Downdraft Cape Quiz and Flashcards

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Questions and Answers

Which index successfully accounts for low-level moisture and/or inversions but fails to consider cold air above 500 mb?

MLI

LI (correct)

Showalter Index

S Index

What type of echo configuration indicates the potential for strong to severe straight-line winds in a bow echo?

Convex downstream

Convex upstream

Concave downstream (correct)

Concave upstream

What is the primary drawback of the Modified Lifted Index (MLI) in predicting severe thunderstorms?

Too warm atmospheric levels

Failure to account for low-level moisture

Poor results when the -20 level ais above 500mb or below LCL. (correct)

Insufficient consideration of cold air aloft

What is a key characteristic of the line echo wave pattern (LEWP) that can indicate the presence of severe weather?

Concave downstream echo configuration Signup and view all the answers

Which geographical region is the S Index specifically tailored for?

Europe Signup and view all the answers

In which area of the comma shape line formed by a weakening downburst may tornadoes still occur?

North end Signup and view all the answers

What characteristic sets the S Index apart from the Total Totals (TT) index?

Addition of 700 mb moisture Signup and view all the answers

What spatial scale characterizes microbursts, making them difficult to predict?

Less than 4 km diameter Signup and view all the answers

What atmospheric condition is ideal for microburst formation according to the text?

Warm and moist low levels, cool and dry mid-levels Signup and view all the answers

Which index is known for its effectiveness as a severe thunderstorm indicator in Europe?

MLI Signup and view all the answers

What weather condition negatively impacts the performance of the Modified Lifted Index (MLI)?

-20°C level above 500 mb (too warm) or below LCL (too cold) Signup and view all the answers

What indicates the presence of a mesocyclone in a comma-shaped line formed by a weakening downburst?

'Hook' shape in the radar echo Signup and view all the answers

What does the Dry Microburst Index (DMI) quantify?

Mid-level lapse rates and moisture content Signup and view all the answers

In the DMI formula, what does (T - Td)700 represent?

700 mb temperature minus dew point Signup and view all the answers

How are environments with large DMI values and instability aloft considered?

Favorable for dry microbursts Signup and view all the answers

What does the Wind Index (WINDEX) help forecast?

Maximum potential wind gusts Signup and view all the answers

Which factor is NOT considered in the WINDEX calculation?

Cloud cover percentage Signup and view all the answers

How does precipitation near 500 mb affect microbursts?

It enhances downdraft magnitude by evaporating into dry air below Signup and view all the answers

What happens to negative buoyancy if mid-level lapse rates are steep?

It increases further Signup and view all the answers

What type of air is associated with large DMI values according to the text?

700mb (dry air) & (moist air) 500mb Signup and view all the answers

What environmental conditions are associated with high values of DCAPE?

Steep low- and mid-level lapse rates and dry low-level air Signup and view all the answers

What is a key factor in diagnosing the magnitude of DCAPE?

The difference in equivalent potential temperature (theta-e) between low- and mid-levels Signup and view all the answers

Why are thunderstorms unlikely to develop if both the low- and mid-levels are dry?

Insufficient moisture for cloud formation and instability Signup and view all the answers

Why does high theta-e in the low-levels and low theta-e aloft indicate high downdraft instability?

Low theta-e indicates unstable air parcels prone to downdrafts Signup and view all the answers

Which scenario is likely to result in convective winds according to the text?

Moist low-levels and dry mid-levels Signup and view all the answers

How does evaporative cooling contribute to increasing negative buoyancy and downdraft strength?

By increasing humidity levels at the surface, enhancing buoyancy Signup and view all the answers

What type of environment is rare for convective winds according to the text?

Moist mid- and low-levels due to precipitation loading Signup and view all the answers

What factors contribute to high downdraft instability according to the text?

Steep lapse rates and dry air at both levels Signup and view all the answers

What type of cloud can dry microbursts occur from?

Any cumuliform cloud with appreciable vertical extent Signup and view all the answers

In the dry microburst forecast technique, what is the depth of the radiational temperature inversion at the surface that should be looked for?

40-50 mb Signup and view all the answers

For dry microburst potential, what should the mixing ratio be below the convective condensation level (CCL)?

Greater than 5 g/kg Signup and view all the answers

What is the minimum relative humidity required above the dry adiabatic layer for dry microbursts to be possible?

More than 70% Signup and view all the answers

How can you determine if dry microbursts are possible based on the convective temperature (CT)?

Reaching CT by mixing down 850 mb temperature to the surface Signup and view all the answers

What should you find in the dry adiabatic layer to potentially indicate a dry microburst?

Strongest wind speeds Signup and view all the answers

Study Notes

Severe Thunderstorm Indices

The Lifted Index (LI) successfully accounts for low-level moisture and/or inversions but fails to consider cold air above 500 mb.
The Modified Lifted Index (MLI) is limited by its inability to predict severe thunderstorms.

Bow Echo and Line Echo Wave Pattern

A bow echo with a strong, curved leading edge indicates the potential for strong to severe straight-line winds.
The line echo wave pattern (LEWP) is characterized by a series of convective cells, and its presence can indicate severe weather.

S Index and Geographical Region

The S Index is specifically tailored for the high plains region of the United States.

Comma-Shaped Line and Tornadoes

Tornadoes may still occur in the area of the comma shape line formed by a weakening downburst, near the boundary of the downdraft and the surrounding air.

S Index and Total Totals Index

The S Index is distinct from the Total Totals (TT) index due to its consideration of low-level moisture and wind shear.

Microbursts and Spatial Scale

Microbursts are characterized by a small spatial scale, making them difficult to predict.

Atmospheric Condition for Microburst Formation

Ideal conditions for microburst formation include a dry and cool air layer near the surface, with warm air aloft.

Severe Thunderstorm Indicator in Europe

The S Index is known for its effectiveness as a severe thunderstorm indicator in Europe.

Weather Condition Affecting MLI

High humidity negatively impacts the performance of the Modified Lifted Index (MLI).

Mesocyclone in Comma-Shaped Line

The presence of a mesocyclone in a comma-shaped line formed by a weakening downburst is indicated by a rotating updraft, known as a mesocyclone.

Dry Microburst Index (DMI)

The DMI quantifies the potential for dry microbursts.
In the DMI formula, (T - Td)700 represents the difference between the temperature and dew point at 700 mb.
Environments with large DMI values and instability aloft are considered conducive to dry microbursts.

Wind Index (WINDEX)

The WINDEX helps forecast the potential for strong winds.
The WINDEX calculation does not consider the low-level wind direction.

Precipitation and Microbursts

Precipitation near 500 mb can weaken microbursts.

Negative Buoyancy and Mid-Level Lapse Rates

If mid-level lapse rates are steep, negative buoyancy increases.

Air Associated with Large DMI Values

Dry air is associated with large DMI values.

Environmental Conditions and DCAPE

High values of DCAPE are associated with environments featuring high instability and dry air.

Diagnosing DCAPE

A key factor in diagnosing the magnitude of DCAPE is the low-level moisture content.

Thunderstorm Development

Thunderstorms are unlikely to develop if both the low- and mid-levels are dry.

Downdraft Instability

High theta-e in the low-levels and low theta-e aloft indicate high downdraft instability.

Convective Winds

Convective winds are likely to occur in scenarios with high instability, dry air, and strong wind shear.

Evaporative Cooling and Downdraft Strength

Evaporative cooling contributes to increasing negative buoyancy and downdraft strength.

Rare Environment for Convective Winds

Environments with high humidity and low wind shear are rare for convective winds.

Factors Contributing to Downdraft Instability

Factors contributing to high downdraft instability include low-level moisture, mid-level lapse rates, and wind shear.

Cloud and Dry Microbursts

Dry microbursts can occur from towering cumulus or altocumulus clouds.

Dry Microburst Forecast Technique

The dry microburst forecast technique involves looking for a radiational temperature inversion at the surface with a depth of at least 100 m.
The mixing ratio should be below the convective condensation level (CCL) for dry microburst potential.
A minimum relative humidity of 20% is required above the dry adiabatic layer for dry microbursts to be possible.
The convective temperature (CT) should be at least 5°C higher than the surface temperature for dry microbursts.
In the dry adiabatic layer, a dry and cool air layer near the surface, with warm air aloft, can indicate a dry microburst.

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Description

Learn about Downdraft Convective Available Potential Energy (DCAPE) values and how they are influenced by different atmospheric conditions. Understand the impact of DCAPE on thunderstorm development, downdraft strength, and convective wind severity.

Downdraft CAPE and Thunderstorms