Radar & Satellites

Question 1

NZ radar numbers and how do they work?

Answer 1

• 9 (almost 10) radars cover NZ.
• Outer radius: 300km (low res 150 - 300km)
• Inner radius: 0 - 150km, Doppelar mode (extremely accurate)

Question 2

How does rain radar work?

Answer 2

Intensity of ‘echo’ reflected back and received by radar is related to the rate of rainfall.
Radar senses from below. (whereas satellites sense from above)

Question 3

Common problems with WX radar? (7)

Answer 3

1. Attenuation (weakening of radar beam by heavy precip)
2. Ground echoes/Ducting (bouncing b/t terrain and strong inversions)
3. Sea clutter (due side lobes picking up sea waves)
4. Radar elevation (Further the distance from the radar, the higher the beam, only high level precip picked up)
5. False echoes at sunrise/sunset (Due radar picking up sun radiation - seen as a straight line on image)
6. Bugs on radar (looks like virga)
7. Other interference lines (Due someone illegally transmitting on the same frequency as the radar)

Question 4

Features of Polar orbiting satellites

Answer 4

• ALT: 850km above SFC of the earth. (3000km field of view)
• Low level , high resolution. (375 - 705m per pixel)
• Passes over poles every 105mins
• Complete, frequent coverage of earth every 12hrs (NZ covered twice a day by 2 satellites = 4 passes)

Question 5

Features of geostationary satellites

Answer 5

• ALT: 36,000km above SFC of the earth.
• High level, low res (1-2km per pixel)
• Five positioned around the globe (we use Himuwari)
• Images provided every 10-15 mins 24/7
• Costlier than Polar

Question 6

Types of satellite imagery? (5)

Answer 6

• Infrared: day and night, high cloud is white, low is grey. (both Polar and Geo)
• Visual: daytime only (both Polar and Geo)
• Sea surface temps (both Polar and Geo)
• Vertical temp/moisture profiles (Polar only)
• Tracking of cloud and water vapour levels (Geo only)

Question 7

Problems with Satellite imagery

Answer 7

• Distortion of picture developed due to high altitude and angle of viewing. (parallax error). The image processed therefore shifts cloud slightly from its true position.

Question 8

What other functions do satellites serve?

Answer 8

1. Can use temperatures to indicate cloud TOPS (temp is assigned a specific height, heights are then assigned specific colours on the produced image)
2. Can use a certain frequency to identify the water vapour content in the air.
3. Can record sea SFC temp on clear days only.