week 2 precipitation

8/12/2019 Week 2 Precipitation

1/45


2/45


3/45

Water falling on earth from atmosphere in liquid orsolid form is called precipi tat ion.

Rain and snow are the two most important forms of

precipitation hydrologically.

Main difference between them being that falling as rain immediately becomes runoff

Whereas snow generally melts after a period of time.

3


4/45

Necessary Conditions for Precipitation:

1. Sufficient vapor should exist in that region of the

atmosphere (The main source of vapor in the atmosphere

is oceans, such that 90% of the precipitation on lands

comes from water evaporated from the oceans).

2. Air mass should be cooled (Cooling of the air reduces itsvapor carrying capacity, and water condenses into liquid

form when the saturation point is reached).

4


5/45


6/45

Necessary Conditions for Precipitation:

4. Drops that are sufficently large(about 1 mm) should beformed so that they can fall down to earths surface

This occurs when ice crystals on which water vapor condense are

present, or when small drops grows by hitting each other. In temperatures below 10C water vapor condenses on ice

crystals to form large drops when sufficient amount of ice crystalsare present in a cloud, because vapor pressure on ice is lower thanvapor pressure on liquid water.

This is not possible in warmer clouds, where the only way larger

drops are formed is the collosion of drops condensed on salt nuclei. Precipitation may not occur although the first three conditions are

satisfied.

6


7/45

Convective precipitation

Cyclonic(Frontal) precipitation Orographic precipitation

7


8/45

An air mass near the earths surface rises when it isheated strongly.

Air is heated rises cooles

Precipitation

8


9/45

When a warm air mass meets a cold air mass along avertical front, warm air will be raised and cold air will

be lowered.

Cold front precipitation occurs when cold air mass

moves along the front by pushing the warm air mass.Warm front precipitation occurs when warm mass

moves by pushing the cold air mass

9


10/45


11/45

Precipitation does not occur when the mechanism to form raindrops that

are sufficient size to fall down to earth is not present. In such cases, raincan be produced artificially.

Crystals of silver iodine: Are scattered over the clouds from airplanes,

Silver iodine vapor produced by burning it on land surface rises to the

clouds(Water condenses on these crystals at temperatures below-5C, and large enough ice crystals are formed.)

11


12/45

Crystals of silver iodine:

1016 number of precipitation seeds are formed from 1 g silver iodine.

This is the sufficient number of seed that precipitation can occur.

The concentration of silver iodine is lower than 0.1 mikrogram/liter in rain

drops.

USA Common Wealth Research Institution has declared that the maximum

concentration of silver iodine in water sources is 50 mikrogram/liter.

This is first applied in Turkey in 1990 in Istanbul by Istanbul Water and

Sanitation Branch (ISKI) .

This method was also used in different time intervals in Ankara and Izmir as

well.

.

12


13/45

Dry Ice(Solid carbondioxide) :

Scattering over the clouds to drop the temperature to -40C so that

water in the clouds form ice crystals.

Thus ice crystals are not otherwise present in the clouds are

formed artificially(cloud seeding). This is possible only when the

temperature is below -5C.

13


14/45

The efficiency of artificial rain depends on such conditions:

Seeding timing

Wind profile,

The velocity of air elevation and other atmospheric conditions,

Super cooled water drops,

Seed concentration,

The collosion of dorps and their direction.

It is shown that artificial rain can be produced when the conditions areright, but the results have been the subject of much discussion.

The rain may start 15 minutes or a couple of hours following theseeding process.

.

14


15/45

Precipitation is expressed as the depth ofprecipitation defined as the height of the watercolumn that accumulates in a certain time interval ona horizontal plane.

It is usually given in milimeters. 1 mm of precipitationis equal to 1 kg/m2.

Recording instruments show the variation ofprecipitation in time and supplies information when it

is not possible to approach the instrument. Non- recording Pluviometers

Recording Pluviographs

15


16/45

Pluviometer : Non- recording

instruments

Any cylinder with vertical sidewall canbe used to measure the rainfall.

Standart vessels must be used so that

measurements are comparable and

errors are of same magnitude.

16


17/45

Pluviograph Types

These instruments record the variation ofthe precipitation depth with time on paper.

1. Weighing Gage

Rainfall is accumulated in a bucket. As

the bucket gets heavier, it moves a penon a rotating paper chart. This gives a

curve showing the variation of

precipitation depth with downwards

17


18/45

Pluviograph Types

2. Tipping bucket gage

Rainfall entering the gageaccumulates in a very small bucket,

which is emptied by tippin when it fills,

moves a pen on a chart by certain

amaount, and is replaced by another

bucket.

This gives a step line on the rotating

chart. Each step equaling to 0.3-0.5

mm depth of precipitation.

18


19/45

Pluviograph Types

3. Float-type gage

As the water level in the vessel rises, a float moves a pen

on a rotating chart. When the vessel is full, it is emptied

rapidly by an automatic siphon.

19


20/45

4. Pulse counting gage

When the cups in cylinder vessel is full, the rainfall

fills the other.

The repositining of the cup generates a pulse.

Each pulse is equal to 0,2 mm depth of

precipitation . These pulses are counted so that the total amount

can be calculated.

Pluviograph Types

20


21/45


22/45

Measurement of snowfall

Measurement of snow cover

Snowfall is measured by gages used for rainfall.

Non-recording gages and weighing gages can beused to measure snowfall.

Antifreeze addititves such as calcium chloride andethylene glycol should be used to prevent thefreezing of accumulated snow.

Gages must be installed at a sufficient distanceabove the ground so that they are not buried in thesnow.

22


23/45

Water equivalent of snow equals the product of

specific gravity of snow cover and its thickness. It varies 100-600 kg/m3

For new snow- 100, for denser snow 300-600.

23


24/45

Most important errors are caused by the wind.

Wind speed increases with the distance of the

gage above the ground, together with the ration

of precipitation that enters the gage.

24


25/45


26/45

A network of precipitation measurement gages must

be set up that is dense enough to determine the

areal distribution of precipitation.

Gages must be placed denser in mountainous

regions affected by air masse comign from the sea,

where the intensity of precipitation varies rapidly with

elevation.

26


27/45

World Meteorological Organization reccomends the

optimum density of one gage per 600900 km2 inplains, and one gage per 100250 km2 in

mountainous regions.

The interval of elevations of gages must be of

recording type to allow the distribution ofprecipitation in time to be measured.

DMI: 545 number of gages are established in Turkey

of the 470 gages are automated.

27

769.600 km2of Turkeys

surface area, of 400 000 km2

are defined as mountainous

area.


28/45

www.tumas.mgm.gov.tr

28


29/45

Precipitation mass curve:P-t curve

P= Precipitation depth(mm) i= Precipitation depth in unit

time- precipitation density(mm/hr)

29


30/45

Hyetograph:The curve showing the variation of precipitation intensity with time.

The time interval is chosen respect to the size of the region, usually in therange 1-6 hours.

Usually the average intesity reduces as the duration increases.

If the precipitation intensity: 7.5mm/hr=Heavy rain

i=Average intensity,T=Return period,

(The average number of yearsbetween two precipitationevents with intensity of at least i)

tp=Duration30


31/45

Homogeneity of a precipitation record is analyzed by doublemass curve method to check whether the location, oreintation

and the method of measurement of the precipitation gage has

been changed.

At least 5 gages must be used toestimate the average.

In case the straight line drawnthrough the plotted points isbroken, then the year when theslope of the line changes is the

year when homogeneity is broken. To homogeneize the

measurements before that date,they are multiplied by the ratio ofthe slopes of the lines before andafter that date.

31


32/45

Missing data in a station can be filled in using the records ofneighboring stations.

NA,NB,NC= Annual average precipitationat stations (mm/year)

PA,PB,PC= Precipitation observed at stations(mm)

Nx= Annual average precipitation (mm/year)

When NA,NB,NC do not differ from Nx by more than 10%, aritmeticaverage can be used instead of weighted average of:

Pi & Di= Readings at 4 stations closest to the

station with missing data and their distances

to the station with missing data 32


33/45

In meteorologically homogeneous region, records atstations that are far enough from each other such

that the precipitation depths are independent, can be

considered together as the record at a single station

along the total years.

Thus 30-year long records at 10 stations can be

brought together as a 300-year year record at a

single station

33


34/45

dA=Element of area

A= Total area of the region

The precision increases with the density of the gagesbecause of the topography, vegetation, lakes and

structures influence the uniformity of precipitation

distribution.

Arithmetic Average Thiessen Method

Isohyetal Method

34


35/45

It can be used in regions smaller than 500 km2 whenthe gages are rather uniformly scattered.

The simplest method is to take the average of thereadings of all instruments

This method may not give good results in

mountainous regions and heavy rainfall, when

precipitation various rapidly over the area.

35


36/45


37/45

Isohyets(curves of equal precipitation depth) are drawn by joiningthe points of equal precipitation.

Areal average precipitation is then calculated by a weightedaverage with weights equal to the average of the isohyets forareas between the two adjacent isohyets.

Isohyetal Method

Ai= The area between two adjacentisohyets

Pi = The average of their precipitationvalues

N= Number of gages

Gives good results especially inmountainous regions. 37


38/45

During a storm average areal precipitation is always smallerthan the point value at the center of the storm.

Areal average precipitation in an area around the center

decreases as the area increases.

The ratio (P/Po ) decreases rapidly for a 30-minute storm, butthe decrease is much slower for a 24-hour storm.

38


39/45

The recution of the precipitation over an area with size of the

area during a certain storm duration is given by a formula due

to Horton.

P0= Depth of precipitation at the storm center,

P=Average precipitation over the area A,

k,n= Coefficients determined seperately for each storm.

tp= Storm duration,

a,m,b= Regional coefficients

39


40/45

In some cases it is required to know not only the areal

distribution of precipitation but also its distribution in time:

1. The area inside each isohyet around each storm center

in the region is measured and average precipitation

depths are calculated.

2. Mass curves are drawn for each recording gage in theregion.

40


41/45

3. 6,12 hour precipitation

depth-area curves are

obtained from the regions

inside each isohyet.4. Areal distribution of

precipitation depth for

each duration is plotted

and envelope curves aredrawn.

41


42/45

Probable maximum precipitation, defined

as the maximum precipitation depth in the

basin that is physically possible for a

certain duration, is used in the design ofthe spillways of large dams that would

cause loss of life when they collapse.

42


43/45

1. Precipitation depth-area-duration curves of the basin areobtained for the heaviest observed storms.

2. The efficiency of the mechanism that converts the water vapor

to rain, the vapor content of the air masses, and the wind

velocities in the observed storms are increased to theirmaximum possible values.

3. Also the heavy storms observed in the neighbouring basins

that are meteorologically similar are transported to the basin

taking the meteorological differences into consideration.

This method should not be used for orographic precipitation,

because the precipitation depth varies rapidly in space.

43


44/45

Precipitation depth has an annual periodicity because of

the meteorological effects.

Variation of the precipitation along the year has an effect

on the water budget of the region.

44


45/45

www.srh.noaa.gov/
http://www.srh.noaa.gov/http://www.srh.noaa.gov/http://www.srh.noaa.gov/http://www.srh.noaa.gov/http://www.srh.noaa.gov/http://www.srh.noaa.gov/

week 2 precipitation

Documents