3.6.1 Squall Lines in the Tropics

Overall, squall lines that occur in the tropics are structurally very similar to midlatitude squall lines. That being said, however, there are a few notable differences. First, because the tropopause is higher in the tropics (as shown by the height scale in these contrasting conceptual models) the leading line convective storms are generally taller than their midlatitude counterparts. Secondly, tropical convection develops in generally lower shear, lower LFC environments, both of which affect the details of system evolution. Specifically, tropical convection is easily triggered and systems tend to move more slowly than midlatitude MCSs.

Third, because the environments are of tropical origin there tends to be much less mid-level dry air to encourage evaporation and thus downdrafts. This means that system cold pools are generally weaker than those in midlatitude squall lines (~ 1 hPa excess pressure in the tropical cold pool as compared to 3 hPa in the midlatitude conceptual model), which is another factor contributing to generally slower moving systems. Fourth, and not shown in these cross sections, low-latitude squall lines display less of a tendency toward asymmetric evolution over time because the Coriolis force is reduced at tropical latitudes. Finally, as we’ll see on the next page, the most obvious distinction between midlatitude and tropical squall lines is their typical system motion throughout their evolution.

3.6.2 Example from South America

Click the image above to view animation.

Most tropical squall lines move from east to west rather than west to east as commonly observed with midlatitude MCSs. This occurs because the vertical wind shear profile in the tropics is dominated by easterly trade winds rather than the westerlies that control storm motion in the midlatitudes. It should be noted that although tropical MCSs tend to move more slowly than their midlatitude counterparts and develop in moister environments, they still move faster than nearby isolated cells and tend to develop in comparatively dry air (for the tropics).

3.6.3 Famous Tropical Squall Line Locations

Other well-studied tropical linear disturbances are the squall lines of West Africa, the Sumatra of Indonesia, and the subtropical squall lines that occur ahead of the Mei-Yu front near Taiwan. Tropical squall lines preferentially initiate over land surfaces, but are also observed to trigger over oceanic areas. The preference for formation over land is likely due to the greater presence of the continental influence on mid-level “dry” layers. Hail rarely occurs in association with tropical squall lines due to the warm nature of the tropical air masses, but heavy “warm” rain and gusty surface winds are usually observed.

End of Section 3.6: Squall Lines - Tropical Squall Lines