Parker, DJ orcid.org/0000-0003-2335-8198, Willetts, P orcid.org/0000-0002-6769-2377, Birch, C orcid.org/0000-0001-9384-2810 et al. (5 more authors) (2016) The interaction of moist convection and mid-level dry air in the advance of the onset of the Indian monsoon. Quarterly Journal of the Royal Meteorological Society, 142 (699). pp. 2256-2272. ISSN 0035-9009
Abstract
The advance of the onset of the Indian monsoon is here explained in terms of a balance between the low-level monsoon flow and an over-running intrusion of mid-tropospheric dry air. The monsoon advances, over a period of about 6 weeks, from the south of the country to the northwest. Given that the low-level monsoon winds are westerly or southwesterly, and the midlevel winds northwesterly, the monsoon onset propagates upwind relative to midlevel flow, and perpendicular to the low-level flow, and is not directly caused by moisture flux toward the northwest. Lacking a conceptual model for the advance means that it has been hard to understand and correct known biases in weather and climate prediction models.
The mid-level northwesterlies form a wedge of dry air that is deep in the far northwest of India and over-runs the monsoon flow. The dry layer is moistened from below by shallow cumulus and congestus clouds, so that the profile becomes much closer to moist adiabatic, and the dry layer is much shallower in the vertical, toward the southeast of India. The profiles associated with this dry air show how the most favourable environment for deep convection occurs in the south, and onset occurs here first.
As the onset advances across India, the advection of moisture from the Arabian Sea becomes stronger, and the mid-level dry air is increasingly moistened from below. This increased moistening makes the wedge of dry air shallower throughout its horizontal extent, and forces the northern limit of moist convection to move toward the northwest. Wetting of the land surface by rainfall will further reinforce the north-westward progression, by sustaining the supply of boundary layer moisture and shallow cumulus. The local advance of the monsoon onset is coincident with weakening of the mid-level northwesterlies, and therefore weakened mid-level dry advection.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2016 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. |
Keywords: | Indian monsoon; onset; deep convection; cumulonimbus; cumulus congestus; dry intrusion; land-atmosphere interaction; water cycle |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Inst for Climate & Atmos Science (ICAS) (Leeds) |
Funding Information: | Funder Grant number Royal Society No External Reference NERC No External Refererence NERC R8/H12/83/006 Met Office L0627 NERC No External Ref |
Depositing User: | Symplectic Publications |
Date Deposited: | 08 Apr 2016 09:27 |
Last Modified: | 14 Mar 2019 15:48 |
Published Version: | https://dx.doi.org/10.1002/qj.2815 |
Status: | Published |
Publisher: | Wiley |
Identification Number: | 10.1002/qj.2815 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:97869 |