The interaction with this dry air promotes evaporation and associated cooling, which, in turn, promotes negative buoyancy and downward accelerations. The RFD develops when dry winds in the middle and upper troposphere (typically southwesterly or westerly) encounter the back side of the updraft, where that precipitation is wrapping around the mesocyclone. How can supercells have two distinct areas of precipitation? Well, some falling precipitation actually gets caught in the mesocyclone's circulation and wraps around to form the hook echo on radar. In addition to the FFD, there's another downdraft associated with precipitation called the rear-flank downdraft (RFD). Because the precipitation particles have been swept away from the main updraft thanks to strong vertical wind shear, the FFD does not interfere with the updraft, and since updraft and the FFD are separated, the stage is set for supercells to be long-lived.īut, many supercells actually have two distinct areas of precipitation, and therefore, two distinct downdrafts, as demonstrated by this radar cross-section of a supercell (from a supercell near Rapid City, South Dakota on July 13, 2009). The FFD is also where most of the heavy precipitation is located within a supercell. Formally, the FFD is the main region of downdraft in the forward (leading) part of a supercell. This cascade of precipitation particles promotes a downdraft (falling precipitation particles drag air down with them) called the forward-flank downdraft (FFD). Precipitation particles swept upward in the rotating updraft are rapidly carried downstream away from the updraft by strong upper-level winds (air movement within the storm is traced out by tan arrows on the cross-section of a supercell below).Īs a result, precipitation particles fall earthward well northeast (or east-northeast) of the updraft typically (note that the highest reflectivity is displaced from the updraft and mesocyclone). But, supercells have unique characteristics when it comes to their updrafts and downdrafts compared to other types of thunderstorms, thanks in large part to strong vertical wind shear. A classic supercell displays a hook echo on images of radar reflectivity, which occurs as precipitation wraps around the mesocyclone (the rotating updraft), and if a tornado forms, it does so within the hook echo. To start, we're going to build off the basic model of a supercell that you learned previously. So, supercells are a triple threat when it comes to severe weather. After all, supercells produce most tornadoes (and nearly all strong tornadoes), are responsible for nearly all reports of hail at least two inches in diameter, and nearly all supercells produce damaging straight-line winds. We're going to continue with a closer look at supercells since they're such prolific severe weather producers.
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