Precipitation

Precipitation refers to any product of the condensation of atmospheric water vapour that is deposited on the Earth's surface.

Precipitation rate (R): rain water falling on ground per unit area per unit time

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R s

m

Mass flux

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sm 6106.31

Ordinary rain gauge

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ratefallrain d

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50 cm high20 cm in diameterCorss-section area of measuring tube 1/10of rain gaugeUp to 2 in rainfall

Tipping bucket rain gauge

Disadvantages: oscillations of buckets in high winds; evaporation, overflow with heavy rain

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r

V

AreaVolume

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resolutiontriald

100

Resolution of the sensor

Calibration (100 tips of rain)

Bucket tips when 0.01 in rain collected

Optical rain gauge (ORG)

A ORG measures the scintillation in an optical beam produced by raindrops falling between a light source and an optical receiver.

Scintillation: rapid variations in brightness or color of a distant luminous object viewed through the atmosphere.

Optical Rain Gauge (ORG)

Disdrometer

Measuring the droplet size distribution and velocity of falling droplets.

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34 DrV D Falling speed of rain droplet

Diameter (mm) Fall speed (m/s) Diameter (mm) Fall speed (m/s)

0.1 = 50μ =r 0.27 2.6 7.57

0.2 0.72 2.8 7.82

0.3 1.17 3.0 8.06

0.4 1.62 3.2 8.26

0.5 2.06 3.4 8.44

0.6 2.47 3.6 8.60

0.7 2.87 3.8 8.72

0.8 3.27 4.0 8.83

0.9 3.67 4.2 8.92

1.0 4.03 4.4 8.98

1.2 4.64 4.6 9.03

1.4 5.17 4.8 9.07

1.6 5.65 5.0 9.09

1.8 6.09 5.2 9.12

2.0 6.49 5.4 9.14

2.2 6.90 5.6 9.16

2.4 7.27 4.8 9.17

Liquid water content: liquid water mass in unit volume

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volume

ii DNDL

The liquid water content of droplets with diameter Di

For droplets of all sizes

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The mass flux of rain fall )()( 36 iiil DVDDNR

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Vertical momentum of an impacting raindrop can be transformed into an electric pulse whose amplitude is a function of drop diameter

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Another type of disdrometers uses a video camera to directly count drops to give distribution of rain droplets N (D) or number density of the drops

This disdrometer uses two, side video to record optical images of each raindrop.

The disdrometers may also be designed based on acoustic signals.

Precipitation radar

Weather radars are both transmitters and receivers.

Relationship between the intensity echo and the size of rain droplets

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6.1200RZ

Echo strength is measured in units of DBZ (decibels).

)(log10 10 ZdBZ

dBZRainrate

(inches per hour)Description

65 16+ Extreme thunderstorms, possible hail

60 8.00Very heavy thunderstorms, possible

hail55 4.00 Heavy thunderstorms52 2.50 Thunderstorms47 1.25 Very heavy rain or sleet41 0.50 Heavy rain or sleet36 0.25 Moderate rain or sleet30 0.10 Light rain or moderate snow20 Trace Very light rain or snow

Doppler precipitation radar

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Doppler effect

Stationary sound source

Sound source moving at a speed slower than sound speed

Sound source is moving at the same speed as the sound speed

This pressure wall is called shock wave

Doppler precipitation radar

Doppler effect

Measuring precipitation from space

Rainfall can be inferred from the temperature of cloud tops. Microwave radiation emitted by clouds can be used to discern rainfall intensity. For TRMM, a weather radar can transmit a beam of microwave energy downward inside clouds; the energy scattered back to the radar by raindrops provides information on rain intensity and its vertical distribution

TRMM has a limited view of the Earth, ranging from 36 N to 36 S, and samples rain relatively infrequently, passing over the same location about once a day. TRMM is insensitive to light rainfall. Also, cloud top temperatures are imperfectly correlated with actual rainfall reaching the surface.