Storms and Precipitation

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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
Mature stage
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
  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.

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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 (??) grows into a cumulonimbus (???).

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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 eastwest 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 linearly-organized 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
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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Troposphere 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.

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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 1200C. These high temperatures are
  generated from the absorption of intense solar
  radiation by oxygen molecules (O2).

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