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DODO RESULTS: Campaign Averages & BAe-146 Nephelometer Findings

This study presents the findings from the DODO campaign, specifically focusing on the BAe-146 nephelometer measurements. It discusses under-sampling in the nephelometer, intercomparison with NASA DC-8, comparison with AERONET, and estimation of the nephelometer cut-off radius. The results suggest a potential underestimation of dust scattering and provide insights for future observations and modeling.

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DODO RESULTS: Campaign Averages & BAe-146 Nephelometer Findings

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  1. DODO RESULTS:Campaign Averages&BAe-146 Nephelometer Findings Claire McConnell Ellie Highwood Acknowledgements: Paola Formenti, Met Office, FAAM

  2. DODO Flights DODO 1 Feb 2006 DODO 2 Aug 2006

  3. DODO Average Properties

  4. Under-sampling in the Nephelometer • Coarse particles are lost in the inlet of the nephelometer • During SHADE (2000) AODs derived from scattering on the C-130 required multiplication of 1.5 to agree with AERONET AODs, implying an inlet cut-off between 1.3 and 1.9μm radius. (Haywood et al., 2003) • Experiments on BAe-146 nephelometer: • The intercomparison between the BAe-146 and the NASA DC-8 during DODO2 allowed a comparison between the nephelometers • Size distributions measured during DODO & Mie code allow an analysis of the under-sampling • Under-sampling can be estimated through comparisons with ground-based sites • These processes allow an estimation of the factor of sampling missed and of the cut-off radius on the BAe-146 nephelometer

  5. Intercomparison with NASA DC-8 Uncorrected nephelometer scattering for 3 straight level runs – bold=BAe-146, light=DC-8 • BAe-146 and DC-8 show same spatial variability in scattering • correlation coefficients vary between 0.93-0.96 • Difference in absolute scattering measured • average ratio of DC-8/BAe-146 uncorrected scattering = 2.1 • Hypothesis: The BAe-146 nephelometer is underestimating the total DODO dust scattering at 550nm by a factor of 2-3, and is only measuring submicron (diameter) scattering

  6. Comparison with AERONET • AODs for DODO2 calculated by integrating BAe-146 profile data • AERONET AODs for the same times are a factor of 3.2 larger than those from the nephelometer profiles (but factor varies between 2 and 4.5) • Results consistent with the DC-8 intercomparison AODs from BAe-146 profiles and from AERONET station at Dakar

  7. Estimation of Nephelometer Cut-off Radius • The size distribution was measured by the PCASP and SEM sizing during DODO • ‘Cut-off test’ – average size distribution successively ‘cut-off’ at decreasing radii to mimic the loss of the coarse mode • Each size distribution run through Mie code to obtain scattering/absorption properties • ‘Cut-off Factor’ (C) calculated – factor required to match the ‘cut-off’ scattering/absorption/extinction with that of the full size distribution Size distributions available for use in the cut-off test

  8. Results from Cut-off Tests • Submicron sampling (r<0.5μm) results in a scaling factor of 3 for the nephelometer • ω0 is considerably lower if the full size distribution is considered

  9. Conclusions on Cut-off Radius on BAe-146 Nephelometer • Submicron sampling on the 146 neph would result in a cut-off factor of 3 • The AERONET-derived scaling factor suggests a cut-off of 0.5μm • DC-8 intercomparison-derived scaling factor suggests a cut off of 0.8-1μm • Suggested cut-off, around r=0.5μm, appears to be smaller than experienced for dust during SHADE 2000 Caveats • Mie work assumes spherical particles • Cut-off factors may change with different size distributions (e.g. more/less coarse mode present)

  10. Future Work Observations • Results from radiometers • Results from Edwards & Slingo radiation code • Chemistry • Iron Content, for deposition • Refractive indices, for comparison with derived values • Specific case studies, comparison to remote sensing Modelling • DODO – dust modelling, comparisons to observations, deposition estimates

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