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Discussion papers
https://doi.org/10.5194/acp-2019-1075
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1075
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Dec 2019

Submitted as: research article | 17 Dec 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Confinement of air in the Asian monsoon anticyclone and pathways of convective air to the stratosphere during summer season

Bernard Legras and Silvia Bucci Bernard Legras and Silvia Bucci
  • Laboratoire de Météorologie Dynamique, UMR 8539, CNRS/PSL-ENS/Sorbonne Université/Ecole Polytechnique, Paris,France

Abstract. We study the transport pathways from the top of convective clouds to the lower

tropical stratosphere during the Asian monsoon, using massive Lagrangian trajectories driven by observed clouds and the two reanalysis ERA-Interim and ERA5 with diabatic and kinematic vertical motions. We find that the upward propagation of convective impact is very similar for the kinematic and diabatic calculations using ERA5 while the two cases strongly differ for ERA-Interim. The separation of descending and ascending motion occurs at a crossover level which is slightly above the all sky zero level of radiative heating rate, except over the Tibetan plateau. The parcels that stay confined within the Asian monsoon anticyclone and reach 380 K are mostly of continental origin while maritime sources are dominating when the whole global 380 K surface is considered. We find that the strong impact of the Tibetan plateau with respect to its share of high clouds is entirely due to its elevated proportion of high clouds above the crossover. We find no trace of a vertical conduit above convection over the Tibetan plateau; parcels are rather entrained into an ascending spiral motion that spans the whole anticyclone. The mean age of parcels with respect to convection exhibits a minimum at the centre of the Asian monsoon anticyclone due to the permanent renewal by fresh convective air and largest values on the periphery as air spirals out. The contrast is reduced by dilution for increasing potential temperature. We find that the confinement above 360 K can be represented, on the average, by a simple 1D process of diabatic advection with loss. The mean loss time is about 13 days and uniform over the range 360 K to 420 K which is to be compared with a total circulation time of two to three weeks around the anticyclone. The vertical dilution is consequently exponential with an e-folding potential temperature scale of 15 K (about 3 km).

Bernard Legras and Silvia Bucci
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Short summary
The Asian monsoon is, during summer, the most active region bringing surface compounds by convection to the stratosphere. We study the transport pathways and the trapping within the upper layer anticyclonic circulation. Above 15 km, the confinement can be represented by a uniform ascent over continental Asia of about 200 m per day and a uniform loss to other regions with a characteric time of two weeks. We rule out the presence of a "chimney" over Tibetan plateau proposed in previous studies.
The Asian monsoon is, during summer, the most active region bringing surface compounds by...
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