Precipitation.pdf

PRECIPITATION Prepared by: Engr. Teofilo Jr U. Sevilla The initial process in the formation of precipitation is the condensation or sublimation of atmospheric moisture. the process by which water vapor in the air is changed into Condensation...

PRECIPITATION Prepared by: Engr. Teofilo Jr U. Sevilla The initial process in the formation of precipitation is the condensation or sublimation of atmospheric moisture. the process by which water vapor in the air is changed into Condensation liquid water the process by which water vapor in the air is changed into Sublimation solid without becoming liquid. all forms of water that forms in the atmosphere and falls Precipitation back to the earth’s surface. The usual forms are rain, snowfall, hail, frost, dew. The initial process in the formation of precipitation is the condensation or sublimation of atmospheric moisture. the process by which water vapor in the air is changed into Condensation liquid water Deposition the process by which water vapor in the air is changed into Sublimation solid without becoming liquid. all forms of water that reach the earth’s surface. The usual Precipitation forms are rain, snowfall, hail, frost, dew. The condensation into water droplets and sublimation into ice crystals of atmospheric water vapor generally occur around condensation or sublimation nuclei. Principal types of these nuclei; Dusts Ocean salt aerosols Products of combustion and oxides of nitrogen Other particulate matter (less than a micron in diameter)  Causes of Condensation or Sublimation  Adiabatic cooling is the primary cause of condensation and hence, is responsible for most precipitation. Adiabatic cooling results mainly from expansion of air mass which results from a decrease in atmospheric pressure. Adiabatic cooling is triggered primarily by the upward or vertical motion of air masses. As a result, storm or precipitation events are usually classified according to the conditions that cause this upward motion.  Causes of Condensation or Sublimation  Mixing of air masses of varying temperatures, radiation cooling and contact cooling. Heat radiating from the earth’s surface at night, cools the bottom air until it reaches saturation. Fog forms first at the surface, thickens as cooling continues. Further radiational cooling at the top of fog layer, deepens it.  In order for Precipitation to form:  The atmosphere must have moisture.  There must be sufficient nuclei present to aid condensation. The nuclei are usually tiny salt particles or products of combustion.  Weather condition must be good for condensation of water vapor to take place.  The products of condensation must fall back to the earth. Precipitation results when water droplets come together and coalesce to form larger drops. In order for Precipitation to form: The collision-coalescence process is when cloud Collision-Coalescence droplets collide and coalesce or stick together.  Forms of Precipitation:  Rain – form of water drops of sizes larger than 0.5 mm.  Light Rain – trace to 2.4 mm/hr  Moderate Rain – 2.5 mm/hr to 7.5 mm/hr  Heavy Rain - >7.5 mm/hr  Snow – consists of ice crystals which usually combine to form flakes.  Drizzle – a fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/hr.  Glaze – when rain or drizzle come in contact with cold ground at around 0 degC.  Sleet – it is a frozen raindrops of transparent grains which form when rain falls through air at subfreezing temperature.  Hail – a showery precipitation in the form of irregular pellets or lumps of ice of size more than 8 mm.  Graupel - is also called snow pellets or soft hail, as the graupel particles are particularly fragile and generally disintegrate when handled.  Types of Precipitation:  Convective Precipitation - results from the differential heating of air masses near the ground surface resulting in the upward movement of warmer air masses. A heated air mass moving upward is cooled down by the surrounding air and by the expansion process. Cooling brings about condensation which may develop into precipitation.  Types of Precipitation:  Orographic Precipitation - precipitation influenced by topography. Moist air masses moving over mountain barriers are cooled, resulting in condensation and precipitation. Such air masses get dry and warm as they move downslope on the leeward side of the mountain barriers. Orographic types of rainfall are usually of very low intensity.  Types of Precipitation:  Cyclonic Precipitation - Are associated with the movement of air masses due to differences in barometric pressure. This type of precipitation is triggered by the presence of a low pressure into which air flow converges resulting in the lifting and subsequent cooling of air masses. In most parts of the Philippines, more than 80% of the total rainfall are attributable to cyclonic precipitation.  Annual Rainfall The mean annual rainfall of the Philippines varies from 965 to 4,064 millimeters annually. Baguio City, eastern Samar, and eastern Surigao receive the greatest amount of rainfall while the southern portion of Cotabato receives the least amount of rain. At General Santos City in Cotabato, the average annual rainfall is only 978 millimeters.  Measurement of Precipitation Precipitation is expressed in terms of the depth of rainfall water would stand on an area if all the rain were collected. The precipitation is collected and measured in a rain gauge. Terms such as pluviometer, ombrometer, and hyetometer are sometimes referred to as rain gauge.  Measurement of Precipitation For setting up a rain gauge:  The ground must level and in an open and the instrument must present a horizontal catch surface.  The gauge must be set near the ground as possible to reduce wind effects, but it must be sufficiently high to prevent splashing, flooding, etc.  The instrument must be surrounded by an open fenced area of at least 5.5 m x 5.5 m. No object should be nearer the instrument than 30 m or twice the height of the obstruction.  Measurement of Precipitation Rain gauges can be broadly classified into two category: (1) Non-recording Gauges; (2) Recording Gauges. Non-Recording Gauges: 1. Symon’s Gauge - 2. Standard 8-inch Rain Gauge  Measurement of Precipitation Recording Gauges: 1. Tipping-Bucket Type – the catch from the funnel falls onto one of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it tips and brings other one in position.  Measurement of Precipitation Recording Gauges: 2. Weighing-Bucket Type – the catch from the funnel empties into a bucket mounted on a weighing scale. The weight of the bucket and its contents are recorded on a clockwork-driven chart. This instrument gives a plot of the accumulated rainfall against elapsed time.  Measurement of Precipitation Recording Gauges: 3. Natural-Siphon Type – also known as float-type gauge. The rainfall is collected by a funnel-shaped collector led into a float chamber using a float to rise. As the float rises, a pen records the elevation of the float on a rotating drum driven by a clockwork mechanism. A syphon arrangement empties the float chamber after reaching the maximum level.  Measurement of Precipitation Recording Gauges: 4. Telemetering Rain Gauges – recording type and contain an electronic units to transmit the data on rainfall to a base station both at regular intervals and on interrogation. 5. Radar Measurement of Rainfall - ground-based radars send out pulses of microwave energy in narrow beams that scan in a circular pattern. When the microwave pulse encounters precipitation particles in the atmosphere, the energy is scattered in all directions, sending some energy back to the radar.  Rain Gauge Network World Meteorological Organization (WMO) recommendation on rain gauge densities; In flat region of temperate, Mediterranean and tropical zones: Ideal – 1 station for 600 to 900 sq. km, Acceptable – 1 station for 900 to 3000 sq. km. In mountainous regions of temperate, Mediterranean and tropical zones: Ideal – 1 station for 100 to 250 sq. km, Acceptable – 1 station for 25 to 1000 sq. km. In arid and polar zones: 1 station for 1500 to 10, 000 sq. km depending on feasibility 10% of rain gauge stations should be equipped with self-recording gauges to know the rainfall intensities.  Adequacy of Rain Gauge Stations If there already rain gauges in a catchment, the optimal number of rain gauge that should exist to have an assigned percent of error is given by, Where N = optimal number of stations; Cv = coefficient of variation; ε = allowable degree of error in the estimate of the mean rainfall at existing m stations (%).  Adequacy of Rain Gauge Stations If there are m existing stations, Cv can be computed as,  Adequacy of Rain Gauge Stations If there are m existing stations, the percentage of error of estimating mean precipitation can be calculated, In calculating the number of rain gauges N for a given level of error it is usual to take as 10%. As the value of is small, the number of rain gauges will be more.  Adequacy of Rain Gauge Stations Let’s try! Example No. 1  Estimation of Missing Data Before using the rainfall data in application, it is necessary to check the data for continuity and consistency. In the estimation of missing data from a station, the performance of the neighboring stations including the one of missing data are considered. Normal Rainfall Is the average rainfall at a particular data, a month or year over a specified 30-year period. If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station X, use a simple arithmetic average procedure, otherwise, use the Normal Ratio Method:  Estimation of Missing Data Let’s try! Example No. 2  Test for Consistency of Record Common causes of inconsistency of record; Shifting of rain gauge station to new location The neighboring station undergo a marked change Change in ecosystem due to calamities Occurrence of observational error Double-Mass Curve Based on the principle that when recorded data comes from the same parent population, they are consistent.  Test for Consistency of Record A group of 5 to 10 base stations in the neighborhood of the problem station X is selected. The data of annual rainfall of the station X and also the average rainfall of the group of base stations covering a long period is arranged in reverse chronological order. The accumulated precipitation of station X and the accumulated values of the average of the group are calculated and plotted.  Presentation of Rainfall Data 1. Mass Curve of Rainfall – plot of the accumulated precipitation against time, plotted in chronological order.  Presentation of Rainfall Data 2. Hyetograph – plot of the intensity of rainfall against the time interval. It is derived from the mass curve and is usually represented in bar chart/graph.  Presentation of Rainfall Data 3. Point Rainfall – also known as station rainfall, refers to the rainfall data of a station. Graphically, these data are represented as plots of magnitude vs chronological time in the form of bar diagram. 4. Moving Average – is a technique for smoothening out the high frequency fluctuations of a time series and to enable the trend, if any, to be noticed. Bar Chart of Annual Rainfall at Station X 3-Year Moving Mean  Mean Precipitation Over an Area 1. Arithmetic Mean Method – when rainfall over a catchment show little to no variation, the average precipitation over an area is taken as the arithmetic mean of the station values.  Mean Precipitation Over an Area 2. Thiessen Polygon – the rainfall recorded at each station is given a weightage on the basis of an area closest to the station.  Mean Precipitation Over an Area 2. Isohyetal Method – the isohyets of various values are drawn, the area of two adjacent isohyets are determined and the average value of rainfall indicated by the two isohyets is assumed to be acting over the inter-isohyet area.  Mean Precipitation Over an Area Let’s try! Example No. 3  Mean Precipitation Over an Area Let’s try! Example No. 4  Mean Precipitation Over an Area Oh My Tasks  Obtain a 10-year period of rainfall data from USeP PAGASA and encode the data in the spreadsheet. Test the consistency of data using double-mass curve and present the data using the different methods of presenting rainfall data.  Mean Precipitation Over an Area Another Tasks  Research for the list of rain gauge station in the Philippines. Summarize the list based on the following table format; Station No. Location Northings Eastings 1 Davao City 7.0736∘ N 125.6110∘ E 2 Tagum City 7.4472∘ N 125.8093∘ E 3 Tacurong City 6.6880∘ N 124.6788∘ E