Updraft strength is maximized for increasing CAPE and intermediate values of wind shear

Document Sample
Updraft strength is maximized for increasing CAPE and intermediate values of wind shear Powered By Docstoc
					T.O. 6.4.1
A) Describe and assess the stability of atmospheric soundings: local parcel stability;
latent instability; potential instability; differential advection causing changes in
stability
Local Parcel instability:
   Most commonly assessed for deep convection
   Air parcel is lifted from the surface (or from a level aloft, important for elevated convection); we
    assume: 1) its pressure adjusts instantly to that of the environmental air; 2) no heat exchange
    between the parcel and ambient air (adiabatic or isentropic). If air is unsaturated, its temperature
    decreases with height at the dry adiabatic lapse rate; If air is saturated, its temperature decreases
    with wet adiabatic lapse rate (smaller than that of dry air, due to the latent heat release)
   At a level, Tparcel > Tenvironment  unstable; Tparcel < Tenvironment  stable; easily seen on a
    tephigram. Usually we compare the environmental lapse rate with dry adiabatic rate and wet
    adiabatic rate:
         < w  absolute stable (flat or slow Tenv decrease, so parcel always cooler)
         > d  absolute unstable (steep or fast Tenv decrease, so parcel always warmer)
        w <  < d  conditional unstable
         = w (or  = d )  neutral equilibrium for saturated (unsaturated) air

Latent instability
   For an air parcel, sufficient lifting is required to achieve convection, i.e., it can acquire a
    temperature warmer than that of the environment at some point above the layer in which it
    originates, it is said to possess latent instability (LI)
   LI is realized by the release of latent heat when the available moisture condenses during the
    parcel's ascent.




                                                                          Tc = convective temperature

    Two ways to diagnoses:
1)   From the latent instability analysis perspective:
      Draw the lowest value wet adiabat (LVWA) tangent to the environmental temperature curve. If
      this line intersects the environmental wet bulb temperature curve at some lower level, all parcels
      in that layer with w values to the right of the LVWA have latent instability.
2)   From the latent instability layer perspective:



                                                                                                       1
     If pseudo-adiabats through some point on the environmental wet bulb temperature curve crosses
     the environmental temperature curve at higher level, then the point is in an unstable layer and
     has latent instability.
   LI occurs most frequently in environments with steep lapse rates aloft and high dew points in
    lower levels; favorable condition for thunderstorm development
   To release LI, a lifting is needed: frontal, topographic, synoptic lift (PVA) etc

Potential instability
   A layer of air, if given sufficient ascent, the layer will acquire an unstable lapse rate, then the
    layer is said to have potential instability (PI). Important for Acc and elevated thunderstorms
   Diagnosis: on the tephigram, compare the lapse rate of Tw curve of the environment to the moist
    adiabats. If the environment Tw decreases with height faster than the moist adiabats, then this
    layer possesses PI
   To realize PI, lift needed by upslope of terrain, synoptic lift etc:
    1) For dry air masses, ascent  dry adiabatic cooling of whole layer  vertical stretching of
         the layer (p between the top and bottom keeps a constant) steeper lapse rate  less
         stable layer




    2)   For saturated air (or air that becomes saturated through ascent), differences in humidity of
         the layer affect its stability after lift (moister air cools slower), so that layers that are moist at
         the bottom will become more unstable than uniform or "top moist" layers




Differential advection causing changes in stability
   Warm advection increases with height (more warming at high level)  stability increase
   Cold advection increases with height (more cooling at high level)  stability decrease
        During night, Radiative cooling of the cloud top  unstable
        During daytime, surface heating  unstable
   Diagnostics: (use hodograph)
    1) warm advection  veering wind with height; cold advection  backing winds with height




                                                                                                             2
    2)   By comparing the thermal advective rates between two different levels in hodograph, one
         can derive the changes in lapse rate and hence, vertical stability.

B) Describe and interpret the vertical wind shear properties of atmospheric
soundings using the hodograph: Vertical wind shear; Storm motion and storm
relative winds; Hodograph curvature
Vertical wind shear
   Vertical wind shear is the change of the horizontal wind between two discrete levels, is a vector
    related to the horizontal T gradient (thermal wind relationship)
   In hodograph, wind vectors at different heights are plotted in a polar coordinate; wind shear is a
    vector joining the tips of two wind vectors (pointing to the wind vector tip of a higher level)
   Total wind shear over a particular depth is calculated by adding up the entire length of the
    segments of the hodograph
   Hodograph shape refers to the degree of both speed and directional wind shear and how the
    hodograph curves with height (Hodograph shape is very important for TS structure and evolution)
   Importance of vertical wind shear:
    1) weak wind shear: no updraft tilting, single-cell short-lived storms, strength mainly depends
          on CAPE, little new-cell regeneration along gust fronts
    2) moderate low-level shear (below 3km), 20kt for new cell generation in downshear side,
          expect slightly longer-lived cells/clusters with cell regeneration.
    3) strong low-level shear: 30kt shear below 700mb and weak shear above, expect persistent
          multi-cell storms with intensity modulated by CAPE.
    4) deep wind shear: ~60kt shear below 500mb, ~40kt below 700mb, expect supercells with
          enough CAPE

Storm motion
   During the early stages, storm generally moves along in the direction and nearly the speed of the
    mean wind of SFC - 6km.
    1) For straight hodograph, the mean wind use the mid-point of the hodograph below 6km.
    2) For a curved hodograph, visually move the origin of the hodograph to the point representing
        the SFC wind with the x' axis passing through the 6km wind. Then average all new x' and y'
        components of the wind to estimate the mean wind




Storm relative wind
   Storm relative wind is "the winds as seen by the storm", i.e., in the storm's frame of reference its
    motion is zero. The storm relative winds are simulated by moving the hodograph origin to the tip
    of storm motion vector and replotting the wind vectors in the storm's frame of reference.
   The wind shear in the storm-relative coordinate will determine how the structure of the storm will
    evolve. For a curved hodograph with storm motion off the wind shear vector the storm relative
    winds and wind shear are increased.

Hodograph curvature


                                                                                                      3
   Hodograph curvature refers to the degree of both speed and directional wind shear and how the
    resulting hodograph may curve with height. Whether the hodograph is relatively straight or has
    clockwise or counterclockwise curvature has implications for thunderstorm evolution.
   Assessment of hodograph curvature allows the forecaster to anticipate the motion and type of
    storms that may be expected to develop.

C) Explain how convection is dependent on factors such as insolation, moisture,
winds, stability and local geography;
Insolation:
    1) provides energy to lift parcels and trigger convection
    2) During the day, the insolation heats the ground and evaporates water into the air. The air
         near the ground becomes warmer and moister than the air above it  positively buoyant and
         statically unstable  upward vertical acceleration of an air parcels rise from the surface
         layer
    3) Lift air parcel to LCL; if the surface air temperature reaches to convective temperature (Tc)
          set off free convection even without mechanical lift (in such case, convective
         condensation level (CCL) = LFC = LCL)
                   LFC – level of free convection
                   LCL – lifted condensation level
                   CCL – convective condensation level

Moisture:
    1) moist air is more buoyant than dry air as water vapor is lighter than air, leading to stronger
        updraft in the convection
    2) An increased amount of water vapor in the lowest layer of the atmosphere results in
        increased potential instability
    3) For an air parcel, more moisture means  more latent instability
    4) Larger moisture increases CAPE and thus the strength of the convection
    5) Saturated air parcel cools more slowly than dry air due to latent heat release that supplies
        energy for continuous convection

Winds:
         horizontal wind convergence produces upward motion, initiate convection
         Vertical wind shear:
         1) Low-level wind shear important for cold pool interactions and new cell generation
              (multicells). In lowest 3km or so can lead to new cell development on downshear side of
              cold pool (>=20kts of shear). In some cases low-level shear can lead to organized
              multicell storms (>=30kt of shear)
         2) Deep shear important for tilting updraft prolonging storm longevity. For supercells, deep
              shear necessary for generation of mid-level rotation and vertical perturbation pressure
              gradients. In lowest 6 km important for tilting storm updraft prolonging lifetime. In some
              cases interactions with updraft can lead to supercell storms (>=30kt of shear)

Stability:
       need instability to release latent energy and to characterize updraft strength and modulates
        storm intensity
       Positive buoyancy (hence, convection) is a result of instability and moisture
       Potential instability is a necessary condition for development of thunderstorms. Ordinary
        conditional instability, based on the lapse rate of temperature with height, is insufficient. If the
        atmosphere becomes unstable in term of the lapse rate, convection develops on a scale too
        small to produce a cumulonimbus.




                                                                                                          4
         If an inversion caps a humid layer near the surface, energy of instability is not released until
          higher values of potential instability are reached. Then the convection develops more
          vigorously and may form thunderstorms.

Local geography: Provide triggers for convection
       Upslope of terrain forces air parcel to rise and lift air parcel to its LFC
       Terrain induced mountain-valley breeze, sea-land breeze, barrier jet, sea-breeze fronts and
        jet change wind shear and advect moisture
       regions of excess surface heating
       Provide moisture for convection: Lakes, oceans are moisture sources of convections

D) State and distinguish between the mechanisms by which latent instability and
potential instability are realized
LI
     The physical importance of a layer of air possessing LI is that although a layer is locally stable
      (the layer may be quite thick as well), parcels of air within it can, if given sufficient lift, become
      positively buoyant with respect to their new local environment. LI is realized by the release of
      latent heat when the available moisture condenses during the parcel's ascent.
     Latent instability is favoured by steep lapse rates aloft and high moisture contents in the low
      levels.
     Mechanisms to realize LI include:
      1) In summer, cloud top cooling, cold pools in mid levels, and Surface heating. For convection
           to occur through a deep layer in the atmosphere (e.g., severe convection), the layer of latent
           instability should be based near the surface and be of significant depth.
      2) In winter, cold air over open water, cloud top cooling, and mid-level steep lapse rates above
           cloud layers and a trend in destabilization.

PI:
     A layer is said to have PI if, given sufficient lift, the layer will acquire an unstable lapse rate
     LI deals with parcel ascent through local lift, while PI tells the potential for decreasing stability
      due to large-scale ascent of a layer or of an entire air mass
     The lifting mechanisms are required for realizing PI include forced ascent over rising terrain,
      PVA ahead of an upper trof, or divergence associated with high level jets.
     LI refers to a parcel’s buoyancy relative to the lapse rate of the environment above it. PI looks at
      the stability of a particular layer after air mass lift, regardless of the sounding above that layer. LI
      requires cold air aloft, PI does not.
     However that if there is no potential instability in any part of a sounding there can be no latent
      instability. Parcels undergoing finite vertical motions may realize LI but a small scale local lifting
      mechanism is required to supply the escape energy. Air mass layers undergoing general lift may
      realize PI but sufficient air mass lift is required.

E) Explain latent and potential instability are important to convection
     LI and PI indicate that there are available instability exist for convection development, once the
      vertical motion of a parcel or a layer is forced upon it, either by local or large-scale synoptic lift,
      the instability will be realized for convection.
     PI is most commonly found in layers aloft and is often correlated with the presence of ACC or
      elevated Cb (this can sometimes include the persistence of nocturnal thunderstorms) assuming
      sufficient moisture is present to achieve moist convection.
     LI associated with steep lapse rates aloft and high dew points in lower levels favor large positive
      buoyant energies and strong updraft, favorable conditions for thunderstorm development. Once
      SFC heating allows the convective temperature to be reached, the LI will be realized and
      thunderstorm development will depend primarily on the degree of local parcel instability above
      LFC.


                                                                                                            5
   PI is a necessary condition for development of thunderstorms. Ordinary conditional instability,
    based on the lapse rate of temperature with height, is insufficient. If the atmosphere becomes
    unstable in term of the lapse rate, convection develops on a scale too small to produce a
    cumulonimbus.
   If an inversion caps a humid layer near the surface, the situation is different. Energy of instability
    is not released until higher values of potential instability are reached. Then the convection
    develops more vigorously and may form thunderstorms.




                                                                                                        6
T.O. 6.4.2
A) Describe the buoyancy processes related to: Updrafts (including Bubble
convection, CAPE, Estimate of updraft strength, CIN, Entrainment); Downdrafts
(including Evaporative cooling, Precipitation loading, Downdraft CAPE, Estimate
of downdraft strength)
Updrafts
  Bubble convection:
   1) Bubble theory views the active convective elements in a cloud as discrete "bubbles". As a
         bubble rises, turbulence mixes environmental air into its interior.
   2) The earliest saturated bubbles (thermals) can only penetrate a limited distance upward
         because the mixing of environmental air into the bubble reduces the bubble's buoyancy.
         With sufficient mixing, the bubble loses its identity and is totally mixed into the
         environment. This is known as parcel detrainment.
   3) The erosion of these first bubbles can result in a warming and moistening of the local
         environment through which they ascend. Clouds may form and dissipate as bubbles rise and
         detrain into the environment.
   4) Later bubbles, produced by continued heating of the ground, following the same ascent path
         will encounter this "enriched" environment and will not erode until encountering drier and
         cooler air at higher heights.
   5) Bubble convection views the growing cumulus cloud as the result of a train of bubbles rising
         through the same channel; each successive bubble rises to greater heights before eroding.
   6) Whether or not cumulus clouds will persist and grow to greater heights depends on the net
         effect of the rate of enrichment of the environment by the rising bubbles and the rate of
         drying by the environment.
   7) There are drag forces which work against bubble ascent. These include:
          Loading by Cloud Water, the downward force due to the weight of suspended cloud liquid
            water decreases the buoyancy of the cloud bubble.
          Turbulent Drag is the retarding influence of frictional (fluid viscosity) and turbulent
            forces. Momentum exchange during entrainment of environmental air which is at rest
            represents a major drag force.

   CAPE (Convective Available Potential Energy)
    1) On a tephi, it refers to the positive area between a lifted parcel T curve and the
       environmental T curve
    2) Integrated from the LFC to the Equilibrium level (EL)
    3) Gives estimate of potential updraft strength
    4)  The strongest updraft occurs at the level where the parcel is its warmest with respect to the
       environmental air, so a "fat" CAPE indicates stronger vertical accelerations than the skinny"
       CAPE, thus produce more hazardous weather

   Estimate of updraft strength
    1) Assume all CAPE are converted to kinetic energy, then the maximum updraft velocity for a
         parcel is Wmax  2CAPE
    2)   This is the upper limit to the updraft velocity arising from buoyancy, because the negative
         buoyancy effects by entrainment and water loading. Are not considered

   CIN (convection inhibition)
    1) To release latent instability a parcel must reach its LFC, CIN measures the amount of energy
        required to reach level free convection (LFC), it prevents convective initiation.




                                                                                                   7
    2)   On a tephigram, CIN is the “negative area” between lifted parcel T curve and the
         environmental T curve integrated between LCL and LFC. Small value of CIN means less lift
         required to achieve LFC. CIN values typically increase when the static stability above the
         unstable layer increases.




   Entrainment
      Entrainment refers to mixing of environmental air into the parcel, and can occur both laterally
       and at top of updraft. A strong updraft will minimize the amount of entrainment. Entrainment
       depends on:
       1) Thermal and moisture profile of environment
       2) Size of the convective cloud (small clouds  more entrainment) as entrainment occurs
           mainly at the periphery of the updraft (large cloud protects updraft core)
      Impacts of environment on parcel
       1) Environmental air is cooler and drier than a rising saturated parcel
       2) Cooler air will lower the sensible heat of the rising parcel ( +ve buoyancy)
       3) A “drier” parcel is less buoyant than a moist one ( +ve buoyancy)
       4) Drier air leads to cooling as cloud water evaporates in a saturated parcel ( +ve
           buoyancy)
      Results of entrainment
       1) Parcel cools at a greater rate than that predicted by parcel theory
       2) As parcel “dries” it becomes more negatively buoyant
       3) Net result is an updraft with less buoyant energy (CAPE) and increased negative
           buoyancy effects impeding upward accelerations
       4) Storms tops are lower than estimated and updrafts are weaker
       5) Important for descending parcels (downdraft)

** Summary of updraft
      1) when wind shear is weak, buoyancy processes are the dominant control on convective
           updrafts. As positive buoyancy increases, upward vertical acceleration increases.
      2) buoyant energy arises from temperature differences between a lifted parcel and its
           environment. Buoyant energy is maximized in environments characterized by steep mid-
           level temperature lapse rates (conditional instability) and sufficient moisture within or
           below the buoyant layer.



                                                                                                    8
       3)     positive buoyancy (upward) through a vertical column can be estimated quantitatively
              by integrating the parcel excess temperatures between the LFC and the equilibrium
              level. This is called CAPE or buoyant energy. Other measures of updraft strength
              include various indices such as the Lifted Index and Showalter Index.
       4)     unstable parcels must be lifted to their Level of Free Convection (LFC) to release
              buoyant energy and produce an updraft.
       5)     Convective Inhibition (CIN) is a measure of the amount of lift (energy) required to reach
              the LFC. As the CIN increases, more lifting energy is required.
       6)     Solar heating is a frequent source of energy for lifting parcels to their LFC. However,
              there are many other sources of lift that must be considered before rendering a
              convective forecast.


Downdrafts
  Why important?
      1) Downdraft can cause damaging surface winds
      2) Associated with microbursts
      3) Significant impact to aviation community
      4) Responsible for “cold pool” that is important for new convective cell development
           and longevity of multicell storms
  Downdraft is enhanced by
      1) evaporation cooling
      2) precipitation loading
      3) momentum transfer

   Evaporative cooling: Evaporation of cloud water in an unsaturated layer cools the air, and
    produces negative buoyancy to air parcels, and enhances downdraft speeds; enhanced by:
       1) availability and entrainment of dry environmental air (increase the evaporation rate)
       2) steep environmental lapse rate air (acts to maintain negative buoyancy as the parcel
            descends)
       3) large liquid water content (provides more precipitation for evaporation)
       4) small drop size (Smaller droplets tend to evaporate more quickly than larger drops)

   Precipitation loading
       1) Downdraft strength is increased by the drag effect of liquid water. Liquid water loading
            adds weight to a column, increasing the downward force of gravity. This force
            eventually dominates the updraft and initiates the downdraft, usually in the mid-levels of
            the storm. As the amount of liquid water in the column increases, water loading
            increases, resulting in potentially stronger downdrafts.
       2) In general, a stronger updraft will have stronger downdrafts due to the effect of
            precipitation loading.
       3) Operationally, air-masses with high liquid water content and high buoyant energy are
            particularly susceptible to strong downdrafts, particularly when dry air is found at mid-
            levels.

   Momentum transfer
            Downward transfer of stronger winds aloft towards the surface can also enhance outflow
            winds. Storms developing in stronger environmental winds may produce stronger outflows
            as a result of downward horizontal momentum transfer.

   Downdraft CAPE (dCAPE)




                                                                                                     9
     1)   Downdraft CAPE (dCAPE) is a theoretical estimate of the max. increase in kinetic energy
          available to a downdraft parcel due to negative buoyancy accelerations
     2)   On Tephi, it is proportional to the area bounded by a downdraft pseudoadiabat and the
          environmental temperature curve below the level at which the downdraft originates. The
          level to use as the origin of the downdraft usually is the lowest valued moist adiabat
          intersecting the environmental wet bulb curve.
     3)   Larger dCAPE implies stronger downdrafts (and outflow winds), and maximized for cold
          dry air aloft and warm moist air near the surface.

   Estimate of downdraft strength
    1) An upper limit to the maximum velocity of the downdraft parcel when it reaches the SFC
          can be found using Wmax  2dCAPE
     2)   Operationally, look for soundings exhibiting dry air at mid-levels and a warm, moist
          boundary layer
     3)   The strength and depth of the cold pool at the SFC will be related to the temperature
          difference between the downdraft parcel and the ambient air at the surface. High dCAPE
          values imply larger negative buoyancy and deeper cold pools.




** Steep lapse rate air contributes to both stronger updrafts and stronger downdrafts (why?)
         For updraft, steep environmental lapse rate  fast environment T decrease  larger (Tparcel
          – Tenv)  larger positive buoyancy  stronger updraft
         For downdraft, buoyancy term doesn’t help explain; but note
     1)   stronger updraft caused by steep lapse rate  precipitation  precip. loading
     2)   steep lapse rate  cool & drier mid-level air  enhanced evaporation cooling
     Both contribute to stronger downdrafts.




                                                                                                  10
B) How cold pool and low level shear interactions control storm evolution and
structure
    Importance
     1) Cold pool and low-level shear interactions responsible for longer lived storms where new
          updrafts develop on a preferred flank of the storm
     2) Dominant process controlling long-lived mesoscale convective systems (multicell storms)
          that can produce widespread severe weather (wind and hail damage)
    Cold pool
     1) downdraft within storm reaches SFC; cold air spreads out in all directions
     2) leading edge acts like cold front providing lift (gust front)
     3) horizontal vorticity roll enhances lift over denser cold air
     4) If parcels can reach LFC new cells develop
    Interaction between cold pool and low-level vertical shear
     1) environmental horizontal vorticity is created by shear
     2) cold pool vorticity interacts with environmental vorticity
     3) upshear side suppressed vertical motion and cell advected over cold pool
     4) lift favoured on downshear side of cold pool and cell advected in direction of gust front
     5) strength of shear impacts new cell development
    Factors for New Cell Development
           Strength of the wind shear vs. that of the new updraft will influence whether cell can
            survive
           Speed of gust front and new cell motion a critical factor: Gust front too fast means cell
            end up over cold pool and is short-lived; assessing low-level shear will alert you to the
            possibility
     1) Weak low level wind shear
            wind speed above cold pool significantly less than speed of outflow boundary
            outflow boundary undercuts updraft of new cell
            no new cell development downshear
     2) Moderate low level wind shear
            wind speed above cold pool and outflow boundary speeds the same
            continuous updraft under new cell with no net shear
            new cell development downshear
            new multicell storms sustain themselves
     3) strong low level wind shear
            wind speed above cold pool faster than speed of outflow boundary
            Enhanced lift on downshear side of pool but cell is “shredded” by wind shear
            no new cell development downshear
    Operationally
            values of 0-3km wind shear of 20kt may be associated with new cell development on the
             downshear side of the outflow boundary.
            Values of 0-3km shear of 40kt can be associated with long-lived multicells or if combined
             with large CAPE can result in bow-echoes, squall lines or derechoes


C) Describe how buoyancy and deep shear interactions control storm evolution and
structure including: Tilting; Hodograph curvature and storm evolution
2)   Tilting
     1) In a deep shear environment (0-6km or 500mb), relative high pressure created on upshear
          side of cloud; relative low-pressure created on downshear side of cloud
     2) Horizontal pressure gradient causes tilting of the updraft (and cloud) tilted downshear




                                                                                                   11
     3)    The degree of tilting will depend on the vertical velocity of the parcel and the strength of the
           wind shear.
     4)    The tilting of updraft downshear allows the precipitation particles not falling right over the
           updraft and allows the updraft to thrive
     5)    perturbation pressure gradient important for supercells
     6)    Strong shear will initially act to inhibit storm development. If updraft persists and is strong
           enough stronger shear will generally lead to more organized storms. The balance between
           the strength of an updraft and the environmental wind shear is critical in determining if a
           convective cell will persist and how organized a resulting thunderstorm will be (i.e., single-
           cell, multicell, supercell).
              This may be quantified via the Bulk Richardson Number: BRN = CAPE/WS.
              BRN ~ 10-50 supercell; 50-350 multicell; > 350 airmass; too small, no storm
     7)    Tilting of the thunderstorm updraft is the first step in the development of a persistent
           convective system. Further organization depends on the hodograph shape (strength,
           direction and depth of the wind shear).

3)    Hodograph curvature and storm evolution
       Vertically wind shear  horizontal vorticity tubes (direction, perpendicular to the wind shear
        vector; strength, proportional to the strength of WS)
      Horizontal vorticity is tilted into the vertical in the vicinity of the updraft; a vertical vorticity
        couplet forms at mid-levels (~3-8km) of the incipient storm, with low pressure at the center of
        rotation (regardless of cyclonic or anti-cyclonic, PGF = centrifugal force, cyclostrophic)
      Stretching of the vertical vorticity in the updraft increases vorticity to a maximum at mid-
        levels and vertical perturbation pressure gradient develops below each of the rotational
        centers
      Dynamically induced vertical pressure gradients beneath the rotation centers induce updrafts
        on the left and right flanks of the storm. Further storm evolution depends on hodograph shape




Straight Hodograph (unidirectional shear)

         Wind shear is unidirectional, the high pressure region on the upshear side extends through the
          depth of the column with no vertical pressure gradient (vertically stacked).
         This region of high pressure acts to separate the vertically stacked vorticity couplet.
         Enhanced by the dynamically induced updrafts on both flanks of the storm the cell separates
          into two distinct mirror image storms that begin to move away from one another. The right-
          moving (left-moving) storm exhibits cyclonic (anti-cyclonic) rotation. This process is
          sometimes aided by the presence of a precipitation-induced downdraft depending on the depth
          of the vertical wind shear and liquid water content of the storm.




                                                                                                        12
       This idealized case for a straight hodograph is a pair of "mirror-image" storms which will
        tend to move away from one another as they propagate to the right and left of the mean wind,
        with continued development on the updraft flank of each storm. In reality it is much more
        common that there is a preferred flank for new development with one storm favoured to
        persist and strengthen




Curved Hodograph

   Physics:

        Wind shear is not unidirectional, high pressure that developed on the upshear side of the
         storm rotates as the WS vector, so not vertically stacked
        An upward directed perturbation pressure gradient may develop on a preferred (right) flank
         of the storm. A rotating updraft (mid-level mesocyclone) is enhanced by both upward
         directed pressure gradients (due to mid-level rotation and veering of the horizontal pressure
         gradients with the wind shear vector); The left flank of the storm develops a downward
         directed pressure gradient force counteracting the updraft induced by the anti-cyclonic
         member of the mid-level vorticity couplet
        The right flank is now strongly associated with the storm updraft and and the left flank is
         associated with the downdraft. The storm begins to deviate to the right of the mean wind
        The "right-motion" storm is further enhanced by
       1) Contributions of streamwise vorticity to the updraft (resulting in an environment more
            conducive to tornadogenesis)
       2) Right deviation allows the storm relative inflow into the storm to avoid the main
            precipitation area and maintain its buoyant properties
        The supercell storm may last for a number of hours. If the cold pool becomes too strong the
         cold air will spread out ahead of the updraft and the original supercell will dissipate. Other
         factors may lead to regeneration of subsequent rotating updrafts.




                                                                                                    13
** Description for the whole processes:

1)   Initially there is an updraft present on both flanks of the storm as in the straight hodograph case.
     The updrafts are initially enhanced by the low pressure associated with the centers of the mid-
     level vorticity couplet.
2)   Due to interactions between the developing storm and the environmental wind shear, an upwards
     (downwards) directed perturbation pressure gradient develops on the right (left) flank of the
     storm. The right flank is now strongly associated with the storm updraft and the left flank is
     associated with the downdraft. The storm begins to deviate to the right of the mean wind. The
     splitting of the storm as in a straight hodograph environment never really occurs as the left flank
     quickly becomes dominated by the storm downdraft.
3)   The combination of dynamic pressure effects due to the mid-level rotation and interaction of the
     storm with the environmental wind shear act to strengthen the updraft and associated vertical
     velocities beyond those attributed to buoyancy considerations alone. The rotation of the updraft
     is enhanced through the stretching of vertical vorticity as the mid-level mesocyclone develops
4)   The storm continues to propagate to the right of the mean winds as new development continues
     on the right flank. The "right-motion" enhances the contributions of streamwise vorticity to the
     updraft resulting in an environment more conducive to tornadogenesis. Right deviation also
     allows the storm relative inflow into the storm to avoid the main precipitation area and maintain
     its buoyant properties. The result is an intense, well-organized, and persistent storm capable of
     producing large hail, devastating winds, heavy rain, and tornadoes.



** Storm relative helicity
    Crosswise Vorticity
       Storm relative inflow is perpendicular to the environmental wind shear induced horizontal
        vorticity tubes
       The main updraft is at the leading edge of the storm, no vorticity contribution to storm (as
        horizontal vorticity is tilted into the vertical vorticity couplets which are located on the flanks
        of the storm)




    Streamwise Vorticity
        The storm-relative inflow and the vorticity vector (arising from wind shear generated
         horizontal vorticity) are parallel
        "spiralling" inflow leading directly to updraft rotation when the inflow (and vorticity) is tilted
         into the vertical



                                                                                                        14
   Storm relative helicity (SRH)
       SRH is a measure of the degree of streamwise vorticity present within the inflow
        environment of the storm, thus an indicator of the potential for updraft rotation
       If crosswise vorticity dominates the storm relative inflow, SRH will be low and there is little
        potential for updraft rotation; Conversely, if streamwise vorticity dominates then the
        potential for updraft rotation is high
       Usually the storm inflow layer is approximated by the 0-3 km layer and SRH is calculated
        through this depth
       SRH may be represented visually on a hodograph as negative twice the area bounded by the
        hodograph and the SFC and 3km storm relative wind vectors




   Development of low-level mesocyclone
       The tilting and stretching of horizontal vorticity generated locally by the storm itself seems to
        play a significant role in low-level mesocyclogenesis (** note tilting and stretching of
        environmental horizontal vorticity result in mid-level mesocyclone)
       In general, horizontal vorticity will be generated where there are horizontal gradients in



                                                                                                      15
      buoyancy and resulting vertical motions in cold pool. Within the near-supercell environment
      such conditions exist behind the leading edge of the cold pool associated with the forward
      flank downdraft (FFD) and rear flank downdraft (RFD)
     Note that the vorticity vectors within the cold pool of the FFD generally point towards the
      updraft; and the magnitude of these vectors is greater than the northwest-pointing vorticity
      vectors (in "warm sector" of wave) associated with environmental wind shear. This
      streamwise vorticity increases probability of updraft rotation, and the dynamically induced
      upward pressure gradient force (located below the mid-level vorticity center) is able to lift
      this air into the storm updraft. This mechanism of streamwise vorticity is thought to play a
      major role in the development of the low-level mesocyclone.




     Note that the vorticity vectors within the cold pool of the FFD generally point towards the
      updraft; and the magnitude of these vectors is greater than the northwest-pointing vorticity
      vectors (in "warm sector" of wave) associated with environmental wind shear. This
      streamwise vorticity increases
     In some cases, low-level rotation may become strong enough to generate a downward-
      directed pressure gradient force below the mid-level mesocyclone. This small downdraft
      enhancement can locally accelerate the RFD, strengthening the outflow winds wrapping
      around the back of the storm. An occlusion of the updraft may occur with possible updraft
      regeneration at the point of occlusion. This process is strongly linked to tornadogenesis.
     Other mechanisms for tornadogenesis: when storm forms form near SFC lows or SFC
      vorticity trofs, or as a storm propagates along a thermal boundary, dryline, or outflow
      boundaries from other storms




** About CAPE and Shear




                                                                                                16
    Initial stage
    Updraft strength is maximized for increasing CAPE while updraft strength decreases for increasing
    wind shear. In general, wind shear has a detrimental effect on initial cell growth.
    Secondary stage
    Updraft strength is maximized for increasing CAPE and intermediate values of wind shear. These
    results highlight the enhanced ability of the cold pool to trigger new cells as wind shear increases
    Supercell stage
    The supercell updraft is maximized for stronger vertical wind shear but again weakens if the shear
    is too strong. Interestingly, if the buoyancy is too strong, relative to the wind shear, the updraft
    weakens due to the cold pool becoming too strong and moving out too quickly away from the
    updraft.




                                                                                                     17
T.O.6.4.3
A) Describe the life cycle of a single cell thunderstorm, its characteristics, associated
weather, and typical pre-storm environment




Life cycle:
        Initial Stage:
           updraft dominated from the surface to cloud top with speed of 10-20 m/s or more

           horizontal convergence maximized near the surface

           entrainment ongoing at cloud boundaries

           cell temperature exceeds the environmental temperature at each level

           positive temperature anomalies in the updraft increasing with time

           greatest concentration of hydrometeors is found at or above the freezing level

           no precipitation reaches the surface

        Early mature stage:
          updraft continues to feed moisture
          precipitation (super-cooled water drops and ice-crystals) forms in upper levels
          anvil forms
          overshooting top and gravity wave may be present
        Late mature stage:
          downdrafts initiated at mid-levels and increase in space and with time

          cold pool begins to develop, surface wind gusts develop

          maximum turbulence occurs in regions of greatest vertical motion,

          cloud grows physically, cloud tops (ice crystals present) reach temperatures < -40°C

          cold anomalies occur in the downdrafts, with a maximum in low levels

          warm anomalies still occur in the updrafts

          low-level precipitation is liquid (solid precipitation is possible)

          horizontal boundary of the surface precipitation roughly marks the downdraft boundary

        Dissipating stage:
          cell is “collapsing”

          downdrafts spread through the entire cell

          downdraft speeds are less than in the mature stage

          surface wind divergence rapidly decreases




                                                                                                   18
          cell T falls below that of the environment at each level (but eventually equal to Tenv)
          ice crystal clouds (cirrus) blow off the cell tops
          shower activity is generally light and decreases

Characteristics:
           Typically short-lived (< 1 hr); Small spatial scales (on the order 1km)
Associated weather:
         Server weather rare but not unheard of
         Rain, hail, damaging winds and small tornadoes can occur with strong type storms (with
            large CAPE and weak shear)
Typical Pre-storm environment:
           In a weakly sheared environment, buoyancy is the dominant physical process driving
            thunderstorm evolution
           Without shear the storms are vertically oriented
           Precipitation has nowhere to go but to fall back through the updraft
           Downdraft forms at mid levels due precipitation loading and forms cold pool at the
            surface cutting off the storm inflow


B) Describe the life cycle of a multicell thunderstorm, its characteristics, associated
weather, and typical pre-storm environment; differentiate it from a non-severe
thunderstorm;
Multicell thunderstorm:
       characterized by a high degree of organization with continued new cell growth on a preferred
        flank of the cold pool (usually downshear with respect to 0-3km shear)
       Organized Multicells result from interactions between cold pool and environmental shear in
        the lowest few km (or orography, fronts, convergence lines or other boundaries)

Isolated multicell storm with mean wind vector in direction of shear vector




Life cycle and structure:
    Mean wind (average wind of 0-6km) and low-level (0-3km) shear vector in same direction (cells
     move in same direction as storm system)
    The growth of new cells occurs on the downshear side of the cold pool. The circulation at the
     leading edge of the cold pool is balanced by the environmental horizontal vorticity produced by
     low-level wind shear. This results in a persistent “vertical jet” along the leading edge of the cold
     pool lifting unstable parcels to their LFC. Cold pool needs to be deep enough and near speed of
     winds at cloud base, LFC needs to be near depth of cold pool
    The upper portion of precipitation core is positioned slightly downshear, called an echo
     “overhang.” This overhang is caused by the differential advection of precipitation by the vertical
     wind shear and by the strong lifting force near the leading edge of the cold pool.



                                                                                                      19
    The area below the overhang is called a “weak echo region” (WER). Often, an overshooting top
     is observed above the WER, further implying the existence of a strong updraft in that location
     and of an organized storm
    Updrafts are created at the leading edge of the spreading cold pool (gust front). Parcels are lifted
     and move rear-ward relative to the system with the anvil developing aloft towards the upshear
     side, well behind the leading edge of the cold pool.
    The downdraft develops at mid-levels within the main precipitation core due to precipitation
     loading and evaporation. Downdrafts can be very strong and persistent, fed by a constant supply
     of moisture in the neighboring updraft.
    The updraft and downdraft remain separated but interact constructively, and can maintain a
     strong circulation for hours.
    If the gust front has spread out substantially, and the layer of cool air spreads out and becomes
     shallower, much of the environmental air is not lifted to its LFC, new cells are no longer likely to
     form, the whole multicell system dissipates

Characteristics:
   Continual regeneration of cells near the leading edge of the cold pool leads to a lifetime of many
    hours
   Much larger than single cell storms (on the order of 100-400km)

Associated weather:
   Severe weather not uncommon including damaging winds, widespread large hail, heavy rain
    (possible flooding if slow moving)


Isolated multicell storm with mean wind direction different than shear vector
    More common wind shear is in different direction than mean wind vector
    New cells develop on the downshear side (0-3km) of the cold pool but move in a different
     direction than the system
    Later a new cell will develop in the location of the previous one




Typical pre-storm environment:
   The primary process responsible for organizing and sustaining multicell systems is the
    interaction of low-level vertical wind shear with the circulation produced by storm-generated
    cold pools.


                                                                                                      20
     When low-level wind shear is sufficient, along with presence of positive buoyancy (CAPE),
      regular cell growth will be favored along the down-shear side of the cold pool.
     The interaction of the gust front with other features such as orography, frontal boundaries,
      convergence lines, or other outflows, also can foster the regeneration of new cells on preferred
      flanks.

Differentiate it from a non-severe thunderstorm: (bold + italic denotes multicell TS)
    Life time: less than 1 hour; several hours
    scale: ~1km; 100-400km
    pre-storm environment: weak wind shear and buoyancy is the dominant physical process; low-
     level vertical wind shear and presence of positive buoyancy, interaction between wind shear
     and cold pool is dominant physics)
    weather: Hail, damaging winds and small tornadoes can occur with strong pulse type storms;
     Severe weather occurs as downbursts, moderate size hail, flash floods, and weak tornadoes.

** The dynamics of a convective storm depend upon the environmental CAPE, and shear. When the environmental
vertical shear is relatively weak, only the CAPE and density-current behavior are dynamically significant, and ordinary
cells occur. When the low-level vertical shear is larger and the CAPE is not too small, a succession of ordinary cells, a
multicell storm, can form. If the CAPE is high and the vertical shear is strong, then vertical perturbation pressure-
gradient forces owing to rotation become dynamically important, and supercell forms.
*The ‘bulk’ Richardson number R, the ratio of CAPE to a quantity proportional to the square of the mean vertical shear
integrated over height (S^2) as an indicator of storm type: R=CAPE/S^2 Where S^2=1/2(u6000m-u500m)^2.

** The behavior of convective storms is highly sensitive to the environmental CAPE and vertical shear, forecasters
must be careful to account for changes in environmental CAPE and vertical shear. On the mesoscale, the shear can
change owing to the passage of shortwaves of jet streaks or fronts and the response of the boundary layer winds to
developing surface cyclones or to mountain-valley or sea-breeze circulations. On the storm scale, shear can change
owing to the environmental response to nearby convection. CAPE can change owing to changes in cloud cover and
differential advection of temperature and moisture.



C) Briefly describe a squall line, a mesoscale convective system (MCS), and a
mesoscale convective complex (MCC)
Squall line:
   Thunderstorms are often observed in organized lines, called squall lines
   Its structure is irregular, with activity dominated by a few large intense storms and numerous
    small, relatively weak ones. Unique phenomena such as bow echoes, line echo wave patterns
    (LEWPs), supercells etc. can be found embedded in a squall line.
   usually coincident with the line of greatest instability and often organized by large-scale forcing
    such as frontal convergence.
   Motion of squall line is a result of the advection of cells within the line by the mean wind and the
    propagation of the line itself due to the triggering of new cells along its length. For squall lines
    larger than about 200 km in length, motion will be perpendicular to its original orientation. For
    smaller squall lines, motion is usually in the direction of the low-level shear vector with the
    squall line oriented perpendicular to its motion. Local changes in CAPE, shear, or LFC height
    can affect line motion as well.
Factors controlling squall line structure and evolution:
   The component of the low-level shear vector perpendicular to the orientation of the squall line
    itself (called line-normal shear). As the line normal shear increases, organization, longevity and
    intensity of system downshear increases.
   The size and depth of the cold pool affects system evolution. Strong pools require more shear to
    sustain squall lines (to produce deep lifting at the leading edge of the cold pool)
   The height of the LFC can influence system strength and longevity. High LFC environments
    require more lifting than lower LFC enviromnets. Hence, for a given shear and cold pool
    strength, a low-LFC environment is more likely to sustain stronger and longer-lived system.


                                                                                                                     21
Life cycle and structure:
With weak to moderate shear
Horizontal structure:
    Initially, squall lines appear as an elongated band of convective cells, each producing their own
     cold pool. Intense convection near leading edge of pool. Convergent along leading edge and
     divergent under heaviest precipitation.
    During the mature stage, the strongest precipitation echoes remain near the leading edge of the
     cold pool and form a continuous line with strong convection along the leading edge. Stratiform
     precipitation behind.
    By the weakening stage, the convection along the outflow weakens, and recedes from the leading
     edge. Cold pool spreads out ahead cutting off inflow. Stratiform precipitation may last for hours.
     New line may be triggered out ahead.
Vertical structure: (with weak to moderate shear)
    Early phase (1-3): cells mostly vertical
    Mature phase (4-5): New cells form along cold pool and advect rearward, updraft tilted upshear
     (rearward) feeding stratiform area. Rear inflow jet develops at mid-level, diverging at the
     surface.
    Dissipating Stage (6-7): The leading cells weaken and the updrafts becomes shallower and tilt
     further rearward. Rear-inflow jet surfaces and cold pool spreads out ahead of updrafts




With moderate to strong shear
Horizontal structure:
   The evolution time between the stages increases
   The leading edge echo gradient is stronger with more intense cells taking on a bowed structure.
   The precipitation line is narrower than in the weaker shear case with the intense echo gradient
    remaining on the leading edge of the cold pool, implying a perfect balance between the strength
    of the shear and cold pool.




                                                                                                    22
   Large swaths of significant wind damage are likely during the middle phase near the bulging
    segments of echoes (bow echoes).
Vertical structure:
     Persistent overhangs on downshear side
     Updrafts remain strong and vertical and near leading edge of cold pool
     As rear inflow jet eventually descends to SFC get sustained strong winds and gust front moves
      out ahead




Mesoscale convective system (MCS)
    MCS is a much larger convective ‘system’ produced by the interactions of individual cells
     (single cells, multicells, supercells) which can appear as a disorganized mass of convective cells
     or a highly organized convective line, such as a squall line.
    MCSs can evolve from an isolated cell, a group of cells, or initiate rapidly along a linear feature
     such as a cold front or surface trough.
    MSCs most often develop during the day of evening and continue well into the night, feeding off
     of elevated instability. MCSs often last for hours or over night.
    All types of sever weather are possible with MCSs, especially damaging winds.
    Most common and highly organized model of MCSs: squall lines, bow echoes, and mesoscale
     convective complexes (MCCs).
    Severity and organization of MCSs increases with increasing values of CAPE and low-level
     wind shear.

Mesoscale convective complex (MCC)
    MCC is a vast mesoscale convection system formed in such a manner that all the cells interact
     and share the same anvil. MCC is often severe weather producers.



                                                                                                     23
    Presence of strong divergence of horizontal flow at the level of cirrus anvil and the top of the
     outflow from the storm (like a tropical cyclone). MCC may rotate in a similar fashion to a
     tropical storm.
    Physical characteristics of a mesoscale convective complex based on analysis of enhanced
     infrared satellite imagery
    MCCs exhibit a nocturnal life cycle and are strongly linked with the nocturnal low level jet and
     have been found to interact with and modify the larger scale environment.

Definition:
SIZE:             A: Cold cloud shield with continuously low IR temperatures (less than -32°C); area
                  greater than 100 000 km2.
                  B: Interior cloud region with temperatures less than -52°C must have an area greater
                  than 50,000 km2.
INITIATE:         Size definitions A and B are first satisfied.
DURATION:         Size definitions A and B must be met for a period greater than 6 hours.
MAXIMUM:          Contiguous cold cloud shield (IR temperatures less than -32°C) reaches its maximum
                  size.
SHAPE:            Eccentricity (minor axis/major axis) is greater than 0.7 at time of maximum extent.

Evolution:
   Genesis: form late in the afternoon as several convective cells. The synoptic situation usually has
    these cells in a region of vertical ascent in association with a short wave trough and pronounced
    low level warm air advection.
   Development: occurs in the evening hours and is associated with the greatest occurrence of
    severe weather. The individual convective cells are at their maximum intensity with the cold
    cloud tops grown to satisfy the MCC condition. It is during this time period that the nocturnal
    low level jet reaches maximum intensity, enhancing both warm air advection and influx of moist
    unstable air.
   Mature: the mid to high level cloud shield reaches its maximum extent. As the MCC modifies
    the environment, vertical wind shear begins to decrease. Convective cells continue to develop,
    however their intensity diminishes reducing severe weather occurrences except for locally heavy
    rainfalls.
   Dissipation: The dissipation stage begins when convective cells no longer form, thus leaving the
    precipitation as strictly stratiform and diminishing with time.

     Typically, the first thunderstorms develop during the afternoon with convective complex organization appearing
      in the evening.
     Under favourable conditions, MCC's persist and reach maximum size around local midnight, that is, about the
      time that the nocturnal low-level jet stream both reaches maximum strength and is located nearest the surface.
     Thus, the nocturnal jet may act primarily in a large scale sense, providing the MCC its maximum influx of moist
      static energy.
     Moreover, mature complexes are often associated with upper-level mesoscale regions of pronounced cooling,
      high pressure, and strong anticyclonic outflow.

Weather:
    Form of local heavy rain, Flash flooding. MCC’s tend to move slowly giving the risk of high
    rainfall amounts over a smaller area than with more rapidly moving thunderstorms.

Motion:
    The motion of an MCC deviates from the environmental mean wind in the direction of the newly
    forming cells. This is generally on the side with the moist low level warm inflow.




                                                                                                                 24
Bow Echoes
   • 20-120km long
   • Long-lived symmetric bow-shaped segment
   • High CAPE environments (>2500J/kg)with strong low-level shear (0-3km > 30kts)
   • Can be isolated or embedded in squall lines
   • Long swaths of damaging straight-line winds
   • Move in direction of low-level shear vector with speed proportional to strength and speed of
      cold pool




Line echo wave pattern (LEWP)
   • Long-lived portions of a squall line composed of bow-echoes and supercells
   • Are a significant threat to life and property (damaging wind, heavy rain, hail and tornadoes)

Derechos
       Large scale damaging wind events due to long-lived bow echoes and LEWPs
       Usually associated with tornadoes
   Criteria
       1) numerous reports of wind damage or wind gusts > 26 m/s along a major axis length of at
            least 400 km.
       2) the reports must show a pattern of progression.
       3) within the area, there must be at least three reports of F1 damage or gusts of at least 33
            m/s separated by at least 40 nm.
       4) no more than 3 hours between successive events.
   Two types
        1)   Progressive pattern: This is the most common type of derecho (75% of derechos). It is a
             single bow-shaped system oriented perpendicular to the mean wind direction with the
             bulge oriented in the direction of the mean flow. This bulging is where the damaging
             wind events occur.
        2)   Serial Pattern: This is a squall line associated with a strong, often rapidly moving, low
             pressure system with a series of smaller bow echoes or LEWPs moving along the line.
             The squall line is typically located in the warm sector of an extra-tropical cyclone, ahead
             but parallel to the cold front.




                                                                                                     25
26

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:16
posted:7/30/2012
language:
pages:26