Downdraft CAPE and Thunderstorms
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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?

    <p>Concave downstream echo configuration</p> Signup and view all the answers

    Which geographical region is the S Index specifically tailored for?

    <p>Europe</p> Signup and view all the answers

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

    <p>North end</p> Signup and view all the answers

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

    <p>Addition of 700 mb moisture</p> Signup and view all the answers

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

    <p>Less than 4 km diameter</p> Signup and view all the answers

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

    <p>Warm and moist low levels, cool and dry mid-levels</p> Signup and view all the answers

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

    <p>MLI</p> Signup and view all the answers

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

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

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

    <p>'Hook' shape in the radar echo</p> Signup and view all the answers

    What does the Dry Microburst Index (DMI) quantify?

    <p>Mid-level lapse rates and moisture content</p> Signup and view all the answers

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

    <p>700 mb temperature minus dew point</p> Signup and view all the answers

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

    <p>Favorable for dry microbursts</p> Signup and view all the answers

    What does the Wind Index (WINDEX) help forecast?

    <p>Maximum potential wind gusts</p> Signup and view all the answers

    Which factor is NOT considered in the WINDEX calculation?

    <p>Cloud cover percentage</p> Signup and view all the answers

    How does precipitation near 500 mb affect microbursts?

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

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

    <p>It increases further</p> Signup and view all the answers

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

    <p>700mb (dry air) &amp; (moist air) 500mb</p> Signup and view all the answers

    What environmental conditions are associated with high values of DCAPE?

    <p>Steep low- and mid-level lapse rates and dry low-level air</p> Signup and view all the answers

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

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

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

    <p>Insufficient moisture for cloud formation and instability</p> Signup and view all the answers

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

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

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

    <p>Moist low-levels and dry mid-levels</p> Signup and view all the answers

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

    <p>By increasing humidity levels at the surface, enhancing buoyancy</p> Signup and view all the answers

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

    <p>Moist mid- and low-levels due to precipitation loading</p> Signup and view all the answers

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

    <p>Steep lapse rates and dry air at both levels</p> Signup and view all the answers

    What type of cloud can dry microbursts occur from?

    <p>Any cumuliform cloud with appreciable vertical extent</p> 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?

    <p>40-50 mb</p> Signup and view all the answers

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

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

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

    <p>More than 70%</p> Signup and view all the answers

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

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

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

    <p>Strongest wind speeds</p> 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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    Chapter 3: Convective Weather PDF

    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.

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