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Section 8.1 Thunderstorm development

NOAA Jetstream:

  • a mechanism is necessary for lifting; this can often take place along a cold front, especially if advancing into warm, moist (mT) air

  • thunderstorm development is tied closely to the stability of the atmosphere

    • stability index: take a parcel and lift it adiabatically (DALR) to saturation, then lift saturated parcel (SALR) to 500-mbar level

    • compare parcel temperature to environmental temperature

    • Lifted Index = T(env) - T(parcel)

    • Lifted Index values below zero are unstable (the parcel is warmer than the environment): that is, the parcel will be buoyant relative to its surroundings

Development of single-cell (local) thunderstorm (or “air-mass” thunderstorm)

NOAA Jetstream:

  • occurs by thermal convection, orographic lifting, or surface convergence

  • developing stage - mature stage - dissipating stage

development stage

  • lifting to LCL and cumulus-type cloud formation that expands vertically and horizontally

  • entrainment of surrounding dry air; evaporation cools the air within the cloud

  • moist air flowing upward begins producing an updraft

  • droplet formation by collision and coalescence

mature stage

  • precipitation begins to fall from the cloud

  • rain, lightning, possibly small hail

  • rain helps produce a cool downdraft

dissipating stage

  • main factor: updraft weakens and collapses (downdraft begins to dominate and cuts of upward supply of moisture)

  • moisture content decreases and supply of latent heat diminished

  • precipitation weakens and cell dissipates