Storm Type as a Function of Buoyancy and Shear:
A Summary

Figure 1: Cape vs. Shear with storm type (Weisman and Klemp)

Figure 1 shows that particular storm types occur for wide ranges of buoyancy (CAPE) and shear values. A given set of buoyancy and shear values may support a range of storm types. It is therefore important to use forecast indices as a means to anticipate the processes that may occur in an environment rather than just as threshold values for determining the likelihood of one storm type or another.

Bulk Richardson Number

The Bulk Richarson Number is defined as:

It represents the ratio of buoyancy (CAPE) to shear. Here U is defined as the vector difference between the 0-6 km mean wind and a representative surface layer wind, usually the 0-500 m mean wind.

Bulk Richardson Number and Storm Type

Figure 2: Bulk Richardson Number and storm type

Figure 2 shows observed and numerically modeled multicell and supercell storms plotted as a function of Bulk Richardson Number. Generally observed multicell storms form for BRN > 50 whilst supercell storms, generally forming in higher shear environments, have BRN in the range 10-50. There is overlap between the two storm regimes where CAPE and shear conditions may support both types of storms.

Defining CAPE and Shear Magnitudes

For this warm-season convection module the shear and buoyancy are non-rigorously characterized in the following manner.

  Weak Moderate Strong
Buoyancy (CAPE) < 1000 J/kg 1000-1500 J/kg > 1500 J/kg
Shear (sfc – 700 hPa)   > 25 kts > 30 kts

Table 1.1 CAPE and Shear Magnitudes

Physical Processes Controlling Storm Evolution


Figure 3: Concept map for processes controlling convection


 Figure 3 above shows that buoyancy processes modulate updraft and downdraft strength. It is the shear interaction with either the storm updraft or storm cold pool that determines the degree of organization and longevity of the convection and hence the probability of severe weather. That is not to say that ordinary cells do not produce severe weather. However, when buoyancy is the dominating factor and the shear is weak the resultant cell lifetime is limited. Any severe weather produced by such cells is therefore relatively short-lived. The dominant controlling processes associated with storm types in Figure 3 are shown in Table 1.2 below.

Cell Type Dominant Process
Ordinary Cell Buoyancy (weak shear)
Multicell Storm Cold pool – shear interactions
Supercell Storm Updraft – shear interactions

Table 1.2 Cell Type and Dominant Physical Processes


A summary below (in Table 1.3) gives parameter ranges that support the dominant physical processes that control convection and the likely associated storm type.

Dominant Physical Process Favourable Conditions Typical Storm Type Forecast Parameters
Buoyancy

Moderate-Strong buoyancy

Weak shear

 Ordinary Cell CAPE (moderate-strong)
Cold-Pool/Shear interactions
Favours new cell development on the down-shear side of cold-pool (for a homogeneous environment)		 Moderate-Strong buoyancy

0-2/3 km shear 20-40 kts

0-2/3 km hodograph length 20 kts

0-2/3 km hodograph length of 40 kts for long-lived systems		 Multi-cell CAPE (moderate-strong)

BRN > 50
Updraft/shear interactions
Invoke processes conducive to updraft rotation		 Moderate-Strong buoyancy

50 kt hodograph length over 0-4/6 km likely to promote supercell development

0-3 km shear of 40-50 kts and a straight-line hodograph – storms may evolve into supercells within 1 hour

(Anti)clockwise curving hodograph favours (left) right-moving storms		 Supercell

CAPE (moderate–strong)

BRN 10-50

0-2/3 km |SREH| > 100

0-2km storm relative inflow >= 20 kts

800-350 hPa winds >=
25 kts for classic type.
Australian warm season supercell climatology   Supercell

(The following ranges of values exclude the lowest 10% and highest 10% of values for the dataset).

Buoyancy

CAPE range:1070-2238 J/kg

SLI range: -3 to -8°C.

Dynamics

Shear to 3 km AGL: 24-43 kts

|SREH|: 78% cases > 102 m2/s2.

Table 1.3 Parameters Used to Infer Physical Processes and Related Storm Type

Table 1.4 shows severe weather types and parameter values that indicate favourable pre-storm conditions.

Weather Type Favourable Pre-storm Environmental Conditions
Large Hail > 2 cm in diameter

CAPE > 1500 (non-supercell environments)?

CAPE > 1000 (supercell environments)?

Large proportion of CAPE from LFC to -10°C?

WBFZL height 1500-3600 m AGL
(preferably 2100-2800 m)?

Boundary layer moisture content high
Strong straight-line winds

Mid-level environment RH < 75% between 800-350 hPa?

Fast Storm Movement (> 40 kts)?

Deep dry-adiabatic layer below cloud base?

High liquid water content in downdraft?
Heavy rain and flash floods
 A combination of strong precipitation and long-lasting convection		 Strong updraft?

Moist environment to 500 hPa or precipitable water significantly above average?

Warm cloud depth > 3 km?

Weak environmental wind shear in cloud layer?

Low cloud base and high sub-cloud base RH to reduce evaporation?

Slow-moving storms (< 10-15 kts?) and long-lived cells?

Succession of cells over one area (train effect)?

Table 1.4 Severe Weather Types and Favourable Parameters


COMET CD-ROM: Anticipating Convective Storm Structure and Evolution