Abstract
Abstract. Understanding the transport path of the solar activity proxy 10Be from source to archive is crucial for the interpretation of its observed variability. The extent of mixing of the strong production signal has been quantified in a previous study (Heikkilä et al., 2009). In this study we perform sensitivity studies to investigate the influence of model resolution on the degree of mixing and transport path of 10Be in the atmosphere using the ECHAM5-HAM aerosol-climate model. This study permits us to choose an acceptable resolution, and so minimum CPU time, to produce reconstructions as physically accurate as possible. Five model resolutions are applied: T21L19: a coarse horizontal and vertical resolution with model top at ca. 30 km, T42L31: an average horizontal and fine vertical one, T42L39: similar vertical resolution than L19 but including the middle atmosphere up to ca. 80 km, T63L31: a fine horizontal and vertical resolution and T63L47: a fine resolution horizontally and vertically with middle atmosphere. Comparison with observations suggests that a finer horizontal and vertical resolution might be beneficial, producing a reduced meridional gradient, although the spread between observations was much larger than between the five model runs. In terms of atmospheric mixing the differences became more distinguishable. All resolutions agreed that the main driver of deposition variability, observed in natural archives, is the input of stratospheric 10Be (total contribution 68%) which is transported into the troposphere at latitudes 30–50°. In the troposphere the model resolutions deviated largely in the dispersion of the stratospheric component over latitude. The finest resolution (T63L47) predicted the least dispersion towards low latitudes but the most towards the poles, whereas the coarsest resolution (T21L19) suggested the opposite. The tropospheric components of 10Be differed less between the five model runs. The largest differences were found in the polar tropospheric components, which contribute the least to total production (≈ 4%). We conclude that the use of the T42 horizontal resolution seems to be sufficient in terms of atmospheric mixing of a stratospheric tracer because no substantial improvement was seen when the resolution was increased from T42 to T63. The use of the middle atmospheric configuration is a trade-off between correctly describing stratospheric dynamics and having to reduce vertical resolution. The use of a high vertical resolution seemed more beneficial than the middle atmospheric configuration in this study. The differences found between the T42L31 and T63L31 resolutions were so small that T42L31 is a good choice because of its computational efficiency.
Highlights
Cosmogenic radionuclides, such as 10Be and 7Be, are commonly used proxies for past cosmic ray intensity and solar activity. They are produced in the atmosphere by primary and secondary particles of cosmic rays interacting with atmospheric nitrogen and oxygen atoms
10Be and 7Be atoms attach to ambient aerosol, mainly sulfate, and are transported through the atmosphere and deposited to natural archives with them
This study addressed the question to what extent the atmospheric transport of the solar proxy 10Be is model resolution dependent
Summary
Cosmogenic radionuclides, such as 10Be (half-life 1.4 million years) and 7Be (half-life 53.2 days), are commonly used proxies for past cosmic ray intensity and solar activity. They are produced in the atmosphere by primary and secondary particles of cosmic rays (protons and neutrons) interacting with atmospheric nitrogen and oxygen atoms. The intensity of cosmic rays in the atmosphere is modulated by solar and geomagnetic activity and observed variations of cosmogenic radionuclide concentrations in natural archives, such as ice cores, can be used to reconstruct cosmic ray intensity in the past. Modelling the atmospheric transport of beryllium isotopes is a difficult task. Their main source in the stratosphere (e.g. Lal and Peters, 1967) and long residence time
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