Characteristics of Isolated Convective Storms Meteorology 515/815 Spring 2006 Christopher Meherin

Convective storms depend on the environment in which it grows Thermodynamic stability Vertical wind profiles Mesoscale forcing influences

How do forecasters identify conditions favoring convection? Balloon soundings Surface observations Satellites Radar Vertical profilers

Identifications allow forecasters Storm motion Longevity Potential severity

Storm dynamics is isolated to smaller scale features Individual thunderstorm cells Squall lines

Key components leading to convection are triggering mechanisms Diurnal heating Frontal lifting

What is a convective cell? A region of strong updrafts (10 m*s -1 ) Horizontal cross section of km 2 Extending in vertical through the most of the troposphere Updraft associated with precipitation easily seen on radar

Types of convective cells Short lived single cell storm The mulitcell storm The supercell storm

Single cell storms Contains a single updraft Updraft brings air through troposphere producing –Liquid water –Ice Rain/ice become too heavy for updraft to support –Falls through updraft creating downdraft –Evaporational cooling accelerates downdraft –Outflow spreads horizontally cuts of updraft

Single cell storms (continued) Storm lasts typically 30 to 50 minutes Associated severe weather –High winds –Hail –Tornadoes are rare

Multicell storms Cluster of short-lived single cells Outflow triggers new updrafts to develop Wind shear gives storms longer life Associated severe weather –Flash flooding –Hail –Short lived tornadoes are possible

Supercell storms Evolves, often, from multicells Damaging winds (excess of 57 mph) Severe hail (> than 0.75”) Rotating updrafts Long lived tornadoes

Supercells dynamically different from ordinary convection After 1 hour radar echo moves in direction of wind shear vector Strongest reflectivity gradient located on southwest flank of storm Strong updraft forms Strom veers to right of mean wind Mature stage reached within 90 min Hook echo appears on southwest flank

Dynamical differences (continued) BWER indicates strong rotating updraft Tornado forms on edge of hook echo New mesocyclone/updraft can form Not all supercells go through this evolution, but many do

Physical mechanisms controlling convective storm growth Thermodynamic instability –Buoyancy Vertical wind shear influences forms convection takes –Single cell convection –Mulitcell convection –Supercell convection

Thermodynamic structure influences vertical acceleration Ways to access vertical acceleration Analysis of skew-t diagrams –Positive/negative buoyancy –Evaluation of lifted index –Calculation of BUOYANT ENERGY

Equation for buoyancy and vertical acceleration B represents buoyant energy of a parcel Theta(z) is temperature of a moist parcel Theta(z)bar is the environments temperature G is the gravitational acceleration Wmax is the vertical acceleration

Moist vs. dry layers Boundary layer moisture needed to support updraft growth Warm layer above boundary layer accelerate downdraft –Downbursts or microbursts occur when updrafts are relatively week

Effects of wind shear Situation in which no wind shear exists –Outflow spreads horizontally –Potentially new cells cut off by cold pool Situation in which significant shear exists –Outflow does not cut off new cells –Outflow is down shear of new updraft

Two types of wind shear Unidirectional shear Curved shear

Unidirectional shear Wind shear vector is strait Wind shear vector increases –Pressure lowers on right/left flanks of original updraft Produces two new mesocyclones Cyclonical mesocyclone Anitcyclonic mesocyclone

Curved shear Wind shear vector curves clockwise Strong shear settings –Lowering of pressures cause the right moving storm to intensify –Left moving storm is suppressed