wind lecture #5

What are Midlatitude Cyclones?

• commonly referred to as a "low pressure centres" or "frontal system"

• major weather makers in the midlatitudes (35° - 60°)

  • not at 30° because there is sinking air and high pressure centres

• most storms in Southern Ontario in the fall, winter, and spring are midlatitude cyclones

• occur approximately every 4-7 days

• 100s to 1000s km in extent

  • larger than hurricanes

• less intense winds

• can have thunderstorms and tornadoes associated with them

• fronts are the ingredients to midlatitude cyclones

Recall: Cyclones and Anti-Cyclones

• cyclones are associated with clouds, precipitation and winds

  • anti-clockwise, low pressure centres

• exhibit a characteristic as they move from west to east

• the first sign of a cyclone approaching from the west is the cirrus clouds that appear roughly 12 to 24 hours in advance of the warm front

• anticyclonic is characterized by clear skies and light winds

  • caused by sinking air that can cause very high temperature in summer

Recall: Climatology of the Midlatitudes

• polar front region: located between Polar and Ferrel cells

  • cold front

• polar jet stream has a link to the polar front

Recall: Air Masses

• midaltitudes are the battle ground between cP (continental polar) and mT (maritime tropical)

  • cP is stable

  • mT is unstable

Fronts

• occur at the division/collision centre between air masses

Stationary Front

• stable

  • due to lack of latent heat which is the fuel for storms

• low pressure trough

• horizontal wind shear

• alternating blue tringles and red semi-circles

Cold Front

• cold air is pushing into a warm air mass

• designated by blue line with triangles facing warm air

• frontal slopes 1:50

• 15-25 knots (7-13 m/s)

  • knot is equal to approx.. 1.25..km

• heavy precipitation along the front where mT air is forced up

  • prec due to warm mT air is rising

• criteria for identification:

  • strong temperature gradient

  • change in moisture or dew point

  • shift in wind direction

  • change in pressure

Warm front

• warm air pushing into a cold air mass

• red semi circle side shows lower temperatures

• designated by red line with semi-circles pointing toward cold air

• slope is much less steep 1:150- 1:300

• gentle precipitation

Occluded Front

• occurs when the cold front catches up w/ the warm front

• most intense part of the storm

• warm air forced above the surface

• warm front-style precipitation

• alternating blue triangles and red semi circles on one side

  • associated with the colour purple

Polar Front Theory

• polar front is a semi-continuous boundary separating cold polar air from more moderate mid-latitude air

• midlatitude cyclone (wave cyclone) forms and moves along polar front in wavelike manner

• low pressure or cyclone is the principle weather maker at midlatitudes

• development of a low pressure begins with a small disturbance along the polar front

Step One

• stationary front (stable) with a strong horizontal wind shear

  • wind shear is a horizontal gradient of wind direction and can be unstable

• cP air (continental polar air) on the top side of the stationary front coming from e.g. Northern Canada and meeting mT air from the e.g. Pacific

• as soon as they come close to the stationary front it creates a disturbance

Step Two

• low pressure centre develops

• cold anti-cyclonic fronts and warm cyclonic fronts develop from opposite sides

• cold air pushes and warm air rises

• collision between cP and mT

• the pivot point is the lowest local pressure and is the low pressure centre

• precipitation starts to begin

Step Three

• warm front and cold front are more intense

• pivot point develops

• circular isobar: indication that the centre pressure is dropping

• fully developed wave

  • moves east or northwest

  • takes 12 -24 hours to reach this stage of development

• centre pressure continues to drop

• large bands of precipitation have formed

• warm sector has formed in the region between fronts

  • fuel centre for storms

  • the warm air brings latent heat for fuel

Step Four

• pivot point is even narrower

• isobars are even more closely spaced

• centre pressure continues to drop

• precipitation band grows even longer

• the faster moving cold front catches up with the warm front

  • reduces the size of the warm sector

Step Five

• the storm is now intensifying

• occluded front develops

  • because the cold front catches up with the warm front

• pressure continues to drop

• most intense part of the storm

  • widespread precipitation

  • comma shaped clouds (signifier of intense precipitation)

Step Six

• storm dissipates after occlusion

• the source of the energy (mT air) has been cut-off

  • because no more latent heating

• storm generally weakens and dissipates

Great Lake Storms

Impacts

• major precipitation events - lake levels

• major cause of erosion, sediment transport

• ship and property damage

Where do the storms come from?

• Isard et al. 2000 studied this issue and named the storms

• most storms here have originated elsewhere

• about 20% of midlatitude cyclones have their origins at the Great Lakes

Perfect Storm

• October, 1991

• also called the Halloween Storm

• cold air from the US West met with warmer air mass from Atlantic

• formed Nor'easter or Hatteras Low

• Hurricane Grace: 25-29 October

• Hurricane #8: October 28 - November 2

  • midlatitude storm formed from remnants of Hurricane Grace

• combination of hurricane and midlatitude cyclones

• later transformed to Hurricane #8 - the "unnamed hurricane"

Effects

• hundreds of millions of dollars damage along the East Coast of the US

• loss of life

Storm of the Century, 1993

• began on March 12, 1993 as a frontal wave off the Texas Coast

• intensified into a deep open wave cyclone over Florida

• moves northeastward and becomes occluded over Virginia

• as it moves northward it crossed a few states in the USA and finally reached Cape Breton Island Nova Scotia

• did not transform into a hurricane

Damages

• caused a deep snow blanket from Alabama to Eastern Canada

• damaged hundreds of homes

•  produced 11 tornadoes in Florida

• caused damage of $3 billion

• claimed 250 lives

Reasons

• within a couple hours many low pressure centres were formed

• warm and cold fronts were developed

• pivot point was created

• centre pressures were dropping and the storm intensified

1998 Ice Storm

• midlatitude cyclone

• category of freezing rain

• mT air mass and cP air masses involved

• when they collided the maximum damage occurred in Quebec

• between January 3-10 an icy storm developed in Eastern Canada

• caused 80mm of rain

Effects

• worst natural disaster in Canada

• 28 lives lost

• 3 million without electricity

• military deployed

• $4.2 billion dollars in economic loss

1999 Blizzard of Toronto

• series of 4 storms passed to the south of Toronto in early January in 1999

• storm centre to the south of Toronto, allows for east to southeast winds

• winds sweep across Lake Ontario gathering moisture and energy

• leads to larger than normal snowfall

• midlatitude cyclone with enhancement from Lake Effect

• it was an unusual event that does not appear to be part of a trend of such events

• excessive snow because of creation of lows

Freezing Rain

• Toronto was among the hardest hit by freezing rain of December 2013 that extended from Northeastern United States to Southern Ontario to Quebec to Maritimes in Canada

• starts as midlatitude cyclone

  • most common type of storm that can develop into freezing rain

Mechanism

• mT air mass pushes poleward/North it overrides over the cP air and precipitation starts to begin

• in such situations precipitation forms aloft

• In the higher areas there is lower temperature and snowflakes are created

• when it comes to the warmer layer the snow melts

• but the ground is cold

• when the melted snow touches the cold ground layer it freezes

• when the ice accumulation is more than 6mm it called ice storm