Lecture by Lars van Galen · After lecture and tutorial, measure snow depth! 14. Fronts 15. ... Go...

Weather Systems

Low pressure areas, fronts and surface weather

Atmospheric rotations in (severe) weather:

A deeper look into hurricanes

Meteorology and Climate, Module 9 (Jan 22, 2019)

Lecture by Lars van Galen

1

On Thursday ex-SWA Meteorology studentJelle Wisse of Marin will talk about his work!

Let it snow...2

Large scale – occlusion passage3

Front in visible imagery4

At 1.5 km altitude (850 hPa)

weak warm band 5

At 1.5 km altitude (850 hPa)

weak warm band (12 UTC) 6

Sounding De Bilt, 12 UTC (in the snow)7


Thick clouds(8 km across)

10

ECMWF-model HIRLAM-model

How much will fall?- Accumulations for today

11


How much will fall?- Accumulations for today

3 cm 4-5 cm

How much will fall?- Accumulations for today12


“Observation” at 11 UTC13

Measurement Hellevoetsluis

After lecture and tutorial, measure snow

depth! 14

Fronts15

Warm and cold front 16

Cold front Warm front

Idealised cross sections through frontal

zones

17

X (longitude)

Z (altitude

Idealised cross sections through frontal zones 18


W

C

C

Frontal lifecycle21

22Classic low pressure system

Immature Mature

Warm sector

Fig 8.31

The 4 stages in the

development of cyclones

according to the

Norwegian school

conceptual model

Red arrows indicate

warm sector air flow and

blue in cold sector

relative to the moving

cyclone center

(red dot)

isobars

isotherms

Grey areas: rain 23

Frontal life cycle - vertical 24

W

C

C

25

W

C

C

Frontal life cycle - vertical

26

W

CC


27

W

CC


The development of a classic low pressure area

06-01-2019 06 UTC 28

Phase 1

29


06-01-2019 12 UTC

Phase 2

30


06-01-2019 18 UTC

Phase 2 - 3

31


07-01-2019 00 UTC

Phase 3

32


07-01-2019 06 UTC

Phase 4

33


07-01-2019 12 UTC

Phase 4

34


07-01-2019 18 UTC

Phase 4


08-01-2019 00 UTC 35

Even more phase 4...

Intermezzo – in what phase is ‘our’ low

pressure area of today? 36

Front quiz!37

Goal = to recognize when fronts have passed.

Example includes a warm and a cold front

13 January 2019

Go to kahoot.it

Enter game pin (to be provided)

Enter username

Start!

Temperature of day of frontal passage

Front identification 38

Recap – front passage 39

W

C

C

Radiation at front –

weather of the day question 40

41

Clear spells (Qnet < 0)

Radiation at front –

weather of the day question

Precipitation at front question

42

Precipitation at front question 43

4,5 mm

4,5 mm

Veering of wind44

Pressure tendency45

What the KNMI says...

Warm front at night (before 06 UTC) 46

47

What the KNMI says...

Cold front around 18 UTC

Warm and cold front 48

• Continuous light rain and drizzle

• Pressure decreases before front, steady after front

• Wind veers during front passage

• Tx and Td increase

• Short, showery and intense precipitation

• Pressure decreases before front, increase after front

• Wind veers during front passage

• Td decreases sharply, Tx not so much

Short excursion – the Polar Low49

Polar low

Small extratropical cyclone

Located to far north (60-80N)

Short lifetime (~1-2 days)

Not driven by airmassdifferences, but by convection!

50

https://www.scisnack.com/2015/03/04/polar-lows-what-fuels-arctic-hurricanes/

Polar low





Cold air over warm sea thunderstorms organization

= polar low

51


Polar low






= polar low

52


Polar low






= polar low

53


Climatology of polar lows near Scandinavia

between 2002 and 2011 54

(Terpstra et al., 2016 )

Break 55

Quantifying atmospheric rotation56

Atmospheric Rotation: Scales and Forces

Dust devil

Tornado

Hurricane

Low Pressure System

57

Atmospheric rotations and horizontal scales58


Non-existent

Non-existent


Non-existent

Non-existent

Polar low


Main question: what forces are important at what

scales?

Gradient wind: recap

Centrifugal force

Vf

r

P

r

Vc

12

This is our fundamental equation

62

1 2 3

1

Pressure gradient force2

Coriolis force3

Cyclindrical coordinates

Vf

r

P

r

Vc

12

V

r

63

L

Cyclone

Gradient wind: in cylindrical coordinates

𝑟 = 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑐𝑦𝑐𝑙𝑜𝑛𝑒

𝜃 = 𝑐𝑜𝑜𝑟𝑑𝑖𝑛𝑎𝑡𝑒

𝑝𝑒𝑟𝑝𝑒𝑛𝑑𝑖𝑐𝑢𝑙𝑎𝑟 𝑡𝑜 𝑟𝑎𝑑𝑖𝑢𝑠

Vf

r

P

r

Vc

12

R

rr

r

64

L





𝑉𝜃 = 𝑤𝑖𝑛𝑑 𝑠𝑝𝑒𝑒𝑑


Vf

r

P

r

Vc

12

R

rr

V

r

65

L





𝑉𝜃 = 𝑤𝑖𝑛𝑑 𝑠𝑝𝑒𝑒𝑑


Vf

r

P

r

Vc

12

V

R

V

rr

V

r

66

L

r

V

V

r


In NH sign of V :

+ cyclonic

- anticyclonic

Vf

r

P

r

Vc

12

V

R

r

V

r


V

r

r

V

r

Centrifugal acceleration introduces the scale of the motion!

67

See MetClip 7b

L

Gradient wind: simplifications

Centrifugal force

Vf

r

P

r

Vc

12


68

1 2 3

1

Pressure gradient force2

Coriolis force3

Can we neglect the Coriolis force?

Rossby number – quantifying length scales

Relative importance of:

Centrifugal force, Fcp

Coriolis force, Fcor

Vf

r

P

r

Vc

12

I II III

69

Carl-GustafRossby

(1898-1957)

Rossby number

Relative importance of:

Centrifugal force, Fcp

Coriolis force, Fcor

Definition:

rf

V

Vf

rV

F

FRo

cccor

cp

/

2

Vf

r

P

r

Vc

12

I II III

Ro decides if (III) can be neglected in approximations

Ro depends on scale!!

70

What balance holds?

1) Tornado

r = 100 m

v = 300 km/h

Lat = 30N

2) Polar low:

r = 300 km

v = 100 km/h

Lat = 80N

71

𝑹𝒐 =𝑽𝜽

𝒇𝒄𝒓

Ro >~5 = cyclostrophic balance: neglect Coriolis force

Ro<~5 = use no approximation; i.e. use gradient wind

Tornadoes (Ro~1000)

“Cyclostrophic” approximation

(ignore Coriolis effect):

r

P

r

V

12

Occurrence: under thunderstormsOrigin of rotation not always clear

72

Polar low

(Ro 15)

r

P

R

V

12


73

Hurricanes: strong tropical cyclone

(Ro 1 - 10)

r

P

R

V

12


74

Synoptic Low’s or High’s (Ro 0 - 0.1)

Either gradient wind or geostrophic approximation

Vfr

Pc

10

Vf

r

P

r

Vc

12

75

Synoptic Low’s or High’s (Ro 0 - 0.1)

Either gradient wind or geostrophic approximation

Vfr

Pc

10

Vf

r

P

r

Vc

12

76

Only in case when isobars are straight (Ro 0) Centrifugal

force can be neglected

Tropical cyclones 77

Hurricanes: strong tropical cyclone

(Ro 1 - 10)

r

P

R

V

12


78

Tropical cyclone formation79

Tropical cyclone formation same

mechanism as polar low! 80

From a tropical disturbance...81

Warm and humid air rises and creates thunderstorms

To a hurricane82

High sea water temperatures enhance evaporation and growth of the system

>

Generation of tropical cyclones83

Rotation of the earth sets the tropical cyclone in motionand allows the system to organize

Tropical cyclone formation areas84

Hurricanes require warm sea water (> 26.5 C) and should not be too close to equator

Tropical cyclone formation areas – exceptions85

Catharina (2002)


Catharina (2002)

Pali (2016)


Catharina (2002)

Ophelia (2017)

Pali (2016)

Hurricane Ophelia88

Non-tropical

Tropical

Ophelia southwest of Spain89

Netherlands


Netherlands

Ophelia

Tropical cyclone impact92

Animation of Hurricane Harvey (2017) satellite93

Impact

Winds

Storm Surge

Rainfall

94

Impact in Texas due to Harvey

Track of hurricane Harvey

Each dot resembles a 6-hr point

Max winds at landfall: 215 km/h

95





96

What caused the most impact?• Flooding due to rain?• Wind damage?• Storm surge?





97

What caused the most impact?• Flooding due to rain• Wind damage?• Storm surge?

Summary

Frontal systems associated with low pressure areas largely determine the day-to-day weather in Western Europe.

Rotation of cyclones on large range of scales can be classified according to Rossby number

Tropical cyclones are high-impact tropical weather systems that have a complex structure and naming

98

There’s more!

More on weather maps and interpretation in

Atmospheric Practical,

Period 4 (Feb) with excursion in last week

(scheduled in BSc-SWA program in third year)

More on theory of atmospheric motions in

MSc course Atmospheric Dynamics

99

Tropical cyclone structure100

Tropical cyclone structure101

Rainbands with drier areas, and a clear eye

Eyewall Eye

>

Structure of tropical cyclones102

Tilted depiction

Example: Hurricane Harvey (2017)

Radar image 103


Radar image 104

Eye


Radar image 105

Eyewall


Radar image 106

Tropical cyclones – the naming chaos 107

Tropical cyclone naming conventions

Atlantic and Eastern Pacific

Tropical depression (winds < 64 km/h)

Tropical storm (64 km/h < winds < 120 km/h)

Hurricane (winds > 120 km/h)

108





Hurricane (120 km/h < winds < 209 km/h)

Major hurricane (winds > 209 km/h

West Pacific



Typhoon (winds > 120 km/h)

Super typhoon (winds > 240 km/h)

109





Hurricane (120 km/h < winds < 209 km/h)

Major hurricane (winds > 209 km/h

West Pacific



Typhoon (winds > 120 km/h)

Super typhoon (winds > 240 km/h)

110

In Southern Hemisphere and Indian Ocean Basin, names are even different!

Lecture by Lars van Galen · After lecture and tutorial, measure snow depth! 14. Fronts 15. ... Go...

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Transcript of Lecture by Lars van Galen · After lecture and tutorial, measure snow depth! 14. Fronts 15. ... Go...