HR: 0800h
AN: A21E-0770    [Abstracts]
TI: Dehydration processes in the Indian monsoon anticyclone : Lagrangian analysis and sensitivity to vertical wind fields.
AU: * James, R
EM: james@lmd.ens.fr
AF: LMD, 24 rue lhomond, paris, 75006, France
AU: Bonazzola, M
EM: Marine.Bonazzola@lmd.jussieu.fr
AF: LMD, 24 rue lhomond, paris, 75006, France
AU: Legras, B
EM: legras@lmd.ens.fr
AF: LMD, 24 rue lhomond, paris, 75006, France
AU: Surbled, K
EM: ksurbled@lmd.ens.fr
AF: LMD, 24 rue lhomond, paris, 75006, France
AU: Fueglistaler, s
EM: S.Fueglistaler@damtp.cam.ac.uk
AF: DAMTP, Centre for Mathematical Sciences, Wilberforce Road, CB3 0WA, Cambridge, United Kingdom
AB: During Asian monsoon season, the large-scale monsoon flux generates a persistent anticyclonic circulation at the tropopause over the sub-tropical part of Asia (15°N to 40°N). The anticyclonic region is associated with a large water vapor maxima, which can hardly be explained by the seasonal variation of tropopause temperatures alone.
In the upper troposphere, the moistening effect of deep convective events in the anticyclone has been pointed out by Randel and Park (JGR, 10.1029/2005JD006490, 2006). However, at higher levels the deep convective area is separated from the anticyclonic region (Park et al., JGR, 10.1029/2006JD008294, 2007). The large-scale circulation (slow ascent and anticyclonic barrier) seems hence essential to explain the water vapor distribution observed at 100 hPa (MLS/AURA or MIPAS).
In this context, the ability of the large-scale wind fields to represent the water vapor distribution has been studied from back-trajectories. The calculations have been performed over three summers (1998, 1999 and 2000) using two representations of the vertical wind in the ERA-40 dataset and the new ERA-Interim: the "classical" wind (from divergence equation) and the diabatic wind (from temperature tendency equation).
Coupled to a simple microphysical model, back-trajectories can reconstruct water vapor maps (Fueglistaler et al., JGR, 10.1029/2004JD005516, 2005). Here, the comparison to MLS/AURA retrievements at 100 hPa shows that : if "classical" wind calculations exhibit large discrepancies, diabatic winds accurately reconstruct the location and the concentration of the indian monsoon water vapor maxima without represented small-scale micro-physic.
Investigating transport and dehydration processes along trajectories (intersection with isentrops, clouds from CLAUS,...) the Lagrangian approach offers a synthetic scenario. After being quickly lifted by deep convection over the Bay of Bengal until 350-360 K, most of the parcels are freezed-dry around 370 K above the Bay of Bengal, experimenting the coldest region of the TTL in summer. In the meantime, parcels trapped in the anticyclone avoid this "cold trap", and are freezed-dry westward of it in warmer regions. Hence, the water vapor anomaly in the anticyclone is not explained by a special convective regime, but by the particular horizontal advection through the TTL in the anticyclone ("warm trap").
Complementary results on the sensitivity of our results to the level of sursaturation, and to variations from ERA- 40 to ERA-Interim will be presented.
DE: 0340 Middle atmosphere: composition and chemistry
DE: 0341 Middle atmosphere: constituent transport and chemistry (3334)
DE: 0368 Troposphere: constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting