MESOSCALE ANALYSIS PARAMETERS
(Material Excerpted From SPC Mesoanalysis Page)
Level of Free Convection (LFC)
The Level of Free Convection (LFC) is the level at which a lifted parcel begins a free acceleration upward to
the equilibrium level. Recent preliminary research suggests that
tornadoes become more likely in supercells when LFC heights are less
than 2000 m (6500 feet) above ground level. The EL (equilibrium level) is the level
at which a lifted parcel becomes cooler than the environmental
temperature and is no longer buoyant (i.e., "unstable" ). The EL is used
primarily to estimate the height of a thunderstorm anvil. The height
difference between this parameter and the LCL is important when
determining convection initiation. The smaller the difference between
the LFC and the LCL, the more likely deep convection becomes. The
LFC-LCL difference is similar to CIN (convective inhibition).
Lifting Condensation Level (LCL)
The Lifting Condensation Level (LCL) is the level at which a parcel becomes saturated. It is a reasonable
estimate of cloud base height when parcels experience forced ascent. The
height difference between this parameter and the LFC is important when
determining convection initiation. The smaller the difference between
the LCL and the LFC, the more likely deep convection becomes. The
LFC-LCL difference is similar to CIN (convective inhibition). LCL
heights from approximately 500 m (1600 ft) to 800 m (2600 ft) above ground level are
associated with F2 to F5 tornadoes. Low LCL heights and low surface dewpoint depressions (high low level RH) suggest a warm RFD which may play a role in tornado development.
Convective Condensation Level (CCL)
The Convective Condensation Level (CCL) is the level at which condensation will occur if
sufficient afternoon heating causes rising parcels of air to reach saturation. The CCL is greater
than or equal in height (lower or equal pressure level) than the LCL. The CCL and the LCL are equal
when the atmosphere is saturated. The CCL is found at the intersection of the saturation mixing ratio line
(through the surface dewpoint) and the environmental temperature.
Equilibrium Level (EL)
The Equilibrium Level (EL) is the level at which a lifted parcel becomes cooler than the environmental temperature and is
no longer buoyant (i.e. unstable). The EL is used primarily to estimate
the height of a thunderstorm anvil. You may notice that the "virtual"
and "non-virtual" lifted parcels both end up with the same EL. This
happens because the virtual temperature converges to the actual
temperature when temperatures are very cold (less than -20C) and
moisture effects become negligible.
Lapse Rates (C/km)
A lapse rate is the rate of temperature change with height. The faster the temperature decreases
with height, the "steeper" the lapse rate and the more "unstable" the
atmosphere becomes. Lapse rates are typically displayed in ranges from
850-500-mb (4,500-18,000-ft above sea level) and 700-500-mb
(10,000-18,000-ft above sea level).
Lapse rates are shown in terms of degrees Celsius change per kilometer in height. Values less
than 5.5-6.0 degrees C/km ("moist" adiabatic) represent "stable"
conditions, while values near 9.5 degrees C/km ("dry" adiabatic) are
considered "absolutely unstable." In between these two values, lapse
rates are considered "conditionally unstable." Conditional instability
means that if enough moisture is present, lifted air parcels could have
a negative LI (lifted index) or positive CAPE.
Surface-Based CAPE (SBCAPE - J/kg)
SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere.
This value represents the total amount of potential energy available to
a parcel of air originating at the surface and being lifted to its level
of free convection (LFC). No parcel entrainment is considered. The CAPE
calculation uses virtual temperatures, but the CIN value does not. SPC
forecasters have noted that non-virtual CIN calculations tend to define
areas of weak cap more accurately than if the virtual temperature was
considered.
Convective Inhibition (CIN - J/kg)
CIN (Convective INnibition) Represents the "negative" area on a sounding
that must be overcome before storm initiation can occur.
Surface-Based Lifted Index (SBLI - C) &
Convective Inhibition (CIN - J/kg)
SBLI (Surface Based Lifted Index & Convective Inhibition) is the Lifted Index at 500-mb, based on the
most unstable parcel, and the convective inhibition for the same parcel.
These fields are meant to identify areas of surface-based CAPE and
minimal convective inhibition, which suggests some threat for
surface-based thunderstorms.
Most Unstable CAPE (MUCAPE - J/kg)
MUCAPE (Most Unstable Convective Available Potential Energy) is a measure of
instability in the troposphere. This value represents the total amount
of potential energy available to the most unstable parcel of air found
within the lowest 300-mb of the atmosphere while being lifted to its
level of free convection (LFC). No parcel entrainment is considered. The
CAPE calculation uses virtual temperatures, but the CIN value does not.
SPC forecasters have noted that non-virtual CIN calculations tend to
define areas of weak cap more accurately than if the virtual temperature
was considered.
LPL Height (m AGL)
The LPL (Lifted Parcel Level) allows for the determination
of the height of the most unstable parcel. This makes it easy to
identify areas where the largest CAPE is "elevated."
100-mb Mixed Layer CAPE/CIN (J/kg)
MLCAPE (Mixed Layer Convective Available Potential Energy) is a measure of instability
in the troposphere. This value represents the mean potential energy
conditions available to parcels of air located in the lowest 100-mb when
lifted to the level of free convection (LFC). No parcel entrainment is
considered. The CAPE calculation uses virtual temperatures, but the CIN
value does not. SPC forecasters have noted that non-virtual CIN
calculations tend to define areas of weak cap more accurately than if
the virtual temperature was considered.
3-km CAPE (J/kg) & Surface Vorticity
CAPE in the lowest 3-km above ground level, and surface relative vorticity.
Areas of large 0-3-km CAPE tend to favor strong low-level stretching,
and can support tornado formation when co-located with significant
vertical vorticity near the ground.
Normalized CAPE (J/kg)
The NCAPE (Normalized CAPE) is CAPE that is divided by the
depth of the buoyancy layer (units of m s**-2). Values near or less than
.1 suggest a "tall, skinny" CAPE profile with relatively weak parcel
accelerations, while values closer to .3 to .4 suggest a "fat" CAPE
profile with large parcel accelerations possible. Normalized CAPE and
lifed indicies are similar measures of instability.
Downdraft CAPE (J/kg)
The DCAPE (Downdraft CAPE) can be used to estimate the
potential strength of rain-cooled downdrafts within thunderstorm
convection, and is similar to CAPE. Larger DCAPE values are associated
with stronger downdrafts.
Surface to 6-km Vertical Shear Vector
(kts)
The Boundary Layer through 6-km above ground level shear vector denotes the change in wind
throughout this height. Thunderstorms tend to become more organized and
persistent as vertical shear increases. Supercells are commonly
associated with vertical shear values of 35-40 knots and greater through
this depth.
Effective Bulk Vertical Shear Vector (kts)
The maximum bulk shear from the most unstable parcel level upward to 40-60% of the equilibrium level
height. This parameter is similar to the 0-6 km bulk shear, though it
accounts for storm depth (LPL to EL) and is designed to identify both
surface-based and "elevated" supercell environments. Supercells become
more probable as the effective bulk shear increases through the range of
25-40 kt and greater.
Bulk Richardson Number Shear (m**2/s**2)
The BRN (Bulk Richardson Number) shear is similar to the
BL-6-km shear, except that the BRN Shear uses a difference between the
low-level wind and a density-weighted mean wind through the mid-levels.
Values of 35-40 m**2/s**2 or greater have been associated with
supercells.
Storm Relative Helicity (m**2/s**2)
SRH (Storm Relative Helicity) is a measure of the
potential for cyclonic updraft rotation in right-moving supercells, and
is calculated for the lowest 1-km and 3-km layers above ground level.
There is no clear threshold value for SRH when forecasting supercells,
since the formation of supercells appears to be related more strongly to
the deeper layer vertical shear. Larger values of 0-3-km SRH (greater
than 250 m**2/s**2) and 0-1-km SRH (greater than 100 m**2/s**2),
however, do suggest an increased threat of tornadoes with supercells.
For SRH, larger values are generally better, but there are no clear
"boundaries" between non-tornadic and significant tornadic supercells.
Effective Storm Relative Helicity
(m**2/s**2)
Effective SRH (Storm Relative Helicity)
is based on threshold values of lifted parcel CAPE (100 J kg-1)
and CIN (-250 J kg-1). These parcel constraints are meant to
confine the SRH layer calculation to the part of a sounding where lifted
parcels are buoyant, but not strongly capped. For example, a supercell
forms or moves over an area where the most unstable parcels are located
a couple of thousand feet above the ground, and stable air is located at
ground level. The question then becomes "how much of the cool air can
the supercell ingest and still survive?" Our estimate is to start with
the surface parcel level, and work upward until a lifted parcel's CAPE
value increases to 100 Jkg-1 or more, with an associated CIN
greater than -250 Jkg-1. From the level meeting the
constraints (the "effective surface"), we continue to look upward in the
sounding until a lifted parcel has a CAPE less than 100 Jkg-1 OR a CIN less than -250 J kg-1. Of the three SRH calculations
displayed on the SPC mesoanalysis page, effective SRH discriminates the
best between significant tornadic and nontornadic supercells.
Surface-1-km Vertical Shear Vector (kts)
Surface-1-km Vertical Shear is the difference between the surface wind
and the wind at 1-km above ground level. These data are plotted as
vectors with shear magnitudes contoured. 0-1-km shear magnitudes greater
than 15-20 knots tend to favor supercell tornadoes.
Surface-2-km Storm Relative Winds (kts)
Low-Level SR (Storm Relative) winds (0-2-km)
are meant to represent low-level storm inflow. The majority of sustained
supercells have 0-2-km storm inflow values of 15-20 knots or greater.
4-6-km Storm Relative Winds (kts)
Mid-Level SR (Storm Relative) winds (4-6-km) are of some use in discriminating
between tornadic and non-tornadic supercells. Tornadic supercells tend
to have 4-6-km SR wind speeds in excess of 15 knots, while non-tornadic
supercells tend to have weaker mid-level storm-relative winds.
Anvil Level/9-11-km SR Winds (kts)
The Anvil Level SR (Storm Relative) winds and SR
winds from 9-11-km are meant to discriminate supercell type. In general,
upper-level SR winds less than 40 knots correspond to "high
precipitation" supercells, 40-60 knots SR winds denote "classic"
supercells, while SR winds greater than 60 knots correspond to "low
precipitation" supercells.
Supercell Composite Parameter
A multi-parameter index that includes effective SRH, muCAPE, and effective bulk shear. Each parameter
is normalized to supercell "threshold" values. Effective SRH is divided
by 50 m2/s2, muCAPE is divided by 1000 J/kg, and effective bulk shear is
divided by 20 m/s in the shear range of 10-20 m/s. Effective bulk shear
less than 10 m/s is set to zero, and effective bulk shear greater than
20 m/s is set to one.
Significant Tornado Parameter
A multi-parameter index that includes effective bulk shear, effective SRH, 100-mb mean parcel CAPE,
100-mb mean parcel CIN, and 100-mb mean parcel LCL height.
When the mlLCL is less than 1000 m AGL, the mlLCL term is set to one,
and when the mlCIN is greater than -50 J kg-1, the mlCIN term
is set to one. Lastly, the ESHEAR term is capped at a value of 1.5, and
set to zero when ESHEAR is less than 12.5 m s-1. A majority
of significant tornadoes (F2 or greater damage) have been associated
with STP values greater than 1, while most non-tornadic supercells have
been associated with values less than in a large sample of RUC
analysis proximity soundings.
Significant Hail Parameter
The Sig. Hail Parameter (SHIP) was developed using a large database of surface-modified,
observed severe hail proximity soundings. It is based on 5 parameters,
and is meant to delineate between SIG (>=2" diameter) and NON-SIG (<2"
diameter) hail environments. It is important to note that SHIP is
NOT a forecast hail size. Since SHIP is based on the RUC
depiction of MUCAPE, unrepresentative MUCAPE "bullseyes" may cause a
similar increase in SHIP values. This typically occurs when bad surface
observations get into the RUC model. Developed in the same vein as the
STP and SCP parameters, values of SHIP greater than 1.00 indicate a
favorable environment for SIG hail. Values greater than 4 are considered
very high. In practice, maximum contour values of 1.5-2.0 or higher will
typically be present when SIG hail is going to be reported.
Craven SigSvr Parameter
The simple product of 100mb MLCAPE and 0-6km magnitude of the vector difference (m/s; often referred
to as "deep layer shear") accounts for the compensation between
instability and shear magnitude. Using a database of about 60,000
soundings, the majority of significant severe events (2+ inch hail, 65+
knot winds, F2+ tornadoes) occur when the product exceeds 20,000 m3/s3.
For example, a 0-6-km shear of 40 knots and
CAPE of 3000 J/kg results in a Craven SigSvr index of 60,000. Units are
scaled to the nearest 1000 on the web plot.
Energy-Helicity Index
The basic premise behind the EHI (Energy-Helicity Index) is
that storm rotation should be maximized when CAPE is large and SRH is
large. 0-1-km EHI values greater than 1-2 have been associated with
significant tornadoes in supercells.
Vorticity Generation Parameter (m/s**2)
The VGP Vorticity Generation Parameter)
is meant to estimate the rate of tilting and stretching of horizontal
vorticity by a thunderstorm updraft. Values greater than 0.2 m/s**2
suggest an increasing possibility of tornadic storms.