Title: Storms and Precipitation
1Storms and Precipitation
- Professor Ke-Sheng Cheng
- Dept. of Bioenvironmental Systems Engineering
- National Taiwan University
2Major 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)
3Convective 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.
4Life cycle of a thunderstorm cell
Cumulus stage
Mature stage
Dissipating stage
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5- 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.
6- 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
point-to-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.
7- 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 (??) grows into a cumulonimbus (???).
8- 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.
9Cumulus cloud
10Cumulonimbus
11Cumulonimbus
12- 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.
13- 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.
14Cumulus stage
15Mature stage
16Dissipating stage
17- 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.
18- 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.
19Mesoscale 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.
20- 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.
21- 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 eastwest orientation.
22- 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.
23Not all MCSs are nearly circular. Included within
the category of MCS is a linearly-organized band
of cold cloud tops. Such a structure is nearly
always associated with a frontal boundary.
24Estimating 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.
25Rationale 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.
26- 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.
27Digital enhancement curve (the Mb curve)
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29Active 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
30Enhanced IR
31Rain 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
32Example
Surface obs
S/O satellite estimate
33Step 1 Finding active areas
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39Operational 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.
40- 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.
41Tropical 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.
42- 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.
43- 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.
44Stratiform (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.
45- 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.
46- 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.
47- 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.
48Orographic 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.
49Orographic rainfall
50Spatial 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.
51Using the variogram to characterize the spatial
variability of rainfall data
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53Atmospheric 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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56Troposphere Tropopause (????)
- The tropopause is the atmospheric boundary
between the troposphere(???) 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.
57- 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.
58- 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.
59- 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.
60Stratosphere
- 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.
61- 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.
62- 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.
63Mesophere 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 1200C. These high temperatures are
generated from the absorption of intense solar
radiation by oxygen molecules (O2).
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