Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-616
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
30 Aug 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics
Matthew Gibbons1, Qilong Min1, and Jiwen Fan2 1Atmospheric Science Research Center, State University of New York, Albany NY 12203, USA
2Earth Systems Analysis & Modeling, Pacific Northwest National Laboratory
Abstract. To better understand the impacts of dust aerosols on deep convective cloud (DCC) systems reported by previous observational studies, a case study in the tropical eastern Atlantic was investigated using the Weather Research and Forecasting (WRF) model coupled with a Spectral Bin Microphysics (SBM) model. A detailed set of ice nucleation parameterizations linking ice formation with aerosol particles have been implemented in the SBM. Increasing IN concentration in the dust cases results in the formation of more numerous small ice particles in the heterogeneous nucleation regime (between −5 °C and −38 °C) compared to the background (Clean) case. Convective updrafts are invigorated by increased latent heat release due to depositional growth and riming of these more numerous particles, which results in increased overshooting and higher convective core top heights. Competition between the more numerous particles for available water vapour during diffusional growth and available smaller crystals/drops during collection reduces particle growth rates and shifts precipitation formation to higher altitudes in the heterogeneous nucleation regime. Homogeneous ice formation is reduced in the dust cases as IN concentration is increased, due to more liquid drops converting to ice by freezing or riming before reaching −38 °C and reduced peak supersaturation values from increased diffusional growth. Local IN activation in the stratiform regime contributes to increased cloudiness in the heterogeneous nucleation regime. A greater number of large snow particles form in the dust cases, which are transported from the core into the stratiform regime and sediment out quickly. Together with reduced homogeneous ice formation, fewer particles form within and/or are transported into the anvil regime. This shifts the stratiform/anvil cloud occurrence frequency to warmer temperatures and reduces anvil cloud extents. Total surface precipitation accumulation is reduced proportionally as IN concentration is increased, due to less efficient graupel formation reducing convective rain rates. Stratiform precipitation accumulation is increased due to greater snow formation and growth, but does not counteract the reduced convective accumulation. Riming efficiency in the dust cases is reduced due to smaller cloud ice crystals, resulting in smaller graupel sizes overall. Ice particle aggregation occurs earlier in the simulation and over a wider temperature range in the dust cases, which increases snow formation in the heterogeneous nucleation regime. Radar reflectivity values are increased in the dust cases at temperatures below 0 °C in both the convective and stratiform regimes due to more large snow particles. More numerous small ice/graupel particles that form in the heterogeneous nucleation regime in the dust cases melt and reduce reflectivity values in the convective core near the surface, consistent with case study observations.

Citation: Gibbons, M., Min, Q., and Fan, J.: Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-616, in review, 2017.
Matthew Gibbons et al.
Matthew Gibbons et al.
Matthew Gibbons et al.

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Short summary
The effects of dust aerosols on ice formation within a tropical Atlantic thunderstorm system was investigated using a 3D weather model and compared with observations. Updated ice formation mechanisms directly connect available dust particles with ice particle formation.

The resulting clouds were lower and narrower and produced less rain at the surface compared to cleaner conditions, due to ice formation occurring at warmer temperatures. These results agree well with observed changes.
The effects of dust aerosols on ice formation within a tropical Atlantic thunderstorm system was...
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