mesoscale

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Quasi-Stationary Convection
6-hour Rainfall Totals for the 28 July 1997 Fort Collins, CO Flood
Mesoscale
M. D. Eastin
Quasi-Stationary Convection
Basic Concept
Conceptual Models
Climatology of Extreme Rainfall Events
Historical Events
• Big Thompson, CO flood – 31 July 1976
• Johnstown, PA flood – 19 July 1977
• Fort Collins, CO flood – 28 July 1997
Forecasting
Mesoscale
M. D. Eastin
Basic Concept
A Offsetting Multi-Scale Process:
Right Moving Supercell
• The motion of a given storm system is the sum of two vectors:
Cell Velocity:
• Motion of individual cells that compose the system
• Roughly equivalent to the deep layer mean synoptic wind
Propagation Velocity:
• Motion due to the formation of new cells on the storm periphery
• Related to the motion of mesoscale boundaries and their
interaction with the storm inflow (e.g., cold pool propagation)
• May occur on any side:
• On the leading edge → system accelerates
• On a side → systems move to the “right” or “left”
• On the rear flank → systems decelerates
Squall Line
Quasi-Stationary Storm
• Quasi-stationary convection occurs when the cell velocity and
propagation velocity are roughly equal and opposite
Zero Storm motion
Mesoscale
M. D. Eastin
Conceptual Models
Synoptic Setting:
• Maddox et al. (1979) examined the synoptic environments
for 151 quasi-stationary convective events that produce
significant flash floods
• Identified two regular situations with many common
characteristics
Frontal Forcing
• Nearly stationary synoptic-scale surface front
Meso-High Forcing
Frontal
Forcing
• Nearly stationary, mesoscale, surface-outflow (cold pool)
boundary generated by previous convective activity
Characteristics Common to Both
• Front/Boundary trigger convection via forced ascent
• Large near-surface CIN inhibits convection except for
those regions with strong forced ascent
• Upper-level winds nearly parallel to the front/boundary
• Low-level flow nearly normal to the front/boundary
• Mostly a summer nocturnal phenomena
Mesoscale
Meso-High
Forcing
M. D. Eastin
Conceptual Models
Mesoscale Organization:
• Schumacher and Johnson (2005) examined radar
data for 116 MCS that produced extreme rainfall
• Identified two regular organization types
Training Line – Adjoining Stratiform
• 65% of cases
• Convective cells develop behind a front or boundary
• Cell motion parallel to the boundary, as new cells
form and move over (or train over) the same location
• Stratiform precipitation forms behind the line but
also moves parallel to the boundary
Backbuilding – Quasi-Stationary
• 27% of cases
• Convective cells repeatedly form upstream from their
immediate predecessor (behind a boundary)
• Mature and decaying cells move slowly downstream
• Cell motion and propagation motion are roughly
equal and opposite (cancel each other)
• Minimal stratiform precipitation located downstream
Mesoscale
From Schumacher and Johnson (2005)
M. D. Eastin
Climatology of Extreme Rainfall Events
Definition and Regional Statistics:
Extreme Rainfall Event → When a surface rain gauge reports a 24-hour total rainfall
greater 125 mm (~ 5 inches)
• Five year period (1999-2003)
• 382 events
• Most common in July
• Only occur in the summer
across the northern U.S.
• Year-round in southern U.S.
• 74% of warm season events
were associated with MCS
(or meso-high) forcing
• 95% of cold season events
were associated with strong
synoptic forcing
• Tropical systems contribute
in the south and east
From Schumacher and Johnson (2006)
Mesoscale
M. D. Eastin
Historical Events
Big Thompson Canyon Flood – 31 July 1976
• Quasi-stationary MCS produced 8-10 inches of rain in 1.5 hrs
• Occurred 8:30 -10:00 pm LST
• Killed 145 people (six bodies never recovered) and produced >$40 million in damages
• Strong low-level easterly (upslope) flow of warm, moist air
• Weak westerly winds aloft (above 700 mb)
Details: http://pubs.usgs.gov/fs/2006/3095/pdf/FS06-3095_508.pdf
Mouth of Big Thompson Canyon
1 August 1976
Mesoscale
Today
M. D. Eastin
Historical Events
Johnstown Flood – 19 July 1977
• Associated with a slow-moving MCC that originated four days prior over South Dakota
• Produced 12 inches of rain in 10 hrs across central Pennsylvania
• Occurred between 7:00 pm and 5:00 am LST
• Extreme precipitation caused multiple dams to overtop and break
• Killed 76 people and produced >$200 million in damages
Details: Bosart, L. F., and F. Sanders, 1981: The Johnstown flood of July 1977: A long-lived
convective system, Journal of Atmospheric Science, 38, 1616-1642
Primary
dam that
broke
Johnstown
Mesoscale
M. D. Eastin
Historical Events
Fort Collins Flood – 28 July 1997
• Quasi-stationary MCS produced >10 inches of rain in ~6 hrs
• Occurred 5:00 -11:00 pm LST
• Killed 5 people and produced >$200 million in damages
Details: http://olympic.atmos.colostate.edu/pdf/Petersen-Flood97.pdf
From Peterson
et al. (1999)
Mesoscale
M. D. Eastin
Historical Events
Fort Collins Flood – Synoptic Conditions
• A 500-mb ridge axis over northeast CO
• Strong low-level easterly (upslope) flow
• Deep layer of moist air
• Modest CAPE at ~850 J/kg
• Weak southwesterly winds aloft
Cold
cloud
tops
Denver Sounding
Edge of warm
moist air
Surface
Winds
500mb
Vorticity
Maxima
From Peterson et al. (1999)
Mesoscale
M. D. Eastin
Historical Events
Fort Collins Flood – Mesoscale Organization
• A quasi-stationary back-building MCS
• New cells repeatedly developed
over southern Fort Collins as
their mature and decaying
predecessors moved slowly north
across the city
From Peterson et al. (1999)
Mesoscale
M. D. Eastin
Forecasting
Guidelines:
• Prior to the development of convection, look for:
• Strong low-level flow normal to a front, outflow boundary, or orographic feature
• Warm and moist low-level air
• Modest CAPE (500-1000 J/kg)
• Large CIN (>200 J/kg)
• Weak mid- and upper-level flow parallel to a pre-existing front or boundary
• Once convection has developed, look for and monitor:
• Surface wind observations roughly equal and opposite middle and upper-level winds
• Cell motion parallel to fronts or outflow boundaries
• Slow moving, back-building systems on radar
• Radar-derived estimates of total precipitation
Mesoscale
M. D. Eastin
Quasi-Stationary Convection
Summary:
Basic Concept
• Cell Velocity
• Cell Propagation
Conceptual Models
• Frontal Forcing
• Meso-high Forcing
• Training Line
• Back-building
Climatology of Extreme Rainfall Events
Historical Events
• Big Thompson, CO flood – 31 July 1976
• Johnstown, PA flood – 19 July 1977
• Fort Collins, CO flood – 28 July 1997
Forecasting Guidelines
Mesoscale
M. D. Eastin
References
Bosart, L. F., and F. Sanders, 1981: The Johnstown flood of July 1977: A long-lived convective system, J. Atmos. Sci., 38,
1616-1642.
Chappell, C., 1986: Quasi-stationary convective events. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed. Amer.
Meteor. Soc., 289-310.
Maddox, R. A., C. F. Chappell and L. R. Hoxit, 1979: Synoptic and mesoscale aspects of flash flood events. Bull. Amer.
Meteor. Soc., 60, 115-123.
Maddox, R. A., L. R. Hoxit, C. F. Chappell, and F. Caracena, 1978: Comparison of the meteorological aspects of the Big
Thompson and Rapid City flash floods. Mon. Wea. Rev., 106, 375–389.
Petersen, W. A., L. D. Carey, S. A. Rutledge, J. C. Knievel, R. H. Johnson, N. J. Doesken, T. B. McKee, T. Vonder Haar,
and J. F. Weaver, 1999: Mesoscale and radar observations of the Fort Collins flash flood of 28 July 1997.
Bull. Amer. Meteor. Soc., 80, 191–216
Schumacher, R. S. and R. H. Johnson, 2005: Organization and environmental properties of extreme-rain-producing
mesoscale convective systems. Mon. Wea. Rev., 133, 961-976.
Mesoscale
M. D. Eastin
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