Chip Helms
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19 December 2012 1 Conclusions Results Methodology Backgroun d Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius Examining the Sensitivity of an Idealized Model to Changes in the Initial Potential Temperature Perturbation Chip Helms
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Examining the Sensitivity of an Idealized Model to Changes in the Initial Potential Temperature Perturbation. Chip Helms. Background. The Idealized Model: CM1. Created by George Bryan (NCAR) 3D, non-hydrostatic, non-linear, cloud-resolving, idealized model No data assimilation - PowerPoint PPT Presentation
Transcript of Chip Helms
19 December 2012
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Examining the Sensitivity of an Idealized Model to Changes in the
Initial Potential Temperature Perturbation
Chip Helms
19 December 2012
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
The Idealized Model: CM1• Created by George Bryan (NCAR)• 3D, non-hydrostatic, non-linear, cloud-resolving, idealized
model– No data assimilation– Uses a horizontally constant field for the base state– Adds perturbations to base state
• e.g. warm bubble, cold blob, forced convergence
• Benefits of using CM1– Conserves mass and energy better than other modern cloud
models– Faster and uses less memory than other models for idealized
studies– Very flexible, can be used for a large variety of studies
Background
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Goal
• Qualitatively examine CM1 sensitivity to:– Initial maximum magnitude of θ'– Initial horizontal warm bubble radius
• Potential implications of study:– Sensitivity of cloud-resolving models to
temperature anomalies• e.g. magnitude and extent of an urban heat island
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Methodology
• Two sets of runs to test sensitivities
Methodology
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Model SettingsMethodology
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Sensitivity to magnitude of θ'Results
Composite Reflectivity
Cell split occurs earlier as θ' increases
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Sensitivity to Warm Bubble Radius
Results
Cell splitting is related to the
interaction between vorticity
and updrafts
Cell split occurs earlier as radius
increases
Look at updraft strength evolution
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Results
0.5
K
Delay in reaching peak updraft
strength is non-linear function of θ'
1.0
K
1.5
K
2.0
K
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Results
Relatively low sensitivity at 7 km radius and above
Relatively high sensitivity at 6 km radius and below
Model Dispersion2-6Δx 2-6 km radii
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Conclusions
• As θ' or warm bubble radius increases, storm cell splits earlier and reaches peak updraft strength earlier– More sensitive to θ' than radius
• Small warm bubble radius runs have resolution and dispersion issues– Impacts 5 km radius most– Very little sensitivity at or above 7 km– Suggests sensitivity is due to model dispersion
Conclusions
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Questions
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Peak Updraft Limit
• Function of CAPE
CAPE = 1946 wmax = 62 m/s• Actual < 55 m/s• Difference due to
assumptions of CAPE
Conclusions
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Initial Background ConditionsMethodology
Hodograph corresponds to
these levels
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Hodograph Refresher
• Trace of wind vector direction (azimuth) and magnitude (radius)
• Straight hodographs suggest cells will split
• Veering (backing) hodographs suggest right (left) cell will be dominant
Methodology
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Model Settings - Continued
• Boundary conditions:– Open-radiative lateral boundary conditions– Zero-flux top/bottom boundary conditions
• Not included in these runs:– Atmospheric radiation– Surface drag– Surface fluxes
Methodology
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Sensitivity to magnitude of θ'
• bfds
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Outlying Model Runs
• θ' = 0.5K lags behind other runs• Radius = 5 km lags significantly behind
other runs– Possibly due to turbulent mixing having a
greater impact on tight gradients • smaller size of anomaly would be diffused faster
– Could also be due to dampening near the 2Δx scale (recall Δx = 2 km)
Conclusions
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Detailed model settings• 5th order horizontal/vertical advection schemes• Negative moisture is corrected by taking moisture from adjacent grid cells• No additional artificial diffusion beyond subgrid turbulence scheme• Sixth order diffusion scheme (coefficient = 0.040)• TKE subgrid turbulence scheme• Zero flux boundary condition for vertical diffusion of winds/scalars at top/bottom of domain• Uses Rayleigh damping at upper levels (e-fold time = 1/300, applied above 14km), but not near
horizontal boundaries• Uses Klemp-Wilhelmson time-splitting, vertically implicit pressure solver (as in MM5, ARPS, WRF),
coeff for divergence damper = 0.10, slight foreward-in-time bias used for vertically implicit acoustic solver (alpha = 0.60)
• Moisture scheme: Morrison double-moment scheme– Hail is used for large ice category– cloud droplet concentration: 250 cm^-3 (marine=100,continental=300)
• No Coriolis force• Includes dissipative heating• No energy fallout term• Open-radiative lateral boundary scheme: Durran-Klemp (1983) formulation• Initial base-state sounding: Weisman-Klemp analytic sounding• Initial base-state wind profile: RKW-type profile• Initial pressure perturbation is zero everywhere
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ConclusionsResultsMethodologyBackground
Chip Helms Sensitivity of CM1 to Initial θ' Magnitude and Radius
Sensitivity to magnitude of θ'Results
Composite Reflectivity
Cell split occurs earlier as θ' increases
Cell splitting is related to the
interaction between vorticity
and updrafts
