Precipitation

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					  An Introduction to

PRECIPITATION


  Mrinmoy Majumder
    www.baipatra.ws
                                                    Precipitation
Precipitation is any form by which condensation of atmospheric water                  Causes of Precipitation or Weather System for Precipitation
vapor is pulled down by gravity and deposited on the Earth's surface.
Precipitation is generally measured with either rain gauge or radars.                 Front : Front is the condition when two distinct mass of air meets with
The data is represented by Mass curve and/or Hyetographs. The                         each other and the warmer air moves up above the cold air.In the time of
                                                                                      ascending, the air is adiabatically cooled and creates cloud and
arial distribution of precipitation is expressed by DAD curves. The
                                                                                      subsequent precipitation.
frequency of precipitation is expressed by Frequency Duration
Curves and/or PMP.                                                                    Cyclone : A large low pressure region is known as Cyclone.

                                                                                      Tropical – When the diameter of the low pressure region is 100-
  Different Forms of Precipitation                                                    200km,centre is 10-50km in diameter and if in the tropics, the cyclone is
                                                                                      known as Tropical Cyclone. Heavy rainfall and wind speed of 119km/hr
  Raindrop : Common in tropical zones and lower elevation areas. Raindrops are        can be observed in the region. The intensity of the storm decreases with
  formed due to the condensation and coalescence process. Water droplets              the distance from the eye or center of the cyclone.
  above freezing temperature, due to turbulence in the air, collides with each
  other to form larger droplets. When mass of the droplets become larger than the     Extra-tropical – Cyclones that form outside the tropical zone and are
  resistance of air to gravity, those droplets fall to ground as raindrops.           associated with a frontal system is called as Extra-tropical Cyclone. The
                                                                                      circulation of wind is counterclockwise in the northern hemisphere.
  Ice pellets : If sub-freezing layer is present in both below and above an above     Though magnitude of precipitation and wind velocity is smaller than the
  freezing layer raindrops are converted to crystals of ice before reaching the       tropical cyclones. The intensity of the precipitation is low but the duration
  ground as the cold layer, below the warm re-freezes the raindrops into ice.         and extent of the low pressure region is larger than the former.

  Hail : Hail forms in storm clouds when super cooled water droplets freeze on        Anticyclones : A region of low pressure with moderate wind speed and
  contact with dust or dirt. The storm's updraft blows the hailstones to the upper    clockwise wind circulation is called as Anti-Cyclone.
  part of the cloud. The updraft dissipates and the hailstones fall down, back into
  the updraft, and are lifted up again. The hailstone gains in ice layer and grows    Convective Precipitation : Localized warming can form a warm air
  increasingly larger with each ascent. Once a hailstone becomes too heavy to be      mass surrounded by a cooler one. The warmer mass moves up and form
  supported by the storm's updraft, it falls from the cloud.                          convective clouds and subsequent precipitation. Areal extent of such
                                                                                      precipitation is small.
  Snowflakes : Snow crystals form when tiny super cooled cloud droplets (about
  10 μm in diameter) freeze.                                                          Orographic Precipitation : The warm, moist air moves up in the
                                                                                      windward side of the mountain barriers and undergo cooling,
  Drizzle : Rain-droplets of size less than 0.5 mm and rain intensity of less than    condensation and Orographic Precipitation. Leeward slopes of the
  1mm/h is known as drizzle.                                                          barrier faces lighter rainfall than the other side.

  Glaze : When rain or drizzle touches ground at zero degree centigrade, glaze or
  freezing rain is formed.
         Measurement of Precipitation
Gauges                                                                      Radars

There are two type of gauges :                                              As amount of precipitation over a large area
                                                                            can be measured with a good degree of
1.Recording                                                                 accuracy radars are now frequently used in
                                                                            measurement of precipitation.
2. Non Recording
                                                                            When electro-magnetic signals of a radar
Recording gauges can be further classified                                  crosses with a drop of precipitation it can be
into:                                                                       observed that :

a. Tipping Bucket                                                           Pr = average echo-power

b. Weighing Bucket                                                          = CZ/r2

c. Natural Syphon                                                           where C = constant,
                                                                            Z = radar echo factor = aIb
d. Optical Type                                                             r = distance to target volume
                                                                            I = rainfall intensity in mm/hr
Telemetry to send data of multiple gauge stations through a radio channel
and advent of optical rain gauges has improved the accuracy and             A,b = coefficients, calibrated with the help of
exclusivity of measurement.                                                 observed gauge data of the same region.
Non-Recording Gauge
                   Rainfall
                                                               This type of gauges do not give
                                                            continuous plot of rainfall against time



                                                                        Collecting Funnel : Rainfall is
                                                                        collected through the funnel




                                                                        Metal Container : Funnel is
                                                                        attached to the bottle with the
                                                                        help of this clamp.
                                                                        Measuring Cylinder : When the
                                                                        collecting bottle overflows the water is
                                                                        collected in the measuring cylinder to
                                                                        measure rainfall

                                                                        Collecting Bottle : The water
                                                                        collected by the funnel is channeled
                                                                        to this bucket. From where water is
                                                                        taken to the measuring cylinder to
                                                                        measure rainfall.


Retrieved from http://www.imdpune.gov.in/surface_instruments/workshop/Instruments/raingauge.html
                               Recording Gauges
                                                              A rain gauge (also known as a udometer or a pluviometer) is a
                                                              type of instrument used by meteorologists and hydrologists to gather
             This type of gauges do give
                                                              and measure the amount of liquid precipitation (as opposed to solid
          continuous plot of rainfall against time
                                                              precipitation that is measured by a snow gauge) over a set period of
                                                              time.A recording rain gauge can measure data continuously or for a
                                                              specific time duration. As mentioned earlier recording type raingauge
                                                              is further classified into tipping bucket,weighing bucket,syphon type
                                                              and optical type rain gauge.

                                                              The optimal number of rain gauge required for a catchment is
                                                              estimated by,

                                                              N = coefficient of variation of the rainfall data in the existing station /
                                                              allowable degree of error in the estimation of mean precipitation

                                                              If data is missing and the rainfall at different gauge station varies less
                                                              than 10% from the mean then the missing rainfall data of the station
                                                              which was unable to measure due to relocation or natural calamities
                                                              will be given by,
Test for Consistency of Record
Performed by double mass curve technique where                Summation of mean precipitation of the            neighboring stations /
accumulated annual rainfall of a station is plotted against   number of neighboring station
accumulated annual rainfall average of a group of 5-10
station in a chronological manner. If the curve deviates      But if rainfall of the adjacent station varies more than 10% then
from the slope of itself the slope of the deviated curve is   normal ratio method is used :
divided from the slope of the original curve. The result is
multiplied with the annual precipitation of the station to    The normal or annual precipitation for 30 years of the station whose
estimate the corrected precipitation which is used to         data was missing is divided by number of stations and the result is
measure the deviation of consistency of the measured          multiplied by the summation of the ratio of precipitation and normal
rainfall data.                                                precipitation of each neighborhood station.
                          Tipping Bucket Type


Clockwise driven chart : When
the buckets become full it tips
and the signal is channelized
through an electric pen for
marking in this chart
                  Electric Pen




 Tipping Bucket : This bucket
 can hold 0.2 mm of water. Once
 full it tips sending a signal to the
 Electric Pen for recording.
Weighing Bucket Type

          Collecting Funnel: The funnel
          is used for collecting rain water




          Collecting Bucket: The bucket
          is filled up by the collected rain
          water.The weight of the bucket
          and the water is recorded on a
          clock work driven chart to
          provide a continuous plot of
          storm.
    Natural-Syphon Type

                               Float Connected Pen
                               A float (not seen in the picture) is attached to this pen
                               which records the rising level in the scale.


                               Recording Scale
                               This scale records the rising of the float through the
                               pen.



Rain Water Collecting Funnel
              Optical Type
• In optical raingauge a laser sensor and a
  photo transducer sensor is attached below
  the collecting bucket.
• When the bucket overflows excess water
  falls into the laser.
• The water diffracts the optical ray which is
  captured by the sensor and recorded to
  measure the rainfall.
                   Data Representation
Mass Curve                                              Hyetograph

A graphical representation of accumulated               Hyetograph is a curve which represents
precipitation against time arranged in a                precipitation intensity or incremental precipitation
chronological manner.                                   against duration of the event.

Represents :                                            Represents :

Duration and Magnitude of Storm                         Characteristic of storm floods
Intensities at various time interval                    Total precipitation in a specific time duration




                                                         Mass curve




                                                                             Hyetograph




               Figure shows a comparative diagram of masscurve and hyetograph for rainfall.Retrieved from
                                                                                                            Contd.
               http://tolstoy.newcastle.edu.au/R/e9/help/10/01/attachment.html
                  Data Representation


Point/Station Rainfall                             Mean Precipitation

The representation of precipitation per            The average precipitation of a catchment can be
hour/day/month for a specific gauge station is     estimated based on the data recorded by the
know as point or station rainfall data.            gauge stations present in that catchment. The
                                                   following methods are widely used :
Represents :
                                                   Arithmetic Mean
Moving average of the curve between                Thiessen Mean Method
precipitation against duration time for three to   Isohyetal Method
five consecutive time steps can represent the
trend of the variable in that station.
                       Arithmetic Mean
What ?

When the variation of precipitation in various stations is small arithmetic mean is used to
estimate mean precipitation of the catchment.



Formula

If Pn is the precipitation recorded by nth gauge station in a specific duration of time and N be
the total number of stations available in the catchment, then arithmetic mean of precipitation is
given by,



Pm = (P1+P2+P3…Pn)/N
                   Thiessen Mean Method
What ?

When the variation of precipitation in various stations is larger than 10% Thiessen Mean method is
used as a method to estimate mean precipitation of the catchment.



Formula

If Pn is the precipitation recorded by nth gauge station in a specific duration of time and N be the total
number of stations available in the catchment and An is the surrounding area, created by the bisects of the
straight lines connecting the gauge stations, of the nth gauge station, then mean precipitation is given by,



Pm =

(A1P1+A2 P2+A3 P3…AnPn)/(A1+A2+A3…An)
                             Iso-hyetal Method
What ?

When the variation of precipitation in various stations is larger than 10% Iso-hyetal method is used as a
method to estimate mean precipitation of the catchment. Iso-hyetal lines join the stations or points
which have equal magnitude of precipitation.(Iso means equal and hyeto means rain,hence Isohyetal.
For example line joining equal temperature points, isotherm; equal cloud cover, isoneph; equal
humidity, isohume; equal pressure, isobar etc.

Formula

If Pn is the precipitation of the nth isohyet in a specific duration of time and N and An-1 is the inter-isohyet
area, then mean precipitation is given by,



Pm =

[{(A1(P1+P2)/2}+{A3(P2+P3)/2}..+{An-1(Pn-1+Pn)/2}]/Total Area of the Catchment

Isohyet method is more accurate than the Arithmetic mean and Thiessen Mean Method of estimating
precipitation within a catchment.
                 Depth Area Duration
•   Depth Area Duration curve is used to represent the relationship between precipitation
    depth and area.

•   DAD curves is analyzed before the hydrological design of large hydraulic structures.

•   To develop DAD :

•   1.The severemost storm is first identified.

•   2.Then Isohyetal maps and mass curve of the storm is compiled.

•   3.From the mass curve the rainfall depth and from Isohyetal map,area of the storm is
    estimated.

•   4.A curve is drawn showing depth of precipitation against area starting from the
    center of the storm

•   From the curve it can be observed that for a given duration precipitation is inversely
    proportional to area and for a given area, precipitation depth is proportional to
    duration of rainfall.
         Frequency of Precipitation
Probability of An Extreme Event is important and         Plotting Position
essential for designing hydraulic structures for
extreme condition. In this regard Probability of
                                                         (Weibull Formula)
precipitation events are calculated and used in the
analysis often called as uncertainty or risk analysis.   A plot of precipitation magnitude with the
                                                         return period can be calculated with the
•   Probability (P) of precipitation X cm of duration    help of plotting position which is
    Y hr occurring r times in n successive years will    calculated by,
    be :

                                                         P
     Pr,n
                                                         = (m/(N+1))             (3)

     = nCrPrq(n-r)       (1)                             If the event of precipitation within a time frame
                                                         is arranged in a descending order based on the
     = {n!/(n-r)!r!}Prq(n-r)                   (2)       magnitude then m in Equation 3 will be the
                                                         order number and N will be the number of
                                                         years included in the time frame. Equation 3
•   Return period or recurrence interval is the          varies for different regions and is an empirical
    inverse of P.                                        equation to estimate probability.
         Frequency of Precipitation
Intensity-Duration-Frequency (IDF)                       Probable Maximum Precipitation (PMP)


     –   Intensity of precipitation decreases with           –   Probability of maximum possible
         increase in duration                                    precipitation for a given catchment area
     –   Increase in return period will make that                which can not be exceeded is known as
         intensity of precipitation rarer                        probable maximum precipitation
     –   Relationship between precipitation                  –   Used in designs of major hydraulic
         intensity(i), duration(D) of the                        structures like reservoir, dams, channels,
         precipitation and return period(T) of the               spillways etc.
         same is represented as :                            –   According to statistical studies PMP can
                                                                 be estimated as :


     i = (KTx)/ (D+a)n                            (4)
                                                             PMP = Pm + Kσ                         (5)
     where x,K,a and n are constants which varies with
        catchments                                           where Pm is the mean precipitation,K is the frequency factor
                                                                 which depends upon statistical distribution of the
                                                                 precipitation time series,number of years of record and
                                                                 return period.

				
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posted:11/17/2012
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