life cycle of warm-season midlatitude convection
DESCRIPTION
Life Cycle of Warm-Season Midlatitude Convection. Stan Trier NCAR (MMM Division). Outline. Diurnal Cycle of Convection Rainfall Episodes - Phase Coherence - Latitudinal Corridors Propagating Nocturnal Convection (Model Composite Study) - Statistics - PowerPoint PPT PresentationTRANSCRIPT
Life Cycle of Warm-Season Midlatitude Convection
Stan Trier
NCAR (MMM Division)
Outline
1. Diurnal Cycle of Convection
2. Rainfall Episodes- Phase Coherence- Latitudinal Corridors
3. Propagating Nocturnal Convection (Model Composite Study)- Statistics- Evolving structure and propagation mechanism- Environmental characteristics
Amplitude and Phase of U.S. Diurnal Cycle of Thunderstorm Occurrence
From Wallace and Hobbs (1977) Atmospheric Science: An Introductory Survey 0
6
12
18LST
Hourly Average Rainfall Frequency (June-August 1996-2004)
On WEB http://locust.mmm.ucar.edu/episodes/Hovmoller
Time/Frequency Diagram of United States Warm-Season Convection (1996-2004)
On WEB http://locust.mmm.ucar.edu/episodes/Hovmoller
NOAA/CMORPH Rain RateBoreal Summer - JJAS 2004
mm/hr
Courtesy of Steve Nesbitt, presented at Warm Season Rainfall Workshop (9 June 2006)
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.
June 20-24 1998 Example of Coherent Rainfall EpisodesT
ime
(d
ay/
hr
UT
C)
Stationary Locally Forced
Propagating withIntermittency
Continuous Propagation
LatitudinalCorridor
115W 75W95W 30N 36N 42N 48N
On WEB http://locust.mmm.ucar.edu/episodes/Hovmoller
Longitude Latitude
Documented Locations of Long-Lived Coherent Precipitation Episodes
Radar+Sat
Sat Only
Courtesy of John Tuttle, presented at Warm Season Rainfall Workshop (9 June 2006)
Study Study Domains & Domains &
PeriodPeriod
Main FocusMain FocusMay - August5-year (1999 to 2003)
2-yearSep-Oct: 1999, 2003
2-yearNov–Dec: 1999, 2003
Meteosat-7 IR, 30min
May - AugMay - Aug
020W 20E
20S
20N
0
40E
0.6 1.2
0
Average Elevation 35S - 20S (km)
Nov - Dec Nov - Dec
Sep-OctSep-Oct
Average Elevation 0-20N (km)
1.0
0
2.0
Courtesy of Arlene Laing, presented at Warm Season Rainfall Workshop (9 June 2006)
Tropical N. Africa: Tropical N. Africa: 16 – 30 June 200316 – 30 June 2003253K 233K 213K
16
18
20
22
24
26
28
30
Change in phase likely due to mesoscale convective vortex formation
Courtesy of Arlene Laing
LATITUDE-TIME LATITUDE – PRESSURE CONVECTION MEAN ZONAL WIND (20W-35E)
JUNE 2003
AEJ
Mean Latitude of convection withzonal wind shear (associated with AEJ)
Shear
S N
Courtesy of Arlene Laing
LATITUDE-TIME LATITUDE - PRESSURE CONVECTION MEAN ZONAL WIND (20W-35E)
AUGUST2003
TEJ
W’ly
AEJ
Mean Latitude of convection with W’ly to E’ly shear (monsoon)
ShearShear
S N
Courtesy of Arlene Laing
Comparing ContinentsRegion
(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
Span vs Duration for Four ContinentsSpan vs Duration for Four Continents
y = 42.8x - 26.952
R2 = 0.90010
1000
2000
3000
4000
5000
6000
0 20 40 60 80 100 120 140Duration (h)
Span
(km
)
Europe, 1999-2003
US Mainland, 1997-2000
East Asia, 1998-2001
Tropical N. Africa, 1999-2003
Courtesy of Arlene Laing
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
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
July-Aug 1998-2002 Radar + RUC Analysis
Radar 900 mb Winds CAPE/Shear (600-900 mb)
300 mb Winds/Heights
Corridors of Precipitation
0
6
12
18
24
TIM
E (
UT
C)
110 100 90 80LONGITUDE WEST
Propagating convection
Locally forced
Initiates at time of max solar heating over higher terrain
Initiates during the night in the central plains
From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.
22 LST Surface Potential Temp/Winds/Reflectivity
In situor
weaklypropagating
Rapidlypropagating
Days with strong LLJ (>12 ms-1) 45 days out of 310
900 mb Hgt Anom/Radar 300 mb Hgt Anom/Radar
900 mb Winds 300 mb Winds/Hgts
+-
-
+
From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.
Days with weak/no LLJ (<5 ms-1) 32/310
900 mb Hgt Anom/Radar 300 mb Hgt Anom/Radar
900 mb Winds 300 mb Winds/Hgts
+
-
-
From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.
Days with persistent corridors lasting 4 or more days
900 mb Hgt Anom/Radar 300 mb Hgt Anom/Radar
900 mb Winds 300 mb Winds/Hgts
+
+0
From Tuttle and Davis (2006) To appear in Mon. Wea. Rev.
Longitude
3-10 July 2003Longitude vs Time Rainfall Frequency
0
3
6
9
12
15
18
21
0
3
6
9
12
15
18
21
0
Tim
e (U
TC
hou
r)
105W 100 95 90 85W
65
56
47
37
28
19
9
0%
From Carbone et al. (2002; JAS)
Diurnal Frequency Diagrams of Convection
July 3-10, 2003
500 hPa
Height
Differing Regimes for Organized Convection
Quasi-Stationary E-W Front Pattern Translating Synoptic Cold Front Pattern
• “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)
• 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
7-Day Simulations Using WRF (00Z 3 July to 00Z 10 July 2003)
• Initial and Boundary Conditions Obtained from ETA Analyses (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
Comparison of Simulated and Observed Precipitation Episodes
From Trier, Davis, Ahijevych, Weisman, and Bryan (2006), To appear in J. Atmos. Sci.
Rainstreak Phase Speed Statistics (03-10 July 2003)
Zonal Phase Speed (m/s)
Fre
quen
cy
Composite System-Relative Flow, Theta (Contours), Theta-e (Colors)
Intensifying Stage (Early Evening) Mature Stage (Overnight)
Weakening Stage (Around Sunrise)
Distance (km) Distance (km)
Distance (km)
Hei
ght (
km A
GL
)
Hei
ght (
km A
GL
)
Five Cases 40-km Along-Line Average
Rainstreak Propagation
• Rainstreak movement cannot be explained by advection by mean environmental flow through storm depth
Rainstreak Propagation (cont.)
z2
z 0
2 dzH
c B
v v cB g q
0grc c u
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
zz 00.62 | H
grc c u
0|gr zc c u
z 10m|grc c u
Composites of the Mesoscale Environment for Mature Stage
Composite Vertical Cross Sections of the Mesoscale Environment
Trajectories from NE Trajectories from SW
20
40
60
80
100
20
40
60
80
100
0.0
1.0
2.0
3.0
0.0
1.0
2.0
3.0
Height (km MSL) Height (km MSL)
Relative Humidity (%)
Relative Humidity (%)
03002118 030021
03002118 030021
Time (hr UTC) Time (hr UTC)
Forward Trajectory Analysis for a Strong Frontal Case Example
0
1000
2000
CA
PE
(J
/ kg
)
+ Convection Location
Intensifying
Mature
Weakening
Diurnal Frequency and Composite Mesoscale Environment of Propagating Convection
850 hPa Temperature/Winds
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?