專業(yè)英語南京信息工程大學段明鏗Unit

II:Frontogenesis

and

frontal

characteristics第二課 鋒生和鋒的特征New

Wordsfront

鋒，鋒面

frontolysis

鋒消frontogenesis(frontogeneses)鋒生frontal鋒面的mid-latitude中緯度high-latitude高緯度

daily逐日的specific

humidity比濕low-latitude低緯度day-to-day逐日的mass氣團，質量humidity濕度discontinuity不連續(xù)性meteorologist氣象學家integral整數的，積分的，完整的 integration積分·

interface交界面，接口interaction

internet

interannual(年際的）中期預報medium(media)介質

medium

range

forecastsnumerical數值的numerical

weather

prediction(NWP)數值天氣預報model模式baroclinic

斜壓的

barotropic

正壓的 barotropic

primitive

equation

model

正壓原始方程模式anticyclone反氣旋氣旋生成cyclone氣旋cyclogenesisdepression低壓low

低壓isothermal等溫的isobaric等壓的iso+..等。。synoptic天氣的Principle

of

Synoptic

Meteorology天氣學原理divergence輻散

peak峰regionconvergence輻合apex頂點，峰尖zone區(qū)域cirrus(cirri)卷云ana-front上滑鋒slope坡度multi-layered多層的kata-front下滑鋒

order量級multi-cell多單體的·

stratus(strati)層云cirrostratus

卷層云nimbostratus

雨層云altostratus

高層云

stratocumulus層積云rainfall降水量ascend上升

high-level高層mid-tropospheric對流drizzle毛毛雨precipitation降水saturate使飽和descend下降medium-level中層

low-level低層層中部的coalescence

（雨滴的）合并airflow氣流meso-scale中尺度的large-scale大尺度的broad-scale大范圍的momentum(momenta)動量convective對流性的

advective平流性的potential位勢，潛在的convection對流

advection平流potential

heat潛熱geopotential

meter位勢米rainbelttopographicupwindkata-typerainband雨帶orographic地形的down-wind在下風方向ana-type上滑型cumulonimbus積雨云downpour傾盆大雨thunder雷duration持續(xù)時間occlusion錮囚occlude錮囚trowal(trough

of

warm

air

aloft)高空暖舌aloft高的，上面的stratiform

層狀的

stratification

層結poleward向極地的phase相位，階段，方面squall颮jet

急流

low

level

jet

低空急流evaporation蒸發(fā)n.geostrophic地轉的green

house溫室效應albedo反照率evaporate蒸發(fā)v.oceanic海洋的lee背風面trough槽ridge脊氣團：氣象要素（主要指溫度和濕度）水平分布比較均勻的大范圍空氣團。氣團的形成：下墊面加熱氣團的變性：移動氣團的地理分類：極地、赤道；大陸性、海洋性氣團的熱力分類：相對性FrontFrontogensis鋒生The

first

real

advance

in

our

detailed

understandingof

mid-latitude

weather

variations

was

made

withthe

discover

that

many

of

the

day-to-day

changesare

associated

with

the

formation

and

movement

ofboundaries,

or

fronts,

between

different

air

masses.Observations

of

the

temperature,

wind

directions,humidity

and

other

physical

phenomena

duringunsettled

periods

showed

that

discontinuities

oftenpersist

between

impinging

air

masses

of

differingcharacteristics.·

The

term

“front”,

for

these

surfaces

of

airmassconflict,

was

a

logical

one

proposed

during

theFirst

World

War

by

a

group

of

meteorologistsworking

in

Norway,

and

their

ideas

are

still

anintegral

part

of

most

weather

analysis

andforecasting

particularly

in

middle

and

high

latitude在對中緯度天氣變化詳細深入理解的過程中，第一次真正意義上的進步就是發(fā)現很多逐日天氣變化是與不同氣團邊界或者說鋒的形成和移動有關的。對不穩(wěn)定期內溫度、風向、濕度和其它物理現象的觀測表明，緊密接觸的不同特征氣團之間通常表現為持續(xù)的不連續(xù)性。氣團之間的交界面“鋒”，這個在第一次世界大戰(zhàn)時由一群挪威氣象學家提出的。這一思想仍然是目前大多數天氣分析和預報（特別是在中緯度地區(qū)）不可或缺的一部分。Frontal

wavesIt

was

observed

that

the

typical

geometry

of

the

airmass

interface,

or

front,

resembles

a

wave

form.Similar

wave

patterns

are,

in

fact,

found

to

occuron

the

interface

between

many

different

media,

forexample,

waves

on

sea

surface,

ripples

on

beachsand,

aeolian

sand

dunes,

etc.Unlike

these

wave

forms,

however,

the

frontal

wavesin

the

atmosphere

are

commonly

unstable:

that

is,they

suddenly

originate,

in

crease

in

size,

andthen

gradually

dissipate.Numerical

model

calculations

show

that,

in

middlelatitudes

waves

in

a

baroclinic

atmosphere

areunstable

if

their

wavelength

exceeds

a

fewthousand

kilometers.Frontal

wave

cyclones

are

typically

1500-3000

kmin

wavelength.鋒面波觀測表明，氣團交界面（鋒）的典型幾何形狀如同波動一樣。事實上，類似的波動形態(tài)還可以在很多不同介質間的交界面上找到，例如海面上的波動、沙灘上的波痕以及風吹形成的沙丘。但是，不同于這些波動形式，大氣中的鋒面波通常都是不穩(wěn)定的，也就是說，鋒面波會突然出現，尺度增加，然后又慢慢消失。數值模式的計算表明，若干波長超過幾千千米，在中緯度地區(qū)的斜壓大氣中，波動是不穩(wěn)定的。鋒面氣旋的波長一般在1500-3000千米。Similar

wave

patterns

are,

in

fact,

found

to

occur

on

thinterface

between

many

different

media,

for

example,

w

on

sea

surface,

ripples

on

beach

sand,

aeolian

sand

dunetc.It

was

observed

that

the

typical

geometry

of

the

air-interface,

or

front,

resembles

a

wave

form.1500-3000

km

in

wavelength.·

Unlike

these

waveforms,

however,

thefrontal

waves

intheatmosphere

arecommonly

unstable:that

is,

theysuddenly

originate,increase

in

size,and

then

graduallydissipate.The

initially

attractive

analogy

betweenatmospheric

wave

systems

and

waves

formed

oninterface

of

other

media

is,

therefore,

aninsufficient-basis

on

which

to

develop

explanationsof

frontal

waves.In

particular,

the

circulation

of

the

uppertroposphere

plays

a

key

role

in

providingappropriate

conditions

for

their

development

andgrowth,

as

will

be

shown

below.A

depression

(also

termed

a

low

or

cyclone)

is

anarea

of

relatively

low

pressure,

with

a

more

or

lesscircular

isobaric

pattern.It

covers

an

area

100-3000

km

in

diameter

andusually

has

a

life-span

of

4-7

days.Systems

with

these

characteristics,

which

areprominent

on

daily

weather

maps,

are

referred

toas

synoptic

scale

features.The

depression,

in

mid-latitudes

at

least,

is

usuallassociated

with

a

convergence

of

contrasting

airmasses.The

frontal

wave

depressionThe

interface

between

these

air

masses

developsinto

a

wave

form

with

its

apex

located

at

the

centreof

the

low-pressure

area.The

wave

encloses

a

mass

of

warm

air

betweenmodified

cold

air

in

front

and

fresh

cold

air

in

therear.The

formation

of

the

wave

also

creates

a

distinctionbetween

the

two

sections

of

the

original

airmassdiscontinuity

for,

although

each

section

still

marksthe

boundary

between

cold

and

warm

air,

theweather

characteristics

found

in

the

neighborhoodof

each

section

are

very

different.·

The

two

sections

of

the

frontal

surface

aredistinguished

by

the

names

warm

front

for

theleading

edge

of

the

wave

and

cold

front

for

that

ofthe

cold

air

to

the

rear.

The

depression

usually

achieves

itsmaximum

intensity

12-24

hours

after

thebeginning

of

occlusion.低壓通常在開始錮囚后12-24小時的時候，強度達到最大。modified

cold

airfreshcold

airwarm

airFrontal

characteristicsThe

activity

of

a

front

in

terms

of

weather

dependsupon

the

vertical

motion

in

the

air

masses.If

the

air

in

the

warm

sector

is

rising

relative

to

thfrontal

zone,

the

fronts

are

usually

very

active

andare

termed

ana-fronts.Whereas

sinking

of

the

warm

air

relative

to

thecold

air

masses

gives

rise

to

less

inactive

kata-fronts.以垂直運動分Ana

frontKata

front（1）至（5）為上滑鋒。其中（3）為標準情形。雲系在鋒後。（6）至（10）為下滑鋒。其中（10）為標準情形。雲系在鋒前。The

warm

frontThe

warm

front

represents

the

leading

edge

of

thewarm

sector

in

the

wave.The

frontal

zone

here

has

a

very

gentle

slope,

ofthe

order

1/2°-1°,

so

that

the

cloud

systemsassociated

with

the

upper

portion

of

the

frontherald

its

approach

some

12

hours

or

more

beforethe

arrival

of

the

surface

front.The

ana-warm

front,

with

rising

warm

air,

hasmulti-layered

cloud

which

steadily

thickens

andlowers

towards

the

surface

position

of

the

front.The

first

clouds

are

thin,

wispy

cirrus,

followed

bysheets

of

cirrus

and

cirrostratus

and

altostratus.The

sun

is

obscured

as

the

altostratus

layerthickens,

and

drizzle

or

rain

begins

to

fall.The

cloud

often

extends

through

most

of

thetroposphere

and

with

continuous

precipitationoccurring

is

generally

designated

as

nimbostratus.Patches

of

stratus

may

also

form

in

the

cold

air

asrain

falling

through

this

air

undergoes

evaporationand

quickly

saturates

it.The

descending

warm

air

of

the

kata-warm

frontgreatly

restricts

the

development

of

medium-andhigh-level

clouds.

The

frontal

cloud

is

mainlystratocumulus,

with

a

limited

depth

as

a

result

ofthe

subsidence

inversions

in

both

air

masses.Precipitation

is

usually

light

rain

or

drizzle

formeby

coalescence

since

the

freezing

level

tends

tobe

above

the

inversion

layer,

particularly

insummer.In

the

passage

of

the

warm

front

the

wind

veers,the

temperature

rises

and

the

fall

of

pressure

ischecked.The

rain

becomes

intermittent

or

ceases

in

thewarm

air

and

the

thin

stratocumulus

cloud

sheetmay

break

up.Forecasting

the

extent

of

rain

belts

associated

withthe

warm

front

is

complicated

by

the

fact

that

mostfronts

are

not

ana-or

kata-fronts

throughouttheirlength

or

even

at

all

levels

in

the

troposphere.For

this

reason,

radar

is

being

used

increasingly

todetermine

by

direct

means

the

precise

extent

ofrain

belts

and

to

detect

differences

in

rainfallintensity.Such

studies

have

shown

that

most

of

theproduction

and

distribution

of

precipitation

iscontrolled

by

abroad

airflow

a

few

hundredkilometers

across

and

several

kilometers

deep,which

flows

parallel

to

and

ahead

of

the

surfacecold

front.這些研究表明，大多數降水的產生和分布是受外圍氣流的控制，這些氣流的水平尺度達幾百公里、垂直尺度為幾公里，且這些氣流與冷風鋒面平行，位于其前方。Just

ahead

of

the

cold

front

the

floe

occurs

as

alow-level

jet

with

winds

up

to

25-30m/s

at

about

1km

above

the

surface.The

air,

which

is

warm

and

moist,

rises

over

thewarm

front

and

turns

southeastward

ahead

of

it

asit

merges

with

the

midtropospheric

flow.This

flow

has

been

termed

a

“conveyor

belt”

(forlarge-scale

heat

and

momentum

transfer

in

mid-latitudes).Broad-scale

convective(potential)

instability

isgenerated

by

the

over-running

of

this

low-level

flowby

potential

colder,

drier

air

in

the

middletroposphere.就是在冷鋒之前，在離地表約1km之上隨著風速增大到25-35m/s，這股氣流形成了低空急流。暖濕空氣在暖鋒上上升，在暖鋒前轉向東南方，同時與對流層中部的氣流合并。這股氣流被稱之為“傳送帶”（因為在中緯度傳輸大尺度的熱量和動量）。大范圍的對流（位勢）不穩(wěn)定是由與對流層中部潛在的干冷空氣在低層氣流上爬越作用而造成的。Instability

is

released

mainly

in

small-scaleconvection

cells

that

are

organized

into

clusters,known

as

meso-scale

precipitation

areas(MPAs).These

MPAs

are

further

arranged

in

bands,

50-100km

wide.

Ahead

of

the

warm

front,

the

bands

arebroadly

parallel

to

the

airflow

in

the

rising

sectionthe

conveyor

belt,

whereas

in

the

warn

sector

theyparallel

the

cold

front

and

the

low-level

jet.In

some

cases,

cells

and

clusters

are

furtherarranged

in

bands

within

the

warm

sector

andahead

of

the

warm

front.不穩(wěn)定主要在形成的對流群---即中尺度降水區(qū)的小尺度對流單體中釋放。這些中尺度降水區(qū)進一步排列進帶狀，大約50---100km寬。在暖鋒前，這條帶狀區(qū)域大致與傳送帶上升區(qū)中的氣流平行，但是暖區(qū)則平行于冷鋒和低空急流。在某些情況下，對流單體和對流群在暖區(qū)內和暖鋒前進一步排列成帶狀。Precipitation

from

warm

front

rainbands

ofteninvolves

“seeding”

by

ice

particles

falling

fromthe

upper

clouds

layers.It

has

been

estimated

that

20%-35%

of

theprecipitation

originates

in

the

“seeder”

zone

andthe

remainder

in

the

lower

clouds.Some

of

the

cells

and

clusters

are

undoubtedly

setup

through

orographic

effects

and

these

influencesmay

extend

well

down-wind

when

the

atmosphereis

unstable.The

cold

frontThe

weather

conditions

observed

at

cold

fronts

areequally

variable,

depending

upon

the

stability

ofthe

warm

sector

air

and

the

vertical

motion

relativeto

the

frontal

zone.The

classical

cold-front

model

is

of

the

ana-type,and

the

cloud

is

usually

cumulonimbus.Over

the

British

Isles

air

in

the

warm

sector

israrely

unstable,

so

that

nimbostratus

occurs

morefrequently

at

the

cold

front.With

the

kata-cold

front

the

cloud

is

generallystratocumulus

and

precipitation

is

light.

With

ana-cold

fronts

there

are

usually

brief,

heavydownpours

sometimes

accompanied

by

thunder.The

steep

slope

of

the

cold

front,

roughly

2°,means

that

the

bad

weather

is

of

shorter

durationthan

at

the

warm

front.

With

the

passage

of

thecold

front,

the

wind

veer

sharply,

pressure

beginsto

rise

and

temperature

falls.The

sky

may

clear

very

abruptly,

even

before

thepassage

of

the

surface

cold

front

in

some

cases,although

with

kata-cold

fronts

the

changes

arealtogether

more

gradual.Occlusions

are

classified

as

either

cold

or

warm,the

difference

depending

on

the

relative

states

ofthe

cold

air

masses

lying

in

front

and

to

the

rear

ofthe

warm

sector.If

the

air

is

colder

than

the

air

following

it

then

thocclusion

is

warm,

but

if

the

reverse

is

so,

it

istermed

a

cold

occlusion.The

air

in

advance

of

the

depression

is

most

likelyto

be

coldest

when

depression

occlude

overEurope

in

winter

and

very

cold

of

air

is

affecting

thecontinent.The

occlusiona

warm

occlusiona

cold

occlusiona

neutral

occlusionThe

line

of

the

warm

air

wedge

aloft

is

associatedwith

a

zone

of

layered

cloud

and

often

ofprecipitation.Hence

its

position

is

indicated

separately

on

someweather

maps

and

it

is

referred

to

by

Canadianmeteorologists

as