Horizontal Convective Rolls

1. Horizontal Convective Rolls MPO 551 Paper Presentation Dan Stern Horizontal 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

2. What are horizontal convective rolls? • Counter-rotating horizontal vortices which commonly occur within the convective boundary layer. AMS Glossary of Meteorology

3. Results of Previous Studies • Both sfc. layer heat flux and vertical wind shear are necessary conditions for roll occurrence. • Roll wavelength is proportional to the depth of the boundary layer. • Roll orientation is along mean CBL wind and/or shear directions.

4. Problems with Previous Studies • Lack of consistent and objective means of defining and classifying rolls. • Few observational platforms for sampling of rolls and of surrounding environment. • Small sample size of roll cases. • Lack of comparison with null cases (non-roll convection, no convection)

5. Motivation and Objectives • Determine environmental parameters which favor roll formation and define their wavelengths and orientation. • Objectively define rolls from radar reflectivity. • Further examine results using 3-D numerical model.

6. Theories for roll formation • Thermal Instability: Energy is obtained from buoyancy, with bands organized so as to minimize shear. • Dynamic Instability: Energy is extracted from the kinetic energy of wind normal to roll axes.

7. Thermal Instability • Past studies have shown that a modest sfc heat flux is necessary. • Rolls are most commonly observed in slightly unstable environments. • But as thermal instability increases, 2D convection becomes less likely, and 3D is preferred.

8. Dynamic Instability • Inflection Point Instability • There must be an inflection point in the cross-roll component of the mean large scale wind profile. Faller, JAS 1965

9. Combination of Instabilities • Monin-Obukhov length: • |L| is approximately the height at which buoyancy dominates over shear in turbulence production. • Convective instability decreases as L increases. • Studies have shown rolls to exist within a specified range of L

10. Objective classification of convective modes • Reflectivity within 15X15km box was interpolated onto a cartesian grid • Spatial Autocorrelation field was calculated and plotted (pattern recognition) • Ratio of major to minor axis of .2 correlation coefficient contour defines the convective mode. • Horizontal Aspect Ratio (HAR) >6 for rolls.

15. Measurement of CBL Characteristics • Winds retrieved from VAD radar routine with highest elevation angle used. • CBL depth determined from the well-mixed potential temperature layer from soundings, when available. Otherwise, the height at which a change in slope of reflectivity occurs is defined as the top of the CBL.

16. Effect of Sensible Heat Flux • No convection cases are less unstable • Cellular cases occur in narrow range of heat flux • Rolls occur in broader range, still limited. • Unorganized convection has broadest range.

17. Model results of varied heat flux • No minimum threshold of heat flux for rolls • Beyond a certain point, increased heating causes convection to become less organized. • Maximum implied by model results.

18. Effect of Wind Shear • All cellular cases occur with shear less than 2x10-3 s-1 • All rolls occur with shear greater than 2x10-3 s-1 • Shear was typically low throughout experiment.

19. Effect of Wind Speed • All rolls occur with mean CBL wind speed greater than 5.5m/s • All rolls occur with 10m wind speeds greater than 3m/s • Cellular convection occurs only at lower speeds while unorganized convection varies over a broad range.

20. Model Results of Varied Wind Speed • Simulation with low wind speed (2m/s) produced unorganized convection. • Higher wind speeds (5m/s, 10m/s) produced linear convection. • This supports the observations that there is a minimum threshold of wind speed for rolls.

21. Sensible Heat Flux vs. Wind Shear • Rolls only occur within a specific range of heat flux and above a threshold value of shear. • Shear magnitude separates cellular from roll convection.

22. Forcing Mechanisms of Rolls • TKE Budget: • Buoyancy dominates for unorganized convection at all levels. • For rolls, buoyancy dominates in the upper boundary layer, but the forcing from shear is comparable to buoyancy at low levels.

23. Roll Wavelength vs. CBL Depth • Wavelength is well correlated with CBL depth (r=.84), in agreement with theory and prior observations. • Wavelength increases with increasing depth. • Average aspect ratio is 5.7

24. Influences on aspect ratio • Previous studies had suggested that aspect ratio is related to CBL wind shear and/or wind speed. This study found them to be uncorrelated however. • Aspect ratio is found to be well correlated with convective instability. • Aspect ratio increases with increasing convective instability.

25. Roll Orientation • Orientation is highly correlated with CBL wind shear direction, mean CBL wind direction, and 10m wind direction. • This is because these variables were all highly correlated with each other in the experiment (very little directional wind shear). • Therefore, it was not possible to determine which variable is most relevant.

26. Summary (this is not yet the end) • Rolls were objectively classified, and characteristics of rolls and their environments were determined from both observations and modeling. • Minimum wind speed and shear criterion, although required shear is quite low and directional shear is unnecessary for roll formation. • Low-level shear is important, but could not be well measured due to limitations of experiment. • There is a preferred roll regime constrained by heat flux and wind. • Wavelength proportional to CBL depth and orientation correlated with wind direction.

27. Why Rolls are Relevant • Rolls are boundary layer convergence zones. • Low-level convergence may lead to thunderstorm initiation. • Intersection of rolls with other boundaries often lead to convective development. • Rolls themselves may initiate thunderstorms, even in the absence of other forcing. • Initiation of deep convection by rolls alone only occurs a fraction of the time. Why?

28. Storm Day

30. No Storm Day

32. Inability of soundings to predict convective potential. • Storm Case: LFC at 2.3km while CBL depth is only .8km; CAPE=644 J/kg; CIN= -30 J/kg • True potential for deep convection is underestimated because the sounding is unrepresentative of the region of initiation. • It is necessary to measure the environment of the roll updraft branches, since this is where thunderstorms form.

33. Sounding modified by aircraft data • Variability of temperature is small, but moisture variability is large. • Using maximum CBL mixing ratio for parcel ascent, LFC=1.2km; CAPE=1665J/kg; CIN=0

34. Sounding for No Storm Day • Original Sounding: LFC=2.3km while CBL depth=.85km; CAPE=966 J/kg; CIN=-44 J/kg • Modified Sounding: LFC=1.85km; CAPE=1847 J/kg; CIN=-18 J/kg

35. CBL Depth vs. LFC

36. CBL Depth vs. LFC continued • Difference between CBL depth and LFC is smaller 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

37. Some parameters which are useless for predicting convection • Using sfc moisture variability to modify soundings incorrectly suggests convection will occur on every day. • No difference between storm and no-storm days was found from surface mixing ratios, RH, temp., wind speed or direction, etc… • Wind shear was always very small, and there was no difference 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.

38. Summary (yes, this is the end) • Most soundings do not sample the updraft branches of rolls. Therefore, soundings by themselves are insufficient for predicting the potential for deep convection due to rolls alone. • Soundings modified by aircraft data are able to indicate the true convective potential. • Surface measurements are useless • In the absence of synoptic forcing, CBL water vapor variability must be measured with rather high spatial resolution (~500m) to accurately forecast the initiation of deep moist convection.