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    Low-Level Monsoon Dynamics Derived from Satellite Winds

    Source: Monthly Weather Review:;1983:;volume( 111 ):;issue: 004::page 774
    Author:
    Stout, John E.
    ,
    Young, John A.
    DOI: 10.1175/1520-0493(1983)111<0774:LLMDDF>2.0.CO;2
    Publisher: American Meteorological Society
    Abstract: The dynamics of low-level summer monsoon flow near 900 mb is studied using daily MONEX (1979) satellite wind data to estimate mechanisms influencing the horizontal momentum. We present an improved estimate of the large-scale monsoon geopotential field near the level of maximum wind, and a more approximate field of friction as well. Average fields for the premonsoon and established monsoon periods of 1.5 months are shown. The evolution of forces and accelerations along different trajectories crossing the western Indian Ocean are compared. The net horizontal force, equal to the pressure gradient plus friction force, is obtained for the two periods by directly estimating the mean Coriolis and relative acceleration vectors. The contribution to mean acceleration by synoptic-scale transient eddies is significant only south of 30°S. Inertial acceleration by the mean flow produces a Rossby number in excess of 0.25 in an equatorial belt which expands to 10°N in the Somali Jet entrance. A method is devised to split the observed net force field into its pressure gradient and friction force components; the method corresponds to solving the vorticity and divergence equations, respectively, and uses the property that pressure gradient is exactly irrotational and the assumption that friction force is mostly non-divergent. It is found that the diagnosed friction force tends to oppose the wind and is distinctly weaker than the pressure gradient force. The calculated geopotential field shows the development of a distinctive ?reversed S? contour connecting the hemispheres and supporting strong cross-equatorial flow. The corresponding trajectories show that the degree of imbalance is greater in the Northern Hemisphere as the air adjusts to the changing Coriolis influence and monsoonal pressure gradient forces which increase and rotate. Northward moving air slows by friction as it approaches the equator, but increases speed in the Northern Hemisphere by flowing toward lower pressure. The assumptions involving frictional estimates for boundary conditions are evaluated using theory and wind data.
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      Low-Level Monsoon Dynamics Derived from Satellite Winds

    URI
    http://yetl.yabesh.ir/yetl1/handle/yetl/4200885
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    • Monthly Weather Review

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    contributor authorStout, John E.
    contributor authorYoung, John A.
    date accessioned2017-06-09T16:04:18Z
    date available2017-06-09T16:04:18Z
    date copyright1983/04/01
    date issued1983
    identifier issn0027-0644
    identifier otherams-60237.pdf
    identifier urihttp://onlinelibrary.yabesh.ir/handle/yetl/4200885
    description abstractThe dynamics of low-level summer monsoon flow near 900 mb is studied using daily MONEX (1979) satellite wind data to estimate mechanisms influencing the horizontal momentum. We present an improved estimate of the large-scale monsoon geopotential field near the level of maximum wind, and a more approximate field of friction as well. Average fields for the premonsoon and established monsoon periods of 1.5 months are shown. The evolution of forces and accelerations along different trajectories crossing the western Indian Ocean are compared. The net horizontal force, equal to the pressure gradient plus friction force, is obtained for the two periods by directly estimating the mean Coriolis and relative acceleration vectors. The contribution to mean acceleration by synoptic-scale transient eddies is significant only south of 30°S. Inertial acceleration by the mean flow produces a Rossby number in excess of 0.25 in an equatorial belt which expands to 10°N in the Somali Jet entrance. A method is devised to split the observed net force field into its pressure gradient and friction force components; the method corresponds to solving the vorticity and divergence equations, respectively, and uses the property that pressure gradient is exactly irrotational and the assumption that friction force is mostly non-divergent. It is found that the diagnosed friction force tends to oppose the wind and is distinctly weaker than the pressure gradient force. The calculated geopotential field shows the development of a distinctive ?reversed S? contour connecting the hemispheres and supporting strong cross-equatorial flow. The corresponding trajectories show that the degree of imbalance is greater in the Northern Hemisphere as the air adjusts to the changing Coriolis influence and monsoonal pressure gradient forces which increase and rotate. Northward moving air slows by friction as it approaches the equator, but increases speed in the Northern Hemisphere by flowing toward lower pressure. The assumptions involving frictional estimates for boundary conditions are evaluated using theory and wind data.
    publisherAmerican Meteorological Society
    titleLow-Level Monsoon Dynamics Derived from Satellite Winds
    typeJournal Paper
    journal volume111
    journal issue4
    journal titleMonthly Weather Review
    identifier doi10.1175/1520-0493(1983)111<0774:LLMDDF>2.0.CO;2
    journal fristpage774
    journal lastpage798
    treeMonthly Weather Review:;1983:;volume( 111 ):;issue: 004
    contenttypeFulltext
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