An Introduction to PRECIPITATION

1. An Introduction toPRECIPITATION Mrinmoy Majumder www.baipatra.ws

2. Precipitation Precipitation is any form by which condensation of atmospheric water vapor is pulled down by gravity and deposited on the Earth's surface. Precipitation is generally measured with either rain gaugeor radars. The data is represented by Mass curveand/or Hyetographs. The arial distribution of precipitation is expressed by DAD curves. The frequency of precipitation is expressed by Frequency Duration Curvesand/or PMP. Causes of Precipitation or Weather System for Precipitation Front : Front is the condition when two distinct mass of air meets with 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 subsequent precipitation. Cyclone : A large low pressure region is known as Cyclone. Tropical – When the diameter of the low pressure region is 100-200km,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 can be observed in the region. The intensity of the storm decreases with the distance from the eye or center of the cyclone. Extra-tropical – Cyclones that form outside the tropical zone and are associated with a frontal system is called as Extra-tropical Cyclone. The circulation of wind is counterclockwise in the northern hemisphere. Though magnitude of precipitation and wind velocity is smaller than the tropical cyclones. The intensity of the precipitation is low but the duration and extent of the low pressure region is larger than the former. Anticyclones : A region of low pressure with moderate wind speed and clockwise wind circulation is called as Anti-Cyclone. Convective Precipitation : Localized warming can form a warm air mass surrounded by a cooler one. The warmer mass moves up and form convective clouds and subsequent precipitation. Areal extent of such precipitation is small. Orographic Precipitation : The warm, moist air moves up in the windward side of the mountain barriers and undergo cooling, condensation and Orographic Precipitation. Leeward slopes of the barrier faces lighter rainfall than the other side. Different Forms of Precipitation Raindrop : Common in tropical zones and lower elevation areas. Raindrops are formed due to the condensation and coalescence process. Water droplets 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 resistance of air to gravity, those droplets fall to ground as raindrops. Ice pellets : If sub-freezing layer is present in both below and above an above freezing layer raindrops are converted to crystals of ice before reaching the ground as the cold layer, below the warm re-freezes the raindrops into ice. Hail : Hail forms in storm clouds when super cooled water droplets freeze on contact with dust or dirt. The storm's updraft blows the hailstones to the upper 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 increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls from the cloud. Snowflakes : Snow crystals form when tiny super cooled cloud droplets (about 10 μm in diameter) freeze. Drizzle : Rain-droplets of size less than 0.5 mm and rain intensity of less than 1mm/h is known as drizzle. Glaze : When rain or drizzle touches ground at zero degree centigrade, glaze or freezing rain is formed.

3. Measurement of Precipitation Gauges There are two type of gauges : 1.Recording 2. Non Recording Recording gauges can be further classified into: a. Tipping Bucket b. Weighing Bucket c. Natural Syphon d. Optical Type Telemetry to send data of multiple gauge stations through a radio channel and advent of optical rain gauges has improved the accuracy and exclusivity of measurement. Radars As amount of precipitation over a large area can be measured with a good degree of accuracy radars are now frequently used in measurement of precipitation. When electro-magnetic signals of a radar crosses with a drop of precipitation it can be observed that : Pr = average echo-power = CZ/r2 where C = constant, Z = radar echo factor = aIb r = distance to target volume I = rainfall intensity in mm/hr A,b = coefficients, calibrated with the help of observed gauge data of the same region.

4. 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

5. 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 and measure the amount of liquid precipitation (as opposed to solid 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, Summation of mean precipitation of the neighboring stations / number of neighboring station But if rainfall of the adjacent station varies more than 10% then normal ratio method is used : The normal or annual precipitation for 30 years of the station whose data was missing is divided by number of stations and the result is multiplied by the summation of the ratio of precipitation and normal precipitation of each neighborhood station. This type of gauges do give continuous plot of rainfall against time Test for Consistency of Record Performed by double mass curve technique where accumulated annual rainfall of a station is plotted against accumulated annual rainfall average of a group of 5-10 station in a chronological manner. If the curve deviates from the slope of itself the slope of the deviated curve is divided from the slope of the original curve. The result is multiplied with the annual precipitation of the station to estimate the corrected precipitation which is used to measure the deviation of consistency of the measured rainfall data.

6. 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.

7. 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.

8. 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

9. 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.

10. Data Representation Hyetograph Hyetograph is a curve which represents precipitation intensity or incremental precipitation against duration of the event. Represents : Characteristic of storm floods Total precipitation in a specific time duration Mass Curve A graphical representation of accumulated precipitation against time arranged in a chronological manner. Represents : Duration and Magnitude of Storm Intensities at various time interval Mass curve Hyetograph Figure shows a comparative diagram of masscurve and hyetograph for rainfall.Retrieved from http://tolstoy.newcastle.edu.au/R/e9/help/10/01/attachment.html Contd.

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

12. 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

13. 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)

14. 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.

15. 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.

16. Frequency of Precipitation Plotting Position (Weibull Formula) A plot of precipitation magnitude with the return period can be calculated with the help of plotting position which is calculated by, P = (m/(N+1)) (3) If the event of precipitation within a time frame is arranged in a descending order based on the 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 varies for different regions and is an empirical equation to estimate probability. Probability of An Extreme Event is important and essential for designing hydraulic structures for extreme condition. In this regard Probability of precipitation events are calculated and used in the analysis often called as uncertainty or risk analysis. • Probability (P) of precipitation X cm of duration Y hr occurring r times in n successive years will be : Pr,n = nCrPrq(n-r) (1) = {n!/(n-r)!r!}Prq(n-r)(2) • Return period or recurrence interval is the inverse of P.

17. Frequency of Precipitation Probable Maximum Precipitation (PMP) • Probability of maximum possible precipitation for a given catchment area which can not be exceeded is known as probable maximum precipitation • Used in designs of major hydraulic structures like reservoir, dams, channels, spillways etc. • According to statistical studies PMP can be estimated as : PMP = Pm + Kσ(5) 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. Intensity-Duration-Frequency (IDF) • Intensity of precipitation decreases with increase in duration • Increase in return period will make that intensity of precipitation rarer • Relationship between precipitation intensity(i), duration(D) of the precipitation and return period(T) of the same is represented as : i = (KTx)/ (D+a)n(4) where x,K,a and n are constants which varies with catchments