7/30/2019 Horizontal Conveadasdastive Rolls
1/38
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
7/30/2019 Horizontal Conveadasdastive Rolls
2/38
What are horizontal convective rolls?
Counter-rotatinghorizontal vortices whichcommonly occur within
the convective boundarylayer.
AMS Glossary of Meteorology
7/30/2019 Horizontal Conveadasdastive Rolls
3/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
4/38
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)
7/30/2019 Horizontal Conveadasdastive Rolls
5/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
6/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
7/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
8/38
Dynamic Instability
Inflection PointInstability
There must be an
inflection point in thecross-roll component ofthe mean large scale
wind profile.
Faller, JAS 1965
7/30/2019 Horizontal Conveadasdastive Rolls
9/38
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
7/30/2019 Horizontal Conveadasdastive Rolls
10/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
11/38
7/30/2019 Horizontal Conveadasdastive Rolls
12/38
7/30/2019 Horizontal Conveadasdastive Rolls
13/38
7/30/2019 Horizontal Conveadasdastive Rolls
14/38
7/30/2019 Horizontal Conveadasdastive Rolls
15/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
16/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
17/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
18/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
19/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
20/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
21/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
22/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
23/38
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
7/30/2019 Horizontal Conveadasdastive Rolls
24/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
25/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
26/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
27/38
7/30/2019 Horizontal Conveadasdastive Rolls
28/38
Storm Day
7/30/2019 Horizontal Conveadasdastive Rolls
29/38
7/30/2019 Horizontal Conveadasdastive Rolls
30/38
No Storm Day
7/30/2019 Horizontal Conveadasdastive Rolls
31/38
7/30/2019 Horizontal Conveadasdastive Rolls
32/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
33/38
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
7/30/2019 Horizontal Conveadasdastive Rolls
34/38
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
7/30/2019 Horizontal Conveadasdastive Rolls
35/38
CBL Depth vs. LFC
7/30/2019 Horizontal Conveadasdastive Rolls
36/38
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
7/30/2019 Horizontal Conveadasdastive Rolls
37/38
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.
7/30/2019 Horizontal Conveadasdastive Rolls
38/38
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.