Wenyi Zhong

Radiative Forcing by Well-Mixed Greenhouse Gases: Estimates from Climate Models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4)

The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO2, CH4, N2O, CFC-11, CFC-12, and the increased H2O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH4 and N2O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.

The role of microphysical and chemical processes in prolonging the climate forcing of the Toba Eruption

The mega-eruption of Toba, Sumatra, occurred around 73 Ka ago, during the onset of a glaciation of the Late Quaternary. This coincidence combined with the unprecedented amount of sulphur released by this volcano has led to the hypothesis that Toba sulphate aerosols caused a transient surface cooling which may have contributed to a shift of the climate system. Because of the self limiting effect of gravitational sedimentation, the climatic impact of extremely large sulphur injections into the stratosphere are thought to be rather limited. Here we present model calculations combining microphysical and chemical feedbacks which show that the eruption could instead have led to the formation of a long-lasting volcanic aerosol layer. Although the concentrations of radiatively active species such as O3 or SO2 could also have been considerably perturbed, the resulting forcings should have only slightly moderated the aerosol cooling effect during the first few years following the eruption. According to our results, extremely high stratospheric sulphur loading could lead to a more prolonged effect on the climate than previously assumed.

The contribution of unknown weak water vapor lines to the absorption of solar radiation

Thousands of unknown water vapour weak lines in the spectral region between 13200 and 22700 cm -1 are deduced from extrapolations of experimental results. These lines are then included in the HITRAN database and used in line-by-line calculations of atmospheric opacity with standard atmospheric profiles. The weak lines predicted by a theoretical model are also used in the line-by-line model to estimate their contribution to the absorption of solar radiation. The additional absorption of solar radiation is of order 1.5 to 2.5 W/m 2 at 45 mm precipitable water, about 8.5% to 14% of the absorption due to HITRAN lines in the spectral region. The effect is also compared with that of a continuum model.

Testing broadband radiation schemes for their ability to calculate the radiative forcing and temperature response to stratospheric water vapour and ozone changes

The radiative forcing and fixed dynamical heating temperature response is calculated for observed changes (1979-1997) in carbon dioxide, stratospheric ozone and stratospheric water vapour, using several radiation schemes. It is found that certain broadband schemes substantially underestimate the radiative forcing and absorption by stratospheric water vapour by up to 50%, for the instantaneous forcing, and 15% for the adjusted radiative forcing. This error in the water vapour absorption leads to a 30% smaller stratospheric temperature response and also causes an underestimate of the adjusted stratospheric ozone radiative forcing of up to 50%. It was found that this error could be corrected by splitting the wavelength band which accounts for strong water vapour absorption into two separate bands. This work illustrates the need to carefully test each new forcing introduced into a broadband scheme, as its designer may not have catered for such eventualities.

Climate forcing by stratospheric ozone depletion calculated from observed temperature trends

The radiative forcing of the surface-troposphere system caused by stratospheric ozone depletion in the 1980s is calculated using observed values of change in ozone, from TOMS data, and temperature, from MSU data. The seasonal variation of the ozone and temperature trends produces strong seasonal and latitudinal variations in radiative forcing. The balance between peak positive solar forcing and maximum negative longwave forcing shifts the peak negative forcing at the South Pole from November to October. The time difference between peak ozone loss and maximum temperature decrease results in net positive values in Southern Hemisphere mid-latitudes in August and September. Positive values also occur at low latitudes in both hemispheres. The globally and annually averaged net radiative forcing is about −0.025Wm−2 between 1979 and 1990, much less than that previously reported using fixed dynamical heating model temperature changes, although inclusion of the ozone 14 µm band would somewhat narrow this gap. Ozone radiative forcing is very sensitive to the vertical profile of temperature change.

Greenhouse gases in the stratosphere

The potential radiative forcing in the stratosphere of changing concentrations of ozone, methane, nitrous oxide and chlorofluorocarbons 11 and 12 is assessed. Significant changes in heating rate in the lower stratosphere are found. The response of a fully interactive radiative-photochemical-dynamical two-dimensional model to such changes in gaseous concentrations is investigated. The inclusion of CH[sub 4], N[sub 2]O and the CFC in the radiation scheme causes a small (1 K) decrease in temperature throughout the stratosphere after 50 model years with a resulting increase in ozone column up to 1% in summer high latitudes. An experiment in which lower stratospheric ozone concentrations were forcibly reduced in line with recent satellite observations results in significant (several degrees) temperature decrease in this region. Such decreases may be very significant in maintaining polar ozone loss. 20 refs., 12 figs., 2 tabs.

First results from a 3-dimensional middle atmosphere model

Initial results are presented from a middle atmosphere extension to a version of the European Centre For Medium Range Weather Forecasting tropospheric model. The extended version of the model has been developed as part of the UK Universities Global Atmospheric Modelling Project and extends from the ground to approximately 90 km. A comprehensive solar radiation scheme is included which uses monthly averaged climatological ozone values. A linearised infrared cooling scheme is employed. The basic climatology of the model is described; the parametrization of drag due to orographically forced gravity waves is shown to have a dramatic effect on the simulations of the winter hemisphere.

Synergistic use of ATSR and ATSR/M satellite data

Abstract The cloud liquid water is derived from the Microwave Radiometer (ATSR/M) on board of the First European Environmental Research Satellite/ERS-1/. A method and results for cloud top height estimation by infrared stereo images from the Along Track Scanning Radiometer (ATSR) are presented. The clouds heating effect is analysed involving ECMWF (European Centre for Medium-Range Weather Forecasts) radiative code. Clouds properties along the ERS-1 track are estimated using liquid water, retrieved from ATSR/M and algorithms from Atmospheric General circulation model (AGCM) of the United Kingdom Meteorological Office (UKMO).