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Storms and Precipitation

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Title: Storms and Precipitation


1
Storms and Precipitation
  • Professor Ke-Sheng Cheng
  • Dept. of Bioenvironmental Systems Engineering
  • National Taiwan University

2
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)

3
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.

4
Life cycle of a thunderstorm cell
Cumulus stage
Mature stage
Dissipating stage
???
??
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.

9
Cumulus cloud
10
Cumulonimbus
11
Cumulonimbus
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.

14
Cumulus stage
15
Mature stage
16
Dissipating 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.

19
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.

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.

23
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.
24
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.

25
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.

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.

27
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

30
Enhanced IR
31
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

32
Example
Surface obs
S/O satellite estimate
33
Step 1 Finding active areas
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39
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.

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.

41
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.

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.

44
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.

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.

48
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.

49
Orographic rainfall
50
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.
51
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.

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.

60
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.

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.

63
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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