1.2.3.6. Backsheared anvil – updraft strength vs. wind shear

  • 10 months ago
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  • Author: SW101

Strong wind shear & upper level winds

When wind shear is present, the updraft becomes tilted and the anvil becomes offset / pushed downwind, in the direction of upper level winds. The stronger the wind shear and upper level winds, the more the updraft is tilted and the more the anvil is sheared.

The shape of the anvil in a sheared environment will strongly depend on the balance between upper level winds and updraft strength. The stronger the updraft, the more it can ‘resist’ upper level winds. The stronger the upper level winds, the more they elongate the anvil. Let us see a couple of examples to illustrate this effect.

Anvil shape (and updraft tilt) in strong wind shear, strong upper level winds when the updraft is very strong. Strong wind shear tilts the updraft and elongates the anvil downwind. However, as the updraft is very strong and the upward momentum transfers into lateral (horizontal) momentum when the updraft hits the tropopause, there is enough momentum for the anvil to spread some distance upwind. The stronger the updraft, the further upwind it can extend (in the same upper level wind). Typically, the leading edge of such an anvil will be sharp and thick. This is called a backsheared anvil, it is often indicative of a very strong updraft and a severe thunderstorm.

When the updraft is strong compared to the wind shear and upper level winds (e.g. strong updraft in moderate wind shear and upper level winds or very strong updraft in strong wind shear and upper level winds) the backsheared anvil extends also some distance upwind. The extent depends on how strong the updraft is compared to upper level winds, the stronger the updraft the larger forward extent. In strong wind shear and upper level winds, the updraft must be very strong: in such case a thick forward edge of the anvil forms.

When wind shear and upper level winds are very strong, the anvil becomes very elongated. There is not enough forward momentum for a strong leading edge to build (i.e. a backsheared anvil) and the anvil extends only downwind.

Anvil and updraft in very strong wind shear. The updraft is strongly tilted and the anvil has very little forward extent, as it is rapidly sheared downwind by strong upper level winds.