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

Abstract

<p><strong>Abstract.</strong> We study the transport pathways from the top of convective clouds to the lower <p>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).</p></p>

Upward transport into and within the Asian monsoon anticyclone as inferred from StratoClim trace gas observations

“…A contribution of ASM anticyclone air at even higher potential temperature levels cannot be ruled out based on our observations. The significant isolation of the ASM anticyclone air up to 400 K and the rapid weakening of this isolation in the range between 400 and 435 K shown by our observations is roughly consistent with recent analyses of ASM confinement using trajectory analyses (Brunamonti et al, 2018;Legras and Bucci, 2020).…”

“…This diabatic "chimney" results from frequent deep convective activity up to around 370 K, although single convective events may reach higher. Legras and Bucci (2020), show high clouds in 2017 to be mostly distributed between 340 and 370 K, with some rare convective events reaching up to 400 K This is very similar to the cloud top height distribution over the ASM region that was shown for 2007 by Ueyama et al (2018).…”

“…This dynamically driven slow upwelling is radiatively balanced and is less geographically confined than the convective uplift. Pure trajectory models show an "upward spiralling motion of air" extending over large parts of the ASM anticyclone (Vogel et al, 2019;Legras and Bucci, 2020).…”

Pollution trace gas distributions and their transport in the Asian monsoon upper troposphere and lowermost stratosphere during the StratoClim campaign 2017

“…In an analysis of the ATLAS trajectories, the influence of the usage of ERA5 or ERA-Interim as meteorological fields, the influence of applied vertical diffusion, and the influence of the usage of kinematic or diabatic trajectories was investigated (not shown). This analysis (and also similar analyses by Legras and Bucci (2019) (Levelt et al, 2006). In this work, we use the troposheric column of nitrogen dioxide (NO 2 ) as a proxy for pollutant emissions (Crutzen, 1979).…”

“…We use here the cloud top products provided by the SAF-NWC (EUMETSAT Satellite Application Facility for Nowcasting) software package (Derrien et al, 2010;Sèze et al, 2015) from MSG1 (Meteosat 8) and Himawari geostationary satellites. More details on the algorithm and its evaluation against trace gas measurements can be found in Bucci et al (2020) and Legras and Bucci (2019).…”