Things to look for on the weather maps Visible and IR satellite images (& radar too): Look at cloud movements and locations - do they correlate with what.

Things to look for on the weather maps

Visible and IR satellite images (& radar too): Look at cloud movements and locations - do they correlate with what you would expect from the surface or 500mb pressure patterns? How can you distinguish between high and low level clouds and between deep and shallow clouds?

VIS

IR

Surface: Look for locations of high and low pressure centers, warm and cold fronts, regions of high winds, rain, snow, drylines, or other significant weather.


850mb: Use this map to look at temperature gradients, and to find regions of warm or cold air advection. It is also important to look at the moisture field and advection at this level if available (low-level moisture).


500mb: Where are the troughs and ridges? Where are the troughs and ridges in relation to the high and low pressure systems on surface maps (Positive Vorticity Advection & Negative Vorticity Advection)?


300mb: Use this map to look for jet streaks, or elongated pockets of very strong winds embedded in the jet stream. Jet streaks generally occur above regions of strong temperature gradients that you would find on the 850mb map. Why?


Forces govern the atmospheric

motionsPressure gradient force

Coriolis force

Gravity

Friction

Forces

latitude. the is φ

Earth. the of velocity angular the is where

Fuφcosgzp

ρ1

tdwd

Fuφsin- yp

ρ1

tdvd

Fwφcosvφsinxp

ρ1

tdud

z

y

x

Ω

Ω2

Ω2

Ω2Ω2

Momentum equations

Coriolis Force

O

B

'B,A

'A

*reference point

(fixed star)

O B

A

*

O

v1 d x

x<<d

v2d

Deflect to the right on the northern hemisphere, and to the left on the southern hemisphere.

ball

Coriolis Force

O

v1 d x

x<<d

v2d

2•2

1= tax

1v/dt

2

12

1

vd

ax

2

11

2

2 2

1

vd

avdω

tdωtvx ,cetandis

Given Coriolis acceleration a, the distance is

Travel time:

12 vωa

N

z

is latitude angle

N

z

is latitude angle

Coriolis Force

φsinΩ=Ω=ω z

1

1

fv=

v)φsinΩ2(=a

parameter) (Coriolis φsinΩ2=f where

If is that due to the earth’s rotation about a local vertical axis, such that

Then

(= Coriolis force if M = 1 kg)

Forces govern the atmospheric

motionsPressure gradient force

Coriolis force

Gravity

Friction

Forces

latitude. the is φ

Earth. the of velocity angular the is where

Fuφcosgzp

ρ1

tdwd

Fuφsin- yp

ρ1

tdvd

Fwφcosvφsinxp

ρ1

tdud

z

y

x

Ω

Ω2

Ω2

Ω2Ω2

Momentum equations

Pressure Gradient & Pressure Gradient Force

xp

ρxp

ρ Δ

Δ11

• Why do we want to know sea level pressure (SLP)?

• Why do we want to know pressure gradient?

960mb 1000mb 960mb1000mb

Positive pressure gradient but negative pressuregradient force in the x direction.

Pressure gradientxp

xp

Δ

Δ

Pressure gradient force

Negative pressure gradient but positive pressuregradient force in the x direction.

s: along the wind directionn: perpendicular to the wind direction, positive to the left.

Nature coordinate

x

y

sn

The nature coordinate can be obtained by rotating the Cartesian Coordinate until the x direction is along the wind direction.

Then, x is in the s direction and y is in the n direction.

And u=V and v=0

V

P1(xo,y1)

P2(xo,y2)

O

P2

P1

Po

L

H

(x0,yo)

Estimate Pressure Gradient

How to calculate pressure gradient at point O?

jyp

ixp

pH Δ

Δ+

Δ

Δ=∇

Δ

Δ+

Δ

Δ=∇

22

H yp

xp

|p|

P1(x1,yo)

P2(x2,yo)


degreeper km 111≈360/km 6371×π2

:lat 1 =>

km 6371=R where

,Rπ2=ncecircumfere

o

o

e

e

How to calculate real distance ?n y, ,x ΔΔΔ

1 o lat = ? km

Re

y

lat = lat2 – lat1y= lat x 111 km per degree


)latcos(Rr e

)φcos(×km 111≈

360/)φcos(×km 6371×π2=long1

rπ2=ncecircumfereo o

x

long = long2 – long1x= long x 111 km x cos

How to calculate real distance ?n y, ,x ΔΔΔ

r

Re

1 o long = ? km

φ

)φcos(km longpp

xxpp

xp

111ΔΔ

Δ 12

12

12

Pressure gradient at point O:


P1(x1,yo) P2(x2,yo)O

P2

P1

Po

L

H

(x0,yo)

km lat

p-py-yp-p

yp

111×Δ==

Δ

Δ 12

12

12

Pressure gradient at point O:


P1(xo,y1)

P2(xo,y2)P2

P1

Po

L

H

(x0,yo)O

Δ

Δ+

Δ

Δ=∇

22

H yp

xp

|p|

Or using the nature coordinates


distance shortest ,yyxxn

npp

p

234

234

12

Δ

Δ

nΔ

P1(x3,y3)

P2(x4,y4)

Ox1 x2

y2

y1

O

P2

P1

Po

L

H

(x4-x3)

(y4-y3)

(x0,yo)

Horizontal pressure gradient vs. vertical pressure gradient

Pressure Gradient

s m kg10 to ~p

.atmosphere lower the in s m kg 10~gρzp

2-2- 3-H

2-2-

210

Vertical pressure gradient force is

much greater than the horizontal

one, but is mostly balanced out by

gravity. So, the motion in the

atmosphere is dominated by horizontal

winds.

Geostrophic flow (Vg)

988

1000

992

996

O

CP

P

P

P

C

C

..

..

gV

np

ρf-V ,

sp

ρ-

dt

dV

φsin2f where ,fV-nρ

gg

g

∂

∂1=0=

∂

∂1=

×Ω== ∂

p ∂1

Not happen often in the real world. If it does, this is seen in large scale.

The horizontal pressure gradient force (P) is balanced by the Coriolis force

(C).

rest

Gradient wind flow (VG)

GV

GV

C > P

P > C

C

C

P

P

0= ∂

∂1==

∂

∂1 2

sp

ρ-

dtdV

;RV

-fV-np

ρGG

G

This is for large scale flow since Coriolis force is important.

Subgeostrophic

fVG >

=> VG > Vg

Supergeostrophic

If one uses geostrophic wind to approximate gradient winds, what happens?

VG < Vg fVg =

np

ρ-

∂

∂1

np

ρ-

∂

∂1

500 mb Wind Vectors

Gradient wind flow (VG)

Surface Weather Map

Any difference from the 500-mb one?

L

Because of what?Friction!

Friction

Friction is proportional to the roughness of

the Earth’s surface and wind speed, and is

opposite to the wind moving direction.

996

1008

1000

1004

geostrophic balance unbalanced flow balance with friction

Wind turns toward to the low pressure side

!!!

P

C

gVV

P

C

VF F

P

C

V

F

P

C

gVV

Flow with friction

This is for large scale flow too since Coriolis force is important.

ConvergenceStormy weather

DivergenceGood weather but potentially bad for air quality

Cyclostrophic wind flow

RV

np

ρc21

cVP

This is for small scale when Coriolis force is not important!

This flow can exist only around a low pressure, and the force needed to change the wind direction is provided by the pressure gradient force.

So, dust devils and tornadoes can turn either cyclonically or anticyclonically.

cV

P

Things to look for on the weather maps Visible and IR satellite images (& radar too): Look at cloud movements and locations - do they correlate with what.

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