The supercell storm Anthony R. Lupo Atms 4310 / 7310 Lab 12

The supercell storm   This is a cell contains an updraft, possibly a downdraft and rain curtain.   This is the most dangerous type of convective storm. It may produce high winds, heavy rain, large hail, and long lived powerful tornadoes.

The supercell storm   The most basic format consists of a large rotating updraft, which may have a lifetime of several hours or more while propagating continuously to the right (sometimes to the left) of the mean wind.

The supercell storm   They typically develop from multicellular convection, however they are dynamically different from ordinary convection. (Slantwise convection as opposed to vertically stacked.)   Develop in a very baroclinic (large amounts of directional speed shear in the vertical) environment.

The supercell storm   They develop features that are easily identifiable on radar (LEWP, BWER, hook). (Courtesy of Norman, NWS)

The supercell storm   May have two or more down drafts which do not cut off the updraft (this gives the supercell it’s long life!)   Form a mesocyclone which rotates rear flank downdraft into front flank (this is similar to the occlusion process for cyclones). Tornado forms at the tip of the “occlusion” (edge of the hook echo) just within the updraft.

The supercell storm   A new meso cyclone and updraft can form at the triple point, and some supercells go through this process several times (families of tornadoes).  Predicting supercell formation  Our synoptic scale checklist provides a good start, but recall they favor strongly sheared or baroclinic environments!

The supercell storm  Richardson Number  Ri between 10 and 30 units:  where Ri = B / (½ U 2 )  and B = bouyant energy

The supercell storm  Then, the Richardson Number augurs for high CAPE values and strong vertical shear.  Some rules of thumb (convection type):  where wind shear = V500 hPa – V sfc

The supercell storm  and * means severe weather likely  Case I:  CAPE 500 – 1000 J /kg  moderate shear > = 15 m/s  (ordinary and supercell convection)

The supercell storm  Case II:  a. CAPE  moderate shear 15 – 25 m/s  (ordinary and supercell*)  b. strong shear > 25 m/s  (super cell*)

The supercell storm  Case III:  a. CAPE > 2500 J/kg  moderate shear 15 – 25 m/s  (ordinary* and supercell*)  b. strong shear > 25 m/s  (supercell*)

The supercell storm  Wind Gusts (Prediction and Algorithms)   Wind gusts from thunderstorms can be forecast using the following empirical formula.  V1 = 13 sqrt(T1) = average wind gusts.

The supercell storm  Where T1 is the dry instability index as follows: if there is an inversion whose top is within 200 hPa of the surface and it’s a radiation inversion, follow the moist adiabat from the top of the inversion to the 600 hPa level. Then T1 = Tmoist 600 – T env. 600 (in degrees C)

The supercell storm  (The dryer the air is at about 600 hPa, more cooling, heavier parcel greater downward acceleration.)   Otherwise,  follow the moist adiabat from the maximum predicted surface temp to 600 hPa  and use the same formula as before! (Same principle)

The supercell storm  For peak gust potential forecast  Add to V1 the following:  1/3 of the mean wind speed in the PBL (850 hPa and lower).  Wind gust direction forecasts:  Wind gust direction in the mean direction of the 800 to 600 hPa winds.

The supercell storm  The End