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 10-100 km2

• 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