1. Heavy precipitation at a location = intensity x longevity

2. Common sources of heavy precipitation in U.S.
Mesoscale convective systems and vortices
Orographically induced, trapped or influenced storms
Landfalling tropical cyclones

3. MCSs & precipitation facts
Common types: squall-lines and supercells
Large % of warm season rainfall in U.S. and flash floods (Maddox et al. 1979; Doswell et al. 1996)
Initiation & motion often not well forecasted by operational models (Davis et al. 2003; Bukovsky et al. 2006)
Boundary layer, surface and convective schemes “Achilles’ heels” of regional-scale models
Improved convective parameterizations help simulating accurate propagation (Anderson et al. 2007; Bukovsky et al. 2006)
Supercells often produce intense but not heavy rainfall
Form in highly sheared environments
Tend to move quickly, not stay in one place

4. U.S. flash flood seasonality
Contribution of
warm season MCSs
clearly seen
Number of events
Maddox et al. (1979)

5. Linear MCS archetypes (e.g., squall-lines)
58%
19%
Note in passing
Parker and Johnson (2000) 5

6. Storm motion matters
Doswell et al. (1996)

7. Forecasting MCS motion
(or lack of motion…)

8. North Plains

9. Some common “rules of thumb” ingredients
CAPE (Convective Available Potential Energy)
CIN (Convective Inhibition)
Precipitable water
Vertical shear - magnitude and direction
Low-level jet
Midlevel cyclonic circulations

10. Some common “rules of thumb”
MCSs tend to propagate towards the most unstable air
mb layer mean RH ≥ 70%
MCSs tend to propagate parallel to mb thickness contours
MCSs favored where thickness contours diverge
MCSs “back-build” towards higher CIN
Development favored downshear of midlevel cyclonic circulations
“nothing succeeds like success”

11. 70% RH rule of thumb Implication: Relative humidity more skillful than
70% layer RH
70% RH rule
of thumb
Implication:
Relative humidity
more skillful than
absolute humidity
RH > 70%
# = precip. category
Junker et al. (1999)

12. MCSs tend to follow thickness contours
Implication: vertical shear determines
MCS orientation and motion.
Thickness divergence likely implies
rising motion

13. Back-building towards higher CIN
Lifting takes longer where there
is more resistance

14. Corfidi vector method Propagation is vector difference P = S - C
Therefore, S = C + P
to propagate = to cause to continue, to pass through (space)

15. Schematic example
System motion as shown

16. Schematic example We wish to forecast system motion
So we need to understand what controls
cell motion and propagation

17. Individual cell motion
“Go with the flow”
Agrees with previous observations (e.g, Fankhauser 1964) and theory (classic studies of Kuo and Asai)
Cells tend to move at
mb layer
wind speed*
*Layer wind weighted towards lower troposphere,
using winds determined around MCS genesis.
Later some slight deviation to the right often appears
Corfidi et al. (1996)

18. Individual cell motion
Cells tend to move at
mb layer
wind speed
Cell direction comparable
To mb layer
wind direction
Corfidi et al. (1996)

19. Composite severe MCS hodograph
Low-level jets (LLJs)
are common
Note
P ~ -LLJ
Bluestein and Jain (1985)

20. Propagation vector and LLJ
• Many storm environments
have a low-level jet (LLJ) or
wind maximum
• Propagation vector often
anti-parallel to LLJ
Propagation vector direction
P ~ -LLJ
Corfidi et al. (1996)

21. Forecasting system motion using antecedent information
Cell motion ~ mb wind
Propagation ~ equal/opposite to LLJ
S = C - LLJ

22. Evaluation of Corfidi method
Method skillful in predicting
system speed and direction
Corfidi et al. (1996)

23. Limitations to Corfidi method
Wind estimates need frequent updating
Influence of topography on storm initiation, motion ignored
Some storms deviate significantly from predicted direction (e.g., bow echoes)
P ~ -LLJ does not directly capture reason systems organize (shear) or move (cold pools)
Beware of boundaries!
Corfidi (2003) modified vector method

24. Composite severe MCS hodograph
Low-level shear
influences
storm organization
& motion
Angle between
lower & upper shear
also important
(Robe and Emanuel 2001)
mb shear of geostrophic wind is close to parallel to system motion
These are severe squall lines, so weak lines already excluded. Lines stronger when shear above 500 mb does not interfere
Bluestein and Jain (1985)

25. Low-level shear

26. Potential vorticity Simplest form (see Holton. Ch. 4):
absolute vorticity/depth is conserved for dry adiabatic processes.
Equivalent to angular momentum conservation;
stretching increases vorticity.
This is a special case of Rossby-Ertel PV

27. Rossby-Ertel potential vorticity q
incorporating:
3D vorticity vector, potential temperature gradient
and Coriolis expressed as a vector (function of z only)
In this formulation,
mass x q is conserved between two isentropes
even (especially!) if diabatic processes are
changing the potential temperature
Haynes and McIntyre (1987)

28. Rossby-Ertel potential vorticity q
Here, we simplify a little bit
and focus only on the vertical direction.
The conserved quantity is mq. Holton’s version is
derivable from Rossby-Ertel’s equation, where A is
horizontal area. (Keep in mind ∆ is fixed between
two isentropes.)

29. Rossby-Ertel PV For a dry adiabatic process, the mass between
two isentropes cannot change. Thus, the only
way to increase the cyclonic vorticity  is to
move the object equatorward (decreasing f)
OR decrease its horizontal area A.
Now, consider a more relevant example…

30. Start with a stably stratified environment,
with no initial horizontal variation.
Define two layers, bounded by these three
isentropes.
We are dealing with horizontal layers.
Horizontal area A is not relevant.

31. m1 and m2 are the initial masses residing in these two layers.
q1 and q2 are the initial PVs.
mq can be transported horizontally but not vertically.
So m1q1 and m2q2 will not change.

32. Introduce a diabatic heat source, representing convection.
The potential temperature in the heated region
increases. This effectively moves the isentrope 2
downward.

33. Now there is less mass in the lower isentropic layer,
and more mass in the upper layer.
Because mq is conserved between any two
isentropes, q has increased in the lower layer
because m has decreased there.
q has NOT been advected vertically.

34. The increased q in the lower layer represents
a positive PV anomaly (+PV). Because q
has increased,  is enhanced and a cyclonic
circulation is induced.
In the upper layer, decreased q means -PV
and an induced anticyclonic circulation.

35. MCVs as PV anomalies Combination: uplift & destabilization on
Here they work together
Combination: uplift & destabilization on
windward side AND downshear side
Raymond and Jiang (1990)

36. Composite analysis of MCV heavy rain events
• Based on 6 cases
poorly forecasted by models
• Composite at time of
heaviest rain (t = 0h)
• Heaviest rain in early
morning
• Heaviest rain south of
MCV in 600 mb trough
600 mb vorticity (color), heights and winds.
Map for scale only
Schumacher and Johnson (2008)

37. Schumacher’s situation
Tends to result in very slow-moving,
back-building convection south of MCV

38. Back-building Ground-relative system speed ~ 0
Schumacher and Johnson (2005)
Doswell et al. (1996)

39. Evolution of the heavy rain event
At t + 6h (morning):
rain decreases as LLJ weakens
600 mb vorticity, 900 mb winds & isotachs
Schumacher and Johnson (2008)

40. South Plains LLJ Enhanced southerly flow over South Plains
Most pronounced at night
Responsible for moisture advection from Gulf & likely a major player in nocturnal thunderstorms and severe weather

41. Bonner (1968)
- LLJ occurences
meeting certain
criteria
most frequent in
Oklahoma
- most frequent at
night

42. Explanations for LLJ
Oscillation of boundary layer friction (mixing) responding to diurnal heating variation
Vertical shear responding to diurnally varying west-east temperature gradients owing to sloped topography
Cold air drainage down the Rockies at night
Topographic blocking of some form

43. Bonner (1968) observations of wind speed vs
Bonner (1968) observations of wind speed vs. height for days in which nocturnal LLJ appeared at Ft. Worth, TX
wind speed max just below 1 km MSL (about 800 m AGL) at midnight and 6AM local time
note increased low-level shear

44. Bonner (1968) observations
of Ft. Worth wind at height
of wind max.
• wind weaker, more
southerly during afternoon
• nighttime wind stronger,
more from southwest,
elevation lower

45. Episodes of MCSs & predictability Hovmoller diagrams reveal westward-
propagating MCSs
Note “envelope” of
several systems
with “connections”
Carbone et al. (2002)

46. MCV role in predictability
Carbone et al. (2002)

47. “Training lines” of cells
• In Asia, stationary front
could be the Mei-Yu
(China), Baiu (Japan) or
Changma (Korea) front
• Motion along the front
and/or continuous back-
building
Schumacher and Johnson (2005)

48. Record 619 mm in 15 h at Ganghwa, KoreaX
shear
Lee et al. (2008)
Sun and Lee (2002)

49. 2-3 April 2006

50. New cell initiation ahead of squall-lines
One possible trapping mechanism: the storm anvil
Fovell et al. (2006)

51. Trapping mechanism
Trapping can occur when a layer of lower l2 resides over a layer with higher values
More general Scorer parameter (c = wave speed)
Lowered l2 can result from decreased stability or creation of a jet-like wind profile
Storm anvil does both

52. New cell initiation ahead of squall-lines
…and can create clouds
Fovell et al. (2006)

53. New cell initiation ahead of squall-lines
…some of which can develop into
precipitating, even deep, convection
Fovell et al. (2006)

54. New cell initiation ahead of squall-lines
14 km
150 km
Fovell et al. (2006)

55. Tropical Storm Erin (2007)

56. Erin’s redevelopment over Oklahoma
Emanuel (2008)

57. Erin inland reintensification
Hot and wet loamy soil can rapidly transfer energy to atmosphere
Previous rainfall events left Oklahoma’s soil very wet
Need to consider antecedent soil moisture and soil type
Emanuel (2008)
see also
Emanuel et al. (2008)

58. Soil T as Erin passed
Emanuel (2008)

59. Summary
A critical view of some ideas, tools relevant to heavy precipitation forecasting
Emphasis on factors operational models do not handle particularly well
CAPE & CIN, MCS development and motion, surface and boundary layer conditions
Philosophical rather than detailed summary