1. Mesoscale Convective Systems 1
Weather Systems – Fall 2017
Outline:
definitions and dynamics

2. Definition
Isolated T-storms are generally classified as one of three types: ordinary cells, multi-cell, or supercells
However, groups of storms often join into larger systems, generally referred to as mesoscale convective systems (MCSs)
From the AMS glossary of meteorology, an MCS produces a contiguous precipitation area greater than 100 km in horizontal scale in at least one direction

3. Definition There are many subclassifications of MCSs, including:
(Markowski and Richardson)

4. Why are they important?
In the Midwest, MCSs provide 30-70% of the warm season rainfall – crucial for agriculture
MCSs cause a number of weather hazards, including damaging hail, winds, lightning, and flash flooding
They contribute significantly to the hydrologic cycle and the general circulation

5. Mesoscale convective complexes
One type of MCS that has been studied in detail is the mesoscale convective complex (MCC)
MCCs are large MCSs with a circular cloud shield
Maddox (1980) first defined MCCs – the exact definition is somewhat arbitrary, but their characteristics when observed by IR satellite include:
Cold cloud shield > 100,000 km2 in area
Large interior cloud shield with temperature < -52°C
Ratio of minor axis to major axis ≥ 0.7 (i.e., nearly circular)

6. MCC examples

7. Global distribution of MCCs
Laing and Fritsch (1997)
MCCs are mostly continental
They tend to occur in the lee of elevated terrain
MCCs worldwide are predominantly nocturnal

8. Fritsch and Forbes (2001)

9. Temp (dashed), θe (thick black), height (thin black), wind vectors
MCCs in the US usually occur when a low-level jet transports warm, moist (high θe) air northward, which destabilizes the environment
The high-θe air glides upward over a baroclinic zone, which provides the necessary lifting
MCCs/MCSs typically occur under relatively weak synoptic forcing
Conditions leading to MCCs in other parts of the world are very similar to these (Laing and Fritsch 2000)
Laing and Fritsch (2000)

10. Cold pool dynamics
Many MCSs (especially squall lines) develop in the warm sector, without frontal lifting – what maintains these MCSs?
Evaporative cooling creates a pool of cold outflow at the surface
The wind shear affects how this cold outflow will spread out
A prominent (and controversial) theory that describes the interactions between cold pools and shear was developed by Rotunno, Klemp, and Weisman (1988), and is known as “RKW theory”

11. RKW theory
Consider a 2-D (x,z) framework, so that the x-axis is perpendicular to the squall line
Horizontal vorticity of this sense is negative:

12. Buoyancy and horizontal vorticity
First, start with the simple situation of a buoyant bubble:
The horizontal vorticity equation describes both the upward motion in the center and the downward motion on the sides of the updraft

13. Cold pool Now, consider a pool of cold air at the surface
It’s possible to describe all of these motions using the hydrostatic and momentum equations
But the vorticity equation nicely captures the flow using just one equation
Lifting occurs on the edges of the spreading cold pool, with return flow and sinking motion in the middle

14. MCS-like structure Now we add a warm pool aloft
There is convergence at midlevels, above the cold pool and below the warm pool

15. Add a sprinkle of shear…
Recall that vertical wind shear is associated with horizontal vorticity of its own +

16. RKW theory
RKW theory: there exists an optimal state where the (positive) horizontal vorticity from the shear exactly balances the (negative) horizontal vorticity from the cold pool
In this situation, the strength and longevity of the squall line will be maximized
Where c is the strength of the cold pool and Δu is the shear over the depth of the cold pool

18. No shear In the case with no shear, updraft is initially upright
When the cold pool forms, its vorticity dominates and the updraft tilts over

19. Strong low-level shear
With strong low-level shear, updraft initially leans downshear
When the cold pool forms, its vorticity balances the ambient vorticity and the system is upright (and can be long-lived)

20. Strong low-level shear
In time, as more downdrafts contribute to the strength of the cold pool, it becomes stronger and starts to overwhelm the shear – the line tilts back over the cold pool

21. Rear-inflow jet
At this mature stage, the latent heating owing to condensation leads to the warm-pool-above-cold-pool structure we saw before: there is low pressure and convergence at midlevels
This leads to the commonly observed rear-inflow jet

22. Rear-inflow jet
The magnitude of the midlevel warming affects the strength of the rear inflow

23. If the warming aloft is relatively weak, the RIJ descends and causes the line to tilt over further
This tends to weaken the system, but the descending RIJ can produce severe winds at the surface
If the warming aloft is strong, the RIJ remains elevated, the line remains upright, and the system strengthens/persists

24. Summary
The essence of RKW theory is that there exists an optimal state for long-lived squall lines—it is when the horizontal vorticity of the shear and the cold pool balance each other
It does not say that severe squall lines can only occur when conditions are optimal – they happen in a variety of conditions (and are not simply 2- dimensional)

25. Recent work
Weisman and Rotunno (2004) revisited RKW theory by considering a wider range of shears and came to generally the same conclusions
Bryan et al. (2006) used multiple models to confirm the general findings

26. Issues/problems/controversy
The theory neglects other sources of vorticity within the larger MCS, or external lifting mechanisms
Recent observational studies (Coniglio et al and others) suggest that the interaction between low-level shear and cold pool are not as important as the shear over a deeper layer
They also find that the c/Δu relationship is not very helpful in explaining observed squall-line structure
Coniglio et al. (2012) study on the 8 May 2009 “superderecho”:
“If cold pool–shear interactions were critical to producing such a strong system, then the extension of the line-normal shear above 3 km also appeared to be critical. It is suggested that RKW theory be applied with much caution, and that examining the shear above 3 km is important, if one wishes to explain the formation and maintenance of intense long-lived convective systems, particularly complex nocturnal systems like the one that occurred on 8 May 2009.”

27. Organization of Linear MCSs
The most common mode of organization is the “leading line, trailing stratiform” squall line, but other modes exist as well
Unique 3D flow fields for each; mid- and upper- level flow determine where stratiform precip is
Systems often evolve between categories
58%
Parker and Johnson looked at 88 MCS cases in , came up with these three types. Convective line + stratiform region
Each has its own three-dimensional flow fields, and surface pressure patterns, associated with it. Where the stratiform region is placed highly depends on what the mid and upper level environmental wind fields are doing.
We still don’t fully understand the flow fields associated with all of them, especially the leading stratiform. How is its inflow not contaminated?
These are not always static categories – an MCS might evolve from one type to another over its lifecycle
19%
19%
From Parker and Johnson (2000)

28. Johnson and Hamilton (1988)
Trailing Stratiform
TS MCSs are most common for several reasons:
Convective lines tend to become oriented perpendicular to the low-level shear
Storm-relative flow in a squall line is generally front-to-rear, so hydrometeors are advected rearward
Trailing stratiform is the most popular and also the most studied.
Surface pressure features for the squall line MCS include mesohigh, wake low, and pre-squall mesolow. Mesohigh beneath the convective line, wake low at the back edge of the stratiform rain area, pre-squall mesolow ahead of conv. line.
Because the systems moves so quickly forward, winds don’t have time to converge/diverge directly in/out of the wake low/mesohigh area. Instead, the axes are shifted behind them.
Johnson and Hamilton (1988)

29. Vertical structure Main features: front-to-rear and rear-to-front flow
Houze et al. (1989)
Main features: front-to-rear and rear-to-front flow
Convective downdrafts lead to mesohigh
Descending rear inflow and rear inflow notch lead to wake low
adiabatic warming with descent, offset by evaporation
feeds into cold pool
Main features are ascending front-to-rear flow and descending rear inflow. Front-to-rear flow initiates the convective updraft at the leading edge of the cold pool, then carries small raindrops/cloud drops backward to the stratiform region, which are the impetus for the stratiform rain there.
The mesohigh is formed through downdrafts of air that gets cooled by evaporation of rainfall, and melting/sublimation of ice/hail. Forms cold pool which expands along the surface. Cold air is denser and heavier – get high pressure.
The descending rear inflow warms adiabatically as it descends. At the back end of the stratiform region, there is no rain (or very little) to evaporte to offset this warming, so air parcels can get relatively warm.
It may not be able to punch all the way through the cold pool to the surface, but does decrease the depth of it. Short cold pool = relatively lower pressure at the back edge of the SP region.
Sometimes the rear inflow does get to descend all the way to the surface, at that point it has warmed so much you get a heat burst.
Also, at the back end of the stratiform region, you will often see a “notch” of weaker or no reflectivity where this rear inflow has evaporated some of the precipitation. Somewhat evident in the previous slide.
Some of the rear inflow descends all the way to the surface through the stratiform precip area, and so is evaporatively cooled all the way down – feeds directly into the strength of the cold pool. Also can cause severe winds.
discuss how RIJ causes mesolow, what causes mesohigh, etc.

30. Trailing Stratiform
Often, these MCSs transition from a “symmetric” to “asymmetric” structure
TS evolves from symmetric to asymmetric over time, can take several hours. In the asymmetric stages, the convective line to the south is stronger, and any new development happens there.
The stratiform region is to the north. (assuming propagation to the east, as is usual). Mesohigh and wake low go north with the stratiform rain.
Loehrer and Johnson (1995)

31. What causes this transition?
SOUTHERLY FLOW, TRANSPORTING HYDROMETEORS TO NORTHERN END OF LINE; CONTRIBUTES TO ASYMMETRY
Why the asymmetric evolution over time?
Coriolis: go back to previous figure. Front to rear flow, which carries the cloud drops for the stratiform region, is turned to the right over time. Rear to front flow, which feeds into the cold pool to strengthen it, and also the lifting, is turned to the right as well. Skamarock ran model simulations in which this was bourne out.
However, H&J 98 noted another fact, that often mean flow has large compenent from the south.

32. Trailing Stratiform Example

33. Leading Stratiform
Pettet and Johnson (2003)
flow here flipped from TS. Note mid- and upper-level winds are rear-to-front flow. This flow now feeds the convective updrafts. explanation for how convective line still works, since if was set up similar to TS flow would have inflow which had been fed through the stable cold pool first.
“Leading” inflow jet acts like rear inflow jet of TS fame.
These can be either “rear-fed”, which is mostly just a mirror image of a TS system, or “front-fed”
The image above is a cross-section of a rear-fed LS MCS

34. Front-fed LS And here’s a front-fed LS MCS
But shouldn’t inflow from the east, which passes through the evaporatively cooled air, be stable and cause the system to dissipate?
Parker and Johnson (2004)

35. Front-fed LS
But shouldn’t inflow from the east, which passes through the evaporatively cooled air, be stable and cause the system to dissipate?
Not if the evaporative cooling increases with height!
Parker and Johnson (2004)

36. Leading Stratiform Example
Storm et al. (2003)

37. Parallel stratiform
Parker (2007b)
flow here flipped from TS. Note mid- and upper-level winds are rear-to-front flow. This flow now feeds the convective updrafts. explanation for how convective line still works, since if was set up similar to TS flow would have inflow which had been fed through the stable cold pool first.
“Leading” inflow jet acts like rear inflow jet of TS fame.
In PS MCSs, there is both along-line and across-line vertical shear, which favors the advection of hydrometeors parallel to the line
Eventually, however, almost all PS systems embark on the “seemingly inexorable march toward TS structure” (Parker 2007b)

38. Parallel Stratiform Example
Parker (2007a)

39. Evolution among the archetypes
An MCS typically won’t stay in the same category for its full lifetime; it’s common for them to evolve from one to another (and another…)