Horizontal Horizontal Convective RollsConvective Rolls

MPO 551 Paper PresentationMPO 551 Paper Presentation

Dan SternDan SternHorizontal Convective Rolls : Determining the Environmental Conditions Supporting their existence and Characteristics. Weckwerth et al. 1997

The Effect of Small Scale Moisture Variability on Thunderstorm Initiation. Weckwerth 2000

What are horizontal convective What are horizontal convective rolls?rolls?

Counter-rotating Counter-rotating horizontal vortices horizontal vortices which commonly which commonly occur within the occur within the convective convective boundary layer.boundary layer.

AMS Glossary of MeteorologyAMS Glossary of Meteorology

Results of Previous Results of Previous StudiesStudies

Both sfc. layer heat flux and vertical Both sfc. layer heat flux and vertical wind shear are necessary conditions wind shear are necessary conditions for roll occurrence.for roll occurrence.

Roll wavelength is proportional to Roll wavelength is proportional to the depth of the boundary layer.the depth of the boundary layer.

Roll orientation is along mean CBL Roll orientation is along mean CBL wind and/or shear directions.wind and/or shear directions.

Problems with Previous Problems with Previous StudiesStudies

Lack of consistent and objective Lack of consistent and objective means of defining and classifying means of defining and classifying rolls.rolls.

Few observational platforms for Few observational platforms for sampling of rolls and of surrounding sampling of rolls and of surrounding environment.environment.

Small sample size of roll cases.Small sample size of roll cases. Lack of comparison with null cases Lack of comparison with null cases

(non-roll convection, no convection)(non-roll convection, no convection)

Motivation and Motivation and ObjectivesObjectives

Determine environmental Determine environmental parameters which favor roll parameters which favor roll formation and define their formation and define their wavelengths and orientation.wavelengths and orientation.

Objectively define rolls from radar Objectively define rolls from radar reflectivity.reflectivity.

Further examine results using 3-D Further examine results using 3-D numerical model.numerical model.

Theories for roll Theories for roll formationformation

Thermal Instability: Energy is Thermal Instability: Energy is obtained from buoyancy, with bands obtained from buoyancy, with bands organized so as to minimize shear.organized so as to minimize shear.

Dynamic Instability: Energy is Dynamic Instability: Energy is extracted from the kinetic energy of extracted from the kinetic energy of wind normal to roll axes. wind normal to roll axes.

Thermal InstabilityThermal Instability

Past studies have shown that a Past studies have shown that a modest sfc heat flux is necessary.modest sfc heat flux is necessary.

Rolls are most commonly observed Rolls are most commonly observed in slightly unstable environments.in slightly unstable environments.

But as thermal instability increases, But as thermal instability increases, 2D convection becomes less likely, 2D convection becomes less likely, and 3D is preferred.and 3D is preferred.

Dynamic InstabilityDynamic Instability

Inflection Point Inflection Point InstabilityInstability

There must be an There must be an inflection point in inflection point in the cross-roll the cross-roll component of the component of the mean large scale mean large scale wind profile.wind profile.

Faller, JAS 1965

Combination of Combination of InstabilitiesInstabilities

Monin-Obukhov length:Monin-Obukhov length: |L| is approximately the height at |L| is approximately the height at

which buoyancy dominates over which buoyancy dominates over shear in turbulence production.shear in turbulence production.

Convective instability decreases as L Convective instability decreases as L increases.increases.

Studies have shown rolls to exist Studies have shown rolls to exist within a specified range of Lwithin a specified range of L

Objective classification of Objective classification of convective modesconvective modes

Reflectivity within 15X15km box was Reflectivity within 15X15km box was interpolated onto a cartesian gridinterpolated onto a cartesian grid

Spatial Autocorrelation field was Spatial Autocorrelation field was calculated and plotted (pattern calculated and plotted (pattern recognition)recognition)

Ratio of major to minor axis of .2 Ratio of major to minor axis of .2 correlation coefficient contour defines the correlation coefficient contour defines the convective mode.convective mode.

Horizontal Aspect Ratio (HAR) >6 for Horizontal Aspect Ratio (HAR) >6 for rolls.rolls.

Measurement of CBL Measurement of CBL CharacteristicsCharacteristics

Winds retrieved from VAD radar Winds retrieved from VAD radar routine with highest elevation angle routine with highest elevation angle used.used.

CBL depth determined from the well-CBL depth determined from the well-mixed potential temperature layer mixed potential temperature layer from soundings, when available. from soundings, when available. Otherwise, the height at which a Otherwise, the height at which a change in slope of reflectivity occurs change in slope of reflectivity occurs is defined as the top of the CBL.is defined as the top of the CBL.

Effect of Sensible Heat Effect of Sensible Heat FluxFlux

No convection cases No convection cases are less unstableare less unstable

Cellular cases occur Cellular cases occur in narrow range of in narrow range of heat fluxheat flux

Rolls occur in Rolls occur in broader range, still broader range, still limited.limited.

Unorganized Unorganized convection has convection has broadest range.broadest range.

Model results of varied Model results of varied heat fluxheat flux

No minimum No minimum threshold of heat threshold of heat flux for rollsflux for rolls

Beyond a certain Beyond a certain point, increased point, increased heating causes heating causes convection to convection to become less become less organized.organized.

Maximum implied by Maximum implied by model results.model results.

Effect of Wind ShearEffect of Wind Shear

All cellular cases All cellular cases occur with shear occur with shear less than 2x10less than 2x10-3 -3 ss-1-1

All rolls occur with All rolls occur with shear greater than shear greater than 2x102x10-3 -3 ss-1-1

Shear was typically Shear was typically low throughout low throughout experiment.experiment.

Effect of Wind SpeedEffect of Wind Speed All rolls occur with All rolls occur with

mean CBL wind speed mean CBL wind speed greater than 5.5m/sgreater than 5.5m/s

All rolls occur with All rolls occur with 10m wind speeds 10m wind speeds greater than 3m/sgreater than 3m/s

Cellular convection Cellular convection occurs only at lower occurs only at lower speeds while speeds while unorganized unorganized convection varies over convection varies over a broad range.a broad range.

Model Results of Varied Wind Model Results of Varied Wind SpeedSpeed

Simulation with low Simulation with low wind speed (2m/s) wind speed (2m/s) produced unorganized produced unorganized convection.convection.

Higher wind speeds Higher wind speeds (5m/s, 10m/s) (5m/s, 10m/s) produced linear produced linear convection.convection.

This supports the This supports the observations that observations that there is a minimum there is a minimum threshold of wind threshold of wind speed for rolls.speed for rolls.

Sensible Heat Flux vs. Wind Sensible Heat Flux vs. Wind ShearShear

Rolls only occur Rolls only occur within a specific within a specific range of heat flux range of heat flux and above a and above a threshold value of threshold value of shear.shear.

Shear magnitude Shear magnitude separates cellular separates cellular from roll from roll convection.convection.

Forcing Mechanisms of Forcing Mechanisms of RollsRolls

TKE Budget:TKE Budget:

Buoyancy dominates Buoyancy dominates for unorganized for unorganized convection at all levels.convection at all levels.

For rolls, buoyancy For rolls, buoyancy dominates in the upper dominates in the upper boundary layer, but the boundary layer, but the forcing from shear is forcing from shear is comparable to comparable to buoyancy at low levels.buoyancy at low levels.

Roll Wavelength vs. CBL Roll Wavelength vs. CBL DepthDepth

Wavelength is well Wavelength is well correlated with CBL correlated with CBL depth (r=.84), in depth (r=.84), in agreement with agreement with theory and prior theory and prior observations.observations.

Wavelength Wavelength increases with increases with increasing depth.increasing depth.

Average aspect ratio Average aspect ratio is 5.7is 5.7

Influences on aspect Influences on aspect ratioratio

Previous studies had Previous studies had suggested that aspect suggested that aspect ratio is related to CBL ratio is related to CBL wind shear and/or wind wind shear and/or wind speed. This study speed. This study found them to be found them to be uncorrelated however.uncorrelated however.

Aspect ratio Aspect ratio isis found to found to be well correlated with be well correlated with convective instability.convective instability.

Aspect ratio increases Aspect ratio increases with increasing with increasing convective instability.convective instability.

Roll OrientationRoll Orientation Orientation is highly correlated with Orientation is highly correlated with

CBL wind shear direction, mean CBL CBL wind shear direction, mean CBL wind direction, and 10m wind direction.wind direction, and 10m wind direction.

This is because these variables were all This is because these variables were all highly correlated with each other in the highly correlated with each other in the experiment (very little directional wind experiment (very little directional wind shear).shear).

Therefore, it was not possible to Therefore, it was not possible to determine which variable is most determine which variable is most relevant.relevant.

Summary (this is not yet Summary (this is not yet the end)the end)

Rolls were objectively classified, and characteristics Rolls were objectively classified, and characteristics of rolls and their environments were determined of rolls and their environments were determined from both observations and modeling.from both observations and modeling.

Minimum wind speed and shear criterion, although Minimum wind speed and shear criterion, although required shear is quite low and directional shear is required shear is quite low and directional shear is unnecessary for roll formation.unnecessary for roll formation.

Low-level shear is important, but could not be well Low-level shear is important, but could not be well measured due to limitations of experiment.measured due to limitations of experiment.

There is a preferred roll regime constrained by heat There is a preferred roll regime constrained by heat flux and wind.flux and wind.

Wavelength proportional to CBL depth and Wavelength proportional to CBL depth and orientation correlated with wind direction.orientation correlated with wind direction.

Why Rolls are RelevantWhy Rolls are Relevant

Rolls are boundary layer convergence zones.Rolls are boundary layer convergence zones. Low-level convergence may lead to Low-level convergence may lead to

thunderstorm initiation.thunderstorm initiation. Intersection of rolls with other boundaries Intersection of rolls with other boundaries

often lead to convective development.often lead to convective development. Rolls themselves may initiate thunderstorms, Rolls themselves may initiate thunderstorms,

even in the absence of other forcing.even in the absence of other forcing. Initiation of deep convection by rolls alone Initiation of deep convection by rolls alone

only occurs a fraction of the time. Why?only occurs a fraction of the time. Why?

Storm DayStorm Day

No Storm DayNo Storm Day

Inability of soundings to Inability of soundings to predict convective predict convective

potential.potential. Storm Case: LFC at 2.3km while CBL Storm Case: LFC at 2.3km while CBL

depth is only .8km; CAPE=644 J/kg; depth is only .8km; CAPE=644 J/kg; CIN= -30 J/kgCIN= -30 J/kg

True potential for deep convection is True potential for deep convection is underestimated because the sounding is underestimated because the sounding is unrepresentative of the region of unrepresentative of the region of initiation.initiation.

It is necessary to measure the It is necessary to measure the environment of the roll updraft environment of the roll updraft branches, since this is where branches, since this is where thunderstorms form.thunderstorms form.

Sounding modified by Sounding modified by aircraft dataaircraft data

Variability of Variability of temperature is temperature is small, but moisture small, but moisture variability is large.variability is large.

Using maximum Using maximum CBL mixing ratio CBL mixing ratio for parcel ascent, for parcel ascent, LFC=1.2km; LFC=1.2km; CAPE=1665J/kg; CAPE=1665J/kg; CIN=0CIN=0

Sounding for No Storm Sounding for No Storm DayDay

Original Sounding: Original Sounding: LFC=2.3km while LFC=2.3km while CBL depth=.85km; CBL depth=.85km; CAPE=966 J/kg; CAPE=966 J/kg; CIN=-44 J/kgCIN=-44 J/kg

Modified Modified Sounding: Sounding: LFC=1.85km; LFC=1.85km; CAPE=1847 J/kg; CAPE=1847 J/kg; CIN=-18 J/kgCIN=-18 J/kg

CBL Depth vs. LFCCBL Depth vs. LFC

CBL Depth vs. LFC CBL Depth vs. LFC continuedcontinued

Difference between CBL depth and Difference between CBL depth and LFC is smaller on storm days (.8km LFC is smaller on storm days (.8km vs. 1.3km)vs. 1.3km)

However, this is not a good predictor However, this is not a good predictor of convection.of convection.

Using modified soundings, there is Using modified soundings, there is good discrimination between storm good discrimination between storm days and no-storm days.days and no-storm days.

LFC-CBL depth for modified storm LFC-CBL depth for modified storm day soundings is only .1kmday soundings is only .1km

Some parameters which are Some parameters which are useless for predicting useless for predicting

convectionconvection Using sfc moisture variability to modify Using sfc moisture variability to modify

soundings incorrectly suggests convection will soundings incorrectly suggests convection will occur on every day.occur on every day.

No difference between storm and no-storm No difference between storm and no-storm days was found from surface mixing ratios, days was found from surface mixing ratios, RH, temp., wind speed or direction, etc…RH, temp., wind speed or direction, etc…

Wind shear was always very small, and there Wind shear was always very small, and there was no difference between storm and no-storm was no difference between storm and no-storm days.days.

Topography and geography had no influence.Topography and geography had no influence. The roll circulation and updraft strength were The roll circulation and updraft strength were

very similar between storm and no-storm days.very similar between storm and no-storm days.

Summary (yes, this is the Summary (yes, this is the end) end)

Most soundings do not sample the updraft Most soundings do not sample the updraft branches of rolls. Therefore, soundings by branches of rolls. Therefore, soundings by themselves are insufficient for predicting the themselves are insufficient for predicting the potential for deep convection due to rolls potential for deep convection due to rolls alone.alone.

Soundings modified by aircraft data are able Soundings modified by aircraft data are able to indicate the true convective potential.to indicate the true convective potential.

Surface measurements are uselessSurface measurements are useless In the absence of synoptic forcing, CBL In the absence of synoptic forcing, CBL

water vapor variability must be measured water vapor variability must be measured with rather high spatial resolution (~500m) with rather high spatial resolution (~500m) to accurately forecast the initiation of deep to accurately forecast the initiation of deep moist convection.moist convection.