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)