Forecasting convective rainfall: convective initiation, heavy precipitation and flash flooding

1
Forecasting convective rainfallconvective
initiation, heavy precipitation and flash flooding

• Robert Fovell
• University of California, Los Angeles
• rfovell_at_ucla.edu

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 19
Parker and Johnson (2000)
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)
10
The multicell storm
Unsteadiness episodic entrainment owing to
local buoyancy-induced circulations.
Browning et al. (1976)
Fovell and Tan (1998)
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
19980714 - 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
• 1000-500 mb layer mean RH 70
• MCSs tend to propagate parallel to 1000-500 mb
  thickness contours
• MCSs favored where thickness contours diverge
• MCSs back-build towards higher CIN
• Development favored downshear of midlevel
  cyclonic circulations

17
70 layer RH
70 RH rule of thumb Implication Relative
humidity more skillful than absolute humidity
RH gt 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
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 850-300 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
Cell direction comparable To 850-300 mb
layer wind direction
Cells tend to move at 850-300 mb layer wind speed
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 motionusing antecedent
information
Cell motion 850-300 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

31
http//locust.mmm.ucar.edu/episodes
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
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
Schumachers situation
Hairpin hodograph Sharp flow reversal above LLJ
41
Schumachers 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, Korea
X
shear
Lee et al. (2008)
Sun and Lee (2002)
51
2-3 April 2006
52
Why did new cells appearahead 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 antecedentsoil moisture conditions
(Generally not captured well by models)
57
Tropical Storm Erin (2007)
http//en.wikipedia.org/wiki/ImageErin_2007_track
.png
58
Erins redevelopmentover Oklahoma
Emanuel (2008) http//www.meteo.mcgill.ca/cyclone/
lib/exe/fetch.php?idstartcachecachemediawed20
30.ppt
59
Erin inland reintensification

• Hot and wet loamy soil can rapidly transfer
  energy to atmosphere
• Previous rainfall events left Oklahomas 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