“Dynamical Effects of Convection” Kathryn Saussy Meteorology 515: Analysis & Pred. of Severe Storms March Bluestein, Howard: Synoptic-Dynamic Meteorology in Midlatitudes, Vol. II. Oxford Press, 594 pp. Johnson, Richard H., Mapes, Brian E, 2001: Mesoscale Processes and Severe Convective Weather. Meteor. Mon. 28 (50), Amer. Meteor. Soc., Boston. Monteverdi, John. Advanced Weather Analysis Lectures, Spring 2005.

Dynamical Effects of Convection I.Development & dynamical consequences of rotation II. Cold pool-shear interactions

I. Development & dynamical consequences of rotation Pressure perturbation forces develop. 2 perturbation dynamic pressures are associated with the wind field: The linear, p´ L, and the nonlinear part, p´ NL. ( p´  perturbation upward-directed pressure gradient force that adds to the synoptic-scale upwards directed force…or subtracts from the perturbation pressure change that would occur at that level. )

Nonlinear pressure perturbation, p´ NL p´ NL    2 This says that pressure falls are proportional & opposite in sign to the square of the vertical vorticity.

Nonlinear pressure perturbation, p´ NL (con’d) Unidirectional shear  Initial stage shows pressure falls (on either flank) that augment the updraft.

Nonlinear pressure perturbation, p´ NL (con’d) Unidirectional shear  Splitting stage shows downdraft forming; equal preference for R and L-moving storms.

Nonlinear pressure perturbation, p´ NL (con’d) On radar, it appears that the storm splits into mirror images. Note the unidirectional shear on the hodograph.

Linear pressure perturbation, p´ L p´ L   v/  z   2 w´ Unidirectional shear  If the wind shear vector (  v/  z) lies transverse a buoyant updraft, then pressures rise on the upshear side and fall on the downshear side. This results in no preferential growth to either of the flanks lying across the shear.

Linear pressure perturbation, p´ L (con’d) Clockwise shear vector  Leads to pressure falls on the right flank of the storm…and pressure rises on the left. New growth is favored on the right.

Linear pressure perturbation, p´ L (con’d) On radar, it appears that new storm growth favors the right flank. Note the clockwise shear indicated on the hodograph.

II. Cold pool-shear interactions Vertical shear  enhances ability of outflow (or cold pool) to trigger new storms Increasing shear  interaction between shear and cold pool enhances lifting on preferred storm flank.

Evolution of a Convective System: Stage 1 The initial updraft leans downshear in response to the surrounding vertical shear. (C is the strength of the cold pool; D u is the strength of surrounding vertical shear. Circular arrows show the most significant horizontal vorticity.)

Evolution of a Convective System: Stage 2 The circulation generated by the storm-induced cold pool balances the surrounding shear, and the system becomes upright.

Evolution of a Convective System: Stage 3 The cold pool dominates the surrounding shear and the system tilts upshear, producing a rear-inflow jet. (The rear-inflow jet is indicated by the thick, black arrow.)

Thank you.