The sensitivity of the Northeast Colorado thunderstorm environment to upstream surface conditions

1
The sensitivity of the Northeast Colorado
thunderstorm environment to upstream surface
conditions

• Rick Grimaldi
• SUNY Oneonta

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Motivation

• More mountain precip. falls as rain
• reduced snowpack
• Earlier runoff
• Drier warm-season landscape/low Bowen ratio

1.5C to 3.5C
Does the memory of a reduced snowpack effect
warm-season climate downstream?
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Data

• Two Colorado Rocky Mountain stream gauges (USGS)
• Denver radiosonde archive
• Severe weather archive (SPC)
• Composite precipitation anomalies

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Methods

• Take upper air climatology (May and June)
• Severe weather climatology (May and June)
• Isolate anomalous years using cumulative
  streamflow discharge
• Compare contrast statistics
• Try to explain the results

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Streamflow Data
High snow
Low snow

• Cumulative discharge
• 1 May to July 31
• 7 year subsets

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Elevated Mixed Layers

• Dry adiabatic layers form in an effective
  convective boundary layer during the peak heating
  of the day
• Strong solar heating and dry surfaces
• PBL pressures near 700mb
• Mixed layer decouples from surface during evening
  to become an elevated residual layer
• Residual layers advect downstream
• Typical southwesterly flow
• Able to maintain its integrity despite undergoing
  vertical displacements (Gd)
• EMLs follow their isentropic surface (about
  320K)

Gd
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Instability
CAPPING INVERSION LAYER
ERL(-2)
320K surface under 240 flow
ERL(-1)
WARM CAP
ERL(0)
Potential range of influence
Parcel
Surface
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Linear correlation (all years)
Subset comparison
Subset comparison

• Warmer at 700mb for low snow conditions
• More stable beneath (stronger cap)
• Less stable above (steeper lapse rates)

warm
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EMLs increase convective inhibition and low-level
CAPE

• Less numerous storms, but those that do form
  may be more severe..
• Large parcel acceleration above the LFC
• Enhanced vortex stretching
• Favorable for tornadogenesis

Fat cape
Gd
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Severe thunderstorm data
Delayed onset of SVRWX reports
Foothills Plains

• A STRONGER CAP CAUSES
• Delayed onset of moist convection
• Increased storm spacing and reduced areal
  coverage for rainfall
• Those cells that do break the cap can become more
  severe due to
• reduced competition for limited low-level
  moisture
• dry adiabatic environmental lapse rates just
  above LFC

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Can a warmer capping plume inhibit rainfall at
synoptic-scale distances downstream?
High Snow years
Low Snow years
drought
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Conclusions

• Dry mountain surfaces tend to form warm
  well-mixed residual layers
• Upon advecting downstream these warm capping
  mixed layers inhibit moist convection and reduce
  convective rainfall
• There is a suggested invigoration of severe
  weather on the plains, 100km east of the front
  range for those storms that do manage to break
  the cap
• The suppression of precipitation appears to
  extend to synoptic scale distances downstream

Grimaldi, R.T., 2008 The sensitivity of the
Northeast Colorado Thunderstorm environment to
Upstream Surface Conditions. Journal of
Hydrometeorology, 9, pp 61-75.