Lecture 18: Map discussion + Atmospheric circulation & pressure distrib’ns (Ch 8)

• map discussion; focus on series of lows forming in trough over Ab this week (powerpoint “animation”)

• Rossby waves

• Chinook and sea breeze – examples of a synoptic-scale and of a meso-scale wind system

• El Nino

We’ve had a week I would characterize as cool, moist, cloudy, calm. We’ve had drizzle, a few flakes of snow, a frost one morning. Let’s focus on Thursday 19 Oct…

19-Oct-06 T RH Td

13:00 Light Rain 3 90 2 SE 9

12:00 Light Rain 3 91 1 SE 15

11:00 Cloudy 2 90 1 SE 13

10:00 Cloudy 2 92 1 SE 9

09:00 Light Rain 2 93 1 ESE 11

08:00 Cloudy 2 94 1 calm

07:00 Cloudy 1 94 0 SSW 5

06:00 Cloudy 1 94 0 calm

05:00 Cloudy 1 97 1 calm

04:00 Mostly Cloudy 1 97 1 calm

03:00 Cloudy 1 97 0 NNE 5

02:00 Cloudy 2 97 1 N 15

01:00 Cloudy 1 95 1 NW 9

00:00 Cloudy 2 94 1 W 9

• at or near saturation• absolutely stable layers• NW wind all levels

• widespread low cloud(not very different Friday)

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

00Z Wed 18 Oct.00Z Wed 18 Oct.

Why? Because science can be said to be all Why? Because science can be said to be all about recognizing, recording and (if about recognizing, recording and (if possible) explaining possible) explaining patternspatterns … no … no patterns? … then not possible to “do” patterns? … then not possible to “do” science. This is one of the Ab weather science. This is one of the Ab weather “patterns”“patterns”

Path of “this” low Path of “this” low

06Z Wed 18 Oct.06Z Wed 18 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

12Z Wed 18 Oct.12Z Wed 18 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

18Z Wed 18 Oct.18Z Wed 18 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

00Z Thurs 19 Oct.00Z Thurs 19 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

06Z Thurs 19 Oct.06Z Thurs 19 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

Edmonton:Edmonton:

(T=) 0(T=) 0

(Td=) 0(Td=) 0

ie. sfc chart gives Td ie. sfc chart gives Td directlydirectly

12Z Thurs 19 Oct.12Z Thurs 19 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

18Z Thurs 19 Oct.18Z Thurs 19 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

00Z Fri 20 Oct.00Z Fri 20 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

06Z Fri 20 Oct.06Z Fri 20 Oct.

Let’s watch the progression of a trough across BC into Alberta…Let’s watch the progression of a trough across BC into Alberta…… … and formation of surface lows…and formation of surface lows…

12Z Fri 20 Oct.12Z Fri 20 Oct.

12Z Thurs 19 Oct.12Z Thurs 19 Oct.

During these events, During these events, consistently low consistently low pressure at all levels in pressure at all levels in eastern Canada eastern Canada

Edmonton:Edmonton:

(T=) -7(T=) -7

(T-Td=) 0(T-Td=) 0

ie. upper charts give ie. upper charts give T and T-TdT and T-Td

12Z Thurs 19 Oct.12Z Thurs 19 Oct.3x5+2.5 m/s3x5+2.5 m/s

Heavy stippling on 700 mb analysis:Heavy stippling on 700 mb analysis:

T-TT-Tdd <= 2 <= 2ooCC

This ridge moves This ridge moves eastward by Frieastward by Fri

12Z Fri 20 Oct.12Z Fri 20 Oct.Edmonton:Edmonton:

(T=) -9(T=) -9

(T-Td=) 1(T-Td=) 1

ie. upper charts give ie. upper charts give T and T-TdT and T-Td

2x5+2.5 m/s2x5+2.5 m/s

Heavy stippling on 700 mb analysis:Heavy stippling on 700 mb analysis:

T-TT-Tdd <= 2 <= 2ooCC

12Z Thurs 19 Oct.12Z Thurs 19 Oct.

• note the weak ridgenote the weak ridge and NW flow onto ABand NW flow onto AB

Long (Planetary/Rossby) Waves Long (Planetary/Rossby) Waves ——• wavelength 1000’s kilometers• typically 3 - 7 around globe (fewer, longer, stronger in winter)• not always unambiguously identifiable - but can be shown mathematically to exist in ideal atmos. due to N-S variation of Coriolis force• (usually) move slowly eastward

Waves in mid-latitude mid/upper troposphere causing Waves in mid-latitude mid/upper troposphere causing convergence and divergence aloft...convergence and divergence aloft...

500 mb500 mbcon

div

condiv

LL

Fig. 8-13

Example of high amplitude Rossby wave over N. AmericaExample of high amplitude Rossby wave over N. America

Sept. 22, 1995Sept. 22, 1995

Edmonton low +4oC, high +20oC

Winnipeg low -2oC, high +11oC

Fig. 8-12

Rossby wave moving eastwardRossby wave moving eastward

Notice the “short wave” here; covered later – are associated with storms

Rossby wave moving eastwardRossby wave moving eastward

Rossby wave moving eastwardRossby wave moving eastward

Rossby wave moving eastwardRossby wave moving eastward

Foehn/Chinook/etc. – synoptic scale wind down mountain slopeFoehn/Chinook/etc. – synoptic scale wind down mountain slope

“when winds warmed by compression descend the eastern slopes of the Rocky Mountains in North America, they are called Chinooks” (native-American term meaning “snow-eater”). Condensation of vapour on the windward slopes also contributes to the warmth of the dry, descending lee-side current

“Low-pressure systems east of the mountains cause these strong winds to descend the eastern slopes…”

Warm lee current may ride above a layer of cold dense air on the lee side… frontal boundary may move back and forth – rapid temperature changes

Chinook conditions in SW AlbertaChinook conditions in SW Alberta

Strong SW 700 mb flow strikes mountains perpendicularly

Warm air at 850 mb in lee of Rockies (adiabatic compression)… “trowal”

Surface lee trough of low pressure; tight contours, windy.

Sea breezeSea breeze

• mesoscale

• caused by differential warming (“thermally driven”)

• shallow circulation

• masked (ie. effect over-ridden) by strong synoptic-scale winds

Fig. 8-24

El Nino/ La NinaEl Nino/ La Nina

• ENSO an internally-generated disruption or instability of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe and giving some basis for long range weather forecasting (though presently with low skill)

• Is connected with the Southern-Oscillation, a pattern of reversing sea-level east-west pressure gradient across the equatorial Pacific… El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO)

So potentially can be forecast by coupled So potentially can be forecast by coupled oc/atmos prediction modeloc/atmos prediction model

Fig. 8-27

• easterly Pacific tradewinds pile up warm surface water in the west Pacific - so that sea surface is about 1/2 meter higher at Indonesia than at Ecuador. A weak surface ocean counter-current then develops

• The sea surface temperature is about 8oC higher in the west, with cool temperatures off South America, due to an upwelling of cold (nutrient-rich) water from deeper levels

Normal conditionsNormal conditions

• strong convection/rain over the warmest water, and the east Pacific is relatively dry

• strong equatorward-flowing coastal current (“Humboldt” or “Peruvian”) sustains the upwelling cold deep-water

Equivalent to Fig. 8-17, for S. hemisphere

• Reversal in sea-level east-west pressure gradient across equatorial Pacific. Abnormal winds (weakened easterly trades, or even westerlies)

• The warm surface water normally found in the western equatorial Pacific “sloshes eastward”, and the warm sea surface temperature anomaly in the eastern Pacific suppresses upwelling of cold deep-water

El Nino phaseEl Nino phase

Fig. 8-29… sea sfc temperature anomaly averaged over eight El-Nino’s (red = + 2oC)

La Nina phaseLa Nina phase

• La Niña phase is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific

Fig. 8-31… sea sfc temperature anomaly averaged over eight La-Nina’s (blues, about = minus 1oC)