Synoptic Meteorology I

Surface Analysis

Surface Plotting

1. obtain METAR reports

2. decode METAR

3. plot METAR

Surface PlottingKIRK 270853Z 20013KT 9SM –RA OVC040 28/17 A2991 RMK A02 SLP129 8/5// T02780172 57020=

82

639

129-20••

Surface Plotting

ddVV ww

TT

TdTdCL

CM

CH

NPPPppp a

fffRRR

N = sky conditions

TT = temperature

TdTd = dew point temp.

dd = wind direction

fff= wind speed

VV = visibility

ww = weather type

CH = prevailing cloud genus in high etage

CM = prevailing cloud genus in middle etage

CL = prevailing cloud genus in low etage

PPP = abbreviated sea level pressure

ppp = pressure change

a = symbol for pressure tendency

RRR = precipitation since last report

Surface PlottingWinds

Direction: shaft - always points into windSpeed: barbs & flags – always to shaft’s left

-short barb = 5 knots-long barb = 10 knots-flag = 50 knotw

Surface PlottingSky Conditions

0/8 CLR or SKC

>0/8 – 2/8 FEW

3/8 – 4/8 SCT

5/8 –7/8 BKN

8/8 OVC

Surface Plotting

TemperaturesConvert 0C to 0F in the U.S.Use T group in RMK section of METAR

If T group is unavailable, convert METAR body oC to oF and round to the nearest degree.

< 88.5 = 88> 88.5 = 89

Surface Plotting

Sea Level PressurePlot what you see

SLP 987SLP 043SLP 321

987

043

321

*But remember that those pressures are•998.7 mb•1004.3 mb•1032.1 mb

Surface Plotting

Cloud typePlot the symbol that corresponds to the number

Pressure Tendency5 8 003

look upsymbol Plot what you see

(except zeros)

-3

Surface PlottingSome notes on surface obs.

Manual obs take ~ 10 minutes to compileTemps from 2m over grassy surface

Instrument shelterStevenson screen Cotton Region shelter

Winds from 10mManual = 2 minute average

Observation time is time last element is observedObserved (manual = pressure)

(should be twice as far from obstacleas they are high)

Surface PlottingYou can now:

Decode METARSPlot them on station circles

It is now time to:Construct maps of sites with METAR obsAnalyze fields of

PressureTemperatureMoisture

*Using Isopleth Analysis*

Scalar Analysiscontours = isopleths

isopleths: Lines that connect all the places on a map where the same numerical value of some variable occurs, separating higher values from lower values.

Scalar Analysiscommon isopleths

isobar = equal pressureisotherms = equal temp.isohume = equal mixing ratioisohyet = equal rainfall amountisohypse = equal elevationisallobar = equal pressure changeisodrosotherm = equal dew points

Contouring Rules

• isopleths have a “high” side and a “low” side

low side of 80o

70o

80o

high side of 80o 90o

• isopleths never cross one another

90o

85o

85o

90o

Contouring Rules

• Try to avoid contouring around single data points.

• 69

• 69

• 69

• 69

• 69• 71

• 8180 70

70Okay to draw the

isotherms

• 67

• 69

• 69 • 69• 68

• 69

• 72

Don’t draw the 70isotherm

Contouring Rules

• Stop your analysis when the data stops - don’t exceed your map.

94

87

92

91

91

89 8787

89

89 88

94

93

89

90o

90o

90o

90o90o

90o NO!

89

U.S.

Gulf

Contouring Rules• How to:

1. begin- draw lightly in pencil

2. interval

a. pressure – every 4 mbi. start at 1000 mbii. solid lines – black

b. Temperature – 5o to 10oFi. Start at 0oFii. Dashed lines – red > 32oF

blue < 32oF

40oF302010

0-10-20-30

1012100810041000

996992

Surface AnalysisWhat is a col?What are the topography of surfaces

Saucier Fig. 4.101“ “ 4.102“ “ 4.103“ “ 4.105Discontinuities

zero orderfirst ordersecond order

Saucier Fig. 4.111Saucier Fig. 4.112

horizontal

vertical

Surface Analysis

H

H

L

L

•1011.5

1032

1024

1016 1012

10161012

1000

1008

1012

1010

10241032

1012 1010

1008

1000

Situation for a col:

Surface Analysis 1. Winds and Pressures

a. Winds in geostrophic balance blow ll to isobars

L PGF f

Surface Analysis

g

g

fuyp

fvxp

fuyp

dtdv

fvxp

dtdu

−=∂∂

=∂∂

−∂∂

−=

+∂∂

−=

ρ

ρ

ρ

ρ

1

1

1

1

0

0

forgeostrophic

flow

1

2

3

4

Surface Analysis1. Winds and Pressures cont.

a. Isobaric spacing should change gradually from one location to the next.

b. Sharp change (and increased winds) may suggest a front.

c. True warm sector of ETC is barotropic (isobars ~ straight; ll to 500 mb contours)Φ

Surface Analysis

barotropic: a fluid where in the surfaces of ρ and p coincide is barotropic.

-usually, p = p(z) and ρ = ρ(z) (fns of altitude only)

In a barotropic environment, ρ = ρ(p)z y

ρ

ρ-1

p

p-1Vertical Horizontalρ = constantp = constant

x ∴T = constant

Surface Analysis

“spot low” – analyze carefully

LL

L

Surface Analysis

Pressure TendenciesBeware of stations with pressure tendencies +/- 0.3 mbNot every “check” tendency indicates a front Not all fronts are followed by a pressure increasePressure may fall behind an occlusion

Surface AnalysisAlso….

Station reports are influenced by a diurnal cycle.Up to ± 0.6 mb/3hrsOver U.S.:

t

P

03 UTC 09 UTC 15 UTC 21 UTC

Surface AnalysisFronts

ideal:

30 40 50 60 70

80

1000

1004

1008

-temp gradient (leading edge)-pressure trough-wind shift-drier air-moisture gradient

Surface AnalysisFront: a zone of transition between

different air masses

Found by:1. Changes is temperature2. Changes in moisture3. Changes in wind direction/speed4. Changes in pressure5. Changes in clouds precipitation

Surface Analysis

Cold Front40 50 60 70

1000

1004

1008

80

•

Classic Signatures-temp falls-dew point falls-wind shift SW to NW-pressure falls before and rises after passage

Surface Analysis

Warm Front

L

996 1000 10041008

80

70

405060

Classic Signatures-temp rises-dew point rises-winds shift SE to SW-pressure falls before,steady or slight riseafter-visibility improves-precipitation-ceiling improves

Surface AnalysisOccluded Front Signatures

-temp rises before, fallsafter passage-humidity rises before, fallsafter-visibility often poor prior to front passage and rapidlyimproved after-sky may clear briefly-pressure tendency unclear,may rise or fall (system deepens)

L ••