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 frontFrontolysis: the decay of a front

dryline

Frontogenesis

Kinematics and thermodynamics of Frontogenesis:

2D frontogenesis (F):

θDtDF p Frontogenesis function

DtDQ

Cpp

DtθD

p

CRo p 1

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,

pθ

yω

yθ

yv

DtθD

y

pθ

yω

yθ

yv

xθ

yuθVyt

θy

yθVy

θt

yθ

DtD

surface p a on ,θDtDF

ppp

pppppp

pp

p

p

y

x

pyyy

vDtDQ

ypp

CFppp

CRo

p

p

∂∂

∂∂

+∂∂

∂∂

+∂∂1

=θωθ

0

Inhomogeneous diabatic heating Confluence/diffluence Tilting effect

0=∂∂x

[ ][ ][ ][ ] [ ][ ]

Frontogenesis(1) Confluence/diffluence

y

x

Frontogenesis, F> 0

y

x

y

x

y

x

Frontolysis. F < 0

pp yθ

yvF

Frontogenesis(1) Confluence/diffluence

Frontogenesis(2) Tilting effect

z

y

Frontogenesis, F> 0

z

y

Frontolysis. F < 0

z

y

z

y

pθ

yωF

p

θDtDF p

NE

Frontogenesis

y

x

(3) Quasi-horizontal variation due to diabatic heating

Frontogenesis, F> 0

Frontolysis. F < 0

Cold, cloudy

Warm solar heating

Day Night

y

x

Cold, cloudy, less cooling

Warm side, longwave radiative cooling, stronger cooling

Dt

DQyp

pCF

pCRo

p

1

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. Multicell3. 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.

ThunderstormsLife time: short-lived cell: ~ 30 min

multicell: ~ 10-15 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 ( ) iR

2v+u

CAPE=ShearCAPE=R 2

T2

Ti

wdzθθθgCAPE EL

LFCp 2

21

Tm500=zm6000=zT v for same ,u - u=u

(an overestimated w)

- Rsupercell ,R10

cell ordinary or multicell ,R

i

i

i

40

45

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

Supercell

Supercell

Shear and Storm Splitting

Uni-directionalshear

Multi-directionalshear

Shear and Storm Moving

Uni-directional shear

Multi-directional shear

Supercell

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

Supercell

Anticyclonic circulationCyclonic circulation

Survival

Uni-directionalshear

Multi-directionalshear

Storms and Floods

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