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:

AOS 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 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). t λ VISIR

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 K 7 km 3 km I1I1 I2I2 I3I3 >> most intense least intense

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 clouds Gray 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 example STRATUS Low cloud VIS IR

CIRRUS High cloud VIS IR

Cumulonimbus VISIBLE

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 t λ WV

Water Vapor Example

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: mm 6 /m 3 ) Z = N x D 6 N = number of droplets of diameter D per m 3 D = diameter of droplets (in mm) Reflectivity is usually reported in dbZ where dbZ = 10 * log 10 Z (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 0.5 o = base reflectivity

Higher Z or dbZ = higher rain rate.

National Composite Radar Image