1. Robert Fovell University of California, Los Angeles rfovell@ucla.edu
Forecasting convective rainfall: convective initiation, heavy precipitation and flash flooding
Robert Fovell
University of California, Los Angeles

2. Heavy precipitation at a location = intensity + longevity

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

4. Mesoscale Convective Systems (MCSs)

5. 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

6. Seasonality of flash floods in U.S.
Contribution of
warm season MCSs
clearly seen
Number of events
Maddox et al. (1979)

7. Linear MCS archetypes (e.g., squall-lines)
58%
19%
Note in passing
Parker and Johnson (2000) 7

8. Squall-lines usually multicellular

9. The multicell storm Four cells at a single time
Or a single cell at four times:
unsteady
Browning et al. (1976)
Also show multiple cells in conceptual model 9

10. The multicell storm Unsteadiness = episodic entrainment owing
to local buoyancy-induced
circulations.
Browning et al. (1976)
Also show multiple cells in conceptual model
Fovell and Tan (1998) 10

11. Storm motion matters How a storm moves over a specific
location determines rainfall
received
Doswell et al. (1996)

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

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

14. North Plains

15. 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

16. 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

17. 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)

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

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

20. Propagation is vector difference
Corfidi vector method
Propagation is vector difference
P = S - C
Therefore, S = C + P

21. Example

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

23. 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)

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

25. Composite severe MCS hodograph
Bluestein and Jain (1985)

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

27. 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)

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

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

30. 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

32. 5 June 2004
X = Hays,
Kansas, USA

33. Mesoscale Convective Vortices (MCVs)

34. Cyclonic vortex following squall line
Not a clean
MCV case

35. Potential vorticity (PV) anomalies
PV anomaly shown drifting in westerly sheared flow
Raymond and Jiang (1990)

36. Potential vorticity (PV) anomalies
Ascent occurs on windward (here, east) side… destabilization
Raymond and Jiang (1990)

37. Potential vorticity (PV) anomalies
Cyclonic circulation itself results in ascent on east side
Raymond and Jiang (1990)

38. Potential vorticity (PV) anomalies
Here they work together
Combination: uplift & destabilization on
windward side AND downshear side
Raymond and Jiang (1990)

39. 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)

40. Schumacher’s situation
“Hairpin” hodograph:
Sharp flow reversal above LLJ

41. Schumacher’s situation
South side of MCV is windward at low-levels
and downshear relative to midlevel vortex

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

43. Evolution of the heavy rain event
At t - 12h (afternoon):
- MCV located farther west
- 900 mb winds fairly light
600 mb vorticity, 900 mb winds & isotachs
Schumacher and Johnson (2008)

44. Evolution of the heavy rain event
At t - 6h (evening):
- MCV drifted west
- 900 mb winds strengthening (LLJ intensifying)
600 mb vorticity, 900 mb winds & isotachs
Schumacher and Johnson (2008)

45. Evolution of the heavy rain event
At time of heaviest rain (midnight):
- 900 mb jet well developed
- LLJ located east, south of MCV
600 mb vorticity, 900 mb winds & isotachs
Schumacher and Johnson (2008)

46. 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)

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

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

49. “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)

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

51. 2-3 April 2006

52. Why did new cells appear ahead of the mature line?

53. New cell initiation ahead of squall-lines
The waves themselves disturb
the storm inflow
Fovell et al. (2006)

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

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

56. Importance of antecedent soil moisture conditions
(Generally not captured well by models)

57. Tropical Storm Erin (2007)

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

59. 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)

60. Soil T as Erin passed
Emanuel (2008)

61. end