Tornadoes review
Two steps in development
- Attain rotation in horizontal - due to vertical wind shear, get rotating "tube" of air
- Updraft pushes up the tube so that rotation axis becomes oriented vertically
Tornadoes in US almost all rotate cyclonically
Not because of Coriolis force
Updraft splits storm and pushes up rotating horizontal tube so that there is rotation in the vertical direction
On right side of storm cyclonic rotation and on the left side anticyclonic rotation
Typically thunderstorm moving west-to-east, and split into right-mover (with cyclonic rotation) veering south and left-mover (with anticyclonic rotation) turning north
Usual vertical wind shear has southerly winds near surface and westerly winds at middle levels
Thus stronger inflow for right-mover causes it to intensify while very weak inflow causes left-mover to weaken
Therefore cyclonic rotation potentially able to become tornadic while anticyclonic rotation usually dies out
Tornado occurrence in US
May happen in all states, but most common depending on season in Central Plains, Southeast, and Midwest
Most frequent in spring since that is optimal time to get optimal jet position, cold & dry air aloft, and warm & moist air near surface (see EOM Fig. 10.31)
Most frequent in late afternoon when surface heating is at maximum making air column most unstable and thus favorable for thunderstorm development
Tornadoes and weather radar
Tornadic storms have recognizable characteristics that may be seen before a tornado is on the ground allowing opportunity for warnings
On ordinary radar that shows reflectivity (intensity of precipitation) a "hook echo" may be evident where rotation causes a rain band to wrap around a region where dry
Doppler radar is able to measure speed of precipitation moving either directly toward or away from radar
Thus Doppler radar can demonstrate rotation in a storm by showing winds of opposite directions in close proximity