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.
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.
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.
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.
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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