Previsão de fenómenos severos ou extremos

Tópico em 'Aprendizagem e Formação' iniciado por Vince 18 Mai 2007 às 13:15.

  1. Vince

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    23 Jan 2007
    Já estava a preparar há uns dias este post para publicar, resolvi finalizá-lo ontem à noite já que se aproximam novamente condições favoráveis a boas trovoadas.

    Quase todos conhecem o que é o CAPE/LI gerado pelo modelo GFS:


    Mas o CAPE só por si não é tudo.
    Há muitos outros dados que são muito úteis e importantes na previsão de fenónomos mais extremos. De trovoadas banais ao granizo extremo, das supercélulas aos downburst, das trombas marinhas aos tornados. Quem esteja habituado a ler os alertas do Estofex já deparou com imensos termos um pouco cripticos e desconhecidos.

    O Oscar Van der Velde, autor do grande site Convective Weather Maps e um dos forecasters do nosso conhecido Estofex publicou há poucos dias um fabuloso guia de interpretação dos muitos e valiosos mapas que disponibiliza no site dele e no servidor do Estofex.

    O site é este:

    E o guia pode ser descarregado aqui:

    Para iniciar este tópico, resolvi pegar no Guia dele, mas adaptá-lo a uma realidade prevista pelos modelos no futuro, a situação do próximo sábado à tarde, em que os modelos indicam potencial. Ou seja, as imagens/output's todos serão as previsões para sábado à tarde (amanhã), e a descrição em cada imagem será o texto do guia.
    Mais tarde colocarei novos post's aqui com outros guias e sites.

    Nota importante:
    1) Obviamente até lá os modelos mudam, estas imagens referem-se aos últimos runs.
    2) Há sempre algum delay entre o último run dos modelos e a actualização destes modelos específicos que estão neste site.
    3) Os textos que acompanham cada imagem são a descrição das mesmas e como devem ser interpretados. Mas pontualmente numa ou noutra pode fazer referência não a esta imagem concreta, mas às que estavam no guia. Mas percebe-se quando tal acontece.

    The three basic ingredients for severe deep convection are instability, lift, and vertical
    windshear. In many situations, without a good source of lift (e.g. a front, trough, sea breeze
    convergence, dryline, forced flow over mountains) a parcel which has conditional instability
    will have trouble to rise out of the boundary layer and form a persisting convective storm.
    Without vertical shear (change of direction and speed of the horizontal wind with height) a
    storm will have trouble to live longer than say 45 minutes and die, instead of organising itself
    into a cluster or MCS, or develop a rotating updraft, with all consequences for severe weather

    1. MLCAPE (and MSL Pressure, 500 hPa Geopotential Heights)
    This map offers a usual view of common heights for a quick overview of pressure systems,
    with the addition of mixed-layer CAPE (Convectively Available Potential Energy).
    CAPE is the potential energy a parcel has when it is lifted to its level of free convection and
    becomes warmer than its surroundings, experiencing upward buoyancy. The potential energy
    can be converted to kinetic energy reflected in upward motion. An vertical speed could in
    principle be calculated from this, but parcel theory is not perfect and does not account for
    things like precipitation drag or dynamic pressure contributions of vertical shear. However,
    higher CAPE typically involves stronger storms with a higher chance of large hail and other
    severe weather. That said, note that CAPE is usually of lesser importance than the vertical
    shear environment for tornadoes, while the probability of large hail increases with CAPE,
    given at least moderate shear (values around 500-1000 J/kg are sufficient).
    Contributors to CAPE are steep temperature lapse rates from low to mid levels and a warm
    and humid boundary layer. The colder the mid levels are compared to the parcel, and the
    higher the parcel experiences upward buoyancy (high equilibrium level), the larger CAPE in
    general. However, warm, dry layers at low levels may function as a cap that prevent
    boundary layer parcels from reaching the level of free convection, and may prevent storms
    from developing (see LFC-LCL map).
    The CAPE used in these maps is calculated for a parcel with mixing ratio and potential
    temperature averaged from the 0-1 km layer, because it reflects the process of mixing
    in the boundary layer. Note that the problem of GFS overestimating low level dewpoints (and hence
    CAPE) in conditions of weak winds and strong insolation in the summer half year is
    somewhat mitigated by not including the 2-meter level in the calculation.
    Finally, be aware that CAPE is very sensitive to small differences in the moisture and
    temperature profiles, as well as the calculation and used parcel. It is therefore fairly useless to
    speak for example of "855 J/kg CAPE" or even "900 J/kg". If the maps indicate 1000 J/kg
    CAPE, be prepared to find in soundings mostly 500-1500 J/kg, a wide margin of at least 50%.

    2. Omega: Advection of 600 hPa Geostrophic Vorticity by the 900-300 hPa Thermal Wind Vectors, 600 hPa Height


    This maps uses the Trenberth method for estimating the resulting vertical motion induced by
    differential vorticity advection and temperature advection, giving a qualitative picture of
    geostrophic vertical motions. It is different from model output vertical motions (which are
    influenced also by convection itself). Use to get a sense of large scale lift and subsidence.
    Cellular convection over sea often is able to maintain itself even under subsidence, if low
    level lapse rates are strong enough, but comma clouds for example would need geostrophic
    lift (usually a vorticity maximum).

    3. Mid-tropospheric Potential Vorticity (400-600 hPa)


    Used to highlight atmospheric processes in a different way. PV is a conserved quantity for
    adiabatic processes, equivalent to momentum. It can be used to trace airmasses. The
    tropopause is usually associated with 2 PV units, with lower PV below. Strong vertical
    motions can stir up the tropopause, such that high PV air enters the troposphere and is
    brought downwards. The presence of a strong PV anomaly in midlevels or lower indicates
    either strong postfrontal subsidence or a bubble of mid-level cold air with steep lapse rates
    and high vorticity. I won't go deeply into PV theory, but for practical use: strong upward
    motions can be expected ahead of a PV maximum, in other words, mid-level lapse rates will
    steepen and mid-level vorticity generating upward motion in the direction a PV maximum
    moves. Especially the dark blue and beyond needs attention. The patterns seen in this map
    often correspond with the dark bands in satellite water vapour images (intrusions of dry air).

    4. Thompson index, Convective Precipitation, 700 hPa Height


    The Thompson thunderstorm index is an ancient index, from the times that every calculation
    had to be done be hand from soundings. The index consists of K Index minus the Lifted
    Index. The latter is simply the difference between the temperature a parcel has at 500 hPa
    and its surrounding air, so a result of parcel theory. The K Index is T850 + Td850 - (T700 -
    Td700) -T500 and thus a sum with no meaning, including a lapse rate, a low level dewpoint
    and a mid level relative humidity. The fixed levels make this physically mean something
    different on high plains than at sea level. But it came out best in a comparison study of a
    number of cases with SFLOCs (lightning reports) I did almost 10 years ago. The 700 hPa
    moisture factor in it can be useful for the reason that it serves to indirectly include a source of
    lift, as usually it is more humid in the mid levels around fronts. I strongly suggest using parcel
    theory parameters though.
    What to use the map for is mainly a view of standard output GFS convective precipitation...
    which actually is often pretty reliable although it may overreact in case the evapotranspiration
    problem (weak wind, strong insolation) shows up.

    5. Equilibrium Level Temperature (most unstable parcel)


    A very useful map in cases of near-neutral environments of very low CAPE, read: mostly in
    the winter half year. Convective cells need to have updrafts reaching sufficiently into the
    mixed-phase temperature region (usually -10 to -30 degrees Celsius), where ice particles in
    the cloud co-exist with liquid water droplets, in order for the non-inductive charging process
    to be effective. The equilibrium level is where the parcel will be at the same temperature as
    the environment after its free convection. It will experience an increasingly negative
    buoyancy force as it ascends further and will slow down. This often corresponds with levels
    near the tropopause, but may also be an inversion lower in the troposhere. The map indicates
    the temperature, not the height. Thunder may be possible with EL temperatures lower than -
    10 degrees, and becomes likely especially beyond -30 degrees. In winter time the
    corresponding heights of the cloud tops is lower and the moisture content lower as well, with
    weaker updrafts so the electrification process is less effective.
    In the summer over large areas parcels can reach very cold temperatures and other indicators
    may be more useful to look at. However, for the calculation the "best layer" is used (i.e. the
    level with the highest theta-e parcel below 600 hPa), and this map is useful in identifying
    elevated convection when ML parcel methods do not show potential.
    Attention: there is currently no map to check the LFC of an elevated parcel.

    6. Lifting Condensation Level, LFC-LCL difference


    The height of the LCL of a 0-1 km mixed parcel is plotted as background. This LCL is similar
    to the Convective Condensation Level, i.e. the cloud base height that cumuliform clouds may
    have. It relates strongly to the relative humidity of the boundary layer, so very low heights
    may associate with low clouds or fog during the night (and in bad cases persist during the day
    and block solar heating required for storms).
    High LCL heights can enhance downburst winds because the downdraft air will be colder
    relative to the surrounding air, the negative buoycancy accelerating downward speeds. High
    LCLs (>2000 m) may also indicate more difficulty for beginning convection to sustain itself,
    due to entrainment in the dry environment. Low LCL heights (under 1000 meters) are
    favourable for tornadoes, as was found by SPC, the reasons of which have not been fully
    explained, but involve downdraft-updraft buoyancy processes.
    The LFC (Level of Free Convection) is the level below which a 0-1 km mixed parcel when
    lifted is colder than its environment, and normally wants to return to where it came from. A
    very strong source of low-level lift may push a parcel to the LFC, so that it becomes warmer
    (lighter) than the surrounding air and experience an upward force. More common is that the
    capping warm layer is adiabatically lifted and removed, or that heating and mixing from
    below will yield a higher LCL and a lower LFC (the convective temperature concept).
    In the form of vectors, the difference between the cloud base and the Level of Free
    Convection is drawn. No vector means no MLCAPE present. Small vectors indicate small
    LFC-LCL differences, so that there is almost no extra heating or forcing required for initiation
    of convection. Longer vectors require more, and thick vectors may indicate too much capping
    inhibiting the formation of thunderstorms. Along the dryline in the USA Great Plains, the
    gradient may be so steep that only a few points with small LFC-LCL are visible on the grid
    points of the model. At night, the LFC-LCL difference may increase again, but usually already
    developed storms will persist for some time, depending on moisture and storm-relative inflow
    above the boundary layer. In general, the lower the LFC-LCL difference, the easier (less
    forcing required) and earlier storms develop. The same goes for lower LCLs because
    entrainment is less of a problem.
    Note that because the model adjusts its environment (weakens lapse rates, lowers LCL) as
    result of convection, the LFC-LCL difference may become larger and may give a counterintuitive
    ‘capped’ impression where there is already convection. Check this by looking at the
    convective precipitation map.

    7. 0-3 km MLCAPE, Spout index


    0-3 km MLCAPE (low-level CAPE) uses the 0-1 km mixed layer parcel, but represents the
    MLCAPE present not all the way to the EL, but only in the lowest three kilometers above the
    surface. This indicates whether a parcel is able to accelerate rapidly above the LFC. A low
    LFC and temperatures dropping rapidly with height in the 0-3 km layer make for a upward
    acceleration in this layer, which is important especially for tornadogenesis. The type of
    generally weak tornadoes (F0-F1) known as 'spouts' (landspouts, waterspouts) happen by
    stretching of vorticity with a vertical axis into an updraft. This process is enhanced by vertical
    acceleration (the same mechanism as the whirl when draining water from a bathtub).
    Prerequisite is a source of vertical vorticity and convergence, such as wind shift lines. In
    addition it seems important that low-level winds are not too strong, otherwise turbulence
    may disturb this process. Steep near-surface lapse rates will also help (next map). Mid/upper
    level cold pools and weak troughs are notorious for outbreaks of spouts. The green
    experimental composite index incorporates these factors, but it is not calibrated or tested, and
    of little use.
    Similarly, tornadoes can be generated by tilting of low-level vorticity with a horizontal axis
    (strong low-level shear) into the vertical by a strong updraft and may also profit from
    stronger 0-3 km CAPE.

    8. Temperature Lapse Rate: 0-500 m AGL


    The temperature difference between the surface (not 2 meters) and 500 m above ground. This
    map contains useful information about the relative temperature of the air compared to the
    surface over which it flows. The dry-adiabatic lapse rate is about 10-11 Kelvin decrease per
    kilometer, while 5-6 degrees decrease per kilometer is moist-adiabatic. Lower values indicate
    inversions. Values higher than 11 K/km indicate superadiabatic conditions which necessarily
    imply turbulent mixing as surface parcels have already positive buoyancy with the minimal
    lift. This is favourable for vertical vortex stretching such as dust devils and spouts.
    One may often easily infer which process is responsible for steep or inverted lapse rates. Large
    bodies of water do not change temperature very quickly, so very steep lapse rates will mostly
    be the result of advection of relatively cold air over the surface. Similarly, inverted lapse rates
    indicate strong warm air advection over the water surface. Land, on the other hand, responds
    quickly to radiative processes. Contrasts between land and adjacent water surfaces may
    induce mesoscale circulations like land/sea breeze. Lapse rates increase quickly during the
    afternoon when the sun shines, while in the evening a ground inversion forms. This makes it
    possible to evaluate if the model produces cloudiness that may inhibit heating of the
    boundary layer during the day, or reflect of long wave radiation to earth at night (>4 K/km
    over land), making it a useful map if also if you need to know possibilities for clear skies at
    night for astronomical observations or sprites.

    9. Temperature Lapse Rate: 2000-4000 m AGL


    This somewhat arbitrarily chosen layer for mid-level lapse rates is often used to identify an
    important contributor to CAPE, independently of moisture availability. In maritime polar
    airmasses behind cold fronts it generally indicates values over 6 K/km. More equatorward it
    often is capable of defining the edge of deep convection rather well, where subsidence
    establishes an inversion. Shallow convection may still occur in these regions.
    Elevated and dry regions such as the Spanish Plateau and the Sahara often create a deep dry
    layer with steep lapse rates, that can be advected away into western Europe (e.g. Spanish
    Plume). On the Great Plains in the USA, very steep lapse rates can be seen developing over
    the Rocky mountains and western High Plains and being transported eastward over a very
    moist airmass, creating 'loaded gun' soundings. Very steep lapse rates (>7 K/km) in this layer
    in warm airmasses are capable to create a 'fat' CAPE, allowing for rapid upward acceleration,
    and is often associated with large hail and more indirectly with severe downburst winds.
    Neutral lapse rates (5-6 K/km) indicate less exciting conditions, often found in saturated
    frontal regions. Note that 2000 and 4000 m temperatures are advected by different winds, so
    the lapse rate itself does not always advect nicely and just pop up and disappear out of

    10. 700 hPa Theta-e, Streamlines (convergence and divergence)
    11. 0-1 km Theta-e, 10 m Streamlines (convergence and divergence)



    Theta-e is the Equivalent Potential Temperature. It is determined on a Skew-T diagram by
    lifting a parcel to its LCL, then removing adiabatically all moisture from it by following the
    moist-adiabat upward and read its potential temperature at 1000 hPa via the dry-adiabat.
    Actually it is equivalent to the Wet-bulb Potential Temperature (theta-w or WBPT), the
    latter is the moist-adiabat from the LCL followed downward to 1000 hPa. Both are displayed,
    theta-e in colours and theta-w in contours.
    The advantage of theta-e over normal temperatures is that the parameter is conserved in
    adiabatic processes, meaning that bringing air to a higher or lower level does not change its
    value. As different origins of airmasses largely determine their own theta-e, one can use this
    parameter as a marker. Fronts are easily seen as steep gradients in theta-e. The boundary layer
    theta-e shows where fronts are located near the surface, while 700 hPa theta-e shows where
    they are near the 3000 m level. In winter it occurs often that warm fronts do not penetrate
    into the heavy, cold airmass near the surface. They are however visible at the 700 hPa layer.
    The maps can be used to determine if the airmass is potentially unstable, which occurs often
    in split cold fronts. When the values at 700 hPa are lower than in the 0-1 km layer (note this
    may not work over very elevated grounds), lifting the layer enough may increase the lapse
    rates and cause development of CAPE. While the model should in principle be able to
    compute all of this by itself and produce CAPE, it occurs regularly that strongly forced
    narrow convective lines develop at sub-grid scale (use also the PV map).
    Both maps feature streamlines. The colour indicates qualitatively the presence of convergence
    (yellow to red) and divergence (light blue to purple). In summer convection cases, one can
    consider low-level convergence in plumes of high theta-e as the most useful indicator of
    where thunderstorms will develop. Convergence near the surface must result in ascending
    motion of air and works as trigger for convection. Only in cases of very small LFC-LCL
    difference storms may also develop outside such convergence regions. At the 700 hPa level
    you may rather want to see divergent (or neutral) winds in the same region, as a reaction to
    low-level convergence. This couplet may be somewhat horizontally displaced. Convergence
    at the 700 hPa level mostly indicates downward motion. Diurnal cycles of sea breeze and
    mountain circulations can often be discovered.
    The combination of the two streamline fields allows you to inspect directional windshear.

    12. 0-1 km average Mixing Ratio, 0-1 km average wind streamlines (moisture advection)


    Mixing ratio is another word for absolute moisture content and is expressed in grams of water
    vapour per kilogram of dry air. A directly related parameter is the dewpoint temperature.
    However a dewpoint temperature cannot be mixed vertically. Mixing ratio is conserved for
    vertical motions until condensation occurs. This parameter is easily compared with observed
    soundings by taking the average over the lowest kilometer on a Skew-T diagram, useful to see
    if the model is on track with its moisture predictions, after all it is the source of the CAPE
    calculations. The streamlines show colours that tell where there is advection of moister or
    drier air, stressing gradients that are advected perpendicular to the wind.
    This map displays the dryline in the United States much better than Theta-e.

    13. Delta Theta-E, Convective Gust, Cold Pool Strength (T2m -Tdowndraft)


    The parameters in this map are somewhat experimental. Delta-theta-e (thick lines, if present)
    is the difference between the boundary layer theta-e (the moist adiabat used for CAPE) and
    the lowest theta-e found in the mid levels (under 400 hPa). The drier and colder the mid
    levels, and the more warmer/more unstable the boundary layer parcel, the stronger updrafts
    and downdrafts and hence the chance of severe convective gusts. Even microbursts (extreme
    local downbursts) are possible especially with values above 20 K (Atkins and Wakimoto,
    The convective gust speed in shaded colours is simply the pressure-weighted average of
    surface to 700 hPa winds, and is intended to give an indication what to expect when a
    downdraft digs down through a layer of high winds, bringing the momentum down to the
    surface. It may already be very windy, but normally over land the ratio between gust speed
    and 10 minute average winds does not exceed 1.7 or so (1.4 over sea) with some margin. A
    gust significantly enhanced by deep convection could well yield higher gust factors (one can
    often use SYNOP or METAR to determine this).
    Cold Pool Strength is a parameter that takes the lowest theta-e from the mid-levels and brings
    it to the surface, where it is compared to the 2m temperature. One may interpret this as the
    worst temperature drop that can be experienced from thunderstorm outflow (if the model did
    not miss colder theta-e levels). In practice this may often be less dramatic. Physically it
    corresponds with the negative buoyancy of the downdraft into the boundary layer. A
    relatively cold downdraft will propagate away from the thunderstorm with a higher stormrelative
    speed (stronger gusts). It may require strong low level storm-relative winds to
    prevent the storm from being cut off from its moisture source. Values higher than 10 degrees
    are a good signal for strong gusts. Low values indicate an almost neutral profile. At night and
    when convection has already produced precipitation in the model this parameter may not be

    14. 0-6 km Shear, 0-1 km Shear, Significant Tornado Parameter


    Displayed in knots (may change this to m/s), the length of the vector difference (bulk vertical
    shear vector) of the winds at 6 km and 1 km above ground level with the 10 m wind. These
    are often called 'deep layer shear' and 'low level shear', respectively. The chosen levels
    originate from those included in American studies, and their relation to severe weather is
    well documented. The way these are plotted reflects the commonly cited 'threshold' levels,
    although there is some margin. Deep layer shear around 20 kts (10 m/s, weak to moderate) is
    often sufficient to sustain redevelopment of new cells at outflow boundaries next to older
    cells, and support multicell storms and mesoscale convective systems (MCS), the latter
    especially when sufficient dynamic forcing is present. More shear will cause a gradual
    transition from discrete (stepwise) renewing cell growth to more steady-state storms, with
    the downdraft less interfering with the updraft so that cells can live longer. 30 kts (15 m/s) or
    more will usually lead to pretty well organised storms with weakly supercellular
    characteristics, and capable of producing large hail. Usually 40 kts (20 m/s) is taken as
    threshold value for supercells, meaning that the storm is able to develop and sustain a rotating
    updraft. Supercells are very capable of producing large hail (>2 cm), severe downdrafts and
    tornadoes. Generally, the product of CAPE and 0-6 km shear correlates well with increasing
    probability of the full spectrum of severe weather from thunderstorms.
    Low level shear over 20-25 kts (10-15 m/s) is favourable for tornadogenesis, as it represents
    horizontal vorticity that can be tilted into the vertical by strong updrafts. Additionally, an
    MCS in a high 0-1 km shear environment may tend to produce bowing segments which are
    capable of causing concentrated damaging winds.
    Significant Tornado Parameter is a composite index based on deep layer and low level shear,
    CAPE, CIN and LCL height. It highlights regions where these ingredients for tornadoes come
    together most, although it does not tell which necessary ingredient may be lacking most.
    Composite indices cannot replace a detailed analysis, but serve well as an alert to the

    15. 0-3 km Storm-relative Environmental Helicity, Supercell Composite Parameter, Bunkers Storm Motion


    In addition to good 0-6 km shear, it is favourable for development of rotating updrafts to have
    a sequence of wind shear vectors over small layers turning clockwise with height. This results
    in a curved hodograph, the line that connects the arrow heads of the wind vectors of a
    vertical profile when presented in a horizontal plane. Curved hodographs are also possible
    with atypical vertical wind profiles, so it is much easier to see this in hodograph plots than
    from the wind barbs adjacent to a sounding.
    In a Lagrangian sense of motion, a storm is affected by its surrounding winds. Low level
    storm-relative winds are ingested into the updraft. Each small layer of vertical shear bears
    horizontal vorticity, which is ingested and tilted into the vertical, increasing the total rotation
    of an updraft. The surface on a hodograph diagram swapped out between the hodograph line
    connecting the 0-3 km winds and the storm motion vector is equivalent to the rotation that is
    gained. It will follow that a storm motion following the hodograph line will not gain much
    rotation, but a deviant motion to the right of the hodograph may. For a more complete
    discussion refer to the MetEd module. In practice, a straight, sufficiently long hodograph (e.g.
    40 kts 0-6 km shear) may produce both left-moving and right-moving supercell storms (as the
    downdraft may force cells to obtain a deviant motion: split cells), while a clockwise-curved
    hodograph favours right-moving supercell storms. The updraft is forced by non-hydrostatic
    vertical pressure gradient forces to occur to the warm side of the hodograph.
    Supercells often are able to develop when 0-3 km SREH is greater than 150 m2/s2, while also
    the chance for tornadoes increases with larger SREH.
    Veering winds with height are also a sign of temperature advection. A windshear vector over
    a layer represents the thermal wind, which blows parallel to thickness lines with the warm
    air to the right. Warm air advection in the low levels and strong temperature gradients favour
    higher SREH. In some cases this may inhibit surface-based convection by a capping layer of
    warm air.

    16. Storm-relative Moisture Flow and Mid/Upper Flow


    For areas with a Lifted Index lower than 2 and 2-km Lifted Index less than 8 degrees
    (unstable and not too much capped), this somewhat experimental map displays 0-2 km stormrelative
    moisture flow. It is the average mixing ratio (g water/kg dry air) transported by the
    difference vector of the lowest 2 km and the storm motion. The parameter represents the flux
    of water vapour into the storm and has the units g/m2/s. Higher mixing ratios and stronger
    low-level storm-relative winds both contribute to higher values. The parameter has not been
    included in any studies so far, but my observations suggest the warmer colours indeed to be
    more often associated with large hail events and development of mesoscale convective
    systems. (note: another version may be invented using column-integrated water content
    instead of an average mixing ratio value, it would then have the units kg/m/s)
    The storm-relative mid/upper level flow is displayed qualitatively on vectors (the length has a
    fixed scale). Larger vectors may imply better evacuation of precipitation out of the updraft,
    and therefore a potentially longer lived storm. There may be some clues for supercell type
    gained from this (low-precipitation stronger, high-precipitation weaker mid/upper flow). The
    vectors point in the direction the bulk of the anvil would blow to.
    In some occasions, GFS shows strongly diverging upper SR wind vectors. This is a good signal
    the model has produced a large convective system with mesoscale updrafts (confirm with
    convective precipitation).
    Finally, this map can be used as yet another alternative way to determine the presence of
    deep instability, because it is only plotted where Lifted Index is smaller than 2.

    17. 1-8 km Shear, ICAPE and ICIN


    This map is convenient to judge where shear crosses areas of instability. The 1-8 km bulk
    shear vector is another version of deep layer shear, but excludes the 0-1 km layer. This can be
    of use besides the 0-6/0-1 km shear map, especially in cases when the hodograph is straight
    and 0-1 km shear strong and thus part of the 0-6 km shear. It then makes sense to look what
    amount of shear is available above 1 km. The 8 km level is found to be of more value than 6
    km by Bunkers et al. (2006) in discriminating between long- versus short-lived supercells.
    (note: I plan to also provide a map of 1-4 km shear soon)
    ICAPE stands for Integrated CAPE, and has the units J/m2, not J/kg. It was first defined by
    Mapes (1993) as the sum of CAPE*dp/g for all parcels which have CAPE>0. This makes it
    independent of the choice of parcel. Instead, a deeper layer of parcels that have CAPE gives
    higher values for the same CAPE than if only a shallow layer has CAPE. For example, a 100
    hPa thick layer giving 500 J/kg CAPE will result in the same ICAPE value as a shallower 50
    hPa layer of 1000 J/kg (about 500 kJ/m2). The parameter is plotted experimentally, as its
    advantage over other versions of CAPE in operational meteorology has never been tested.
    It makes sense that if as a storm develops, all air from low levels will be taken into the storm,
    and the total energy released by all parcels in a column of one square meter diameter is
    ICAPE. In practice, the map will look very similar to MLCAPE, except when parcel layer
    thickness differs over the area, or when elevated parcels over a stable boundary layer have
    CAPE, where MLCAPE may be absent. So, this parameter has characteristics of both
    Similarly, ICIN is the integrated negative buoyancy counterpart, a sum of all CIN of all
    parcels in a column that have positive CAPE. This is topped in this implementation at 600
    hPa (and parcels to 700 hPa due for computing resource reasons). In the map, the smallest
    vectors indicate very small ICIN sums, while larger and thicker vectors imply higher ICIN.
    Because this is a column value, this may mean that lower parcels have CIN while higher
    parcels not necessarily have CIN (use caution in case of elevated instability). However, the
    thicker the layer of CAPE>0 parcels with CIN, and the stronger the inversion causing the
    CIN, the higher the value and total resistance to lifting.
    An environment with high ICAPE (especially greater than 1000 kJ/m2) is potentially able to
    free a lot of energy from a deep layer and may sustain storms for a long time, while high ICIN
    indicates the total resistance to releasing this energy. Use in combination with MLCAPE,
    MU-EL, and LFC-LCL maps.

    © Oscar van der Velde, 2007
  2. Vince

    Expand Collapse

    23 Jan 2007
    Re: Análise e previsão da fenónomos severos ou extremos

    Outros sites e documentos úteis neste tema:

    Neste manual em espanhol existe uma boa introdução ao CAPE, ao Lifted Index (LI), ao Storm-Relative Helicity, e por aí fora.

    Uso de modelos numéricos para la predicción operativa de conveccion severa
    Instituto Nacional de Meteorología (INM)

    Spain Severe Weather - Formación
    Nesta secção do site espanhol TiempoServero há dezenas de link's para documentos e estudos interessantes:
  3. Rog

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    6 Set 2006
    Norte Madeira (500m)
    Re: Análise e previsão da fenónomos severos ou extremos

    Artigo de peso, bem pensado e completo. Escareceu-me algumas dúvidas que tinha.
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  4. actioman

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    15 Fev 2008
    Elvas (~300m)
    Este fórum é um poço de sabedoria! E tem tanta mas tanta informação que nem nos passa pela cabeça! :eek:

    Faço aqui um mea culpa, pois desconhecia a existência deste post! :o

    Para os que aqui postam, novos e mais antigos, dá alguma pena ver por vezes o desinteresse aparente (espero que apenas isso) com que trabalhos tão bem feitos e de enorme pesquisa, como este, passam assim desapercebidos e sem ser realmente lidos e estudados.
    Eu sei que o que muitos fazemos é apenas dar testemunho das nossas condições meteorológicas locais e pouco mais. Uns porque não sabemos, outros por vergonha ou até mesmo preguiça. Mas a verdade é que se ler-mos posts como estes, iremos realmente ficar muito mais ricos em sabedoria meteorológica e muitos dos seus fenómenos.

    Ao Vince o meu agradecimento sincero pelo esforço e dedicação, que a par dos seus colegas de Administração e equipa de Moderação, nos mostram a cada dia. Fazendo deste fórum um local amador, mas com muita ciência e seriedade, ao ponto de hoje ser-mos respeitados e acarinhados pelo organismo oficial da meteorologia em Portugal, o IM.

    Abraço MeteoPT! [​IMG]
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  5. Vince

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    23 Jan 2007
  6. Knyght

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    10 Mai 2009
    Madeira - Funchal
    Vou ler atentamente obrigado Vince :thumbsup:
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  7. actioman

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    15 Fev 2008
    Elvas (~300m)
    Não tens nada que agradecer, nós sim e MUITO a todos vocês!

    Fica então o registo. Pois ao que parece este será um tema de grande interesse nestes dias tão profícuos em estados de tempo severos! ;)

    Assim para além de observarmos e registarmos os possíveis eventos, ficaremos a compreende-los melhor!
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  8. Jota 21

    Jota 21
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    20 Set 2007
    Porreiro. Há aqui muito que ler. Não dava com este tópico.
  9. Paulo H

    Paulo H
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    2 Jan 2008
    Castelo Branco 386m(489/366m)
    Aqui vai uma sugestão para encontrar melhor este tópico:

    Mudar o nome de "Previsão de fenónomos severos ou extremos" para "Previsão de fenómenos severos ou extremos".

    Lol :)
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  10. Vince

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    23 Jan 2007
    Corrigido. É verdade, parece que durante anos e anos escrevi essa palavra mal, até me recordo que foi o Gilmet que há uns 2 ou 3 anos me chamou a atenção :D

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