Question

Difference between linear and nonlinear effects in supercells. Explain if/how these effects lead to dynamical forcings and encourage lateral updraft propagation. Explain how hodograph shape and associated SRH (or lack thereof) is a factor in differentiating the effects

Answer 1

Linear effects:

The linear forcing produces pressure maxima at the upshear sides of the updraft. Since the shear vector does not change direction with height, the high-pressure region is vertically stacked at the upshear side. The vertical motion in a thunderstorm is maximized somewherehat mid-levels.This implies that the horizontal gradient of the vertical motion, ?hw is also maximized at mid-levels. Thus, there is a perturbation low at midlevels at the downshear side and a perturbation high at midlevels at the upshear side of the updraft. This configuration prevents the cell from being sheared apart. This effect does not contribute to the deviant motion

Non linear effects:

Tilting of vorticity does merely explain the formation of vertical shear vorticity at the flanks of the updraft in case the inflow is purely crosswise. Since spin and splat forcing can be shown to cancel one another in this case, no pressure perturbation would result.As soon as curvature vorticity has been generated, however, spin forcing dominates and a pressure drop occurs in the centers of the counter-rotating vortices. Since the vortices are strongest at mid-levels initially, the perturbation lows are also most intense at midlevels. As a result, the storm propagates normal to the mean shear towards the direction of the vortex centers. The vortices, however, remain at the flanks of the cells and thus continuously “drag” the cell towards them (like a dog which is trying to catch its tail).As the cells continue to propagate off the hodograph, the storm-relative winds begin to veer (right-moving, cyclonically-rotating member) or back (left-moving, anticyclonically-rotating member) with height. This means that the inflow gains streamwise vorticity, and the vertical vorticity centers move closer to the updraft centers, so that the effect of non-linear spin-forced propagation is ultimately canceled (what happens, in terms of the above analogy, when the dog eventually manages to catch its tail).

Factor in differentiating the effects:

Hodographs help a lot. They use a polartype plot with the tails of all vectors imagined at the center point, and a line is drawn then to connect the heads of the vectors. This connecting line is usually the only thing plotted, along with elevations or pressures of some of the points

SRH • SRH = VSR (V x V)

• SRH also happens to be twice the area under the hodograph curve, between it and the storm motion point.

• SRH is computed in a layer, usually 0-3 km, but increasingly often 0-1 km

Doing the math, SRH becomes… SRH = ??(V – c) (V x V) which is approximately

SRH = ? [VSR (?u/?z) – USR (?v/?z)] ?z

Where SR represents storm relative flow (c is the storm motion)

Traditionally, people use 0-3 km SRH where SRH > 150 m2s2 is usually needed for tornadoes.More recently, it has been found that 0-3 km values work better to indicate supercell spin, but many of these storms do not produce tornadoes.

• 0-1 km SRH > 100 m2s2 may work better to predict tornadoes

Problems with SRH

• Note that in the formula, one has to assume a storm motion. This is easy once a storm has formed, but how do you do it as a forecast?

• Also, some problems with storms changing directions during their lifetimes, or storms on a given day moving in different directions

• SRH is very sensitive to assumed storm motion

Storm motion

• Non supercell storms may move with the mean wind in the 0-6km layer (if data is limited, this might be similar to 700mb wind)

• Supercells slow down and deviate toward the right due to the rotation and interaction with the shear • Bunkers Technique is a fancy way to determine predicted storm motion, but it is a bit complicated

• Simpler older rule said supercells move at 75% of the speed and 30 degrees to the right of the mean wind.