Next Week: QUIZ 1

• One question from each of week:– 5 lectures (Weather Observation, Data

Analysis, Ideal Gas Law, Energy Transfer, Satellite and Radar)

– Over main topic of lecture and homework

• Multiple choice, short answer, matching, map question

• Powerpoints: http://www.aos.wisc.edu/~ahulme/aos101/

AOS 101-304

February 26/28

Satellites and Radar

RADAR• Observes

precipitation• Detect small-scale,

local weather (e.g. thunderstorms)

SATELLITES• Observes areas of

cloud cover• Detect large-scale,

regional weather (e.g. cyclones)

Satellites (two types)• 1. Polar orbiting

– Orbit nearly pole-to-pole every 102 min.

– Altitude of 850 km = high resolution– Good coverage of poles, but not good

for tropics (only twice-daily)

• 2. Geostationary– Fixed above the equator at a certain

longitude– Altitude of 35000 km = lower resolution– Continual monitoring of the same area

(loop)– 5 placed at different longitude to provide

global coverage (GOES E/W for U.S.)

How do satellites work?

• Detect outgoing radiation from the planet.

• Make use of “atmospheric windows”:– certain wavelengths of light that are not

absorbed easily by the atmosphere (i.e. high transmittivity).

tλ

VIS IR

Visible (VIS) satellite

• Detects the amount of solar radiation reflected from earth

• Clouds will appear white, land will appear dark

• Can pick up things besides clouds:– Dust, smoke, snow cover, sunglint

• VIS example 1

Infrared (IR) satellite

• Two facts needed:• For a certain wavelength, warmer

objects will emit more intense light (more photons) than cooler objects.

• In the troposphere, where most clouds occur, the temperature usually decreases with height.

GROUND (T = 300 K)

T = 280 K

T = 250 K7 km

3 km

I1 I2 I3> >

most intens

e

least intens

e

• Satellite detects intensity of IR radiation emitted by a cloud, which infers the temperature and height of a cloud.

• On an IR image,– Brighter pixels = less intensity =

cooler temperatures ≈ higher heights.– Duller pixels = more intensity =

warmer temperatures ≈ lower heights.

• Low clouds are dark, high clouds are bright.

Purple area = very cold cloud tops = very high cloudsGray area = warm cloud tops = low clouds

Detecting cloud type from comparing VIS and IR

• Stratus (low) clouds and fog will appear white/gray on VIS, but dark on IR

• Cirrus (high, thin) clouds will appear dark gray on VIS, but bright on IR

• Cumulonimbus clouds (tall thunderheads) will appear bright in both VIS and IR.

Stratus exampleSTRAT

USLow cloud

VIS

IR

CIRRUS

High cloud

VIS

IR

Cumulonimbus

VISIBLEVISIBLE

Water Vapor (WV)• Uses wavelengths that are readily

absorbed by water vapor (6-7 μm).• Only sees topmost water vapor in an

atmospheric column.• Allows observation of large scale patterns

without clouds being present

tλWV

Water Vapor Example

http://mapmaker.aos.wisc.edu/scr3/sat/g8/g8wv.fli

RADAR

• First used to detect airplanes, however anomalous objects would block radar.

• Objects were rain clouds, radars were refined to detect precipitation

• Use microwaves with frequency of 3-30 GHz or wavelength of 1-10 cm

• NWS maintains 158 sites across the U.S.– Wis. sites are Milwaukee, Green Bay and

LaCrosse

How does radar work?• Water droplets are

a good scatterer of microwave light.

• Larger droplets are better scatterers

• Radar sends out a pulse of microwave radiation

• These waves scatter off a water droplet• The amount scattered back to the sensor is

proportional to the diameter of the water droplet.

Reflectivity (Z)• The amount of radiation scattered back is

measured by the quantity Z (units: mm6/m3)

Z = N x D6

• N = number of droplets of diameter D per m3

• D = diameter of droplets (in mm)• Reflectivity is usually reported in dbZ where

dbZ = 10 * log10Z (logarithmic scale)

• Note: snow/hail have different scattering properties so this relationship will be different.

How do you get an image…

• Sensor scans both horizontally and vertically

0.5o = base reflectivity

Higher Z or dbZ = higher rain rate.

National Composite Radar Image