Instability Baroclinic instability needs vertical shear i e

Instability Baroclinic instability (needs vertical shear, i. e. , horizontal T gradient) Kinetic Energy through the lifting of relatively warm, light air and the sinking of relatively cold, dense air Barotropic instability (needs horizontal shear) What kind of instability is required for mid-latitude cyclone to develop? Baroclinic instability!

Cyclone (mid-latitude cyclone or extratropical cyclone): A region of low pressure, which has a scale of ~ 1000 km and is surrounded by strong wind rotating in the counterclockwise direction in NH and clockwise in SH. 1006 cold air mass 1002 998 direction of propagation accompanying with cold front and warm front (and maybe with occluded front as well) cold front warm air mass warm front Low pressure center at surface is around 1000 mb Life cycle is about several days and moves following steering flow.

Cyclogenesis – A fall in pressure of greater than a certain magnitude One potential formation of a cyclogenesis - Lee cyclogenesis Another potential formation of a cyclogenesis C W e. g. , continental polar air mass e. g. , maritime warm tropical air mass C C L W Stationary front W Cyclogenesis

Cyclone evolution Norwegian model (developed during and shortly after World War I) By T. Bergeron and H. Solberg, based on surface observations

Cyclone and Comma-shaped Cloud Comma cloud (mature stage)

Mature Cyclone By Richard Grotjahn

Cyclogenesis preferred locations of cyclone formation in US (i) east of the Rockies (Lee cyclogenesis) (ii) off the east coast of N America (iii) Gulf of Mexico (iv) mid-west initiated in continental polar mass from Canada

Symmetrical thermal structure low

Asymmetrical thermal structure Cyclones slant back into colder air masses and strengthen with height, sometimes exceeding 9 km in depth

Cyclone Development Favorable conditions for cyclone to develop: 1. Low level warm temperature advection (WTA) 2. Low level moisture advection (often come with WTA) 3. Positive vorticity advection at 500 mb or upper level diffluent flow (so downstream of a diffluent trough is really good for surface cyclone to develop!) Note: The wave should not be too long (beta effect is too strong) or too short. The static stability should not be too large (suppress w) 4. The right side of the entrance or the left side of the exit of the 200/300 mb Jet streak

Cyclone Development

Cyclone Development • Most cyclones deepen underneath downstream of a diffluent trough • The rest deepen underneath downstream of a regular trough • Very few deepen underneath downstream of a confluent trough - This may have other mechanisms at different level to help. Enhance divergence 500 mb surface Divergence + diffluence

Cyclone Development 1002. 7 mb X X Surface map Divergence + diffluence 300 mb map

Cyclone Development 1000 mb 48 h forecast 999 mb 24 h forecast

Latent Heat & Sensible Heat Sensible heat : temperature gradient in vertical Latent heat : moisture gradient in vertical Height tendency

Bomb – Explosive cyclogenesis (slightly smaller in size than regular cyclones) Average pressure deepening rate : 1 mb/hr for at least 24 h! Most bombs occur over the ocean during the cold season, downstream from diffluent upper level troughs & near the strongest sea surface temperature gradient. 500 mb surface Divergence + diffluence

Bombs

Ice Storms By Richard Grotjahn

Cold-Air Outbreaks By Richard Grotjahn

Cold-Air Outbreaks By Richard Grotjahn