1. Multicells, Lines, and Mesoscale Convective Systems
METR 1004: Introduction to Meteorology Adapted from Materials by Dr. Frank Gallagher III and Dr. Kelvin Droegemeier School of Meteorology University of Oklahoma

3. Multicellular Thunderstorms
So far we have discussed the structure of air mass or single cell thunderstorms.
We can think of these types of storms as a single “cell” where each cell is:
Independent
Has a complete life cycle
Has a life cycle of 30 minutes to an hour
Is usually weak

4. Multicellular Thunderstorms
We know that many thunderstorms can persist of longer periods of time.
These storms are made up of many cells.
Each individual cell goes through a life cycle but the group persists.
These storms are called multicellular thunderstorms, or simply multicells
Multicellular storms consist of a series of evolving cells with each one, in turn, becoming the dominant cell in the group.

5. Multicellular Thunderstorms
With air mass storms, the outflow boundaries are usually too
weak to trigger additional convection. In multicell storms, the
outflow boundary does trigger new convection.
Cell #1
Mature

6. Multicellular Thunderstorms
With air mass storms, the outflow boundaries are usually too
weak to trigger additional convection. In multicell storms, the
outflow boundary does trigger new convection.
Cell #2
Cumulus
Cell #1
Mature

7. Multicellular Thunderstorms
After about 20 minutes or so, the second cell becomes the
dominant cell. Cell #1 is now dissipating, and a new cell (#3)
is starting.
Cell #2
Mature
Cell #1
Dissipating
Cell #3
Cumulus

8. Multicellular Thunderstorms
After about 20 minutes or so, the third cell becomes the
dominant cell. This process may continue as long as
atmospheric conditions are favorable for new convection.
Cell #2
Dissipating
Cell #3
Mature
Cell #4
Cumulus
Cell #1
Almost
Gone

9. Multicellular Thunderstorms
A cluster of short lived single cells.
Cold outflow from each cell combines to form a much larger and stronger gust front.
Convergence along the gust front tends to trigger new updraft development. This is the strongest in the direction of storm motion.
New cell growth often appear disorganized to the naked eye.

11. Structure of a Multicell Storm

13. Multicell Storm on Radar

14. Multicellular Thunderstorms
Conditions for development
Moderate to strong conditional instability
Once clouds form, there is a significant amount of buoyant energy to allow for rapid cloud growth
Low to moderate vertical wind shear
Little clockwise turning

16. Importance of Vertical Wind Shear
Single cell
Weak shear - storm is vertically stacked
Outflow boundary may “outrun” the motion of the storm cell
New storms that develop may be too far from the original to be a part of it
Multicell
Weak to moderate shear keeps gust front near the storm updraft – triggers new cells
New development forms adjacent to the older cells and connects with the old cell

17. Cell and Storm System Motion

18. Multicellular Thunderstorms
On the previous diagram, there are two arrows that show the “cell motion” and the “storm motion.”
Notice that they are different. Why?
New cells tend to form on the side of the storm where the warm, moist air at the surface is located.
In the central Plains, this is often on the south or southeast side. 1 1 2
Average Wind
Warm, Moist Surface
Air (inflow)

19. Cell and Storm System Motion

20. Cell and Storm System Motion

21. Cell and Storm System Motion

22. Multicellular Thunderstorms
Individual cells typically move with the mean (average) wind flow
The storm system moves differently – by discrete propagation
Multicell storms may last a long time. They constantly renew themselves with new cell growth.

23. Multicellular Thunderstorm Hazards
Heavy rain -- Flooding
Wind damage
Hail
Lightning
Tornadoes -- Usually weak
Multicell storms are notorious for heavy rain and hail

24. Types of Thunderstorms
Squall lines
Storms form in a line
Can last for several hours
Occur all year long
Can produce strong winds and hail
Rarely produce tornadoes, except at southern end

25. Mesoscale Convective Systems
Thunderstorm complexes that are larger in horizontal scale than multicellular or supercellular storms.
Examples
Squall Line Thunderstorms
Mesoscale Convective Complexes

26. Squall Line Thunderstorms
Lines of thunderstorms that tend to act as a single mesoscale system.
May be a continuous line of storms or the storms may be broken into segments.
We classify squall lines according to their development.

27. Squall Line Thunderstorms
© 1993 Oxford University Press -- From: Bluestein , Synoptic-Dynamic Meteorology in Midlatitudes

28. Radar Views of Squall Lines

29. Squall Line on Radar Indianapolis, Indiana 25 July 1986 -- 1714 CST
© 1993 Oxford University Press -- From: Bluestein , Synoptic-Dynamic Meteorology in Midlatitudes

30. Satellite View of a Squall Line

31. Structure of a Squall Line

32. Squall Line Passage

33. Squall Line Cross-section

34. Arcus Cloud Examples
© 1980 The Pennsylvania State University Press -- From: Ludlam , Clouds and Storms
Patrick AFB
Norman, OK May 1977
© 1993 Oxford University Press -- From: Bluestein , Synoptic-Dynamic Meteorology in Midlatitudes

35. Gustnado Produced by a Squall Line

36. Squall Line Cross-section
Precipitation falls into the dry mid-level inflow. The evaporational cooling aids in sustaining a strong downdraft.
Often there is a region of lighter precipitation that follows the passage of the squall line > stratiform precipitation.
Stratiform Precip.

37. Precipitation Types
Convective
(Leading
Edge of
Line

38. Precipitation Types
Stratiform
(Well
behind the
line)

39. Stratiform Precipitation
Requires rising motion behind the convective towers.
The cloud tops radiate energy to space, thereby cooling the top of the stratiform region.
Radiation from the ground is absorbed by the cloud bases of the stratiform region.
This combination acts to destabilize the atmosphere in the stratiform region.

40. Stratiform Precipitation
Updraft
Convective towers: m s-1
Stratiform region: m s-1
Much slower ascent in the stratiform region. The result is much lighter precipitation.

41. Mesoscale Pressure Centers
Just behind the gust front is an area of high pressure. This is called a “mesohigh.”

42. Mesohigh The mesohigh forms just behind the gust front.
A rapid pressure jump occurs.
Caused by the combined effects of:
Evaporation of precipitation
Melting of precipitation
These two effects cool the atmosphere to form a dense pool of cold air just behind the gust front.

43. Mesolow
Behind the stratiform precipitation, a weak low pressure region develops.
Forms probably because of the slowly sinking air behind the stratiform rain.
This sinking air warms slightly, creating the weak low pressure.

44. Conditions Necessary for Squall Line Development
Warm, moist air at low levels
Relatively cold air aloft
Mid-level dry air that can enhance the downdraft and the vigor of the line
Environmental cap
Prevents “widespread” convection
“Linear” forcing
Cold fronts, dry line, outflow maxima
Orography, valley wind currents

45. Conditions for Long-Lived Squall Lines
No Environmental Shear – Updraft is Vertical

46. Conditions for Long-Lived Squall Lines
Low-Level Shear Creates a Bias and a Tilt

47. Conditions for Long-Lived Squall Lines
No Environmental Shear – Updraft is Vertical
Add Surface Cold Pool – Updraft is Biased and Tilted

48. Conditions for Long-Lived Squall Lines
Low-Level Shear Creates a Bias and a Tilt
Low-Level Shear Counters the Cold Pool Bias – Optimal Condition

49. Conditions for Long-Lived Squall Lines

50. The Squall Line Bow Echo

51. Bow Echo on Radar

52. Visual Siting of a Bow Echo

53. Mesoscale Convective Complexes
Classification Information
Infrared cloud-top temperatures are colder than -32oC and cover an area larger than 100,000 km2
Interior cloud-top temperatures are colder than -52oC and cover an area larger than 50,000 km2 for 6 hours or more.
Eccentricity greater than 0.7 at the time of the maximum areal extent of the cloud top.

54. Mesoscale Convective Complexes
MCC

55. Mesoscale Convective Complexes
Often start as a line of thunderstorms that form along the Rocky Mountains.
Move off into the Plains and organize into a much larger system.
Often reach maximum strength at night.
Provide a significant portion of the summertime (warm season) rainfall to the Great Plains.

56. Mesoscale Convective Complexes
MCC’s may form structures that resemble smaller scale cyclones. If given sufficient time, they may rotate.
The figure on the left shows the
remnants of a MCC. The cyclonic
rotation is clearly evident on this
visible satellite image. Note that
the size of the vortex covers almost
all of Kansas and Oklahoma!

57. Mesoscale Convective Complexes
As with squall lines, MCCs have strong convective updrafts and regions of slowly ascending air that creates the stratiform precipitation.
The difference is that in the MCC the slowly ascending updrafts tend to surround a central core of convective updrafts.

58. Mesoscale Convective Complexes
MCCs are producers of
Heavy Rain
Flash Floods
Hail
Frequent Lightning
Possibly a tornado if the environmental shear is sufficient
MCC storms may last for 12 or more hours.
They act to restabilzed a conditionally unstable atmosphere.
They may set up surface boundaries that can influence weather a day later.