Convective Parameters
Weather Systems – Fall 2015
Outline:
a. Stability Indices
b. Wind Shear and Helicity
c. How to relate to predicted / observed convective weather
Convective Parameters
Weather Systems – Fall 2015
COMET Skew-T Tutorial:
http://www.meted.ucar.edu/mesoprim/skewt/intro.htm
Lifting, Convection, and Condensation Parameters
Simple definitions
Lifting Condensation Level (LCL)
The level at which condensation occurs when a mechanical process forces a
parcel to rise to its saturation level. Forcing example: mountain forcing.
Convective Condensation Level (CCL)
The level at which condensation occurs when a thermal process forces a
parcel of air to rise and become saturated. Requires that the parcel be
warmed to its convective temperature (see below). Forcing example: surface
heating.
Convective Temperature (Tc)
Tc is the minimum T to which a parcel must be warmed so that it is buoyant
enough to penetrate high enough through the overlying environmental air to
reach its condensation level (CCL), relying only on its positive buoyancy to
get there.
Level of Free Convection (LFC)
The lowest level at which a rising parcel becomes more buoyant than it’s
surroundings and is free to continue rising.
Equilibrium Level (EL)
The level at which a buoyant parcel becomes neutrally buoyant and is no
longer free to continue rising, except perhaps due to residual upward
momentum.
Lifting, Convection, and
Condensation Parameters
LCL, LFC, EL on a
thermodynamic diagram
Ta is the environmental lapse rate;
Tl is the parcel lapse rate
Figure from Weather Analysis, D. Djuric, 1994
Lifting, Convection, and
Condensation Parameters
CCL, CT (Tc), EL on a
thermodynamic diagram
Ta is the environmental lapse rate;
Tl is the parcel lapse rate
Figure from Weather Analysis, D. Djuric, 1994
Lifting, Convection, and Condensation Parameters – Determining Them
LIFTING CONDENSATION LEVEL (LCL)
Def: The LCL is the height at which a parcel of air becomes saturated when it
is lifted dry adiabatically. The LCL for a surface parcel is always found at or
below the CCL. When the lapse rate is, or once it becomes, dry adiabatic from
the surface to the cloud base, the LCL and CCL are identical.
Procedure:
The LCL is located on a sounding at the intersection of saturation mixing-ratio
line through the surface dewpoint temperature with the dry adiabat through the
surface temperature.
Td
T
Lifting, Convection, and Condensation Parameters – Determining Them
CONVECTIVE CONDENSATION LEVEL (CCL)
Def: The height to which a parcel of air, if heated sufficiently from below, will
rise adiabatically until it is just saturated (condensation starts). In the
commonest case, it is the height of the base of cumuliform clouds which are, or
would be, produced by thermal convection solely from surface heating.
Procedure:
To determine the CCL on a plotted sounding, proceed upward along the
saturation mixing-ratio line through the surface dew-point temperature until this
line intersects the T curve on the sounding. The CCL is the height of this
intersection.
Td
Lifting, Convection, and Condensation Parameters – Determining Them
CONVECTIVE TEMPERATURE (Tc)
Def: The convective temperature (Tc) is the surface temperature that must be
reached to start the formation of convection clouds by heating of the surface
layer air.
Procedure:
Determine the CCL on the plotted sounding. From the CCL point on the T
curve of the sounding, proceed downward along the dry adiabat to the surface
pressure isobar. The temperature read at this intersection is the convective
temperature.
Stability Indices
Indices predicting convective potential [Lifted Index
(LI); Showalter stability Index (SSI); Total Totals
(TT); K index (KI); Severe Weather Threat (SWEAT)
Index (SWI)] . These are in addition to parameters
such as CAPE and CIN
CIN / CAPE
CIN / CAPE
The energy that a parcel has for ascent, or needs to get from an external lifting process
in order to ascend, is usually described in terms of energy per unit mass with units of J /
kg.
CIN value
> 50
25-50
10-25
< 10
Impact on Convection
convection inhibited, unless dynamic forcing is extreme
convection inhibited, but moderate dynamic forcing or
heating can overcome this inhibition
some forcing required to initiate convection
convection can be initiated with only minimal forcing
CAPE value
Convective potential
< 300
Little or none
300-1000
Weak
1000-2500
Moderate
2500 and up
Strong
CAPE
 Advantage:
 CAPE is a robust indicator of the potential for deep convection and
convective intensity
 CAPE provides a measure of stability integrated over the depth of the
sounding, as opposed to other indices
 Disadvantages:
 The computation of CAPE is extremely sensitive to the mean mixing ratio in
the lowest 500 m. For instance, a 1 g/kg increase can increase CAPE by
20%
 Since the computation of CAPE is based on parcel theory, it does not take
into account processes such as mixing, water loading and freezing.
 Surface layer based CAPE computations may underestimate the convective
potential in situations with elevated convection.
 Since CAPE, by itself, does not account for wind shear, it may
underestimate the potential for severe convection where strong wind shear
is present.
CIN
 Advantage: a good indicator of the amount of forcing
necessary for an ‘air parcel’ to tap into environmental
buoyancy
 Disadvantages:
 Since the computation of CIN is based on parcel theory, it
does not take into account processes such as mixing, water
loading and freezing.
 One caveat is that if the CIN is large but storms manage to
form, usually due to increased moisture and/or heating
overcoming the CIN, then the storms are more likely to be
severe
CIN / CAPE
11 hr Forecast from HRRR run Initialized at 10 UTC
CIN / CAPE
Storm Prediction Center
Convective Outlook
11 hr Forecast from HRRR run Initialized at 10 UTC
Lifted Index (LI)
Lifted Index (LI) is a simple parameter used to characterize
the amount of instability in a given environment
Lifted Index (LI)
Lifted Index (LI)
 Advantage: easy to compute from a Skew-T diagram
 Disadvantages:
 limited because it relies on only 3 sounding inputs
(temperature and dewpoint of the boundary layer and the
temperature at 500 hPa). Thus, important sounding features
may be obscured, such as dry layers and/or inversions.
 LI also does not take into account vertical wind shear, which
is often an important element in the severe convective
environment.
Lifted Index (LI)
Lifted Index from HRRR
Lifted Index (LI)
CAPE
Lifted Index
Showalter Stability Index (SSI)
Similar to the LI. While LI starts with a near-surface air
parcel, the SSI uses a parcel lifted from 850 hPa environment
Showalter Stability Index (SSI)
Showalter Stability Index (SSI)
 Advantage: easy to compute from a Skew-T diagram
 Disadvantages:
 It may under-represent the instability if the top of the moist
layer falls below 850 hPa
 It is intended for use at locations with a station elevation up
to about 1000 feet
 It does not take into account vertical wind shear, which also
affects storm potential
K Index (KI)
The K index (KI) is particularly useful for identifying convective
and heavy-rain-producing environments. It does not require a
skew-T diagram; it is simply computed from temperatures at
850, 700, and 500 hPa, and dewpoints at 850 and 700 hPa. The
higher the moisture and the greater the 850-500 temperature
difference, the higher the KI and potential for convection.
K Index (KI)
KI
Thunderstorm Probability (%)
0-15
~0
18-19
20; thunderstorms unlikely
20-25
35; isolated thunderstorms
26-29
50; scattered thunderstorms
30-35
85; numerous thunderstorms
> 36
~100
K Index (KI)
 Advantage: Its computation takes into account the
vertical distribution of both moisture and temperature.
 Disadvantages:
 it can't be used to infer the severity of convection
 last, like several other severe weather indexes, it does not
take into account wind shear, which is a critical factor in
many severe convective environments
Total Totals (TT)
It is computed using the temperature and dewpoint at 850
hPa and the temperature at 500 hPa. The higher the 850 hPa
dewpoint and temperature and the lower the 500 hPa
temperature, the greater the instability and the resulting TT
value.
Total Totals (TT)
Total Totals (TT)
 Advantage: easy to compute
 Disadvantages:
 it is limited in that it uses data from only two mandatory
levels (850 and 500 hPa) and thus does not account for
intervening inversions or moist or dry layers that may occur
below or between these levels.
 In addition, it does not work for areas in the western Great
Plains or the Rocky Mountains, where 850 hPa is near the
surface or below ground.
 Last, like several other severe weather indexes, it does not
take into account wind shear, which is a critical factor in
many severe convective environments.
Severe Weather Threat Index (SWEAT)
The SWEAT index differs from many of the other severe
weather indices in that it takes into account the wind profile
in assessing severe weather potential. Inputs include:
- Total Totals index (TT)
- 850 hPa dewpoint
- 850 hPa wind speed and direction
- 500 hPa wind speed and direction
Severe Weather Threat Index (SWEAT)
Severe Weather Threat Index (SWEAT)
Severe Weather Threat Index (SWEAT)
 Advantage: advantageous for diagnosing severe
convective potential since it takes into account many
important parameters including low-level moisture,
instability and the vertical wind shear (both speed and
direction)
 Disadvantages:
 a limitation is that the inputs are from only 850 and 500 hPa
levels, obscuring any inversions, dry layers etc. that may be
present in intervening layers.
 it can also be somewhat cumbersome to compute, in the
absence of an automated sounding routine such as the
interactive skew-T.
Stability Indices
 CAPE = 1622 J/kg (moderate)
 CIN = -38 J/kg (convection inhibited,
but moderate dynamic forcing and/or
heating can overcome)
 LI = -6.1 (strong SVR Wx potential)
 KI = 38 (100% T-storm probability)
 TT = 53 (SVR T-storm possible)
 SWEAT = 330 (SVR possible)
12 UTC Buffalo
TORNADO
HAIL
WIND
Wind Shear
Wind shear plays a large role in determining what
form convection is likely to take
M = mesoscylone, no tornado
T = storm with 1 tornado
TT = storm with >1 tornado
D = derecho
Rasmussen and Wilhelmson (1983)
Bulk Richardson Number
Represents the ratio of buoyancy (as measured by CAPE)
and the vertical wind shear. As we have noted, CAPE
relates to updraft strength. Storm structure and movement
are related to the vertical wind shear.
Static Stability Indices – CAPE and Vertical Velocity
maximum is seldom realized
due to entrainment and water
loading
Bulk Richardson Number
• BRN < 10 ~ much more shear
than buoyancy and storms tend
to be torn apart by the shear
• exception: in strongly forced,
high-shear, low-CAPE
environments where supercells
are observed with BRN < 10
• 10 < BRN < 35 ~ balance
between shear and buoyancy
favor supercells
• BRN > 50 ~ buoyancy
dominates over shear and
single- or multi-cell storms are
more likely to be observed
Storm Relative Environmental
Helicity (SREH)
SREH provides an indication of an environment that favors
the development of thunderstorms with rotating updrafts.
Storm Relative Environmental
Helicity (SREH)
High values of SREH (usually >150 m2/s2) are usually
associated with long-lived supercells with rotating updrafts,
capable of producing tornadoes.
NOTE: Buffalo sounding
SREH = 88 m2/s2
SREH from HRRR
Storm Relative Environmental
Helicity (SREH)
• when we compute helicity, it is
most appropriate to use stormrelative winds
• to find the storm-relative wind,
we subtract the anticipated or
observed storm speed and
direction from the wind at every
level of the sounding
• this process requires a hodograph
analysis of the wind profile to
predict the storm motion
Storm Relative Environmental
Helicity (SREH)
• on the hodograph, SREH is
proportional to the area swept out by
the storm relative wind vector over
the depth of the inflow (typically 3
km AGL)
• SREH > 0 ~ right-moving storms,
characterized by clockwise-curving
hodographs (as shown here) and
cyclonic rotation
• SREH < 0 ~ left-moving,
anticyclonic-rotating storms with
counterclockwise-curving
hodographs
Storm Relative Environmental
Helicity (SREH)
Storm Relative Environmental
Helicity (SREH)
 Advantage: SREH is perhaps the parameter most widely
used to provide a good diagnosis for the potential for
tornado-producing supercells
 Disadvantages:
 like CAPE values, there is no magic value of (positive)
helicity over which rotating thunderstorms will develop.
 Furthermore, the calculation of SREH is quite sensitive to
assumptions about storm motion and the environmental
wind shear.
 SREH, like other parameters, must be used with caution,
especially with rapidly changing environmental conditions.