No abstract

[1] The quantitative interpretation of isotope records (d 18 O, dD, and d excess) in ice cores can benefit from a comparison of observed meteorology with associated isotope variability. For this reason we studied four isotope records from snow pits in western Dronning Maud Land (DML), Antarctica, covering the period 1998-2001. Timing and magnitude of snowfall events on these locations were monitored using sonic height rangers. For the distinguished snowfall events we evaluated the isotopic composition of the moisture during transport by combining backward trajectory calculations with isotopic modeling, using a Rayleigh-type distillation model (MCIM). The initial isotope ratio of the moisture was determined from monthly mean isotope fields from a general circulation model (ECHAM4). The trajectory analysis showed that the southern Atlantic Ocean is the major moisture source for precipitation in DML. Modeling results along the trajectories revealed that most of the isotopic depletion occurred during the last day of the transport. Finally, a diffusion model was applied to describe the diffusion in the firn layer such that the modeled isotopes could be compared with the observed isotope records. The resulting modeled isotope profiles were mostly in good agreement with the observed seasonal variability in the snow. However, at low temperatures (especially on the Antarctic interior), magnitude of the total distillation was underestimated. Regarding the d excess parameter, our results show a large influence of advection height on the final value of d excess in precipitation. This in turn points to the importance of the vertical structure of d excess over the oceanic source region, which obscures the classical interpretation of this parameter in terms of temperature and relative humidity in the moisture source region.

Three three‐dimensional trajectory models (LAGRANTO, TRAJKS and FLEXTRA), all driven with analysis wind fields from the European Centre for Medium‐Range Weather Forecasts, are intercompared. The comparison has three parts: first, a case study of strong ascent in a warm conveyor belt is performed; second, a large set of back trajectories from the tropopause region over Europe and the mid‐latitude Atlantic Ocean is investigated; third, a set of low‐level trajectories is compared. The intercomparison shows that all three models have been implemented correctly. The degree of model accordance depends on the interpolation methods used. Deviations between the results from a single model using different interpolation schemes are of the same magnitude as the deviations of different models. If all models use linear spatial interpolation, their respective trajectories closely agree with each other, with deviations of 2% or less for the average distance between the starting and the ending positions in the free atmosphere after 48 h. Close to the surface, where the differences in the model formulations are largest, average horizontal position deviations may be up to 10%. Compared with other sources of errors, such as inaccuracies in the wind fields or insufficient temporal and spatial resolution of the data set, these differences are much smaller. Non‐linear spatial interpolation leads to stronger vertical motions than linear interpolation and, in the case study, enhanced the quality of the results. Copyright © 2001 Royal Meteorological Society

No abstract

Abstract. We have performed simulations using a 3-D global chemistry-transport model to investigate the influence that biogenic emissions from the African continent exert on the composition of the troposphere in the tropical region. For this purpose we have applied two recently developed biogenic emission inventories provided for use in large-scale global models (Granier et al., 2005; Lathière et al., 2006) whose seasonality and temporal distribution for biogenic emissions of isoprene, other volatile organic compounds and NO is markedly different. The use of the 12 year average values for biogenic emissions provided by Lathière et al. (2006) results in an increase in the amount of nitrogen sequestrated into longer lived reservoir compounds which contributes to the reduction in the tropospheric ozone burden in the tropics. The associated re-partitioning of nitrogen between PAN, HNO 3 and organic nitrates also results in a ∼5% increase in the loss of nitrogen by wet deposition. At a global scale there is a reduction in the oxidizing capacity of the model atmosphere which increases the atmospheric lifetimes of CH 4 and CO by ∼1.5% and ∼4%, respectively. Comparisons against a range of different measurements indicate that applying the 12 year average of Lathière et al. (2006) improves the performance of TM4 AMMA for 2006 in the tropics. By the use of sensitivity studies we show that the release of NO from soils in Africa accounts for between ∼2-45% of tropospheric ozone in the African troposphere, ∼10% in the upper troposphere and between ∼5-20% of the tropical tropoCorrespondence to: J. E. Williams (williams@knmi.nl) spheric ozone column over the tropical Atlantic Ocean. The subsequent reduction in OH over the source regions allows enhanced transport of CO out of the region. For biogenic volatile organic C1 to C3 species released from Africa, the effects on tropical tropospheric ozone are rather limited, although this source contributes to the global burden of VOC by between ∼2-4% and has a large influence on the organic composition of the troposphere over the tropical Atlantic Ocean.