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

What is a NCFR? Typically form in neutrally stable air mass Large Gaps Precipitation Cores 0 20 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

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 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 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 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

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

Wakimoto and Bosart, 2000

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

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

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

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

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)

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

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

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

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