Download Convective Precipitation

Definition
 All types of moisture reaching the surface of earth from
atmosphere.
Precipitation is the basic input to the hydrology
 Factors determining
precipitation or the
amount of atmospheric
moisture over a region
 Climate
 Geography
 Ocean surfaces is the
chief source of moisture
for precipitation
.
Forms of precipitation
Rain
 Rain is the most common type of
precipitation in our atmosphere. Rain is
when liquid droplets fall to the surface of
the Earth.
 There are two different forms of rain,
either in the form of
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
showers
drizzles
 Showers are heavy, large drops of rain
and usually only last a period of time.
 Drizzles however usually last longer
and are made up of smaller droplets
of water.
 Rain can either be formed as ice crystals
melt or it can be smaller water droplets.
Light
I = 2.5mm/hr
Moderate
I = 2.8-7.6mm/hr
Heavy
I > 7.6 mm/hr
Snow
 Snow is the second most common precipitation in the
North East.
 Snow forms when water vapor turns directly into ice
without ever passing through a liquid state. This
happens as water condenses around an ice crystal.
Density of freshly fallen
snow varies between 125500mm of snow required
to equal 25mm of liquid
water
Average density (specific
gravity) = 0.1
Hail
 Hail is created when moisture and wind are together. Inside
the cumulonimbus clouds ice crystals form, and begin to fall
towards the surface of Earth. When this starts to happen
wind gusts start to pick up the ice crystals pushing them up
high into the clouds. As they start to fall down again they
continue to grow in size. A wind gust might catch the hail
stone again which will push it back up into the cloud. This
whole process gets repeated several times before the hail
stone becomes so big that it is too heavy for the wind to carry
so it must fall towards Earth.
Shapes of hail particles
1. Spherical
2. Conical
3. Irregular
Diameter range 5 to 125 mm
Specific gravity = 0.8
Average density (specific gravity) =
0.1
Fog
 There is really no different between fog and the
clouds that are high in the sky. In simple terms
fog is; a cloud that has formed near the surface of
the Earth.
 There are four main types of fog,




radiation fog
advection fog
upslope fog
evaporation fog
Dew
 The small drops of water which can be found on cool
surfaces like grass in the morning.
 This is the result of atmospheric vapor condensing on
the surface in the colder night air.
 Dew Point is the temperature in which condensation
starts to take place or when dew is created.
Mist
 Mist is a bunch of small droplets of water which are in the
air. This occurs with cold air when it is above a warm surface,
for example water.
 Fog and mist are very similar, the only difference is their
visibility.
 If you cannot see 1 kilometer or less you know you're dealing
with fog.
 You can see visuals through mist and it is more haze looking
than a thicker substance.
Diameter range between 0.1
and 0.5 mm
Glaze
 Glaze is the ice coating, generally clear and smooth,
formed on exposed surfaces by the freezing of
super cooled water deposited by rain or drizzle.
Specific gravity may be as high as 0.8-0.9
Rime
 Rime is the white opaque deposit of ice granules
more or less separated by trapped air and formed
by rapid freezing of super cooled water drops
impinging on exposed objects.
Specific gravity may be as low as 0.2-0.3
Sleet
 Sleet consists of transparent, globular, solid grains
of ice formed by the freezing of raindrops or
freezing of largely melted ice crystals falling
through a layer of sub freezing air near the earth’s
surface.
Lapse rate
 The lapse rate is defined as the rate of decrease with height for
an
atmospheric
variable.
The
variable
involved
is temperature unless specified otherwise.
 The terminology arises from the word lapse in the sense of a
decrease or decline; thus, the lapse rate is the rate of decrease
with height and not simply the rate of change. While most
often applied to Earth's atmosphere.
 In general, a lapse rate is the
 where γ is the lapse rate given
in units of temperature divided by
units of altitude, T = temperature,
and z = altitude. Average lapse rate
is about 2°C/1000ft
Altitude (m)
negative of the rate of temperature
change with altitude change, thus:
Temperature (C)
Formation of precipitation
• Convective
system resulting
from unequal
Radiative
heating and
cooling
of earth
Large
scale
surface and
cooling
atmosphere
needed
• Convergence
caused by
Orographic
barriers
Saturation
• Formation of
precipitation
 Moisture is always present
in the atmosphere, even on
the cloudless day.
 Saturation however does
not necessarily lead to
precipitation.
Necessary mechanism to form Precipitation
1. Lifting mechanism to cool the air
2. Formation of cloud elements
(Droplets/Ice crystals)
3. Growth of cloud elements
4. Sufficient accumulation of cloud elements
1. Lifting mechanism to cool the air
Lifting mechanism gives the three main types of
Precipitation.
 Cyclonic Precipitation (Frontal /non Frontal)
 Convective Precipitation
 Orographic Precipitation
Cyclonic Precipitation
(Frontal/Non frontal)
Frontal precipitation results when the leading edge( front) of a
warm air mass meets a cool air mass. The warmer air mass is
forced up over the cool air. As it rises the warm air cools,
moisture in the air condenses, clouds and precipitation result.
Convective Precipitation
Convectional precipitation results from the heating of the earth's
surface that causes air to rise rapidly. As the air rises, it cools and
moisture condenses into clouds and precipitation
Orographic Precipitation
It results when warm moist air of the ocean is forced to rise by
large mountains. As the air rises it cools, moisture in the air
condenses and clouds and precipitation result on the windward
side of the mountain while the leeward side receives very little.
This is common in British Columbia.
Formation of cloud elements
(Droplets/Ice crystals)
 For droplets, hygroscopic nuclei ,small particles
(0.1-10µm) having affinity for water must be
available in upper troposphere.
 For ice crystals, Freezing Nuclei are required.
 Source of condensation nuclei are particles of sea
salts, products of sulphurous and nitric acid
 Source of freezing nuclei are clay minerals, usually
kaolin, silver iodide etc
Growth of cloud elements
For occurrence of precipitation over an area it is necessary that cloud
elements must be grown in size to over come
 Coalescence of cloud droplets
Cloud droplets are usually smaller than 50µm in diameter, due to different
diameters of droplets they fall with varying fall velocities. As the bigger cloud
elements are heavier , having more fall velocity, hence they collide with smaller
droplets. Smaller droplets join the bigger droplets and in this way the size of
cloud droplets increases.
 Co-existence of cloud droplets & ice crystals
If in a layer of clouds there is mixture of water droplets and ice crystals. As the
saturation vapour pressure over ice is lesser than over water. As a result of this
difference , there results evaporation of water drops and condensation of much of
this water on ice crystals. Causing their growth and ultimate fall through clouds.
The ice crystals will further grow as they fall and collide with water drops.
Growth of droplets and ice crystals
For the occurrence of precipitation over an area
necessary conditions are :
 Cloud elements must increase in size until their falling
speeds exceed the ascending rate of air
 Cloud elements should be large enough in size not to
get evaporated completely before reaching the ground
Measurement of Precipitation
 1. Amount of precipitation
 2. Intensity of precipitation
 3. Duration of precipitation
 4. Arial extent of precipitation
Measurement Methods
 Measurement of precipitation (Rain and Snow) can be
done by various devices. These measuring devices and
techniques are;
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Rain Gauges
Snow Gauges
Radars
Satellites
Scratching of snow packs
Water equivalent in snow packs
RAIN GAGES
 Rain gages are most commonly used for the
measurement of precipitation, both in terms of rain
fall and snow.
Types of rain gages
 There are two main types of rain gages which are used
to measure the precipitation. These are;
 1. Non recording rain gages
 2. Recording rain gages
Non recording rain gauges
 It is a rain gage which does not provide the
distribution of amount of precipitation in a day. It
simply gives the amount of precipitation after 24
hours (daily precipitation).
Recording rain gauges
 These rain gauges are also called integrating rain
gauges since they record cumulative rainfall. In
addition to the total amount of rainfall at a station, it
gives the times of onset and cessation of rains (thereby
gives the duration of rainfall events)
Types of recording Rain gauges
 There are three main types of recording rain gauges
 1. Float type rain gages
 2. Tipping bucket type rain gages
 3. Weighing type rain gages
1. Tipping bucket gauges
tipping bucket rain gauge is used for
measurement of rainfall. It measures the rainfall
with a least count of 1 mm and gives out one
electrical pulse for every millimeter of rainfall
A
2. Weighing type gauges
 It consists of a storage bin, which is weighed to record the
mass. It weighs rain or snow which falls into a bucket, set
on a platform with a spring or lever balance. The increasing
weight of the bucket and its contents are recorded on a
chart. The record shows accumulation of precipitation.
3. Float recording gauges
 The rise of float with increasing catch of rainfall is recorded. Some
gauges must be emptied manually while others are emptied
automatically using self starting siphons. In most gauges oil or mercury
is the float and is placed in the receiver, but in some cases the receiver
rests on a bath of oil or mercury and the float measures the rise of oil or
mercury displaced by the increasing weight of the receiver as the
rainfall catch freezes. Float may get damaged by rainfall catch freezer
Errors in precipitation measurement
by Rain Gauges
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Instrumental errors
Errors in scale reading
Dent in receivers
Dent in measuring cylinders
About 0.25mm of water is initially required to wet the surface of
gauge
Rain gauges splash from collector
Frictional effects
Non verticality of measuring cylinders (10° inclination gives 1.5%
less precipitation)
Loss of water by evaporation
Leakage in measuring cylinder
Wind speed reduces measured amount of rain in the rain gauges.
Measurement of snow
In case of snow fall following two properties of more
interest are measured.
 1. Depth of snow at a particular place in mm/inches
 2. Equivalent amount of water in mm
1. Depth of snow
Depth of snow fall at a particular place can be
measured by the following methods.
 a. Standard rain gauges without collectors
 b. Snow gauges
 c. By scratching snow packs
Depth of snow methods
 Standard rain gauges can also be used for measuring
the snow depth, with some alterations in the
arrangement of rain gauges, particularly, the collectors
are not used
 On a paved surface with snow over it, scratching that
snow layer with some scrapper helps to measure the
depth of snow fall with a tape. Visual observation and
with help of measuring tape helps to measure the
depth of snow
Snow gauges
 A snow gauge is a
type of instrument
used to measure the
solid
form
of
precipitation.
2. Equivalent water in snow
 Snow Water Equivalent (SWE) is a common snow pack
measurement. It is the amount of water contained
within the snow pack. It can be thought of as the
depth of water that would theoretically result if you
melted the entire snow pack instantaneously.
Equipment used is;
 Standard rain gages without receivers
 Weighing type rain gages
 Snow gages
Measurement of equivalent amount of water in
a snow pack
 The equivalent amount of water in a snow pack can be
measured by
 1. Heating
 2. Weighing
 3. Adding measured amount of hot water
1. By Heating
 The equivalent amount in mm of water can be
obtained by heating the cylinder. it will melt the snow
and the depth of the liquid water can be measured
with a measuring stick but this approach is adjustable
because some water may get evaporated during the
heating.
2. By Weighing
Weight is measured either by weighing type rain gauges or by using a snow gauge.
W=W1-W2
W1= weight of snow + empty cylinder
W2= Weight of empty cylinder
W= Weight of snow
By using weight volume relationship
Γ = Weight/ Volume
Γ = W/ A.h
h = W/A Γ
Where,
h = Equivalent amount of water in snow.
3. By scratching snow packs
A measured amount of hot water is added into the
cylinder which will melt the snow. Now measure the
total depth of water in the cylinder “h1”
h = h1-h2
Where,
h2 = measured amount of hot water
h = equivalent amount of water
Radar Measurements
 A weather radar is a type of radar used to locate precipitation,
calculate its motion, estimate its type (rain, snow, hail, etc.), and
forecast its future position and intensity. Weather radars are
mostly Doppler radars, capable of detecting the motion of rain
droplets in addition to intensity of the precipitation. Both types
of data can be analyzed to determine the structure of storms and
their potential to cause severe weather.
Satellite Measurements
 A weather satellite is a type of
satellite that is primarily used
to monitor the weather and
climate of the Earth.These
meteorological
satellites,
however, see more than clouds
and cloud systems, like other
types
of
environmental
information collected using
weather satellites.
Interpretation Of Precipitation Data
Interpretation of missing precipitation data includes;
 1. Estimating missing precipitation data at a station
 2. Checking inconsistency in particular data at a station
 3. Averaging precipitation over an area
1- Estimating missing precipitation data at a
station
Missing precipitation data is estimated by two commonly
used methods.
 Arithmetic Mean Method
 Normal Ratio Method (NRM)
Arithmetic Mean Method
 Arithmetic mean method is used when normal annual
precipitation is within 10% of the gauge for which data
are being reconstructed. This method is least accurate
however.
Example
Normal Ratio Method (NRM)
 Normal ratio method (NRM) is used when the normal annual
precipitation at any of the index station differs from that of the
interpolation station by more than 10%. In this method, the
precipitation amounts at the index stations are weighted by the ratios
of their normal annual precipitation data in a relationship of the form:
Where:
Pm = precipitation at the missing location
Pi = precipitation at index station
Nm = average annual rain at ‘missing data’ gauge
Ni = average annual rain at gauge
N = number of rain gauges
2- Checking inconsistency in a particular data
record at a station
 By a technique called Double Mass Curve Analysis.
 It is used to check the consistency of many kinds of
hydrologic data by comparing date for a single station with
that of a pattern composed of the data from several other
stations in the area
 The double-mass curve can be used to adjust inconsistent
precipitation data
Double Mass Curve Analysis
The theory of the double-mass curve is
based on the fact that a plot of the two
cumulative quantities during the same
period exhibits a straight line so long
as the proportionality between the two
remains unchanged, and the slope of
the line represents the proportionality.
This method can smooth a time series
and suppress random elements in the
series, and thus show the main trends
of the time series.
3- Averaging precipitation over area
It is the amount of precipitation which can be
assumed uniform over an area. If the average
precipitation over an area is known than total rain
volume of water can be computed for that area.
Rain volume = Pavg × A
Methods for computing average precipitation
There are some widely used methods to compute
average precipitation over an area, but the most
common of these used are:
 Arithmetic mean method
 Theissen polygon method
 Isohytal method
Theissen Polygon Method
 Divide the region (area A) into
sub-regions centred about each
rain gauge;
 Determine the area of each subregion (Ai) and compute subregion weightings (Wi) using:
Wi = Ai/A
 Compute total aerial rainfall
using Rainfall recorded at each
station is given a weight age
based on the area closest to the
station.
Theissen Polygon Method
Consider a catchment area with 3 rain gauge stations.
Let there be 3 stations outside the catchment, but in
its neighborhood.
Catchment area is drawn to scale and position of these
6 stations is plotted on it. Stations are joined to get a
network of triangles. Perpendicular bisectors are
drawn to each of the sides of these triangles.
These bisectors form a polygon around each station. If
the boundary of catchment cuts the bisectors, then
boundary is taken as outer limit of polygon. These
bounding polygons are called Thiessen Polygons.
The area of these polygons is measured with a
planimeter or by grid overlay.
Isohytal Method
 Plot gauge locations on a
map;
 Subjectively
interpolate
between rain amounts
between gauges at a
selected interval;
 Connect points of equal
rain depth to produce lines
of equal rainfall amounts
(isohyets);
Isohytal Method
Compute aerial rain using Isohyets. It is a line
joining points of equal rainfall magnitude.
The catchment area is drawn to scale and the rain
gauge stations are marked on it. The recorded
rainfall values for which aerial average is to
determined are marked at the respective
stations.
Neighboring stations outside the catchment are
also considered. Taking point rainfall values as
the guide, isohyets of different rainfall values are
drawn (similar to drawing contours based on
spot levels.
The area between adjacent isohyets is measured
using a planimeter. If isohyets go out of the
catchment, the catchment boundary is used as
the bounding line.
It is assumed that the average value of rainfall
indicated by two isohyets acts over the inter
isohytal area
Intensity of precipitation
It is the total amount of precipitation falling on
a particular area per unit time
OR
Intensity (I) is defined as the rate of change of
precipitation per unit time
[mm per hour, mm per year, etc…].
Duration
 we need to specify the length of time over which the
rainfall occurred: one year - in the case of annual rainfall;
one month (for many climate purposes); or so many days,
hours or minutes. This period of time over which the rain is
measured is called the duration .
Frequency (f)
 The number of times, during a specified period of years, that
precipitation of a certain magnitude or greater occurs or will
occur at a station; numerically, the reciprocal of the frequency
is usually given .
 What is the rainfall depth over one hour exceeded, on average,
once in ten years?". This "once in ten years" is a FREQUENCY
IDF Curves
TC
 The Tc is generally defined as the time required for a drop of water to
travel from the most hydrologically remote point in the sub catchment
to the point of collection
 It is defined as the time needed for water to flow from the most remote
point in a watershed to the watershed outlet.
 The time of concentration equals the summation of the travel times for
each flow regime. There are numerous methods used to calculate the
travel time for each of the flow regimes. Here, we will discuss a few of
the most prevalent methods. E.g. NRCS Method (Tc = Lo + Lsc + Lc )
 Overland Flow – Lo
 Shallow Concentrated Flow – Lsc
 Channel Flow - Lc
Overland Flow – Lo
 Seelye Method:
Travel time for overland flow can be
determined by using the Seelye chart
 Kinematic Wave Method:
This method allows
for the input of rainfall intensity values.
Where:
Tt = travel time
L = length of overland flow in feet
n = Manning's roughness coefficient
i = rainfall intensity
S = slope in feet/foot
 Manning kinematic formula:
Where:
Tt = travel time (hr.)
n = Manning's roughness coefficient (Table 3)
L = flow length (ft.)
P2 = 2-year, 24-hour rainfall (in.) (Diagram 5)
s = slope of hydraulic grade line (feet/foot)
Shallow Concentrated Flow - Lsc
Where:
Tt = travel time (minutes)
L = length of shallow concentrated flow (feet)
V = velocity (feet per second)
Channel Flow - Lc
 Kirpitch Method:
Tc= 0.0078 (L³/h)⁰∙³⁸⁵
Where:
L= length of the channel in (ft)
h=relief along main channel
 Manning's equation
Where:
V = average velocity (ft./sec.)
r = hydraulic radius (ft.) and is equal to a/Pw
a = cross sectional flow area (ft.2)
Pw = wetted perimeter (ft.)
s = slope of the hydraulic grade line (ft./ft.)
n = Manning's roughness coefficient for open channel flow
Rational Equation for flow estimaton
 When runoff is computed using the rational method tc is the appropriate storm
duration and in turn determines the appropriate precipitation intensity for use in
the rational method equation.
 When runoff is computed using the hydrograph method, tc is used to compute
rainfall-runoff parameters for the watershed. tc is also used as an input to define the
appropriate storm duration.
 Rational method is used to estimate the surface runoff in small watersheds.
Q = CIA
Where:
Q = Discharge (m³/sec)
C = Surface runoff coefficient
I = Rainfal Intensity (mm/hr)
A = Area (ha)
Runoff Coefficients