Short Term Convective Mode Evolution along Synoptic Short Term Convective Mode Evolution along Synoptic BoundariesBoundaries

Greg L. Dial, Jonathan P. Racy and Richard L. ThompsonGreg L. Dial, Jonathan P. Racy and Richard L. Thompson

Storm Prediction Center Storm Prediction Center

Norman, OklahomaNorman, Oklahoma

MotivationMotivation

Convective mode evolution is one of the most difficult aspects of severe Convective mode evolution is one of the most difficult aspects of severe storm forecasting.storm forecasting.

Initial motivation arose prior to availability of routine convective allowing Initial motivation arose prior to availability of routine convective allowing model output. Since then, SPC Spring Experiments indicate high model output. Since then, SPC Spring Experiments indicate high resolution non-convectively parameterized models such as the 1km resolution non-convectively parameterized models such as the 1km WRF can often demonstrate skill in predicting how storms will evolve. WRF can often demonstrate skill in predicting how storms will evolve. Nevertheless, it is still desirable to have a means to evaluate when Nevertheless, it is still desirable to have a means to evaluate when model forecasts are likely to be correct.model forecasts are likely to be correct.

Common tornado forecast problem - Discrete supercells evolve into Common tornado forecast problem - Discrete supercells evolve into lines more quickly than anticipated.lines more quickly than anticipated.

When a rapid evolution to lines is expected, a greater emphasis might be When a rapid evolution to lines is expected, a greater emphasis might be placed on damaging wind than tornadoes and large hail, though qlcs placed on damaging wind than tornadoes and large hail, though qlcs tornadoes may still occur.tornadoes may still occur.

Investigate parameters that might exhibit skill discriminating Investigate parameters that might exhibit skill discriminating between when storms remain discrete versus when storms evolve between when storms remain discrete versus when storms evolve into lines into lines within 3 hourswithin 3 hours after initiation. after initiation.

Storms investigated were initiated along Storms investigated were initiated along pre-frontal troughspre-frontal troughs, , cold cold frontsfronts and and drylines ( drylines (accounts for a fraction of all convective initiation accounts for a fraction of all convective initiation cases).cases).

We will also briefly discuss other forms of thunderstorm initiation We will also briefly discuss other forms of thunderstorm initiation and implications for mode including storms initiating in warm sector and implications for mode including storms initiating in warm sector and along boundary mergers.and along boundary mergers.

ObjectiveObjective

Criteria For Selecting Cases For Storms Initiated Along Synoptic Criteria For Selecting Cases For Storms Initiated Along Synoptic BoundariesBoundaries

Surface based storms initiated along a Surface based storms initiated along a cold frontcold front, , pre-frontal troughpre-frontal trough oror dryline dryline and persisted for at least 3 hours. and persisted for at least 3 hours.

The storms produced one or more reports of large hail, damaging The storms produced one or more reports of large hail, damaging wind, and or tornadoes.wind, and or tornadoes.

The 0-6 km AGL shear was 30 kt or greater.The 0-6 km AGL shear was 30 kt or greater.

Geographical And Seasonal Distribution (2003-2007)Geographical And Seasonal Distribution (2003-2007)

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Events included were east of Rockies but most were west of the Events included were east of Rockies but most were west of the Mississippi river and focused around the plains and Mississippi Valley Mississippi river and focused around the plains and Mississippi Valley regionregion

Geographical And Seasonal DistributionGeographical And Seasonal Distribution

Distribution Of Cases By Month

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Total of 169 cases collected, most during spring and autumn.Total of 169 cases collected, most during spring and autumn.

DataData

Observed (~20%) and RUC (~80%) model proximity soundingsObserved (~20%) and RUC (~80%) model proximity soundings

Radar, Satellite, Surface ObservationsRadar, Satellite, Surface Observations

Objective analysis based on RUC and observed dataObjective analysis based on RUC and observed data

169 cases collected so far169 cases collected so far

Explanation of ModeExplanation of Mode

By mode we are trying to distinguish between discrete versus By mode we are trying to distinguish between discrete versus linear evolutions for storms initiated along a boundary. linear evolutions for storms initiated along a boundary.

Storms were classified as discrete when maximum reflectivity Storms were classified as discrete when maximum reflectivity between identifiable cells did not exceed 25 dbz.between identifiable cells did not exceed 25 dbz.

Storms classified as linear when 35 dbz or greater reflectivity Storms classified as linear when 35 dbz or greater reflectivity pattern showed a length to width ratio of at least 5 to 1.pattern showed a length to width ratio of at least 5 to 1.

Examples Of Mode Types ConsideredExamples Of Mode Types Considered

What Determines Rate Of Upscale Linear Growth For Storms What Determines Rate Of Upscale Linear Growth For Storms Initiated Along A Boundary?Initiated Along A Boundary?

Ability of rain cores and outflows to merge and generate new updraftsAbility of rain cores and outflows to merge and generate new updrafts Number of storms initiatedNumber of storms initiated

• Determined in part by degree of external forcing, low-level parcel Determined in part by degree of external forcing, low-level parcel trajectories, residence time of developing storms within external forcing trajectories, residence time of developing storms within external forcing zone, and movement of convective outflows relative to external zone, and movement of convective outflows relative to external boundariesboundaries

Residence time of storms within zone of external forcing determined Residence time of storms within zone of external forcing determined primarily by boundary relative mean cloud layer windprimarily by boundary relative mean cloud layer wind

Ability to continually generate updrafts along front or consolidating Ability to continually generate updrafts along front or consolidating outflows determined by magnitude and depth of ascent along front or outflows determined by magnitude and depth of ascent along front or convective outflows, trends in inversion strength and microphysical convective outflows, trends in inversion strength and microphysical characteristics of the outflow boundaries.characteristics of the outflow boundaries.

Distribution of precipitation around the updraftDistribution of precipitation around the updraft Determined in large part by orientation of cloud layer vertical shear.Determined in large part by orientation of cloud layer vertical shear.

Kinematic and Thermodynamic Parameters Relevant to ModeKinematic and Thermodynamic Parameters Relevant to Mode

Forcing Parameters:Forcing Parameters: Mean low-level convergence along boundaries (40 km RUC grids)Mean low-level convergence along boundaries (40 km RUC grids) Forcing through a deeper layer (upper jet streaks and shortwave troughs)Forcing through a deeper layer (upper jet streaks and shortwave troughs) Forcing/Vertical motion modifies thermodynamic environmentForcing/Vertical motion modifies thermodynamic environment

Kinematic ParametersKinematic Parameters:: Boundary relative normal component of mean cloud layer windBoundary relative normal component of mean cloud layer wind Component of cloud layer shear normal to boundary (Component of cloud layer shear normal to boundary (appears to be somewhat appears to be somewhat

relevant but not as significantrelevant but not as significant))

Thermodynamic ParametersThermodynamic Parameters:: Capping Inversion StrengthCapping Inversion Strength Mid-Level RH (Effects Of Entrainment)Mid-Level RH (Effects Of Entrainment)

Kinematic ParametersKinematic ParametersNormal Component of 2-6 Km or 2-8 Km Shear (m/s) Versus Normal Component of 2-6 Km or 2-8 Km Shear (m/s) Versus

Mode EvolutionMode Evolution

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Kinematic ParametersKinematic ParametersBoundary Relative Normal Component Of Mean Cloud Layer Boundary Relative Normal Component Of Mean Cloud Layer

Wind Versus Storm Attachment To boundaryWind Versus Storm Attachment To boundary

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Boundary Relative Normal Component Of Mean Cloud Layer Boundary Relative Normal Component Of Mean Cloud Layer Wind Versus Mode At 3hrWind Versus Mode At 3hr

Related To Residence Time Of Storms Along BoundaryBest Mode Discrimination Potential Of Any KinematicKinematic Parameter Considered

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Lowest 100 mb convergence Lowest 100 mb convergence (measure of initiation potential, (measure of initiation potential, and likely plays a role in number of storms initiatedand likely plays a role in number of storms initiated))

Forcing ParametersForcing Parameters

Role of convergenceRole of convergence

Number of storms initiated depends partly on depth and strength of UVV: Number of storms initiated depends partly on depth and strength of UVV: modulates CINH and mid level RH (entrainment). UVV can moisten and modulates CINH and mid level RH (entrainment). UVV can moisten and cool the layer.cool the layer.

Low-level convergence can often infer upward vertical motion. Low-level convergence can often infer upward vertical motion.

When boundaries are accompanied by very strong deep layer forcing for When boundaries are accompanied by very strong deep layer forcing for ascent…rapid evolution to lines may occur regardless of the orientation ascent…rapid evolution to lines may occur regardless of the orientation of vertical wind and shear orientations.of vertical wind and shear orientations.

Limitations:Limitations: Does not always correspond to vertical motion through a Does not always correspond to vertical motion through a deepdeep layer. layer. Convergence magnitude is scale dependent (smaller grid spacing would Convergence magnitude is scale dependent (smaller grid spacing would

result in stronger convergence values).result in stronger convergence values).

Lowest 100 mb Convergence (x10Lowest 100 mb Convergence (x10-5-5/s) Versus Mode Evolution/s) Versus Mode Evolution40 KM Grid40 KM Grid

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Convective InhibitionConvective Inhibition

Lid strength = Mean Theta (w)Lid strength = Mean Theta (w) boundary layer – boundary layer – Theta (w)Theta (w) on warmest on warmest part of inversion.part of inversion.

Isolated discrete storms are expected if cap is not removed completely Isolated discrete storms are expected if cap is not removed completely within zone of forcing. Supercells can persist for longer periods of time within zone of forcing. Supercells can persist for longer periods of time than multicells in environments that maintain some convective inhibition.than multicells in environments that maintain some convective inhibition.

For cases where storms move off boundary:For cases where storms move off boundary:

Mean warm sector inversion strength found to be 2.5C for discrete Mean warm sector inversion strength found to be 2.5C for discrete modes and 1.5C for linear modesmodes and 1.5C for linear modes

For cases where storms remain on boundary:For cases where storms remain on boundary:

Mean warm sector inversion strength found to be 2.3C for discrete Mean warm sector inversion strength found to be 2.3C for discrete modes and 1.6C for linear modes. modes and 1.6C for linear modes.

In both instances mean warm sector inversion strength was found to In both instances mean warm sector inversion strength was found to be weaker for cases where storms evolved into lines versus those be weaker for cases where storms evolved into lines versus those where storms remained discrete.where storms remained discrete.

Mode Frequency Versus Boundary Type (3 hr after initiation)Mode Frequency Versus Boundary Type (3 hr after initiation)

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Boundary Point Merger Example (Apr 03 2011)

Examples (APR 03 2011)

Thunderstorm Initiation and Horizontal Convective RollsThunderstorm Initiation and Horizontal Convective RollsWarm Sector InitiationWarm Sector Initiation

Warm sector thunderstorm initiation along horizontal convective rolls in environments with moderate to high instability and strong vertical shear is often associated with persistent discrete supercells. Do you think the compensating subsidence surrounding intense supercell updrafts would have an influence convective mode? If so how?

Evolution to lines or mixed modes within a few hours is favored Evolution to lines or mixed modes within a few hours is favored whenwhen

• Normal component of boundary relative mean cloud layer wind Normal component of boundary relative mean cloud layer wind is small < 6 m/s such that storms do not move off of boundary.is small < 6 m/s such that storms do not move off of boundary.

• The component of cloud layer shear normal to boundary is small The component of cloud layer shear normal to boundary is small (< 8 ms)(< 8 ms)

• Strong convergence and deep forcing accompanies the Strong convergence and deep forcing accompanies the initiating boundary. initiating boundary. Forcing often appears to play a more Forcing often appears to play a more dominant role in mode evolution than wind and shear dominant role in mode evolution than wind and shear orientations.orientations.

• Initiating mechanism involves slab ascent associated with Initiating mechanism involves slab ascent associated with moderate to strong progressive cold fronts.moderate to strong progressive cold fronts.

SummarySummary

• Very rapid (within minutes) upscale linear growth is often Very rapid (within minutes) upscale linear growth is often observed during boundary mergers or collisions such as observed during boundary mergers or collisions such as when a cold front overtakes a dryline.when a cold front overtakes a dryline.

• Upscale linear growth will be delayed or may not occur at all Upscale linear growth will be delayed or may not occur at all if frontal circulation is not deep enough to remove capping if frontal circulation is not deep enough to remove capping inversion.inversion.

• On the other hand in very weakly capped, moist On the other hand in very weakly capped, moist environments, upscale linear growth might occur even if environments, upscale linear growth might occur even if storms move off the boundary.storms move off the boundary.

• Phasing of frontal circulation with deeper forcing for ascent Phasing of frontal circulation with deeper forcing for ascent such as that associated with upper jet streaks or shortwave such as that associated with upper jet streaks or shortwave troughs increases likelihood of upscale linear growth. troughs increases likelihood of upscale linear growth.

SummarySummary

• When the normal component of cloud layer shear is When the normal component of cloud layer shear is very weak (< 6 ms) or directed toward the cool side of very weak (< 6 ms) or directed toward the cool side of the boundary, development of a TSR may occur more the boundary, development of a TSR may occur more rapidly (within 1-3 hours of initiation).rapidly (within 1-3 hours of initiation).

• Cursory observational evidence suggests lines that Cursory observational evidence suggests lines that have developed a TSR are less prone break up into have developed a TSR are less prone break up into discrete cells than lines that do not have a TSR discrete cells than lines that do not have a TSR assuming storms remain in a relatively uniform assuming storms remain in a relatively uniform thermodynamic environment.thermodynamic environment.

SummarySummary

Type of initiating boundary correlated with mode evolution.Type of initiating boundary correlated with mode evolution.

Faster evolution to lines were more often associated with cold Faster evolution to lines were more often associated with cold fronts (fronts (Stronger forcing/slab ascentStronger forcing/slab ascent))

Persistent discrete modes were more often associated with Persistent discrete modes were more often associated with drylines and pre-frontal troughsdrylines and pre-frontal troughs

SummarySummary

Thermodynamic ParametersThermodynamic Parameters CINHCINH Mid Level RHMid Level RH

Environments where CINH is strong and Mid-Level RH is low reduces the Environments where CINH is strong and Mid-Level RH is low reduces the likelihood of upscale linear growth if the boundary circulation is not deep likelihood of upscale linear growth if the boundary circulation is not deep enough to remove the CINH.enough to remove the CINH.

More work is needed with thermodynamic parameters. Need more More work is needed with thermodynamic parameters. Need more frequent upper observations to adequately sample changes in the frequent upper observations to adequately sample changes in the thermodynamic environment where storms are developing.thermodynamic environment where storms are developing.

Because variables cannot be held constant or isolated in observational Because variables cannot be held constant or isolated in observational

studies, it is often very difficult to determine the studies, it is often very difficult to determine the relative influence of relative influence of forcingforcing versus versus kinematic parameterskinematic parameters versus versus thermodynamic parametersthermodynamic parameters on mode on mode evolution. Supplementation with modeling studies (where variables can be evolution. Supplementation with modeling studies (where variables can be independently controlled) is needed to gain better understanding.independently controlled) is needed to gain better understanding.

SummarySummary

It must be emphasized that correlation does not equal causation. Upscale growth to lines along synoptic boundaries appears to be correlated with small components of boundary relative mean wind. However, the effect that boundary and mean wind and shear orientation have on convective mode evolution has not been proven or quantified, and as mentioned previously, forcing often appears to play a more dominant role.

Therefore, mean wind and shear orientations with respect to the initiating boundary cannot be used exclusively when predicting convective mode evolution. The nature of forcing and thermodynamic environment must be incorporated into the process.

Cautions and RecommendationsCautions and Recommendations

Hypothetical graph of the relative influence of the mean wind and shear orientations versus forcing. When forcing is weak and some cap exists, too few storms may develop for them to interact. When forcing is strong, numerous storms may develop and interact, leading to upscale growth regardless of wind and shear orientations.

Cautions and RecommendationsCautions and Recommendations

Cautions and RecommendationsCautions and Recommendations

Results from previous spring experiments indicate convective allowing models at 1 km resolution often demonstrate reasonable skill simulating convective mode, though they are not always correct.

Due to the complexity and many variables involved in convective mode evolution, future mode work is best suited for simulation studies using numerical models where the effects of microphysics on outflow generation and subsequent mode evolution can be incorporated. Also these types of studies would allow for independent control of certain variables to better determine the relative influence of forcing versus wind and shear orientations.

Weather and Forecasting March 2010

Exercise

Determine the predominant convective mode and primary severe threats through 00Z in the region given. The dominant mode

can be discrete, linear or mixed (a combination of both).

For this exercise use all information given and consider:

1.Nature of forcing at the surface and aloft including boundary types.2.Wind orientations.3.Changes in thermodynamic environment.4.On the answer sheet use 1 for discrete, 2 for linear and 3 for mixed.

18Z

23Z

Website ReviewWebsite Review

http://www.spc.noaa.gov/exper/http://www.spc.noaa.gov/exper/ma_archive/index2.htmlma_archive/index2.html

http://www.spc.noaa.gov/exper/archive/http://www.spc.noaa.gov/exper/archive/event.php?date=20010406event.php?date=20010406