|HABYTIME MINI LECTURE 48: HELICITY
METEOROLOGIST JEFF HABY
Helicity is one of those famous meteorology vocabulary words. Many pets have been named Helicity and perhaps even some people! Severe weather
forecasters are typically familiar with the concept of Helicity since it is an important component to producing tornadic thunderstorms. This
writing aims at descriptively explaining what Helicity is.
Helicity is a mathematical quantity based on three factors which are 1) speed shear, 2) directional shear and 3) inflow. Two of these
components (speed and directional shear) are directly related in producing horizontal vorticity which was explained in mini-lecture 46. Another
factor in producing Helicity is inflow. Inflow (storm relative inflow) is the wind flow that is available to be ingested by a storm. Inflow
as it relates to Helicity is the low level wind field that is orientated directly into the tube of horizontal vorticity. The higher the
low level wind then the higher the inflow. In thinking of a storm as an engine, inflow is the oxygen that supplies mass, momentum and
moisture to a storm engine. A storm updraft can quickly transport inflow directly into the updraft.
The diagram below shows an example of these three factors combining to produce Helicity. Like CAPE, Helicity has energy per mass units. Helicity
can be thought of as inflow flowing directly though horizontal vorticity as shown in the diagram below. A twisting corkscrew tube
of air can be imagined from this wind flow. When this tube is ingested by a developing thunderstorm, then a tornadic circulation
could develop. Many other factors and the right balance of the factors are needed in order to produce a tornado such as the height
of the LFC, orientation of flanking downdrafts from a storm and the instability present. Helicity is certainly an index value that
is studied when forecasting the threat for tornadoes. A strong inflow (typically greater than 20 knots) with strong speed and
directional wind shear all within the lower troposphere are ideal for produces strong values of Helicity.
Helicity index number and interpretation:
(150-300 J/kg) Possible supercell
(300-400 J/kg) Supercells favorable
(400+ J/kg) Tornadic possible