Transcript Precipitation - Laboratory for Remote Sensing Hydrology and

Applied Hydrology
Storms and Precipitation
Professor Ke-Sheng Cheng
Dept. of Bioenvironmental Systems Engineering
National Taiwan University
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Major storm types in Taiwan
Convective storms or thunderstorms (July –
October)
Tropical cyclones or typhoons (July –
October)
Frontal rainfall systems (November – April)
Mei-Yu (May – June)
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Convective storms
 Thunderstorm cells are the basic organizational
structure of all thunderstorms.
 Each cell goes through a definite life cycle which
may last from 20 minutes to one or two hours,
although a cluster of cells, with new cells forming
and old ones dissipating, may last for 6 hours or
more.
 Individual thunderstorm cells typically go through
three stages of development and decay. These are
the cumulus, mature, and dissipating stages.
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Life cycle of a thunderstorm cell
Cumulus
stage
積雲
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Mature
stage
積雨雲
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Dissipating
stage
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Cumulus Stage
The cumulus stage starts with a rising column
of moist air to and above the condensation level.
The lifting process is most commonly that of
cellular convection characterized by strong
updraft. This may originate near the surface or
at some higher level. The growing cumulus
cloud is visible evidence of this convective
activity, which is continuous from well below the
cloud base up to the visible cloud top.
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The primary energy responsible for initiating the
convective circulation is derived from converging
air below. As the updraft pushes skyward, some of
the cooler and generally drier surrounding air is
entrained into it. Often one of the visible features
of this entrainment is the evaporation and
disappearance of external cloud features.
The updraft speed varies in strength from pointto-point and minute-to-minute. It increases from
the edges to the center of the cell, and increases
also with altitude and with time through this stage.
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The updraft is strongest near the top of the cell,
increasing in strength toward the end of the
cumulus stage.
Cellular convection implies downward motion as
well as updraft. In the cumulus stage, this takes the
form of slow settling of the surrounding air over a
much larger area than that occupied by the
stronger updraft. During this stage, the cumulus
cloud (積雲) grows into a cumulonimbus (積雨雲).
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Cloud droplets are at first very small, but they
grow to raindrop size during the cumulus stage.
They are carried upward by the updraft beyond
the freezing level where they remain liquid at
subfreezing temperatures. At higher levels, liquid
drops are mixed with ice crystals, and at the
highest levels, only ice crystals or ice particles are
found.
During this stage, the raindrops and ice crystals
do not fall, but instead are suspended or carried
upward by the updraft.
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Cumulus cloud
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Cumulonimbus
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Cumulonimbus
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Mature stage
The start of rain from the base of the cloud
marks the beginning of the mature stage. Except
under arid conditions or with high-level
thunderstorms, this rain reaches the ground.
Raindrops and ice particles have grown to such
an extent that they can no longer be supported
by the updraft. This occurs roughly 10 to 15
minutes after the cell has built upward beyond
the freezing level.
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The convection cell reaches its maximum height in
the mature stage, usually rising to 25,000 (8 km) or
35,000 feet (11 km) and occasionally breaking
through the tropopause (see atmospheric layers)
and reaching to 50,000 (15 km) or 60,000 feet (18
km) or higher. The visible cloud top flattens and
spreads laterally into the familiar "anvil" top. A
marked change in the circulation within the cell
takes place.
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Cumulus
stage
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Mature
stage
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Dissipating
stage
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As raindrops and ice particles fall, they drag air
with them and begin changing part of the
circulation from updraft to downdraft.
The mature stage is characterized by a downdraft
developing in part of the cell while the updraft
continues in the remainder.
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 Dissipating Stage
As the downdrafts continue to develop and spread
vertically and horizontally, the updrafts continue to
weaken. Finally, the entire thunderstorm cell becomes
an area of downdrafts, and the cell enters the dissipating
stage.
As the updrafts end, the source of moisture and energy
for continued cell growth and activity is cut off. The
amount of falling liquid water and ice particles available
to accelerate the descending air is diminished. The
downdraft then weakens, and rainfall becomes lighter
and eventually ceases.
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Mesoscale Convective Systems (MCS)
The convective system begins as a number of
relatively isolated convective cells, usually
during the afternoon. By late evening, the
anvils of the individual cells merge, and the
characteristic cold cloud shield develops
toward maturity sometime after midnight,
local time. Dissipation then occurs typically
sometime in the morning.
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 Although by no means restricted to the nocturnal
hours, MCSs most frequently reach maturity after
sunset.
 MCS circular cloud tops often mask a linear
structure of the convective cells when viewed on
radar.
 Occasionally, MCSs can produce a persistent
mesoscale circulation that can persist well after the
convection dissipates. These circulations have been
observed to be associated with redevelopment of
another MCS, so that the system as a whole can
live longer than 24 h.
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Mei-Yu rainfalls are produced by surface
frontal systems which advance southeastward
from southern China to Taiwan from mid to
late spring through early to mid summer each
year. The fronts are usually accompanied by a
synoptic-scale cloud band with embedded
mesoscale convective systems (MCSs),
extending several thousand kilometres from
southern Japan to southern China with an
approximately east–west orientation.
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During the passage of a Mei-Yu frontal system,
a few very active mesoscale convective cells
may develop repeatedly, causing heavy and
localized rainfall for the area. Although the
synoptic-scale frontal system may last for a
few days, the MCSs generally have lifetime of
a few hours to 1 day only.
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Not all MCSs are
nearly circular.
Included within
the category of
MCS is a linearlyorganized band of
cold cloud tops.
Such a structure
is nearly always
associated with a
frontal boundary.
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Estimating the convective rainfall
using weather satellite images
The Scofield-Oliver method was originally
developed for estimating half-hourly
convective rainfall by analyzing the changes
in two consecutive GOES satellite images.
It estimates convective rainfall at interested
locations while not estimating the rain
volume of the cloud systems.
Useful for early warning of flash flood.
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Rationale of the Scofield-Oliver
method
Bright clouds in the visible imagery produce
more rainfall than darker clouds.
Brighter clouds in the visible and clouds
with cold tops in the IR imagery which are
expanding in areal coverage (in early and
mature stages of development) produce
more rainfall than those not expanding.
Decaying clouds produce little or no rainfall.
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 Clouds with cold tops in IR imagery produce more
rainfall than those with warm tops.
 Clouds with cold tops that are becoming warmer
produce little or no rainfall.
 Merging of cumulonimbus (Cb) clouds increases
the rainfall rate of the merging clouds.
 Most of the significant rainfall occurs in the
upwind (at anvil level) portion of a convective
system. The highest and coldest clouds form where
the thunderstorms are most vigorous and the rain
heaviest.
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Digital enhancement curve (the
Mb curve)
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Active clouds
Tight area of IR gradient within more uniform
anvil
Overshooting tops
Bright or textured part of anvil
Slower moving anvil edge
Upwind area of anvil (200-500mb wind)
Low-level inflow
Radar echoes
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Enhanced IR
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Rain rate assigned based on
Rain rate assigned based on
Coldness of cloud top (colder: more rain)
Cloud growth (growing: more rain)
Getting colder
Getting bigger
Divergence aloft or low-level inflow
Takes account of speed of storm motion
Available atmospheric moisture
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Example
Surface obs
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S/O satellite estimate
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Step 1 – Finding active areas
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Operational rainfall estimates
 Since the early 1980's, the Satellite Analysis
Branch (SAB) of the National Oceanic and
Atmospheric Administration/National
Environmental Satellite Data and Information
Service (NOAA/NESDIS) has been producing
satellite rainfall estimates using the Interactive
Flash Flood Analyzer (IFFA).
 The IFFA uses the McIDAS system which was
developed by the University of Wisconsin. Special
software is used to draw lines of satellite rainfall
estimates. They are saved and then added for
whatever time period is needed.
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 The IFFA is a man-machine interactive system and
is very labor intensive requiring much manual
input. The Scofield Convective Technique is used
by the SAB Meteorologists for the estimated
amounts every half-hour.
 The technique uses GOES Infrared and visible
imagery. The estimates are automatically corrected
for parallax (viewing angle of the satellite), and an
orographic correction can be done for short
periods like the past 3 to 6 hours.
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Tropical cyclones
 A tropical cyclone is a storm system characterized
by a large low-pressure center and numerous
thunderstorms that produce strong winds and
heavy rain.
 They also carry heat and energy away from the
tropics and transport it toward temperate latitudes,
which makes them an important part of the global
atmospheric circulation mechanism. As a result,
tropical cyclones help to maintain equilibrium in
the Earth's troposphere, and to maintain a
relatively stable and warm temperature worldwide.
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All tropical cyclones are areas of low
atmospheric pressure near the Earth's
surface. The pressures recorded at the
centers of tropical cyclones are among the
lowest that occur on Earth's surface at sea
level.
Tropical cyclones are characterized and
driven by the release of large amounts of
latent heat of condensation, which occurs
when moist air is carried upwards and its
water vapour condenses.
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This heat is distributed vertically around the
center of the storm. Thus, at any given
altitude (except close to the surface, where
water temperature dictates air temperature)
the environment inside the cyclone is
warmer than its outer surroundings.
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Stratiform (or frontal) rainfall
There are three distinct ways that
rain can occur. These methods
include convective, stratiform (or
frontal), and orographic rainfall.
Stratiform rainfall is caused by
frontal systems.
When masses of air with different
density (moisture and temperature
characteristics) meet, warmer air
overrides colder air, causing
precipitation.
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Warm fronts occur where the warm air
scours out a previously lodged cold air mass.
The warm air 'overrides' the cooler air and
moves upward. Warm fronts are followed by
extended periods of light rain and drizzle,
because, after the warm air rises above the
cooler air, it gradually cools due to the air's
expansion while being lifted, which forms
clouds and leads to precipitation.
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Cold fronts occur when a mass of cooler air
dislodges a mass of warm air. This type of
transition is sharper, since cold air is more
dense than warm air. The rain duration is
less, and generally more intense, than that
which occurs ahead of warm fronts.
The stability of the warm air mass
determines the type of precipitation
generated by a cold front.
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If the warm air is stable the clouds are of
stratiform form. The clouds are of the
cumuliform type and precipitation
convective, if the warm air is unstable.
Frontal systems in UK.
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Orographic rainfall
 Orographic or relief rainfall is caused when masses
of air pushed by wind are forced up the side of
elevated land formations, such as large mountains.
 The lift of the air up the side of the mountain
results in adiabatic cooling, and ultimately
condensation and precipitation. In mountainous
parts of the world subjected to relatively consistent
winds (for example, the trade winds), a more moist
climate usually prevails on the windward side of a
mountain than on the leeward (downwind) side.
Moisture is removed by orographic lift, leaving
drier air on the descending, leeward side where a
rain shadow is observed.
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Orographic rainfall
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Spatial variability of hourly rainfall
The influence range of hourly rainfall varies
with storm type. In particular, hourly rainfall of
Mei-Yu has the smallest influence range of 24
km, suggesting the highest spatial variation
among all storm types.
The small influence
range of Mei-Yu
rainfall may be
attributed to
redevelopment of
MCSs.
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Using the variogram to characterize
the spatial variability of rainfall data
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Atmospheric Layers
 The atmosphere is divided into five layers. It is
thickest near the surface and thins out with height
until it eventually merges with space.
The troposphere is the first layer above the surface and
contains half of the Earth's atmosphere. Weather occurs
in this layer.
Many jet aircrafts fly in the stratosphere because it is
very stable. Also, the ozone layer absorbs harmful rays
from the Sun.
Meteors or rock fragments burn up in the mesosphere.
The thermosphere is where the space shuttle orbits.
The atmosphere merges into space in the extremely thin
exosphere. This is the upper limit of our atmosphere.
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Ozone
layer
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Troposphere & Tropopause (對流層頂)
The tropopause is the atmospheric
boundary between the troposphere(對流層)
and the stratosphere. Going upward from
the surface, it is the point where air ceases to
cool with height, and becomes almost
completely dry.
About 80 % of the total mass of the
atmosphere is contained in troposphere. It is
also the layer where the majority of our
weather occurs.
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The exact definition used by the World
Meteorological Organization is:
the lowest level at which the lapse rate
decreases to 2 °C/km or less, provided that the
average lapse rate between this level and all
higher levels within 2 km does not exceed 2
°C/km.
The troposphere is the lowest of the Earth's
atmospheric layers and is the layer in which
most weather occurs.
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The troposphere begins at ground level and
ranges in height from an average of 11 km
(6.8 miles/36,080 feet at the International
Standard Atmosphere) at the poles to 17 km
(11 miles/58,080 feet) at the equator.
It is at its highest level over the equator and
the lowest over the geographical north pole
and south pole.
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Measuring the lapse rate through the
troposphere and the stratosphere identifies
the location of the tropopause. In the
troposphere, the lapse rate is, on average,
6.5 °C per kilometre. In the stratosphere,
however, the temperature increases with
altitude.
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Stratosphere
This stratosphere contains about 19.9 % of
the total mass found in the atmosphere.
Very little weather occurs in the
stratosphere. Occasionally, the top portions
of thunderstorms breach this layer.
In the first 9 kilometers of the stratosphere,
temperature remains constant with height. A
zone with constant temperature in the
atmosphere is called an isothermal layer.
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From an altitude of 20 to 50 kilometers,
temperature increases with an increase in
altitude.
The higher temperatures found in this
region of the stratosphere occurs because of
a localized concentration of ozone gas
molecules. These molecules absorb
ultraviolet sunlight creating heat energy that
warms the stratosphere.
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Ozone is primarily found in the atmosphere
at varying concentrations between the
altitudes of 10 to 50 kilometers. This layer of
ozone is also called the ozone layer. The
ozone layer is important to organisms at the
Earth's surface as it protects them from the
harmful effects of the sun's ultraviolet
radiation. Without the ozone layer life could
not exist on the Earth's surface.
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Mesophere & thermosphere
 In the mesosphere, the atmosphere reaches its
coldest temperatures (about -90° Celsius) at a
height of approximately 80 kilometers. At the top
of the mesosphere is another transition zone
known as the mesopause.
 The thermosphere has an altitude greater than 80
kilometers. Temperatures in this layer can be as
high as 1200°C. These high temperatures are
generated from the absorption of intense solar
radiation by oxygen molecules (O2).

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