Brian Freitag 1 Udaysankar Nair 1 Yuling Wu – University of Alabama in Huntsville

Introduction In regions with complex topography, orographic forcing can lead to catastrophic flash floods. Geographic locations in North America impacted by such floods include: Appalachians (VA), Rockies (CO, Alberta, Canada) and Cascades (WA/OR). Current operational models do not adequately resolve terrain features that force such events and thus have difficulty in forecasting these events

Introduction: Prior research Flesch and Reuter (2012); Chung et al. (1976); Shi et al. (2010); Deb et al. (2008); Colle and Mass (2000); Lin and Colle (2009) – Model simulated maximum precipitation was generally < 80 % of observations. – Maximum precipitation location was offset by > 50 km compared to observations – Model performance improved with horizontal resolution.

Objectives With increase in computational efficiency, it is feasible to conduct operational numerical model simulations that better resolve such events (grid spacing of 1km or less). In this context, this study examines the sensitivity of such simulations to initial conditions, choice of parameterization schemes and numerical model configurations.

Study Area: Black Hills of SD The Black Hills region experiences a significant flooding event about 2 times per decade (Driscoll et al., 2010). Events are associated with high precipitable water and shortwave disturbance that initiates convection From Driscoll et al.(2010)

Case Study (08/17 – 08/18/2007) Convective severe thunderstorms persist from 21z – 03z Supercell development for nearly an hour before storm merging occurred. – Reports of 3 inch hail in Piedmont, SD Localized heavy rainfall ranging from 3-10 inches STP. Precipitation footprint similar to historic 9-10 June 1972 event

Models: – WRF-ARW Run from 00z 17 August – 00z 19 August Initialized at 00z on both days using GFS (1°) and NAM (12 km) Analysis Data 3 nested grids (27 km (parent), 9 km, 3 km, 1 km) Varied vertical resolution (discussed later) – 24  33  42 vertical levels < 3 km above the surface.

Models: – Ocean Land Atmosphere Model (OLAM) Initialization: GFS 12UTC and 18UTC of August Global hexagonal grid of ~200 km with regional refinement over Black Hills ~1.5km spacing. Multiple combinations of convective parameterization, diffusion schemes considered.

Topography

Results – Radar Reflectivity ml=55 ml=75 ml=55 ml=65 GFS output NAM output KUDX Radar

Results – Radar Reflectivity ml=55 ml=75 var ml=55 var ml=65 var GFS output NAM output KUDX Radar

Results – Accumulated Rainfall ml=55 ml=75 ml=55 var ml=65 GFS output NAM output KUDX Radar mm8.35 mm mm 8.35 mm mm29.75 mm 7.36 mm 25.49mm29.75 mm 8.29 mm mm mm

Results – Synoptic Setup

Results – OLAM Simulations Initial = 18Z, Emanuel, Smagorinsky Initial = 18Z, Emanuel, ACM2 Initial = 18Z, Grell, Smagorinsky Initial = 12Z, Grell, Smagorinsky

Results – Synoptic Setup

Summary Numerical simulation of flash flood events are very sensitive to vertical grid spacing, initial conditions and choices of parameterization schemes. Shortwave disturbance is modulated by convection initiated over complex terrain upwind of Black Hills, which potentially contributes to the sensitivity. Use of finer resolution NAM analysis improves the simulation. Due to enhanced sensitivity to choice of parameterization schemes, ensemble approach for forecasting is recommended.

Future Work Increase model resolution over Bighorn national park in Wyoming. – Identify why the model is unable to represent the atmospheric flow around the terrain. Compare WRF results with OLAM results. – Vary parameterization schemes Perform a probabilistic analysis of the precipitation patterns and heights.

Questions?