Mesoscale Convective Systems 1 Weather Systems – Fall 2015 Outline: definitions and dynamics

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

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

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

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 km 2 in area −Large interior cloud shield with temperature < -52°C −Ratio of minor axis to major axis ≥ 0.7 (i.e., nearly circular)

MCC examples

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

Fritsch and Forbes (2001)

MCCs in the US usually occur when a low-level jet transports warm, moist (high θ e ) air northward, which destabilizes the environment The high-θ e 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) Temp (dashed), θ e (thick black), height (thin black), wind vectors

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”

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:

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

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

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

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

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

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

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)

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

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

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

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

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)

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

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.”

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 From Parker and Johnson (2000) 58% 19%

Trailing Stratiform Johnson and Hamilton (1988) 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

Vertical structure 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 Houze et al. (1989)

Trailing Stratiform Often, these MCSs transition from a “symmetric” to “asymmetric” structure Loehrer and Johnson (1995)

What causes this transition? Hilgendorf & Johnson (1998) SOUTHERLY FLOW, TRANSPORTING HYDROMETEORS TO NORTHERN END OF LINE; CONTRIBUTES TO ASYMMETRY

Trailing Stratiform Example

Leading Stratiform 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 Pettet and Johnson (2003)

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)

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)

Leading Stratiform Example Storm et al. (2003)

Parallel stratiform 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) Parker (2007b)

Parallel Stratiform Example Parker (2007a)

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…)