Life Cycle of Warm-Season Midlatitude Convection

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Life Cycle of Warm-Season Midlatitude Convection

• Stan Trier
• NCAR (MMM Division)

2
Outline

• Diurnal Cycle of Convection
• Rainfall Episodes
• - Phase Coherence
• - Latitudinal Corridors
• Propagating Nocturnal Convection (Model Composite
  Study)
• - Statistics
• - Evolving structure and propagation mechanism
• - Environmental characteristics

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Amplitude and Phase of U.S. Diurnal Cycle of
Thunderstorm Occurrence
12
LST
18
6
From Wallace and Hobbs (1977) Atmospheric
Science An Introductory Survey
0
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Hourly Average Rainfall Frequency (June-August
1996-2004)
On WEB http//locust.mmm.ucar.edu/episodes/Hovmoll
er
5
Time/Frequency Diagram of United States
Warm-Season Convection (1996-2004)
On WEB http//locust.mmm.ucar.edu/episodes/Hovmoll
er
6
NOAA/CMORPH Rain Rate Boreal Summer - JJAS 2004
mm/hr
Courtesy of Steve Nesbitt, presented at Warm
Season Rainfall Workshop (9 June 2006)
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• From TRMM Tropics-wide observations
• Over ocean, all types of precipitation features
  produce the most rainfall at night around 6 AM,
  mainly controlled by MCSs
• Over land, the total rainfall peaks in the
  afternoon when the atmosphere is least stable,
  however MCS rainfall peaks later at night, around
  midnight, due to their longer life cycle

Nesbitt and Zipser (2003), Mon. Wea. Rev.
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June 20-24 1998 Example of Coherent Rainfall
Episodes
Latitudinal Corridor
Propagating with Intermittency
Time (day/hr UTC)
Stationary Locally Forced
Continuous Propagation
115W
75W
95W
30N
36N
42N
48N
Longitude
Latitude
On WEB http//locust.mmm.ucar.edu/episodes/Hovmoll
er
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(No Transcript)
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Documented Locations of Long-Lived Coherent
Precipitation Episodes
Courtesy of John Tuttle, presented at Warm Season
Rainfall Workshop (9 June 2006)
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Study Domains Period
20N
May - Aug
Main Focus May - August 5-year (1999 to
2003) 2-year Sep-Oct 1999, 2003 2-year NovDec
1999, 2003 Meteosat-7 IR, 30min
0
Sep-Oct
20S
Nov - Dec
0
20W
40E
20E
Courtesy of Arlene Laing, presented at Warm
Season Rainfall Workshop (9 June 2006)
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Tropical N. Africa 16 30 June 2003
Courtesy of Arlene Laing
253K
233K
213K
16 18 20 22 24 26 28 30
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LATITUDE-TIME LATITUDE
PRESSURE CONVECTION
MEAN ZONAL WIND (20W-35E)
JUNE 2003
AEJ
Shear
Mean Latitude of convection with zonal wind shear
(associated with AEJ)
S
N
Courtesy of Arlene Laing
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LATITUDE-TIME LATITUDE - PRESSURE
CONVECTION MEAN ZONAL WIND
(20W-35E)
AUGUST 2003
TEJ
AEJ
Shear
Wly
S
Mean Latitude of convection with Wly to Ely
shear (monsoon)
N
Courtesy of Arlene Laing
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Comparing Continents
Region (Longitude of Domain) Span (km) Duration (h) Phase Speed All episodes (ms-1)
Contiguous US (37deg) 838 (1 per day mean) 18.5 (1 per day mean) Median 13.6
East Asia (50deg) 620( 1 per day mean) 11.6 (1 per day mean) Mean 12.4
Europe (50 deg) Mean 469.16 Mean 8.56 Mean 14.88 Median 13.6
Africa (60deg) Mean - 1066 Median - 700 Mean 25.5 Median 18.0 Mean 12.0 Median 11.2
Courtesy of Arlene Laing
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Span vs Duration for Four Continents
Europe, 1999-2003
Tropical N. Africa, 1999-2003
East Asia, 1998-2001
US Mainland, 1997-2000
Courtesy of Arlene Laing
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Common Features of Episodes

• Global phenomenon (on all continents with deep
  convec)
• Genesis along and immediately downstream of
  significant topography
• At least moderate vertical shear (10 m/s) in
  environment
• Most frequent and longest-lived at height of warm
  season
• Movement at speeds greater than synoptic
  disturbances (e.g., baroclinic waves) or
  low-middle tropospheric steering flow

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Candidate Mechanisms for Long-Lived Coherent
Propagating Convective Episodes

• Density currents
• Trapped gravity waves
• Gravity-inertia waves in the free troposphere
• Balanced circulations associated with and/or
  modified by convection (e.g., MCVs)

Discussed by Carbone et al. (2002) J. Atmos. Sci
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Corridors of Precipitation
July-Aug 1998-2002 Radar RUC Analysis
CAPE/Shear (600-900 mb)
900 mb Winds
Radar
300 mb Winds/Heights
Initiates during the night in the central plains
Locally forced
TIME (UTC)
Propagating convection
Initiates at time of max solar heating over
higher terrain
110 100 90 80
LONGITUDE WEST
From Tuttle and Davis (2006) To appear in Mon.
Wea. Rev.
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22 LST Surface Potential Temp/Winds/Reflectivity
In situ or weakly propagating
Rapidly propagating
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Days with strong LLJ (gt12 ms-1) 45 days out of 310
-
-


From Tuttle and Davis (2006) To appear in Mon.
Wea. Rev.
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Days with weak/no LLJ (lt5 ms-1) 32/310
-

-
From Tuttle and Davis (2006) To appear in Mon.
Wea. Rev.
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Days with persistent corridors lasting 4 or more
days

0

From Tuttle and Davis (2006) To appear in Mon.
Wea. Rev.
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Diurnal Frequency Diagrams of Convection
3-10 July 2003 Longitude vs Time Rainfall
Frequency
0
3
0
6
9
9
12
15
19
18
21
28
Time (UTC hour)
0
3
37
6
47
9
12
56
15
18
65
21
0
105W
100
95
90
85W
Longitude
From Carbone et al. (2002 JAS)
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July 3-10, 2003
500 hPa Height
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Differing Regimes for Organized Convection
Quasi-Stationary E-W Front Pattern
Translating Synoptic Cold Front Pattern

• Supports both MCSs and long narrower linear
  convection
• Convection primarily afternoon and early evening
• Large CAPE both along and ahead (south and east)
  of frontal
• zone

• Classic MCS pattern (e.g., week-long BAMEX
  Case)
• Convection primarily nocturnal and early morning
• Large CAPE confined to frontal zone (restricts
  scale
• of convection)

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7-Day Simulations Using WRF (00Z 3 July to 00Z 10
July 2003)

• Initial and Boundary Conditions Obtained from ETA
  Analyses (D t 3h)
• Yonsei University PBL Scheme with Noah LSM
• Long and Shortwave Radiation Parameterization
• 4-km Simulation
• - Central US Regional Domain (625 x 445 x
  35)
• - Explicit Convection (No Cumulus
  Parameterization)
• - Lin et al. (1983) based Microphysical
  Scheme

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Comparison of Simulated and Observed
Precipitation Episodes
From Trier, Davis, Ahijevych, Weisman, and Bryan
(2006), To appear in J. Atmos. Sci.
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Rainstreak Phase Speed Statistics (03-10 July
2003)
Frequency
Zonal Phase Speed (m/s)
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Composite System-Relative Flow, Theta (Contours),
Theta-e (Colors)
Intensifying Stage (Early Evening)
Mature Stage (Overnight)
Height (km AGL)
Distance (km)
Distance (km)
Weakening Stage (Around Sunrise)

• Five Cases
• 40-km Along-Line Average

Height (km AGL)
Distance (km)
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Rainstreak Propagation

• Rainstreak movement cannot be explained by
  advection by mean
• environmental flow through storm depth

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Rainstreak Propagation (cont.)
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Rainstreak Propagation (cont.)

• Estimates of rain streak zonal phase speed based
  on mature stage cold-pool
• negative buoyancy (left) are systematically
  high
• Similar estimates based on the 16-km deep
  integrated buoyancy anomaly
• (right) are much closer to observed rain streak
  zonal phase speeds

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Composites of the Mesoscale Environment for
Mature Stage
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Composite Vertical Cross Sections of the
Mesoscale Environment
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Forward Trajectory Analysis for a Strong Frontal
Case Example
Trajectories from NE
Trajectories from SW
3.0
3.0
Height (km MSL)
Height (km MSL)
2.0
2.0
1.0
1.0
0.0
0.0
03
21
18
03
21
00
00
100
100
Relative Humidity ()
80
80
60
60
40
40
Relative Humidity ()
20
20
03
21
03
00
21
18
00
Time (hr UTC)
Time (hr UTC)
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Diurnal Frequency and Composite Mesoscale
Environment of Propagating Convection
850 hPa Temperature/Winds
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Some Remaining Questions

• Are mechanisms for nocturnal propagation (a major
  component of long-lived episodes) similar on
  other continents?
• - e.g., poleward low-level jets (many
  continents, not Africa)
• Initiation of many major episodes in central U.S.
  tied to both topography and mobile short waves.
  Are they related?
• What governs intermittency (redevelopment along
  approximate same phase line in next heating
  cycle)?
• - amplification or refocusing free-tropospheric
  disturbance by convection?
• - density current dynamics/trapped gravity
  waves?