Model Performance

Based on comparisons to the local aerosol optical depth statistics of sixteen AERONET-sites, all tested models have a tendency to underestimate aerosol optical depths. Although there are a few exceptions, underestimates are much more frequent (and larger) than overestimates. Based on compa risons of component mass and component optical depth characteristic tendencies of each model are summarized.

ECHAM4 underestimates dust, carbon and sulfate optical depth. The model is very sensitive to the (predicted) ambient relative humidity. This is demonstrated by larger optical depths sulfur and sea-salt during northern hemispheric winters. Larger sulfur mass conversions of GOCART, which are based on the same assumptions for sulfur size and humidification, suggest that ECHAM4 is often too dry. This is at least a contributing factor in underestimates to optical depths of sulfate, sea-salt and organic carbon aerosol. The removal of cloud scenes (with column water exceeding 33g/m3) in the ECHAM4-clr data-subset usually reduces the aerosol optical depths. Smaller optical depths are mainly the result of a reduced water uptake under smaller ambient relative humidities. However, often a lower aerosol dry mass in ECHAM4-clr appears to be a contributing factor.

MIRAGE stands out with the largest sulfate optical depths, while all other component optical depths are usually low. Small dust optical depths are most likely due to weak emissions. A lack in aerosol mass is also the major reason for underestimates in carbon optical depths near tropical biomass burning. Carbon mass at high latitude, however, is one of the largest among models. The large sulfate optical depths are attributed to sulfate mass overestimates, because the mass conversion is one of the smallest among models.

GOCART is the only model whose dust aerosol optical depths seem sufficient to match the AERONET statistics near dust sources. Away from dust sources, dust optical depths are usually too large, most likely due to the lack of removal processes. This offsets in part the lack in carbon and sulfate optical depth, however, it also delays summer-time maxima at northern hemispheric urban-industrial sites. Low sulfate optical depths are related to sulfate mass, because mass conversions for sulfate are the largest among models.

CCSR carbon optical depths are largest among models, however AERONET averages near biomass burning are rarely reached. Aerosol transport appears too weak (or removal processes too strong) as optical depths for dust and especially for carbon decline more rapidly away from sources than in other models. Sulfate aerosol optical depths are low primarily due to a relatively large assumption for sulfate aerosol size.

GISS dust and carbon optical depths are too low, despite mass conversions that are large in comparisons to other models. These large values in part compensate for the lack in mass. Overestimates for sulfur aerosol size cause a smaller sulfur mass conversion, which in part is responsible for underestimates in sulfate optical depth. One of the smallest sea-salt masses among models is compensated by the largest sea-salt mass conversion.

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