Lecture 11

1

Gradient windAtmospheric boundary layer

Geostrophic Wind when the isobars are straight, parallel lines, and the

only two forces acting on a parcel are the PGF and the

CF, then the wind is called geostrophic wind• PGF and CF are equal in magnitude and

opposite in direction• Geostrophic winds are always parallel to

the isobars

Surface500 mb

500 mb

475 mb

CF

PGF Wind

H

L

2

Strength of Geostrophic Wind

geostrophic conditions are that PGF and CF are equal in magnitude and opposite in direction

2 singPGF CF V

2 sing

PGFV

500 mb

475 mb

CF

PGF Wind

H

L

3

strength of geostrophic wind is determined by PGF not by CF, why?

The Coriolis force can only change a parcel's direction, it CAN NOT affect its speed.

4

2 sing

PGFV

Centripetal force and

gradient wind

5

6

Gradient Wind

Geostrophic Wind

counter lines are relatively straight

Counter lines are curvature

Gradient WindCentripetal forceif you attach a string to a ball and swing it in a circular manner, then the force required to keep the ball moving in a circular path is called centripetal force

string

Cent

ripe

tal f

orce

V

2

V

centripital forcer

magnitude of the centripetal force is

V=speed of a parcel (object)r= distance from the center

direction of the centripetal force is directed inward, towards the axis of rotation 7

recall geostrophic wind

500 mb

475 mb

CF

PGFWind

H

L

8

Gradient Wind Centripetal force

Gradient wind is produced by the balance between PGF, CF and centripetal force

PGF

PG

F

PG

F

PGF PGF

PG

F

9

Free troposphereand

Boundary layer

10

above 850 mb level, the flow is either geostrophic wind or gradient wind(free troposphere)

near the surface about 1-2 km (boundary layer), we must include the effect of friction and therefore, the flow is no longer geostrophic or gradient balance

11

Free troposphere without friction

Boundary layer with friction

Q: As a result, which force becomes smaller, the PGF or the CF?

friction has a significant impact on wind in boundary layer (1-2 km)

the atmosphere above the boundary layer is called free troposphere

12

FrictionSlows down the wind, causes winds across isobars. Direction of friction is always in the opposite direction to the wind direction.

Four forces in the atmosphere

1. Pressure gradient force2. Coriolis force3. Centripetal force4. Friction

13

distancehigh lowP P

2 sin( )V 2V

r

kV

which force is not caused by wind?

PGF

which two forces do not affect wind speed?Coriolis force and centripetal

force

at the center of a surface low, the air converges, and then must rise

at the center of a surface high, the air diverges, and must come from aloft due to sinking motion

14

H L

Why temperature is the most important to

understand weather?

What is temperature definition?

Why earth’s radiation is infrared while solar radiation is short-wave?

Wien’s Law

max

3000( )

( )m

T K

The earth’s radiation energy is determined by what?

Stefan-Boltzmann Law4E T

The capacity of air for holding water vapor is determined by what?

0 5 10 15 20 25 30 35 40 45 500102030405060708090

100110120

Temperature (oC)

Satu

rati

on

vap

or

pre

ssu

re (

mb

)

T=30 oC

T=44 oC

T=10 oC

This curve indicates how much water vapor can be held in the air at a given temperature

2

*H OP

Temperature determines the capacity of holding water vapor in the air

2

17.27* 273.3( ) 0.6108

T

TH OP T e

Thermal circulation

Thermal circulation is generated by pressure gradient

Pressure gradient is produced by different heating

RuleCool surface air sinking high surface pressure

Warm surface air rising low surface pressure

Wind is blowing from high pressure to low pressure

CoolHigh P

WarmLow P

Wind

Where is clear sky?

Thermal circulation

1. sea breeze2. land breeze3. Monsoon4. valley breeze5. Katabatic winds6. Chinook7. Santa Ana winds8. Haboos

Scales of Atmospheric Motions

Time and space scale of atmospheric motions

Global scale 5000 km

Synoptic scale 2000 km

Mesoscale 20 km

Microscale 2m

Typical size

Typical life span

23

Sea breeze

Sea breeze1) assume land and sea have same temperature

in the morning (7 AM)2) also assume synoptic phenomena absent

Q: As solar radiation increases in the morning, which surface heats quicker and why?

Q: What is the resultant air flow?

3 PM

90oF 70oF

Wa

rm

Co

ld

Sea breeze

860 mb

900 mb

820 mb

isobars

780 mb

3 PM

90oF 70oF

Wa

rm

Sea breeze

860 mb

900 mb

820 mb

isobars

780 mb

H

LH

L

Co

ld