Convective ParametersWeather Systems – Fall 2015
Outline:a. Stability Indices
b. Wind Shear and Helicityc. How to relate to predicted / observed convective weather
Convective ParametersWeather Systems – Fall 2015
COMET Skew-T Tutorial: http://www.meted.ucar.edu/mesoprim/skewt/intro.htm
Simple definitionsLifting 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
Figure from Weather Analysis, D. Djuric, 1994
Lifting, Convection, andCondensation Parameters
Ta is the environmental lapse rate;Tl is the parcel lapse rate
LCL, LFC, EL on a thermodynamic diagram
Figure from Weather Analysis, D. Djuric, 1994
Lifting, Convection, andCondensation Parameters
CCL, CT (Tc), EL on a thermodynamic diagram
Ta is the environmental lapse rate;Tl is the parcel lapse rate
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.
Lifting, Convection, and Condensation Parameters – Determining Them
TTd
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.
Lifting, Convection, and Condensation Parameters – Determining Them
Td
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.
Lifting, Convection, and Condensation Parameters – Determining Them
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
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 Impact on Convection> 50 convection inhibited, unless dynamic forcing is
extreme 25-50 convection inhibited, but moderate dynamic forcing or
heating can overcome this inhibition10-25 some forcing required to initiate convection < 10 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
CIN / 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.
CAPE
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
CIN / CAPE
11 hr Forecast from HRRR run Initialized at 10 UTC
CIN / CAPE
11 hr Forecast from HRRR run Initialized at 10 UTC
Storm Prediction Center Convective Outlook
Lifted Index (LI)
Lifted Index (LI) is a simple parameter used to characterize the amount of instability in a given environment
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 (LI)
Lifted Index from HRRR
Lifted Index (LI)
Lifted IndexCAPE
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)
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
Showalter Stability Index (SSI)
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 ~ 018-19 20; thunderstorms
unlikely20-25 35; isolated
thunderstorms26-29 50; scattered
thunderstorms30-35 85; numerous
thunderstorms> 36 ~100
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
K Index (KI)
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)
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.
Total Totals (TT)
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)
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.
Severe Weather Threat Index (SWEAT)
Stability Indices
12 UTC Buffalo
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)
TORNADO
WINDHAIL
Wind Shear
Wind shear plays a large role in determining what form convection is likely to take
Rasmussen and Wilhelmson (1983)
M = mesoscylone, no tornadoT = storm with 1 tornadoTT = storm with >1 tornadoD = derecho
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.
SREH from HRRR
NOTE: Buffalo sounding SREH = 88
m2/s2
Storm Relative Environmental Helicity (SREH)
• when we compute helicity, it is most appropriate to use storm-relative 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)
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.
Storm Relative Environmental Helicity (SREH)
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