJA, similar improvements are noted. The inclusion of convective momentum transport in CAM3 reduces many of the model's biases in the representation of surface winds, as well as in the representation of tropical convection. In an annual mean, the Tropical easterly bias, subtropical westerly bias, and the bias in the 60S jet are improved. Representation of convection is improved along the Equatorial belt, with decreased precipitation in the Indian Ocean and increased precipitation in the Western Pacific. The improvements of representation of tropospheric climate are greater with the implementation of the Schneider and Lindzen (1976) parameterization.

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G. G. Pfister, P. G. Hess, L. K. Emmons, P. J. Rasch, F. M. Vitt, Impact of the Summer 2004 Alaska Fires on TOA Clear-Sky Radiation Fluxes, 2007, JGR-Atmospheres, in press.

Abstract

Based on a case study of the record wildfire season in Alaska in 2004 we present a method to constrain emissions of different aerosol types by integrating information about aerosol loading and aerosol-related radiative impacts. . We compare model simulations and satellite observations of aerosol optical depth and of top of the atmosphere (TOA) shortwave and longwave fluxes for the summer 2004 to the summer of 2000 when fire activity in the boreal zone was low. Both observations and model show a decrease in clear-sky fluxes over the Alaska fire region during summer 2004 of -7±6 W m^-2 and -10±4 W m^-2, respectively. About 2/3 of the change occurs in the longwave and 1/3 in the shortwave spectral range. Based on detailed model analysis we estimate that the changes in the longwave fluxes are predominantly explained by a higher surface temperature in summer 2004 compared to 2000. The shortwave effect is largely caused by scattering of solar radiation on organic carbon aerosols emitted from the 2004 fires. The negative effect is somewhat mitigated by the positive effect of absorbing black carbon aerosols emitted from the fires and to a lesser extent by the positive effect of ozone and other greenhouse gases produced and released from the fires. Sensitivity studies with varying aerosol emission scenarios indicate that the ratio of black to organic carbon aerosol emissions of the boreal fires used in this study needs to be increased considerably to match both observations of aerosol optical depth and TOA radiation fluxes or the biomass burning aerosols must be considerably more absorbing than parameterized in the model. While we cannot resolve the cause of this discrepancy in this study the presented technique is a powerful way of constraining aerosol emissions and will benefit from future improvement in measurements and modeling techniques.