Mesoscale Convective Systems

Definition Mesoscale convective systems (MCSs) refer to all organized convective systems larger than supercells Some classic convective system types include: squall lines, bow echoes, and mesoscale convective complexes (MCCs)

MCSs occur worldwide and year-round In addition to the severe weather produced by any given cell within the MCS, the systems can generate large areas of heavy rain and/or damaging winds Definition

Mid-latitude Squall Lines A squall line is any line of convective cells. It may be a few tens of km long or 1000 km long (>500 nm); there is no strict size definition

Mid-latitude Squall Lines Generally mod - strong vertical wind shear in lowest 2.5km perpendicular to the gust front. Can be self-sustaining. Most likely severe weather  wind gusts. Heavy rain is possible.

Initial Organization squall lines may either be triggered as a line, or organize into a line from a cluster of cells

Importance of shear For a given CAPE, the strength and longevity of a squall line increases with increasing depth and strength of the vertical wind shear For midlatitude environments we can classify Sfc. to 2-3 km AGL shear strengths as weak 18 m/s In general, the higher the LFC, the more low- level shear is required for a system’s cold pool to continue initiating convection

Which Shear Matters? It is the component of low-level vertical wind shear perpendicular to the line that is most critical for controlling squall line structure and evolution

Matching vorticity regions of opposite sense Interactions of Vorticity Regions

Non-matching vorticity regions of opposite sense Interactions of Vorticity Regions

Sources of horizontal vorticity Updraft2. Vertical Shear3. Cold Pool

Vorticity interaction #1: updraft tilt = + Matching updraft + shear

+= Strong updraft + weak shear Vorticity interaction #1: updraft tilt

+ = Weak updraft + strong shear Vorticity interaction #1: updraft tilt

Updraft Tilt

+ = Strong cold pool + weak shear Vorticity interaction #2: cold pool lift

Numerical Simulation Weak shear Numerical grid: Vertical Res: 700m Horizontal Res: 2000m Environment Shear: 5ms -1 per 2.5km (weak) Cold pool speed: 20ms -1 Lifting: Dominated by cold pool circulation

+ = Weak cold pool + strong shear Vorticity interaction #2: cold pool tilt

+ = Matching cold pool + shear LFC Vorticity interaction #2: cold pool lift

Cold pool

Squall line motion is controlled by the speed of the system cold pool

Squall Line Motion Segment of a long squall lineA short squall line

The characteristic squall line life cycle is to evolve from a narrow band of intense convective cells to a broader, weaker system over time Classic Evolution (with weak shear)

Classic Evolution (with strong shear) Stronger shear environments produce stronger long-lived lines composed of strong leading line convective cells and even bow echoes

The Rear-Inflow Jet (RIJ)

Non-matching vorticity regions of opposite sense Interactions of Vorticity Regions

The Rear-Inflow Jet (RIJ)

Squall Lines

Bow Echoes Bow echoes are forward bulges in a line. Severe downbursts are often associated with the forward edge of a bow echo.

Bow Echo Evolution

Reasons for Bow Echoes Intensity

Bow Echoes and Bookend Vortices

Mesoscale Convective Complexes long lived “blob” shaped, or round several convective cells under one cloud shield. MCCs usually form in regions with Deep moisture Good low level inflow Weak upper level flow MCCs typically peak overnight or early morning.

MCCs are sustained by a mid-level meso- scale low. At upper levels there will be anti- cyclonic outflow. The downdrafts and cooling below the MCC induce a meso-scale high pressure system. MCCs may be described as warm cored structures. Mesoscale Convective Complexes

The deep moisture and weak upper level flow, combined with a long life time means that heavy rain and flash flooding is a threat. The rainfall will be typified by intense falls within a longer period of light to moderate rainfall. Mesoscale Convective Complexes

MCS Summary MCS structure and evolution depend on the characteristics of the environment The strength and the degree of organization increases with shear The most significant unifying agent for boundary-layer-based MCSs is the surface cold pool Long lived  Coriolis effect plays a role

Conservation of angular momentum

The Rear- Inflow Jet (RIJ)

Supercell Locations in Squall Lines Supercells within lines tend to become bow echoes, but cells at the ends of squall lines can remain supercellular for long periods of time

Tropical Squall Lines Overall, squall lines in the tropics are structurally very similar to midlatitude squall lines. Notable differences include: Develop in lower shear, lower LFC environments Taller convective cells system cold pools are generally weaker less of a tendency toward asymmetric evolution AND Most tropical squall lines move from east to west rather than the west to east

Numerical Simulation Strong Shear Numerical grid: Vertical Res: 700m Horizontal Res: 2000m Environment Shear: 20ms -1 per 2.5km (strong) Cold pool speed: 20ms -1 Lifting: Deep – efficient lifting

Vorticity Interaction #3: MISC Updraft Updraft + Cold Pool Updraft + Shear Updraft + Cold Pool + Shear Bluestein Fig. 3.19

How much shear is needed to create maximum lift along the gust front?

h c u(h) u(0)

Cold Pool/Updraught Interaction with Vertical Wind Shear Buoyancy processes alone cannot explain how convective storms can become organised into long-lived systems – vertical wind shear is needed The interaction of horizontal vorticities from updraughts, cold pools and the vertical shear profile is a simple approach to understanding interactions between these phenomena

The lift created by the cold pool circulation alone may not be sufficient for a surface parcel to reach the LFC Weak low-level shear  new cells trigger due to environmental inhomogeneities Strong low level shear  new cells trigger downshear Deepest lift when horizontal vorticity from cold pool is nearly equal in magnitude and has opposite rotation to the horizontal vorticity from low-level wind shear. Cold-pool shear interactions are instrumental in multi-cell storms. Cold Pool/Updraught Interaction with Vertical Wind Shear References: H.B.Bluestein: Synoptic-Dynamic Meteorology in Midlatitudes (Volume II); Oxford University Press 1993