precipitation and its types presentation

Precipitation: process, types, monsoon and jet streams, mechanism of Indian
monsoon and rainfall pattern, southern ocean oscillation and influence on monsoon,
cyclones, measurement, assessment of precipitation in gauged and un-gauged
basins, hydrological data. Global climate change and influence on precipitation

1. Precipitation
•Precipitation: water falling from the
atmosphere to the earth.
–Rainfall
–Snowfall
–Hail, sleet
•Requires lifting of air mass so that it cools
and condenses.

2. Mechanisms for air lifting
1.Frontal lifting
2.Orographic lifting
3.Convective lifting

3. Definitions
•Air mass : A large body of air with similar temperature
and moisture characteristics over its horizontal extent.
•Front: Boundary between contrasting air masses.
•Cold front: Leading edge of the cold air when it is
advancing towards warm air.
•Warm front: leading edge of the warm air when
advancing towards cold air.

4. Frontal Lifting
•Boundary between air masses with different properties is
called a front
•Cold front occurs when cold air advances towards warm air
•Warm front occurs when warm air overrides cold air
Cold front (produces cumulus cloud) Cold front (produces stratus cloud)

5. Orographic lifting
Orographic uplift occurs when air is forced to rise because of the physical
presence of elevated land.

6. Convective lifting
Hot earth
surface
Convective precipitation occurs when the air near the ground is heated by the Convective precipitation occurs when the air near the ground is heated by the
earth’s warm surface. This warm air rises, cools and creates precipitation. earth’s warm surface. This warm air rises, cools and creates precipitation.

7. Condensation
•Condensation is the change of water vapor into
a liquid. For condensation to occur, the air must
be at or near saturation in the presence of
condensation nuclei.
•Condensation nuclei 
are small particles or
aerosol upon which water vapor attaches to
initiate condensation. Dust particulates, sea salt,
sulfur and nitrogen oxide aerosols serve as
common condensation nuclei.
•Size of aerosols range from 10
-3
to 10 m.

8. Precipitation formation
•Lifting cools air masses
so moisture condenses
•Condensation nuclei
–Aerosols
–water molecules
attach
•Rising & growing
–0.5 cm/s sufficient to
carry 10 m droplet
–Critical size (~0.1
mm)
–Gravity overcomes
and drop falls

9. Forces acting on rain drop
F
d
F
d
F
b
F
g
D•Three forces acting on
rain drop
–Gravity force due to weight
–Buoyancy force due to
displacement of air
–Drag force due to friction
with surrounding air
3
6
DVolume


2
4
DArea


3
6
DgF
wg


3
6
DgF
ab


242
2
2
2
V
DC
V
ACF
adadd

 

10. Terminal Velocity
•Terminal velocity: velocity at which the forces acting on the raindrop
are in equilibrium.
• If released from rest, the raindrop will accelerate until it reaches its
terminal velocity
3
2
23
6246
0
Dg
V
DCDg
WFFF
wada
DBvert





 

33
2
2
6624
DgDg
V
DC
WFF
wa
t
ad
BD





 









 1
3
4
a
w
d
t
C
gD
V


•Raindrops are spherical up to a diameter of 1 mm
•For tiny drops up to 0.1 mm diameter, the drag force is specified by
Stokes law
F
d
F
d
F
b
F
g
D
V
Re
24

d
C
a
aVD


Re
At standard atmospheric pressure (101.3 kpa) and temperature (20
o
C),

w
= 998 kg/m3 and 
a
= 1.20 kg/m3

11. Precipitation Variation
•Influenced by
–Atmospheric circulation and local factors
•Higher near coastlines
•Seasonal variation – annual oscillations in some
places
•Variables in mountainous areas
•Increases in plains areas
•More uniform in Eastern US than in West

12. Rainfall patterns in the US

13. Global precipitation pattern

14. Spatial Representation
•Isohyet – contour of constant rainfall
•Isohyetal maps are prepared by
interpolating rainfall data at gaged points.
Austin, May 1981 Wellsboro, PA 1889

15. Texas Rainfall Maps

16. Temporal Representation
•Rainfall hyetograph – plot of rainfall
depth or intensity as a function of time
•Cumulative rainfall hyetograph or
rainfall mass curve – plot of summation
of rainfall increments as a function of time
•Rainfall intensity – depth of rainfall per
unit time

17. Rainfall Depth and Intensity
Time (min) Rainfall (in)Cumulative 30 min 1 h 2 h
Rainfall (in)
0 0
5 0.02 0.02
10 0.34 0.36
15 0.1 0.46
20 0.04 0.5
25 0.19 0.69
30 0.48 1.17 1.17
35 0.5 1.67 1.65
40 0.5 2.17 1.81
45 0.51 2.68 2.22
50 0.16 2.84 2.34
55 0.31 3.15 2.46
60 0.66 3.81 2.64 3.81
65 0.36 4.17 2.5 4.15
70 0.39 4.56 2.39 4.2
75 0.36 4.92 2.24 4.46
80 0.54 5.46 2.62 4.96
85 0.76 6.22 3.07 5.53
90 0.51 6.73 2.92 5.56
95 0.44 7.17 3 5.5
100 0.25 7.42 2.86 5.25
105 0.25 7.67 2.75 4.99
110 0.22 7.89 2.43 5.05
115 0.15 8.04 1.82 4.89
120 0.09 8.13 1.4 4.32 8.13
125 0.09 8.22 1.05 4.05 8.2
130 0.12 8.34 0.92 3.78 7.98
135 0.03 8.37 0.7 3.45 7.91
140 0.01 8.38 0.49 2.92 7.88
145 0.02 8.4 0.36 2.18 7.71
150 0.01 8.41 0.28 1.68 7.24
Max. Depth 0.76 3.07 5.56 8.2
Max. Intensity9.12364946 6.14 5.56 4.1
Running Totals

18. Incremental Rainfall
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5101520253035404550556065707580859095100105110115120125130135140145150
Time (min)
I
n
c
r
e
m
e
n
t
a
l

R
a
i
n
f
a
l
l

(
i
n

p
e
r

5

m
i
n
)
Rainfall Hyetograph

19. Cumulative Rainfall
0
1
2
3
4
5
6
7
8
9
10
0 30 60 90 120 150
Time (min.)
C
u
m
u
l
a
t
i
v
e

R
a
i
n
f
a
l
l

(
i
n
.
)
30 min
1 hr
2 hr
3.07 in
5.56 in
8.2 in
Rainfall Mass Curve

20. Arithmetic Mean Method
•Simplest method for determining areal average
P
1
P
2
P
3
P
1
= 10 mm
P
2
= 20 mm
P
3
= 30 mm
•Gages must be uniformly distributed
•Gage measurements should not vary greatly about the mean



N
i
i
P
N
P
1
1
mmP 20
3
302010




21. Thiessen polygon method
P
1
P
2
P
3
A
1
A
2
A
3
•Any point in the watershed receives the same
amount of rainfall as that at the nearest gage
•Rainfall recorded at a gage can be applied to
any point at a distance halfway to the next
station in any direction
•Steps in Thiessen polygon method
1.Draw lines joining adjacent gages
2.Draw perpendicular bisectors to the lines
created in step 1
3.Extend the lines created in step 2 in both
directions to form representative areas for gages
4.Compute representative area for each gage
5.Compute the areal average using the following
formula



N
i
ii
PA
A
P
1
1
P
1
= 10 mm, A
1
= 12 Km
2
P
2
= 20 mm, A
2
= 15 Km
2
P
3
= 30 mm, A
3
= 20 km
2
mmP 7.20
47
302020151012




22. Isohyetal method
P
1
P
2
P
3
10
20
30
•Steps
–Construct isohyets (rainfall
contours)
–Compute area between each
pair of adjacent isohyets (A
i)
–Compute average
precipitation for each pair of
adjacent isohyets (p
i)
–Compute areal average
using the following formula



M
i
ii
pAP
1
A
1
=5 , p
1
= 5
A
2=18 , p
2 = 15
A
3=12 , p
3 = 25
A
4
=12 , p
3
= 35
mmP 6.21
47
35122512151855






N
i
ii
PA
A
P
1
1

23. Inverse distance weighting
P
1
=10
P
2
= 20
P
3
=30
•Prediction at a point is more influenced
by nearby measurements than that by
distant measurements
•The prediction at an ungaged point is
inversely proportional to the distance to
the measurement points
•Steps
–Compute distance (d
i) from ungaged
point to all measurement points.
–Compute the precipitation at the
ungaged point using the following
formula



















N
i i
N
i i
i
d
d
P
P
1
2
1
2
1
ˆ
d
1=25
d
2=15
d
3
=10
mmP 24.25
10
1
15
1
25
1
10
30
15
20
25
10
ˆ
222
222




p
  
2
21
2
2112
yyxxd 

24. Rainfall interpolation in GIS
•Data are generally
available as points with
precipitation stored in
attribute table.

25. Rainfall maps in GIS
Nearest Neighbor “Thiessen”
Polygon Interpolation
Spline Interpolation

26. NEXRAD
NEXRAD Tower
•NEXt generation RADar: is a doppler radar used for obtaining weather
information
•A signal is emitted from the radar which returns after striking a rainfall
drop
•Returned signals from the radar are analyzed to compute the rainfall
intensity and integrated over time to get the precipitation
Working of NEXRAD

27. NEXRAD data
•NCDC data (JAVA viewer)
–http://www.ncdc.noaa.gov/oa/radar/jnx/
•West Gulf River Forecast Center
–http://www.srh.noaa.gov/wgrfc/
•National Weather Service Animation
–http://weather.noaa.gov/radar/mosaic.loop/DS.p19r0/ar.us.conus.shtml