Short Term Convective Mode Evolution along Synoptic Boundaries Greg L. Dial, Jonathan P. Racy and Richard L. Thompson Storm Prediction Center Norman, Oklahoma

Motivation  Convective mode evolution is one of the most difficult aspects of severe storm forecasting.  Initial motivation arose prior to availability of routine convective allowing model output. Since then, SPC Spring Experiments indicate high resolution non-convectively parameterized models such as the 1km WRF can often demonstrate skill in predicting how storms will evolve. Nevertheless, it is still desirable to have a means to evaluate when model forecasts are likely to be correct.  Common tornado forecast problem - Discrete supercells evolve into lines more quickly than anticipated.  When a rapid evolution to lines is expected, a greater emphasis might be placed on damaging wind than tornadoes and large hail, though qlcs tornadoes may still occur.

 Investigate parameters that might exhibit skill discriminating between when storms remain discrete versus when storms evolve into lines within 3 hours after initiation.  Storms investigated were initiated along pre-frontal troughs, cold fronts and drylines (accounts for a fraction of all convective initiation cases).  We will also briefly discuss other forms of thunderstorm initiation and implications for mode including storms initiating in warm sector and along boundary mergers. Objective

Criteria For Selecting Cases For Storms Initiated Along Synoptic Boundaries  Surface based storms initiated along a cold front, pre-frontal trough or dryline and persisted for at least 3 hours.  The storms produced one or more reports of large hail, damaging wind, and or tornadoes.  The 0-6 km AGL shear was 30 kt or greater.

Geographical And Seasonal Distribution ( ). Events included were east of Rockies but most were west of the Mississippi river and focused around the plains and Mississippi Valley region

Geographical And Seasonal Distribution  Total of 169 cases collected, most during spring and autumn.

Data  Observed (~20%) and RUC (~80%) model proximity soundings  Radar, Satellite, Surface Observations  Objective analysis based on RUC and observed data  169 cases collected so far

Explanation of Mode  By mode we are trying to distinguish between discrete versus linear evolutions for storms initiated along a boundary.  Storms were classified as discrete when maximum reflectivity between identifiable cells did not exceed 25 dbz.  Storms classified as linear when 35 dbz or greater reflectivity pattern showed a length to width ratio of at least 5 to 1.

Examples Of Mode Types Considered

What Determines Rate Of Upscale Linear Growth For Storms Initiated Along A Boundary?  Ability of rain cores and outflows to merge and generate new updrafts Number of storms initiated Number of storms initiated Determined in part by degree of external forcing, low-level parcel trajectories, residence time of developing storms within external forcing zone, and movement of convective outflows relative to external boundariesDetermined in part by degree of external forcing, low-level parcel trajectories, residence time of developing storms within external forcing zone, and movement of convective outflows relative to external boundaries Residence time of storms within zone of external forcing determined primarily by boundary relative mean cloud layer wind Residence time of storms within zone of external forcing determined primarily by boundary relative mean cloud layer wind Ability to continually generate updrafts along front or consolidating outflows determined by magnitude and depth of ascent along front or convective outflows, trends in inversion strength and microphysical characteristics of the outflow boundaries. Ability to continually generate updrafts along front or consolidating outflows determined by magnitude and depth of ascent along front or convective outflows, trends in inversion strength and microphysical characteristics of the outflow boundaries. Distribution of precipitation around the updraft Distribution 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 Mode  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 environment Forcing/Vertical motion modifies thermodynamic environment  Kinematic Parameters: Boundary relative normal component of mean cloud layer wind Boundary relative normal component of mean cloud layer wind Component of cloud layer shear normal to boundary (appears to be somewhat relevant but not as significant) Component of cloud layer shear normal to boundary (appears to be somewhat relevant but not as significant)  Thermodynamic Parameters: Capping Inversion Strength Capping Inversion Strength Mid-Level RH (Effects Of Entrainment) Mid-Level RH (Effects Of Entrainment)

Kinematic Parameters Normal Component of 2-6 Km or 2-8 Km Shear (m/s) Versus Mode Evolution Related to Distribution of Precipitation

Kinematic Parameters Boundary Relative Normal Component Of Mean Cloud Layer Wind Versus Storm Attachment To boundary

Boundary Relative Normal Component Of Mean Cloud Layer Wind Versus Mode At 3hr Related To Residence Time Of Storms Along Boundary Kinematic Best Mode Discrimination Potential Of Any Kinematic Parameter Considered

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

Role of convergence  Number of storms initiated depends partly on depth and strength of UVV: modulates CINH and mid level RH (entrainment). UVV can moisten and cool the layer.  Low-level convergence can often infer upward vertical motion.  When boundaries are accompanied by very strong deep layer forcing for ascent…rapid evolution to lines may occur regardless of the orientation of vertical wind and shear orientations.  Limitations: Does not always correspond to vertical motion through a deep layer. Does not always correspond to vertical motion through a deep layer. Convergence magnitude is scale dependent (smaller grid spacing would result in stronger convergence values). Convergence magnitude is scale dependent (smaller grid spacing would result in stronger convergence values).

Lowest 100 mb Convergence (x10 -5 /s) Versus Mode Evolution 40 KM Grid

Convective Inhibition Lid strength = Mean Theta (w) boundary layer – Theta (w) on warmest part of inversion. Isolated discrete storms are expected if cap is not removed completely within zone of forcing. Supercells can persist for longer periods of time than multicells in environments that maintain some convective inhibition. For cases where storms move off boundary: Mean warm sector inversion strength found to be 2.5C for discrete modes and 1.5C for linear modes For cases where storms remain on boundary: Mean warm sector inversion strength found to be 2.3C for discrete modes and 1.6C for linear modes. In both instances mean warm sector inversion strength was found to be weaker for cases where storms evolved into lines versus those where storms remained discrete.

Mode Frequency Versus Boundary Type (3 hr after initiation)

Boundary Point Merger Example (Apr )

Examples (APR )

Thunderstorm Initiation and Horizontal Convective Rolls Warm 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 when Evolution to lines or mixed modes within a few hours is favored when Normal component of boundary relative mean cloud layer wind is small < 6 m/s such that storms do not move off of boundary.Normal component of boundary relative mean cloud layer wind 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 (< 8 ms)The component of cloud layer shear normal to boundary is small (< 8 ms) Strong convergence and deep forcing accompanies the initiating boundary. Forcing often appears to play a more dominant role in mode evolution than wind and shear orientations.Strong convergence and deep forcing accompanies the initiating boundary. Forcing often appears to play a more dominant role in mode evolution than wind and shear orientations. Initiating mechanism involves slab ascent associated with moderate to strong progressive cold fronts.Initiating mechanism involves slab ascent associated with moderate to strong progressive cold fronts. Summary

Very rapid (within minutes) upscale linear growth is often observed during boundary mergers or collisions such as when a cold front overtakes a dryline.Very rapid (within minutes) upscale linear growth is often observed during boundary mergers or collisions such as when a cold front overtakes a dryline. Upscale linear growth will be delayed or may not occur at all if frontal circulation is not deep enough to remove capping inversion.Upscale linear growth will be delayed or may not occur at all if frontal circulation is not deep enough to remove capping inversion. On the other hand in very weakly capped, moist environments, upscale linear growth might occur even if storms move off the boundary.On the other hand in very weakly capped, moist environments, upscale linear growth might occur even if storms move off the boundary. Phasing of frontal circulation with deeper forcing for ascent such as that associated with upper jet streaks or shortwave troughs increases likelihood of upscale linear growth.Phasing of frontal circulation with deeper forcing for ascent such as that associated with upper jet streaks or shortwave troughs increases likelihood of upscale linear growth. Summary

When the normal component of cloud layer shear is very weak (< 6 ms) or directed toward the cool side of the boundary, development of a TSR may occur more rapidly (within 1-3 hours of initiation).When the normal component of cloud layer shear is very weak (< 6 ms) or directed toward the cool side of the boundary, development of a TSR may occur more rapidly (within 1-3 hours of initiation). Cursory observational evidence suggests lines that have developed a TSR are less prone break up into discrete cells than lines that do not have a TSR assuming storms remain in a relatively uniform thermodynamic environment.Cursory observational evidence suggests lines that have developed a TSR are less prone break up into discrete cells than lines that do not have a TSR assuming storms remain in a relatively uniform thermodynamic environment. Summary

 Type of initiating boundary correlated with mode evolution. Faster evolution to lines were more often associated with cold fronts (Stronger forcing/slab ascent) Faster evolution to lines were more often associated with cold fronts (Stronger forcing/slab ascent) Persistent discrete modes were more often associated with drylines and pre-frontal troughs Persistent discrete modes were more often associated with drylines and pre-frontal troughsSummary

 Thermodynamic Parameters CINH CINH Mid Level RH Mid Level RH 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 enough to remove the CINH. 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 enough to remove the CINH. More work is needed with thermodynamic parameters. Need more frequent upper observations to adequately sample changes in the thermodynamic environment where storms are developing. More work is needed with thermodynamic parameters. Need more frequent upper observations to adequately sample changes in the thermodynamic environment where storms are developing. Because variables cannot be held constant or isolated in observational studies, it is often very difficult to determine the relative influence of forcing versus kinematic parameters versus thermodynamic parameters on mode evolution. Supplementation with modeling studies (where variables can be independently controlled) is needed to gain better understanding. Because variables cannot be held constant or isolated in observational studies, it is often very difficult to determine the relative influence of forcing versus kinematic parameters versus thermodynamic parameters on mode evolution. Supplementation with modeling studies (where variables can be independently controlled) is needed to gain better understanding.Summary

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 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 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

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