Transcript Precursors to the Initiation of Nocturnal Convection in the Eastern Plains Matthew Dux

Precursors to the Initiation of
Nocturnal Convection in the
Eastern Plains
Matthew Dux
March 1, 2006
WFO Pleasant Hill, MO
Objectives
• Understand the impacts of forecasting
nocturnal convection
• Review synoptic trends prior to nocturnal
MCC initiation as based on previous studies
• Review pre-initiation synoptic conditions as
apparent in localized studies
• Put it all together in the end!
Nocturnal Convection Impacts/Concerns
• Usually a smaller staff base during the late evening and overnight
hours
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Calling people in to work?
Will you need to coordinate with your awareness network?
• Spotter network shrinks once the overnight hours are reached
• Reduced awareness of meteorological impacts of weather
• Heavily rely on law enforcement, media, and emergency managers
• Normally an advanced forecast can be issued
• Uncertainty in strength and type of convection leads to large portions of an area
• Television, radio, and severe weather sirens are usually less effective past 2200 LT.
Precursors to MCC Development
• One of the most significant nocturnal events
– Primary threat of intense rainfall and flash flooding
– Can produce hail, wind, and even tornadoes.
• Looking at synoptic and mesoscale models of
MCC development (Maddox 1980) forecasters
are guided to look at:
• Large low-level moisture content
• Weak low-level warm-air advection
• High equivalent potential temperature advection
• These ingredients are commonly noted as
intensifiers for nocturnal convection.
MCC Study Details
10 total MCC cases into composite maps
Maddox Pre-MCC Initiation Graphics
850 hPa
700 hPa
Pre-MCC Initation Graphics Cont.
500 hPa
200 hPa
Summary of Signals
• Ongoing convection lies ahead of a weak
mid-level shortwave trough
• Long wave ridge pattern dominates flow aloft
• However, strong low-level WAA is the predominate
source for sustaining and organizing convection
• Abundant moisture usually pooled into the
area by a strong low-level jet at 850 hPa
• Jet continues to veer and strengthen as night
progresses sustaining convection
• 10 g/kg average common prior to development
Part 2: A Localized Study of Nocturnal
Convection
Area of interest
• Convection must develop between 94 - 101 W
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and 42 and 46 N.
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Using lightning data
• Obtained lightning data January 1998 –
September 2003
• Divided study area into 0.25 x 0.25 grid
• Every hour counted the number of
lightning strikes in each bin
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– Data written to GEMPAK files
– If less than 10 strikes across the area, hourly
data was not saved.
Example of lightning plot
Criteria for convective initiation
• Must initiate between 0200 UTC and 1400 UTC.
• 10+ lightning strikes in 0.75 x 0.75 box.
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• Majority of must be within the area of interest.
– No lightning strikes in adjacent boxes during the current hour
and the previous hour that can be traced from convection that
developed outside the box or prior to 01Z.
– There can be no lightning strikes in the same box the previous
two hours.
• Continuity
– Must produce at 10 strikes in 0.75 x 0.75 for 2
consecutive hours.
• The 0.75 x 0.75 box must be adjacent for consecutive hours.
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0700 UTC
0800 UTC
0900 UTC
Climatology Results
• 262 days identified
– 43 days per year
• For each storm, following were tracked
– Time of initiation
– Location of initiation to nearest 1 degree
– Time of dissipation
– Number of severe weather reports
– Number of flash flood reports
When does convection initiate?
Where does convection initiate?
Creating composite maps
• Observed soundings collected for each case.
• Soundings objectively analyzed for each case
– Used a Barnes analysis to a 1 x 1 degree lat-lon grid.
• Data quality controlled by examining grids.
– If bad data found, it was removed and data
reanalyzed.
• Pre-initiation data averaged to create a
composite grid
• 0200-0700 UTC period uses pre-event 1200 UTC sounding
• 0800-1400 UTC period uses pre-event 0000 UTC sounding
Pre-Nocturnal Convection
Composites
Focusing on the big three aspects of convection:
1. Lift
2. Moisture
3. Overall Stability
850 hPa
700 hPa
500 hPa
200 hPa
Temperature Advection
850 hPa
700 hPa
700 to 500 hPa Stability
Lapse rate
Differential temperature advection
850 to 700 hPa Stability
Lapse rate
Differential temperature advection
850 hPa Moisture transport
Equivalent potential temperature advection
By comparing 850 hPa theta-e to 700 hPa theta-es one can estimate cap
strength.
•850 hPa theta-e dependent on temperature and moisture.
•700 hPa theta-es dependent on temperature only.
Low-level stability
•Difference in 850 hPa theta-e advection and 700 hPa theta-es
advection shows best area to overcome capping inversion
•Positive values indicate an area where lower levels of the atmosphere can
overcome capping inversion at 700 hPa (more unstable)
•Negative values show the opposite (more stable air dominates)
Conclusions
• Initiation of nocturnal convection is common in the
eastern plains.
– 262 nocturnal events in a 6 year period
– 43 days per year of new convection
• Two peaks in convective initiation
– Late evening and late night.
• Convective initiation favored near the Missouri River near
in south central South Dakota
– No apparent change in location by time.
• Many of the same signals of MCC enhancement are
initiators to new nocturnal convection
Synoptic scale conditions prior to new initiation
• Long wave ridge moving into Great Lakes downstream of study area
• 850 hPa trough developing in lee of Rockies, ridge over the SE
– Low-level southerly flow increasing (nocturnal jet)
• Convection commonly initiates at the nose of the LLJ
– Low level flow increasing moisture into the eastern Plains
• Convection initiates on the northern edge of the moisture surge
• Increasing mid-level instability
– Thermal advection acting to increase mid-level lapse rate (7H-5H)
– Weak mid-level shortwave progressing through the area
• Strong low-level thermal advection
– Synoptic scale forcing for lift seen prior to convective development
• 850 hPa theta-E advection strong enough to overcome strengthening
cap at 700 hPa.
Things to Think About…
• As time progresses, how does nocturnal
convection alter the environment?
– How will the future evolve?
• What makes daytime convection different
than that of nocturnal convection?
References
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Maddox, R. A., 1980: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61, 1374-1387.
Maddox, R. A., 1983: Large-scale meteorological conditions associated with midlatitude,
mesoscale convective complexes. Mon. Wea. Rev., 111, 1475-1493.