Typical Appearance In Vertical Cross Sections
In MCC events, an upper-level jet stream decreases tropospheric stability and intensifies the formation of frontal systems. A dynamical connection between upper and lower levels operates trough vertical thermal circulations. The circulations can be analyzed using Riehl's model, which couples vertical velocities with meridional wind components.
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Thermal Circulations markings overlayed at divergence field at 200hPa - November, 09th 2008 00UTC
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Meteosat-9 Airmass RGB Image 9 November 2008 / 00 UTC - the locations of the Cross Sections A-A' and B-B' are shown.
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Locations of the thermal circulation cross sections at 200 hPa on 9 November 2008 at 00:00 UTC. AA': thermally direct circulation over 64°W, between 25°S and 35°S, BB': thermally indirect circulation over 50°W, between 25°S and 35°S.
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Riehl's model for a high-level jet core and the locations of convergence and divergence accompanying it.
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Thermally direct circulation accompanying a jet core.
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Thermally indirect circulation accompanying a jet core.
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In the the cross section of vertical velocities along the line AA', there is descending air between 33°S and 35°S and ascending air between 22°S and 25°S (blue arrows). In the the cross section of meridional wind components along the line AA', the wind direction is mainly from south to north in the mid-troposphere (green arrow at 500 hPa), whereas the upper-level jet (green line at 250 hPa) and low-level jet (red line at 850 hPa) have an opposite direction.
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Vertical velocity (Pa/s) across AA' along 64°W on 9 November 2008 at 00:00 UTC.
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Meridional wind component (m/s) across AA' along 64°W on 9 November 2008 at 00:00 UTC.
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In the the cross section of vertical velocities along the line BB', there is ascending motion between 31°S and 35°S and descending motion between 22°S and 24°S (blue arrows). In this region the meridional wind components (green arrows) are not so strong as along AA', but a weak southerly component in the upper levels and a northerly component in the lower levels can be seen. The Upper Jet Stream couples the low and high level air streams through vertical circulations, which intensify the convective surface systems.
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Vertical velocity (Pa/s) across BB' along 50°W on 9 November 2008 at 00:00 UTC.
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Meridional wind (m/s) across BB' along 50°W on 9 November 2008 at 00:00 UTC.
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