1. ORGANIZED MESOSCALE CONVECTIVE SYSTEMS

2. Mesoscale Convective Systems
(MCSs) refer to all organized convective systems larger than supercells
Some classic convective system types include:
Squall Lines
Bow Echoes
Derechoes
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

3. ORGANIZED MESOSCALE CONVECTIVE SYSTEMS
Large systems of organised convection : Mesoscale convective systems or MCSs
The largest and longest lasting are approximately circular systems known as mesoscale convective complexes or MCCs
Most common over continental mid-latitude areas in spring and summer.
Severe weather (heavy rain, hail, lightning, strong winds, tornadoes) is usually associated with such systems.
MCSs are often long- lived, with a lifetime of typically 6—12 hours, well beyond that of their individual convective cells.
MCS formation is often accompanied by synoptic-scale forcing and considerable thermodynamic instability.
Depending on the amount of instability, the vertical distribution of moisture and the characteristics of the vertical wind , it may take variety of appearances in Sat imagery.

4. Types of mesoscale convective systems
Organized MCSs may take on a variety of appearances in both satellite and radar imagery. Factors controlling their appearance and the types of weather that occur with them are:-
Instability
Vertical Distribution of Moisture
Charecteristics of the vertical wind profile
Depending on the vertical wind shear relative to a storm, its anvil may extend out ahead or trail behind it; this was discussed by Ludlam (1963).

5. Types of mesoscale convective systems
Since multicell systems are composed of numerous single-cell storm in different stages of development, they can have a lifetime of many hours
With sufficient instability and vertical wind shear, super- cell storms may form; these have a strong quasi-steady rotating updraught and may last for several hours.
Supercells generally propagate to the right of the mean tropospheric wind, and may split in two with a left- moving counterpart.

6. Squall Line Definition
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

7. Initial Organization Squall lines may either
be triggered as a line, or
organize into a line from
a cluster of cells
Mesoscale convective systems appear in many forms, ranging from a relatively disorganized mass of convective cells, to highly organized convective lines.
MCSs may evolve from an isolated cell or small group of cells, or may be triggered as a large convective system from the onset, such as a squall line along or ahead of a cold front or along a dryline.
One study of the modes of severe squall line formation in Oklahoma classified them into “broken line,” “back building,” “broken areal,” and “embedded areal” cases, with broken line and backbuilding cases being the most common.

8. Squall Line Motion Segment of a long squall line
A short squall line, < 55 nm long
Within squall lines composed of primarily ordinary cells, each cell will generally move with the 0-6 km mean wind and new cells triggering in the downshear direction of the low-level vertical wind shear vector, along the cold pool gust front. For all types of squall lines, line motion is a result of both the advection of individual cells within the line and discrete propagation due to the triggering of new cells.
Left:
For very long squall lines (greater than 200 km [110 nm] in length), individual cells may move at an angle to the line, but the net motion of the line usually stays perpendicular to its initial orientation, independent of the direction of the mean wind or mean wind shear vector.
Right:
For shorter lines (less than 100 km [55 nm] long), the systems tend to reorient themselves over time to be perpendicular to the direction of the mean low-level vertical wind shear vector. The lines then propagate in the direction of the shear vector as new cells are triggered more readily along the downshear gust front.

9. Differences Tropical vs Mid LatitudeSquall 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 due lower evoporation in the middle tropospheric level
Most tropical squall lines move from east to west rather than the west to east
Although tropical MCSs tend to move slower than their mid-latitude counterparts and develop in moister environments, they still move faster than nearby isolated cells
Differences worth mentioning:
First, because the tropopause is higher in the tropics the leading line convective storms are generally taller than their midlatitude counterparts.
Secondly, tropical convection develops in generally lower shear, lower LFC environments both of which affect the details of their lifecycle— specifically that tropical convection is easily triggered and systems tend to move more slowly than midlatitude MCSs.
Third, because the environments are of tropical origin there tends to be much less mid-level dry air to encourage evaporation and thus the downdrafts and system cold pools are generally weaker than those in midlatitude squall lines.
Fourth, owing to a reduced influence of the Coriolis force, low latitude squall lines display less of a tendency toward asymmetric evolution over time.
And lastly, the most obvious distinction between midlatitude and tropical squall lines is the system motion throughout their evolution. Most tropical squall lines move from east to west rather than the west to east motion commonly observed with midlatitude MCSs. This occurs because the vertical wind shear profile in the tropics is dominated by easterly trade winds rather than the westerlies that control storm motion in the midlatitudes.
It should be noted that although tropical MCSs tend to move slower than their midlatitude counterparts and develop in moister environments, they still move faster than nearby isolated cells and tend to develop in comparatively dry air (for the tropics).

10. Mobile Squall Lines
When the atmosphere is unstable and there is strong veering and an increase in wind speed with height in conjunction with strong eastward-moving troughs, well organized squall lines with strong convection may develop.
Prior to squall line formation, organized Cu development within a surface convergence zone is usually detectable in satellite imagery.
The convergence zone may be associated with dry lines, frontal zones, or areas of pre-frontal convergence

11. Signatures in Satellite & Radar
In satellite imagery, thunderstorms along squall lines often have:
elongated anvils that stream out ahead of the active core;
overshooting tops in VIS images, a characteristic of the core region
for very intense updraughts, a cold 'V notch' with down­stream warming in IR imagery
In radar imagery, such storms are often characterized by: intense echoes in reflectivity images;
hook echoes associated with mesocyclones and super­cells;
mesocyclone and tornado vortex signatures in Doppler velocity images

12. Examble : Supercells Supercells within lines tend to become bow
echoes, but cells at the
ends of squall lines can
remain supercellular for
long periods of time

13. Slow-moving squall lines
When the atmosphere is unstable and a deep slow-moving or stationary upper-level trough is present, moisture may extend through a deep layer; the trough may come into phase with sub-tropical systems, resulting in a deep stream of moisture into the squall line region
Squall lines forming under these conditions are slow-moving, although in certain situations individual cells may move fairly rapidly along the line.

14. Examble : Slow moving Squall line
Fig GOES images at 1201 UTC on 30 October 1991, showing a squall line AB associated with a tropical WV plume BC:
(a) WV (b) IR

15. Mobile squall lines
When the atmosphere is unstable and there is strong veering and an increase in wind speed with height in conjunction with strong eastward-moving troughs, well organized squall lines with strong convection may develop.
Prior to squall line formation, organized Cu development within a surface convergence zone is usually detectable in satellite imagery.
The convergence zone may be associated with dry lines, frontal zones, or areas of pre-frontal convergence

16. Dry Line A surface boundary between warm, moist air and hot, dry air.
Found in the western Great Plains in the United States
Also found in China, India, and over Central West Africa

17. Dry Line
After sunrise, the sun will warm the surface which will warm the air near the ground.
This air will mix with the air above the ground.
Since the air above the moist layer is dry (and is much larger than the moist layer), the mixed air will dry out.
The dry line boundary will progress toward the deeper moisture.
The dryline tends to “jump” in discrete steps

18. Typical Dryline
Temperatures in
degrees Celsius
©1993 Oxford University Press -- From: Bluestein, Synoptic-Dynamic
Meteorology in Midlatitudes, Volume II

19. Dry Line -- Southern Plains USA
Behind the Dry Line
Westerly winds (often strong)
Clear Skies
Warm Temperatures
Low Moisture
In Front of the Dry Line
S or SE winds
Hazy or Cloudy Skies
Warm Temperatures
High Moisture

20. Example
Fig Storms over Kansas and Oklahoma on 26Apr (a) GOES 1 km resolution VIS image at 1146 local time (1746 UTC). (b) 500 hPa heights (in gdm) at 0600 local time. (c) Surface pressure (in hPa) and temperature (in °C) at 120(1 local time.

21. Example
(e) (e) Wind profile at 1536 local time near point P in (a), showing vertical shear in the direction of storm motion (2350 at this time) relative to storm speed of 35 kn.
(d) Cloud drift winds at 1246 local time (from 5-minute interval, 1 km resolution, GOES images) relative to storm motion of 215° 35 kn, showing stron shear from Cu-level (cyan) to Ci-level (yellow).

22. Explanation
Fig. (a) is a typical example of how a cloud field associated with a dry line appears prior to the formation of an organized squall line.
On this day there was an organized surface convergence zone at the dry line (Fig. (c)) and strong upper-level support for convection (Fig. (b)).
The strong vertical shear and increase in south westerly wind with height is detectable using both surface-based wind profiler data (Fig. (e)) and satellite cloud drift winds (Fig. (d)).
There was a transition from clear skies to Cu across the dry line.
Cu cloudiness was enhanced along the dry line convergence zone.
A well defined convective line was detectable along a surface convergence zone a few hours prior to squall line development.

23. Bow Echoes

24. Bow Echo Definition LEWP
One of the more significant and intriguing forms of mesoscale convective organization is the bow echo.
They are observed both as relatively isolated convective systems and as sub-structures within much larger convective systems. Bow echoes that develop within a squall line are often referred to as Line Echo Wave Patterns (LEWPs).
Bow echoes are relatively small ( km [10-65 nm] long), bow-shaped systems of convective cells noted for producing long swaths of damaging surface winds.
LEWP
One of the more significant and intriguing forms of mesoscale convective organization is the bow echo.
They are observed both as relatively isolated convective systems and as sub-structures within much larger convective systems. Bow echoes that develop within a squall line are often referred to as line echo wave patterns (LEWPs).

25. Bow Echo Evolution
Fujita was among the first to document bow echo evolution.
Cyclonic and anticyclonic line-end (or bookend) vortices are evident behind the northern and southern ends of the bow, respectively, in the early phases. This symmetric structure becomes more asymmetric during the comma echo phase when the cyclonic vortex begins to be dominant.
This conceptual model shows a weak echo area behind the core of the bow, referred to as a “rear-inflow notch” (RIN). This notch usually signifies the location of a strong rear-inflow jet.
Fujita was among the first to document bow echo evolution.
Cyclonic and anticyclonic line-end (or bookend) vortices are evident behind the northern and southern ends of the bow, respectively, in the early phases. This symmetric structure becomes more asymmetric during the comma echo phase when the cyclonic vortex begins to be dominant.
This conceptual model shows a weak echo area behind the core of the bow, referred to as a “rear-inflow notch” (RIN). This notch usually signifies the location of a strong rear-inflow jet.
Just like a shorter squall line, if a bow echo persists for more than 3 to 4 hours, the Coriolis forcing intensifies the northern
vortex at the expense of the southern vortex creating the often observed asymmetric evolution.

26. DEFINITION : DERECHOES
If the cumulative impact of the severe wind from one or more bow echoes covers a wide enough and long enough path, the event is referred to as a derecho.
To be classified as a derecho, a single convective system must produce wind damage or gusts greater than 26 m/s (50 kts) within a concentrated area with a major axis length of at least 400 km (250 nm).
The severe wind reports must exhibit a chronological progression and there must be at least 3 reports of F1 damage and/or convective wind gusts of 33 m/s (65 kt) or greater separated by at least 64 km (40 nm). Additionally, no more than 3 hours can elapse between successive wind damage or gust events.
Deh-RAY-cho is a Spanish word, which can mean “straight ahead."

27. MCCs

28. MCC : IR SATELLITE IMAGERY
To be a true MCC, the system must have a general cloud shield with continuously low IR temperatures less than -32°C over an area > = 100,000 km2, with an interior cold cloud region with temperatures less than -52°C having an area >= 50,000 km2
MCCs often last for 6-12 hours, and are especially known for producing heavy rain over a large area, although severe winds, hail, and tornadoes can also occur during the early phases of MCC evolution.
MCCs are most often observed at night, in areas in which the boundary layer is stable. The source of energy for such systems is often found in an elevated layer above the boundary layer, north of a weak surface warm front.
Case studies have shown that MCC initiation is usually associated with a weak large scale frontal zone and the eastward progression of weak short waves in the middle troposphere.
MCCs tend to occur on the anticyclonic side of a broad weak westerly jet stream.

29. MCC
MCCs often last for 6-12 hours, and are especially known for producing heavy rain over a large area, although severe winds, hail, and tornadoes can also occur during the early phases of MCC evolution.
MCCs are most often observed at night, in areas in which the boundary layer is stable. The source of energy for such systems is often found in an elevated layer above the boundary layer, north of a weak surface warm front.
Case studies have shown that MCC initiation is usually associated with a weak large scale frontal zone and the eastward progression of weak short waves in the middle troposphere.
MCCs tend to occur on the anticyclonic side of a broad weak westerly jet stream.

30. MCC Evolution
During the early phases of evolution, the convective structures that make up an MCC may include multiple squall lines, bow echoes, or isolated convective cells, each evolving through its own lifecycle, with each system contributing to the expanding MCC anvil as depicted on a satellite image.
During the later stages of evolution, however, a large stratiform precipitation region dominates the MCC, as it does in the later stages of squall line evolution.

31. Examples of MCS
Fig.1. Meteosat IR image for 2100 UTC on 21 August 1987, showing an MCS at A with a steep cloud-top temperature gradient at B. Cloud-top temperatures (in °c) shown are: white < -40, red -40 to -30, pink -30 to -20, purple -20 to -10, light blue -10 to 0, green ° to + 10, yellow + 10 to +20, black> +20.

32. Characteristic Cloud Features of Organized Convective Systems
There is a conspicuous shield of cold cloud tops A: a Ci canopy formed by the merging of anvils from an ensemble of convective cells.
Some parts of the cloud shield have sharp edges, with steep cloud-top temperature gradients (e.g. at B); the portion of the system with active convection is usually closely related to these edges.
The circular cloud shields have typical diameters of km, while the long axis of elliptical shields can be up to 1000 km in length.

33. Features in the pre-storm environment
Moisture information in IR imagery
Atmospheric moisture structure, characterized by both horizontal and vertical mesoscale variations, is an important factor in storm development
Moist plumes often serve as the 'fuel' for MCSs. With proper enhancement tables standard 11.2 µm (A VHRR channel-4) IR imagery may be used to help delineate low-level moist (and dry) regions. Energy received by a satellite at 11.2 µm is not from a 'clean' window: water vapour absorption has an effect on what is observed.
Radiances received by the 11.2 µm and 12.7 µm IR channels on GOES, which approximately correspond to A VHRR channels 4 and 5, receive different amounts of water vapour contamination.

34. Features in the pre-storm environment
Using the difference in energy received from these channels, an image product called split window (Chesters, Uccellini and Mostek, 1982) may be derived to depict low-level water vapour.
Split window images are useful over land during the afternoon when significant surface heating has occurred and there is a large difference in the signal between the two channels.
However, when the land surface temperature and that of the overlying moist boundary layer air are nearly the same, the signal difference is small and a meaningful product is difficult to derive.

35. Features in the pre-storm environment
Clouds as a clue to stability
Satellite imagery may also be used to locate regions of stable boundary layer air.
For example, the clear skies behind an arc cloud line,, show where air has been cooled and stabilized by subsidence and the evaporation of rain. Low­level, fog and St are often associated with stable air poleward of warm fronts or in regions that have been stabilized by heavy night-time thunderstorms.
Whereas fog and St reflect a cool near-saturated low-level air-mass, wave cloudiness is found near the top of the stable layer and reflects the vertical shear across the layer.
Locating these features is important in analysing severe weather potential across a region.

36. Features in the pre-storm environment
Water vapour imagery
Distinct patterns of moist and cooler areas (light tones) and warmer and drier areas (dark tones) are readily detected.
These features are related to areas of both synoptic and mesoscale advection and vertical motion.
Strong baroclinic regions such as jet streams and vorticity maxima can often be easily identified in cloud-free regions by the sharp moisture gradient detected in WV images
Depending on the synoptic situation, WV images may be used to help locate likely areas of MCS development. Once the forecaster knows the type of situation, WV imagery may be used to
identify plumes of mid-level moist air, or
identify zones of dry air associated with jet streaks.

37. Features in the pre-storm environment
Fig.2. (a) GOES WV image at 1100 UTC on 23 July 1987 showing a moist plume.
Fig.2. (b) GOES enhanced IR image at 0000 UTC on 24 July 1987 showing an MCS (at M) that developed in the plume. (c) Favoured area for MCS development in relation to a WV plume

38. Features in the pre-storm environment
Fig.2. (c) Favoured area for MCS development in relation to a WV plume
KEY
Plume in WV imagery
Low-level e or w ridge
Upper-level lifting mechanisms Deep layer of moisture

39. Examples
Fig.3. (a) Fig Meteosat WV images for (a) 0000, (b) 0300, and (c) 0600 UTC on 27 June 1986, showing a dry zone DD and a developing MCS at X; the jet position (large arrow) is shown on (a).
Fig.3. (b) Moisture levels shown are, from highest to lowest: white, yellow, green, light blue, purple, black.

40. Example
Fig.3. (d) w at 900 hPa (solid lines) and potential instability portrayed as  w (900 hPa) -  w (600 hPa) (dotted lines), in °C, for 0000 UTC on 27 June 1986; differences> 2°C are hatched.

41. Theta-e
Theta-e is used operationally to map out which regions have the most unstable and thus positively buoyant air. The Theta-E of an air parcel increases with increasing temperature and increasing moisture content. Therefore, in a region with adequate instability, areas of relatively high Theta-e (called Theta-e ridges) are often the burst points for thermodynamically induced thunderstorms and MCS's. Theta-e ridges can often be found in those areas experiencing the greatest warm air advection and moisture advection.

42. Example: Explanation
the leading edge of a dry zone DD was associated with a cyclonically curved jet (Fig. 3(a));
the dry zone started to overrun a tongue of relatively high  w air at low levels, giving high potential instability (hatched on Fig. 3(d));
potential instability was released, and an area of mid-level cloud and moisture appeared at X on the leading edge of the dry zone (Fig. 3(b));
this area developed quickly into the distinctive shield of mid-level cloud and moisture associated with an MCS (Fig.3(c))

43. Mesoscale Convective Complexes (MCC)
MCCs often last for 6-12 hours, and are especially known for producing heavy rain over a large area, although severe winds, hail, and tornadoes can also occur during the early phases of MCC evolution.
MCCs are most often observed at night, in areas in which the boundary layer is stable. The source of energy for such systems is often found in an elevated layer above the boundary layer, north of a weak surface warm front.
Case studies have shown that MCC initiation is usually associated with a weak large scale frontal zone and the eastward progression of weak short waves in the middle troposphere.
MCCs tend to occur on the anticyclonic side of a broad weak westerly jet stream.

44. Mesoscale Convective Complexes (MCC)
MCCs represent a larger form of mesoscale convective system (MCS) organization.
A system is identified as an MCC based on its characteristics as depicted on IR satellite imagery.
The physical characteristics include a general cloud shield with continuously low IR temperatures less than -32° C over an area >=100,000 km2, with an interior cold cloud region with temperatures less than -52° C having an area >=50,000 km2.
MCCs often last for 6-12 h, and are especially known for producing heavy amounts of rain, although severe winds, hail, and tornadoes can also occur during the early phases of MCC evolution.
Because of their greater size and generally longer duration than isolated convective storms, as well as their propensity to occur year round and worldwide (much so more than supercells) MCSs are a greater threat to military operations than severe isolated storms

45. MCC Evolution
The flow field in the later stages of an MCC is characterized by divergent, anticyclonic outflow near the surface and aloft within the anvil, with convergent cyclonic flow at mid-levels.
Like the northern line-end vortices of squall lines that sometimes grow quite large, this mid-level cyclonic flow is often referred to as a mesoscale convective vortex (MCV).
MCCs are most often observed at night, in areas in which the boundary layer is stable. The source of energy for such systems is often found in an elevated layer above the boundary layer, north of a weak surface warm front.
Observational studies suggest that MCC structure and evolution is more dependent on interactions with large-scale forcing features than the boundary-layer-based mesoscale convective systems (MCSs) such as squall lines and bow echoes.

46. Summary
MCS structure and evolution depend on the characteristics of the environmental buoyancy and shear, as well as the details of the initial forcing mechanism.
The strength and the degree of organization of most MCSs increases with increasing environmental vertical wind shear values.
The most significant unifying agent for boundary-layer-based MCSs is the surface cold pool.
MCS evolution is heavily controlled by the interaction between the cold pool and the low-level vertical wind shear.
Since MCSs usually last for > 3 hrs, the Coriolis effect significantly impacts system evolution.
Because of their greater size and generally longer duration than isolated convective storms, as well as their propensity to occur year round and worldwide (much so more than supercells) MCSs are a greater threat to military operations than severe isolated storms.
Because of their greater size and generally longer duration than isolated convective storms, as well as their propensity to occur year round and worldwide (much so more than supercells) MCSs are a greater threat to military operations than severe isolated storms.

47. MCC