horizontal conveadasdastive rolls

7/30/2019 Horizontal Conveadasdastive Rolls

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Horizontal Convective

RollsMPO 551 Paper Presentation

Dan 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


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What are horizontal convective rolls?

Counter-rotatinghorizontal vortices whichcommonly occur within

the convective boundarylayer.

AMS Glossary of Meteorology


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Results of Previous Studies

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

Roll wavelength is proportional to the depth ofthe boundary layer.

Roll orientation is along mean CBL wind and/orshear directions.


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Problems with Previous Studies

Lack of consistent and objective means ofdefining and classifying rolls.

Few observational platforms for sampling ofrolls and of surrounding environment.

Small sample size of roll cases.

Lack of comparison with null cases (non-rollconvection, no convection)


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Motivation and Objectives

Determine environmental parameters whichfavor roll formation and define theirwavelengths and orientation.

Objectively define rolls from radar reflectivity.

Further examine results using 3-D numericalmodel.


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Theories for roll formation

Thermal Instability: Energy is obtained frombuoyancy, with bands organized so as tominimize shear.

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


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Thermal Instability

Past studies have shown that a modest sfc heatflux is necessary.

Rolls are most commonly observed in slightlyunstable environments.

But as thermal instability increases, 2Dconvection becomes less likely, and 3D ispreferred.


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Dynamic Instability

Inflection PointInstability

There must be an

inflection point in thecross-roll component ofthe mean large scale

wind profile.

Faller, JAS 1965


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Combination of Instabilities

Monin-Obukhov length:

|L| is approximately the height at whichbuoyancy dominates over shear in turbulenceproduction.

Convective instability decreases as L increases.

Studies have shown rolls to exist within aspecified range of L


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Objective classification of convective

modes

Reflectivity within 15X15km box wasinterpolated onto a cartesian grid

Spatial Autocorrelation field was calculated andplotted (pattern recognition)

Ratio of major to minor axis of .2 correlationcoefficient contour defines the convectivemode.

Horizontal Aspect Ratio (HAR) >6 for rolls.


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Measurement of CBL Characteristics

Winds retrieved from VAD radar routine withhighest elevation angle used.

CBL depth determined from the well-mixedpotential temperature layer from soundings,when available. Otherwise, the height at which achange in slope of reflectivity occurs is defined

as the top of the CBL.


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Effect of Sensible Heat Flux

No convection cases areless unstable

Cellular cases occur in

narrow range of heat flux Rolls occur in broader

range, still limited.

Unorganized convection

has broadest range.


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Model results of varied heat flux

No minimum thresholdof heat flux for rolls

Beyond a certain point,

increased heating causesconvection to becomeless organized.

Maximum implied bymodel results.


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Effect of Wind Shear

All cellular cases occurwith shear less than2x10-3 s-1

All rolls occur with sheargreater than 2x10-3 s-1

Shear was typically lowthroughout experiment.


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Effect of Wind Speed

All rolls occur with meanCBL wind speed greater than5.5m/s

All rolls occur with 10mwind speeds greater than3m/s

Cellular convection occursonly at lower speeds while

unorganized convectionvaries over a broad range.


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Model Results of Varied Wind Speed

Simulation with lowwind speed (2m/s)produced unorganizedconvection.

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

This supports theobservations that there isa minimum threshold of

wind speed for rolls.


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Sensible Heat Flux vs. Wind Shear

Rolls only occur within aspecific range of heatflux and above a

threshold value of shear. Shear magnitude

separates cellular fromroll convection.


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Forcing Mechanisms of Rolls

TKE Budget:

Buoyancy dominates forunorganized convection at alllevels.

For rolls, buoyancydominates in the upper

boundary layer, but theforcing from shear iscomparable to buoyancy atlow levels.


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Roll Wavelength vs. CBL Depth

Wavelength is wellcorrelated with CBLdepth (r=.84), in

agreement with theoryand prior observations.

Wavelength increaseswith increasing depth.

Average aspect ratio is5.7


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Influences on aspect ratio

Previous studies hadsuggested that aspect ratio isrelated to CBL wind shearand/or wind speed. This

study found them to beuncorrelated however.

Aspect ratio is found to bewell correlated withconvective instability.

Aspect ratio increases withincreasing convectiveinstability.


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Roll Orientation

Orientation is highly correlated with CBL windshear direction, mean CBL wind direction, and10m wind direction.

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

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


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Summary (this is not yet the end)

Rolls were objectively classified, and characteristics of rolls andtheir environments were determined from both observations andmodeling.

Minimum wind speed and shear criterion, although required

shear is quite low and directional shear is unnecessary for rollformation.

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

There is a preferred roll regime constrained by heat flux andwind.

Wavelength proportional to CBL depth and orientationcorrelated with wind direction.


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Storm Day


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No Storm Day


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Inability of soundings to predict

convective potential.

Storm Case: LFC at 2.3km while CBL depth isonly .8km; CAPE=644 J/kg; CIN= -30 J/kg

True potential for deep convection is

underestimated because the sounding isunrepresentative of the region of initiation.

It is necessary to measure the environment of

the roll updraft branches, since this is wherethunderstorms form.


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Sounding modified by aircraft data

Variability oftemperature is small, butmoisture variability is

large. Using maximum CBL

mixing ratio for parcelascent, LFC=1.2km;

CAPE=1665J/kg;CIN=0


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Sounding for No Storm Day

Original Sounding:LFC=2.3km while CBLdepth=.85km;

CAPE=966 J/kg; CIN=-44 J/kg

Modified Sounding:LFC=1.85km;

CAPE=1847 J/kg;CIN=-18 J/kg


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CBL Depth vs. LFC


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CBL Depth vs. LFC continued

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

However, this is not a good predictor of

convection. Using modified soundings, there is good

discrimination between storm days and no-

storm days. LFC-CBL depth for modified storm day

soundings is only .1km


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Some parameters which are useless

for predicting convection

Using sfc moisture variability to modify soundingsincorrectly suggests convection will occur on every day.

No difference between storm and no-storm days wasfound from surface mixing ratios, RH, temp., windspeed or direction, etc

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

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

similar between storm and no-storm days.


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Summary (yes, this is the end)

Most soundings do not sample the updraft branches ofrolls. Therefore, soundings by themselves areinsufficient for predicting the potential for deepconvection due to rolls alone.

Soundings modified by aircraft data are able to indicatethe true convective potential. Surface measurements are useless In the absence of synoptic forcing, CBL water vapor

variability must be measured with rather high spatialresolution (~500m) to accurately forecast the initiationof deep moist convection.

horizontal conveadasdastive rolls

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