Frontogenesis Frontogenesis: The generation of intensity of a front Warm air merged onto colder air Temperature gradient amplified at least one order of magnitude A good example of non-frontal zone boundary is. Mesoscale fronts: land-sea breeze, storm outflow (a few hours) Synoptic scale fronts: fronts on the weather maps (many days) Frontogenesis: the formation of a front Frontolysis: the decay of a front dryline

Frontogenesis Kinematics and thermodynamics of Frontogenesis: 2D frontogenesis (F): Frontogenesis function First law of thermodynamics Diabatic heating (e.g., latent heat, radiation)

Frontogenesis Assume that winds do no vary along the front and x axis //  lines, y x    0 Inhomogeneous diabatic heating Confluence/diffluence Tilting effect [ ]

Frontogenesis (1) Confluence/diffluence y x    Frontogenesis, F> 0 y x    y x    y x    Frontolysis. F < 0

Frontogenesis (1) Confluence/diffluence

Frontogenesis (2) Tilting effect z y    Frontogenesis, F> 0 z y    Frontolysis. F < 0 z y    z y    N E

Frontogenesis y x    (3) Quasi-horizontal variation due to diabatic heating Frontogenesis, F> 0Frontolysis. F < 0 Cold, cloudy Warm solar heating Day Night y x    Cold, cloudy, less cooling Warm side, longwave radiative cooling, stronger cooling

Front Passing

Thunderstorm Frequency Thunderstorm frequency map for the United States

Thunderstorms The upper part usually composes ice and is spread out as anvils. Types: 1.Short-lived cell 2.Multicell 3.Suepercell or split cell (can have hails and tornados) Short-lived cell : when shear is weak, shear < 10 ms -1 below 6 km, Multicell : moderate shear, 10 ~ 20 ms -1, Supercell : strong shear, shear > 20 ms -1. Storms propagation speed = mean wind speed + propagation due to new formation of cell.

Thunderstorms Life time: short-lived cell: ~ 30 min multicell: ~ min for each cell supercell:~ nearly steady state (several hours) Storm dissipates because of: water loading, cut of energy supply, dry air entrainment, mixing, etc.

Thunderstorms Storm types are strongly related to the Bulk Richardson number Parameters: Bulk Richardson number ( ) (an overestimated w) Reference for what type of storms but not their severity.

Why CAPE? Need energy to develop a storm (no help from large scales, like upper level trough to winter storms) Thunderstorms Why shear? 1.The ability of a gust front to trigger a new cell (for multicell) 2.The ability of an updraft to interact with environment wind shear to produce an enhanced quasi-steady storm structure. (supercell)

Shear and Storm Types

Supercell Isolated convetive storms (life time - several hours) Usually requires large CAPE and strong wind shear Low level moist, upper level dry ( - strong downdraft) Shear too strong is not good either (destroy the storm structure) Can potentially produce tornados

Supercell

Shear and Storm Splitting Uni-directional shear Multi-directional shear

Shear and Storm Moving Uni-directional shear Multi-directional shear

Supercell

Note: The wind vectors in the middle latitude of the northern hemisphere usually turn clockwise with height (Coriolis force effect). So, usually the split right-moving storm survives.

Supercell

Anticyclonic circulation Cyclonic circulation Survival Uni-directional shear Multi-directional shear

Storms and Floods For multicell and supercell, if the system is quasi-stationary or slowly moving,  Produce heavy rainfall  Flashflood can occur