Previsão de fenómenos severos ou extremos

Vince

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23 Jan 2007
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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:

capewt1.jpg


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:
http://lightningwizard.com/maps/

E o guia pode ser descarregado aqui:
http://lightningwizard.estofex.org/ConvectiveWeatherMaps.pdf

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.



Introdução:
...
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
potential.

1. MLCAPE (and MSL Pressure, 500 hPa Geopotential Heights)
01fr7.gif

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

02dl8.gif


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)

03sr7.gif


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

04av0.gif


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)

05uo0.gif


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

06iy9.gif


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

07tg8.gif


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

08cp2.gif


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

09sl6.gif


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
nowhere.


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


10zr2.jpg


11yj3.jpg


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)

12cj3.jpg


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)

13zk5.jpg


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,
1991).
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
representative.


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

14xc4.gif


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
forecaster.


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

15yg5.gif


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

16wo6.gif


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

17ta0.gif


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
MLCAPE and MUCAPE.
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
http://www.lightningwizard.com
http://lightningwizard.estofex.org
 

Vince

Furacão
Registo
23 Jan 2007
Mensagens
10,624
Local
Braga
Re: Análise e previsão da fenónomos severos ou extremos

Outros sites e documentos úteis neste tema:

MANUAL DEL BUEN KAZATORMENTAS (MBK)
Neste manual em espanhol existe uma boa introdução ao CAPE, ao Lifted Index (LI), ao Storm-Relative Helicity, e por aí fora.
http://www.tiemposevero.es/ver-reportaje.php?id=96

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

http://www.ame-web.org/JORNADAS/elizaga.pdf

Spain Severe Weather - Formación
Nesta secção do site espanhol TiempoServero há dezenas de link's para documentos e estudos interessantes:
http://www.tiemposevero.es/formacion-spain-severe-weather.php
 

actioman

Cumulonimbus
Registo
15 Fev 2008
Mensagens
2,052
Local
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!
MeteoPT.gif
 

actioman

Cumulonimbus
Registo
15 Fev 2008
Mensagens
2,052
Local
Elvas (~300m)
Este tópico é antigo, num outro algures foi referido que desde o ano passado este manual tem uma tradução/adaptação para espanhol, sempre é mais acessível para os que não se dão bem com inglês:

http://www.tiemposevero.es/ver-reportaje.php?id=265
http://www.tiemposevero.es/ver-reportaje.php?id=260

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!
 

Vince

Furacão
Registo
23 Jan 2007
Mensagens
10,624
Local
Braga
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 :)

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