Meteorology M.Sc. Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences

MAPH40410 Synoptic Meteorology

Meteorology M.Sc.

Peter LynchMeteorology & Climate Centre

School of Mathematical Sciences

University College Dublin

Synoptic MeteorologyMAPH 40410

Peter Lynch

Meteorology & Climate CentreSchool of Mathematical Sciences

University College Dublin

Forecasting Module Lecture 1

Refresher:Airmasses, Fronts &Frontal Depressions

PolarMaritime

ReturningPolar Maritime

TropicalMaritime

TropicalContinental

PolarContinental

Arctic Maritime

Air Masses

Norwegian Model

167 181

Mean sea level pressureLast 3 figures of the MSL Pressure

e.g... 181 = 1018.1hPa

Present weatherSymbols

DrizzleSnow

Showers

Thunderstorm

Phenomena occurred in past hour.

Visibility01 to 50 add 00 for visibility in metres56 to 89 subtract 50 for visibility in km

Cloud amount and height ( Low, Medium and High)Amount of cloud in eighths /height of cloud in feet

01 to 50 add 00 for height in feet56 to 89 subtract 50 for height in thousands of feet

Past weatherSymbols as Present weather

Period covered0000,0600,1200,1800 last 6hrs0300,0900,1500,2100 last 3hrs

All other hours last hour

Type of low cloud

Dew point in tenths of a degreee.g.... 045 = 4.5 C

Screen temperature in tenths of a degreee.g... 167 = 16.7 C

Type of medium and high cloud.

Decode over page

Change in pressure (tendency)Difference in pressure from 3 hours

in tenths of hPa and direction ofup or down. Decode over page

Windspeed and directionWind from direction of the shaft

Triangle = 50 ktsFull feather =10 ktsHalf feather = 5 kts

Just shaft < 3kts

The Station Circle

Observations

11,4510

50 3500199711

30 159978

00706 20

Tephigrams & Stability

Any point on an ascent

AbsolutelyUnstable

AbsolutelyStable

ConditionallyUnstable

The position of the ELR gives the stability of the ascent at that point

DALR > SALR > ELR

ELR > DALR > SALR

DALR > ELR > SALR

EXTRATROPICAL CYCLONES

They form along the line between the polar air mass and the maritime tropical air mass.

Shearing produces cyclonic motion.

Under suitable conditions frontal surface will assume a wave shape.

EVOLUTION OF A FRONTAL DISTURBANCE

initial stage open wave cyclone

occluding fully occluded (mature)

stationary polar front

INITIAL DISTURBANCE

OPEN WAVE DEPRESSION

OCCLUSION BEGINNING

FULLY OCCLUDED

LIFE-CYCLE OF A AN EXTRATROPICAL CYCLONE

Lifecycle of frontal

disturbances

Surface isobars (solid) temperature (dashed)

MAPH40410 Synoptic Meteorology2

Upper-level Height

Contours

Note displacement of upper-level trough to west of surface low

RELATIONSHIP BETWEEN SURFACE RELATIONSHIP BETWEEN SURFACE CYCLONE AND UPPER LEVEL FLOWCYCLONE AND UPPER LEVEL FLOW

500 mb height (thick lines), SLP isobars (thin lines), and layer-mean

temperature (dashed). The deflection of the upper-level wave contributes to deepening of the surface low.

CYCLOGENESISCYCLOGENESIS

Formation of cyclones

Close relationship between surface disturbances and flow of air aloft.

For a mid-latitude cyclone to form: cyclonic flow must be established

Inward flow of air near surface must be supported by outflow aloft.

Total spin / cyclonic height = constant

The jet stream wind is subgeostrophic in troughs, and supergeostrophic in ridges

fast fast

Mid-latitude Frontal Disturbances:Mid-latitude Frontal Disturbances:interaction between low-level and jet-level interaction between low-level and jet-level

flowflow SL pressure and precipitation 300 mb height and wind speed

MAPH40410 Synoptic Meteorologysurface lowupper-level trough

Developing frontal lows tilt westward with height

fast fast

Note the advection of cold and warm airmasses

Satellite Views of Wave Cyclones

Open stage, with clouds over warm and coldfronts, with clear warmsector

Occluding stage

Mature stage

mature stage

occluding stage

Locate the fronts and surface low

IR image

MAPH40410 Synoptic MeteorologyWater vapour image

Finding Surface Fronts

Using plotted surface charts

Finding Surface Fronts What is a front? A boundary between different

types of air If we can define what those

differences typically are, or what we expect them to be …

… we can define the things that we should look for in order to find a front

FrontsFronts

warm aircold air cool air

MAPH40410 Synoptic MeteorologyASXX MSLP Analysis 1200 UTC 20 Apr 2000

Fronts on a Pressure Chart

Station Circles

By studying different elements on a station circle, the position of the surface front can be found

So, what’s available on a station circle? temperature and dew point wind speed and direction, gusts present and past weather, visibility, cloud

amount, cloud type, cloud base pressure tendency

What happens to the weather as a front

passes?

Using the information from station circles

Fronts on a Pressure Chart

>200km ahead of the warm >200km ahead of the warm frontfront

In the warm sector, 20km ahead of the cold front

1886 08

20km behind the cold front

2571 12

>100km behind the cold front

Positioning a Surface Front

Temperature and Dewpoint Windspeed and Direction, Gusts Present Weather and Past Weather Visibility Cloud Amount, Type and Lowest

Cloud Base Pressure Tendency

Temperature and Temperature and DewpointDewpoint Temperature isn’t such a good

guide as dewpoint The dewpoint within an airmass

tends to remain the same The dewpoint in the warm sector

will be constant and fairly high The dewpoint behind the cold

front may fall gradually

Wind speed and direction

The wind will veer on the passage of a front

Veer = turn clockwise Back = turn anticlockwise Gusts are likely to be strongest on the cold

Present and Past Weather

The heaviest rain is often at the front

It is likely to rain ahead of a warm front

Drizzle is most likely in the warm sector

Showers develop behind a cold front

Visibility

Visibility decreases when it is raining Drizzle will decrease the visibility

more than rain (the smaller drop size is more effective at scattering light)

Visibility in the warm sector will generally be poorer

Visibility in the cold air will be very good, up to 30km or more

Clouds

Amount and type High clouds >200km ahead of the

warm front Lowest cloud is associated with

the heaviest rain, often on the front

Cloud base can be affected by orography

Pressure Tendency

The pressure falls just ahead of a front and rises just behind it, giving a pressure ‘kick’

Pressure

Positioning a Surface FrontPositioning a Surface Front

Dewpoint - constant in the warm sector Winds veer as a front passes Drizzle in the warm sector Moderate or heavy rain on the cold front Showers behind the cold front Overcast in the warm sector Broken cumulus behind the cold front Pressure ‘kick’ as a front passes

Conclusion.

Questions welcome

Meteorology M.Sc. Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences

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