pure or mixed organic or inorganic particles is their surface structure and wetting behavior. Surface tension plays an important role in the indirect effect of aerosol particles, potentially providing an important determining factor for the particles to activate. Organic compounds in particles may significantly alter the efficiency with which particles can serve as cloud condensation nuclei (Facchini et al., 2000; Feingold and Chuang, 2002; Ming and Russell, 2004). The transformation timescale from hydrophobic to hydrophilic states is a seriously uncertain parameter in current models.

ATMOSPHERIC REANALYSIS AND DATA ASSIMILATION

Atmospheric reanalysis involves using models to interpolate observations in order to construct physically consistent estimates of atmospheric structure and dynamics. The National Centers for Environmental Prediction (NCEP) Reanalysis (Kalnay et al., 1996) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40) (Bengtsson et al., 2004) are two global analyses that extend across several decades and will continue into the future. Reanalyses can be used to assess the change over time of selected space- and time-integrated climate metrics, such as the 1000-500 mb thickness, the 200 mb heights, tropopause height, and the 200 mb winds (Chase et al., 2000b; Pielke et al., 2001; Santer et al., 2003b).

It remains difficult, however, to estimate reliable, small-amplitude trends from reanalyses (Bengtsson et al., 2004), owing mainly to temporal variations in input data quantity and quality. Given these heterogeneities in reanalyses, it is essential to determine the magnitude of trends that must occur before they can be determined to be statistically significant (Chase et al., 2000b). The use of metrics that integrate atmospheric structure and dynamics represents another effective procedure to utilize reanalyses for trend assessments in that the effect of heterogeneities in the data record may be reduced. Examples include the thickness between pressure surfaces, tropopause height, or the vertical wind shear across the troposphere. The first two provide vertically integrated measures of the warming of the troposphere in response to radiative heating. The third provides an integrated measure of the horizontal gradient in tropospheric mean temperature.

Future reanalyses should strive for as homogeneous a dataset as possible to monitor temporal and spatial changes in tropospheric heat content. This information would be valuable in relating to observed temporal and spatial changes in ocean heat content. For example, can the atmospheric reanalyses help explain the observed focusing of ocean warming in the midlatitudes of the Southern Hemisphere, and will this continue into the future? Accurate reanalyses can also address the question of whether the



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