Sensitivity of Supercell Tornado Simulations to Variations in Microphysical Parameters

1
Sensitivity of Supercell Tornado Simulations to
Variations in Microphysical Parameters

• Nathan Snook and Ming Xue
• School of Meteorology, University of Oklahoma
• February 14, 2006

2
Motivation

• Tornadoes spawned by supercell thunderstorms are
  a major severe weather hazard in the central
  United States, causing multiple fatalities and
  millions of dollars in damage each year.
• Accurate numerical simulation of tornadic
  supercells remains a challenge, as the solution
  is affected by grid resolution and model
  parameters, such as microphysics.
• Most models assume a Marshall-Palmer inverse
  exponential dropsize distribution.
• Observational studies of Marshall-Palmer
  intercept parameters for rain, snow, and hail
  have yielded values that vary by several orders
  of magnitude (Gilmore et al., 2004).

3
Goals

• Investigate the sensitivity of supercell storm
  dynamics to variation in Marshall-Palmer
  intercept parameters for rain, hail, and snow
  dropsize distributions, and hail density.
• Cold Pool Intensity
• Organizational Mode
• Precipitation Distribution and Intensity
• Explore the impacts of these effects on tornado
  potential and tornado formation.

4
Methods

• Idealized modeling studies using the Advanced
  Regional Prediction System (ARPS).
• 13 simulations at 1 km horizontal resolution
• 7 simulations at 100 m horizontal resolution
• Varied Marshall-Palmer intercept parameters for
  rain, hail, and snow, as well as hail density.
• Horizontally homogeneous base state using
  composited sounding from May 20, 1977 Del City,
  Oklahoma supercell case.

5
Results

• 1 km runs revealed rain and hail intercept
  parameters to have most influence on supercell
  dynamics

6
Results

• Of the seven high resolution runs, two had
  intense, sustained tornadic vortices.

N0r 8 x 105 m4, N0h 4x104 m4
Large raindrops
7
Conclusions

• There is a tremendous sensitivity of storm mode,
  cold pool strength, and tornadogenesis potential
  to microphysics.
• Changing intercept parameters alone is sufficient
  to determine the success or failure of
  tornadogenesis.
• Simulations favoring large raindrops, using the
  current ice physics, were more favorable for
  tornadogenesis.
• Weak cold pool due to reduced evaporational
  cooling.
• Better positioning of gust front allowing for
  sustained, intense, vertically-oriented updraft.
• Better microphysics with reduced uncertainty in
  e.g., intercept parameters, will be necessary for
  reliable simulation and prediction of tornadoes
  and their parent storms.