Post on 13-Dec-2015
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
sm
kg2
w
R s
m
Mass flux
hmm
sm 6106.31
Ordinary rain gauge
22III dRgdR
2
2
II
I
ratefallrain d
dRRg
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
22
r
V
AreaVolume
dOH
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.
36
3
2343
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
)(36 ii
densityl
volume
ii DNDL
The liquid water content of droplets with diameter Di
For droplets of all sizes
03
6)( dDDDNL l 3
6 iil DNL
The mass flux of rain fall )()( 36 iiil DVDDNR
3/:Unit mkg
36
DV
6)( ii DDNR
Vertical momentum of an impacting raindrop can be transformed into an electric pulse whose amplitude is a function of drop diameter
0 )( oNdDDN
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
63 )()()( DDNDVDDNZ
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
0c
u
0c
u
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.