Modeling of a Narrow Cold Frontal Rainband to Assess the Mechanisms Responsible for the Core-Gap Str...
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Modeling of a Narrow Cold Frontal Rainband to Assess the Mechanisms Responsible for the Core-Gap Structure. Daniel M. Alrick 14 th Cyclone Workshop Monday, September 22, 2008. Synoptic- Scale Front. Mesoscale Front. Precipitation Cores. Gaps. Velocity of Precipitation Cores. 30 dBz.

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Daniel M. Alrick 14 th Cyclone Workshop Monday, September 22, 2008

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Daniel m alrick 14 th 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


What is a ncfr

Synoptic-

Scale Front

Mesoscale

Front

Precipitation

Cores

Gaps

Velocity of

Precipitation

Cores

30 dBz

-1

10 m s

Large

Gaps

0 20

km

Velocity of

Synoptic-Scale

Front

0612 UTC 8 Dec 1976

(Hobbs and Biswas 1979;

Hobbs and Persson 1982; Parsons and Hobbs 1983)

What is a NCFR?

  • 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


Motivations for research

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


Formation mechanisms

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


Case study 12 13 january 1997

Case Study: 12-13 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


Mesoscale model configuration

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


Daniel m alrick 14 th cyclone workshop monday september 22 2008

2.4-km domain, 18Z

12-km domain, SLP, Surface winds, and 1-km vertical velocities (red is positive)


Daniel m alrick 14 th cyclone workshop monday september 22 2008

  • Similarity: Stronger horizontal velocity gradient in core regions in both observations and model results

  • Difference: Gradient is much stronger in observations

Reflectivity (shaded) and horizontal velocities (black contours)


Daniel m alrick 14 th cyclone workshop monday september 22 2008

Core spacing predicted by HSI

  • HSI predicts spacing of observed cores, but not cores in model simulation


Daniel m alrick 14 th cyclone workshop monday september 22 2008

  • 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

Reflectivity (shaded) and vertical velocities (black contours)


Daniel m alrick 14 th cyclone workshop monday september 22 2008

  • 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

5km

Reflectivity (shaded), vertical velocities (black contours), and core-relative winds (black vectors)


Sensitivity test

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)


Daniel m alrick 14 th cyclone workshop monday september 22 2008

Control Run

No Evaporative Cooling, After Three Hours

  • 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

Reflectivity (shaded), vertical velocity (black contours), and horizontal wind (black vectors), at 400m AGL, both plots at 18Z


Daniel m alrick 14 th cyclone workshop monday september 22 2008

18Z

Control Run

No Evaporative Cooling, After 3 Hours

  • Frontal discontinuity weakens without evaporative cooling

  • Shear zone becomes weaker and wider

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)


Daniel m alrick 14 th cyclone workshop monday september 22 2008

  • Core spacing increases over time in sensitivity test

  • Some cores are dying out without evaporative cooling feedback

Core spacing predicted by HSI


Concluding thoughts

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


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