1. Daniel M. Alrick 14th Cyclone Workshop Monday, September 22, 2008
Modeling of a Narrow Cold Frontal Rainband to Assess the Mechanisms Responsible for the Core-Gap Structure
Daniel M. Alrick
14th Cyclone Workshop
Monday, September 22, 2008

2. What is a NCFR? Typically form in neutrally stable air mass
Large
Gaps
Precipitation
Cores km
Velocity of
Synoptic-Scale
Front
Synoptic-
Scale Front
Mesoscale
10 m s -1
30 dBz
0612 UTC 8 Dec 1976
(Hobbs and Biswas 1979;
Hobbs and Persson 1982; Parsons and Hobbs 1983)
Typically form in neutrally stable air mass
Can have a corrugated structure – band of alternating precipitation cores and gap regions
Weather conditions can vary rapidly along a NCFR, occasionally producing severe weather

3. Motivations for Research
What are the dominant dynamical and microphysical mechanisms that produce these structures?
Orientation of cores relative to the front
Shape of cores and updrafts
Spacing of cores
Observational studies of NCFRs are numerous, but modeling studies are limited
NCFRs are a good test phenomena for mesoscale models

4. Formation Mechanisms
Horizontal Shear Instability (Haurwitz, 1949; Matjeka, 1980)
Wind shear along cold front due to low-level jet
Predicts core spacing of 7.5 times shear zone width
Precipitation/Cloud Microphysics (Locatelli, 1995)
Diabatic cooling due to precipitation in cores enhances frontal discontinuity
Positive feedback amplifies perturbations along the front
Hydrometeors are advected downstream with prevailing flow, producing elliptical core structure
Trapped Gravity Waves (Brown, 1999)
Updrafts act as barriers to along-front flow

5. Case Study: January 1997
Wakimoto and Bosart (2000) provided 3-D airborne Doppler wind observations of a NCFR with unprecedented coverage and detail
Wakimoto and Bosart, 2000

6. Mesoscale Model Configuration
NCFR modeled using WRF-ARW and MM5
MM5 solution better matched observations
Three nested domains
36-km
12-km
2.4-km
Initialized with NCEP/NCAR re-analysis data
Bulk microphysics with 5 hydrometeor types, MRF PBL, 37 levels
Model Domains

7. 2.4-km domain, 18Z
12-km domain, SLP, Surface winds, and 1-km vertical velocities (red is positive)

8. Wakimoto and Bosart, 2000

9. Difference: Gradient is much stronger in observations
Wakimoto and Bosart, 2000
Reflectivity (shaded) and horizontal velocities (black contours)
Similarity: Stronger horizontal velocity gradient in core regions in both observations and model results
Difference: Gradient is much stronger in observations

10. Core spacing predicted by HSI
HSI predicts spacing of observed cores, but not cores in model simulation

11. Observations place cores north of updrafts
Wakimoto and Bosart, 2000
Reflectivity (shaded) and vertical velocities (black contours)
Updrafts and precipitation cores are elliptical in both cases, not circular
Observations place cores north of updrafts
MM5 solution places cores co-located, or slightly west, of updrafts

12. 5km
Wakimoto and Bosart, 2000
Reflectivity (shaded), vertical velocities (black contours), and core-relative winds (black vectors)
Air parcel trajectories from observations indicated elliptical-shaped core is due to northeastward advection of hydrometeors
Hydrometeor trajectory from model results show precipitation falling down through updraft, moving slightly southwestward

13. Sensitivity Test
Test importance of diabatic processes on NCFR formation, corrugation, maintenance, and cell shape and spacing
Ramp down evaporative cooling ~1hr before NCFR broke into core/gap structure, ~3hr before time of analysis (18Z)

14. Core/gap structures still form without diabatic heating
Control Run
No Evaporative Cooling, After Three Hours
Reflectivity (shaded), vertical velocity (black contours), and horizontal wind (black vectors), at 400m AGL, both plots at 18Z
Core/gap structures still form without diabatic heating
Little change in updraft shape, core shape, speed, and direction
Updrafts and reflectivity values are weaker in sensitivity test

15. Frontal discontinuity weakens without evaporative cooling
18Z
Control Run
No Evaporative Cooling, After 3 Hours
Cross sections perpendicular to front, through core regions – Potential temperature (black contours), equivalent potential temperature (color), front-relative winds (black vectors), along-front wind magnitude (blue contours)
Frontal discontinuity weakens without evaporative cooling
Shear zone becomes weaker and wider

16. Core spacing predicted by HSI
Core spacing increases over time in sensitivity test
Some cores are dying out without evaporative cooling feedback

17. Concluding Thoughts
Core-gap structure follows HSI theory; discrepancies in model run likely due to diffusion
Elliptical core shape is caused by elliptical updraft shape
Displacement of observed core from updraft due to advection of hydrometeors
Corrugated structure still formed in absence of evaporative cooling
Diabatic processes seem important in maintaining strength of front and rainband
Sensitivity test showed core separation increased as shear zone widened