1. INFRASOUND PROPAGATION CALCULATION TECHNIQUES USING MESOSCALE ATMOSPHERIC AND TERRAIN SPECIFICATIONS
Robert Gibson1, Douglas Drob2 and David Norris1
1BBN Technologies
Arlington, VA, USA
2Naval Research Laboratory
Washington, DC, USA
Infrasound Technology Workshop, Tokyo, Japan
November 2007

2. Motivation
Calculation of infrasound propagation paths is necessary for event identification, phase association and source location
Explaining details of infrasound propagation relies on:
Propagation models that capture the fundamental physical processes
Accurate characterization of the propagation medium
Linkages to near-real-time atmospheric characterizations enable higher-fidelity infrasound propagation modeling
Infrasound propagation at regional ranges
Characterized by higher frequencies than at global ranges
Effects of the boundary conditions of the propagation domain, in particular the variable terrain elevation, become increasingly important
Model validation and calibration using observed events and ground truth is necessary

3. Recent Efforts Development of techniques for utilizing:
accurate, high-resolution regional atmospheric specifications
terrain elevation databases
Mesoscale atmospheric models that:
focus on the meteorology of a specific region
account for and resolve wind and temperature phenomena relevant to regional and local infrasound propagation
provide atmospheric profiles that are consistent with the variable terrain elevation in a region
Investigation of realistic atmospheric and terrain specifications at a range of resolutions to determine appropriate spatial and temporal scales necessary to improve infrasound predictions at relevant frequencies and ranges

4. Enabling Technologies and Tools
InfraMAP tool kit
Infrasonic Modeling of Atmospheric Propagation (BBN)
NRL-G2S
Naval Research Laboratory Ground to Space Specification
Numerical Weather Prediction (NWP) models
Operational physics-based atmospheric models
Including regional (mesoscale) models, such as:
Weather Research and Forecasting Model (WRF)
Collaborative effort: National Center for Atmospheric Research (NCAR); NOAA/ National Weather Service/ National Centers for Environmental Prediction (NCEP); Air Force Weather Agency; Naval Research Lab; Oklahoma Univ.; FAA, etc.
Output is being integrated into G2S-mesoscale framework
1-hour intervals, 0-20 km altitude, up to 10-km resolution (with higher resolutions possible if necessary)

5. InfraMAP Modeling Schematic

6. Mesoscale Integration Techniques
Framework for high-resolution regional specification
Atmosphere and topography considered
Incorporating synoptic and mesoscale numerical weather analyses into NRL Ground-to-Space global specification

7. Mesoscale Atmospheric Models
Regional Numerical Weather Prediction (NWP)
Physics-based synoptic models; domain is smaller than global.
Assimilation of atmospheric data from many sources.
Accounts for terrain and surface effects on the atmospheric dynamics.
Nested grid models, interpolating and enhancing global models.
Relocatable grids, user-defined dimensions and resolutions.
Terrain-following sigma-coordinate system at the lower boundary.
Must be merged with global specifications at higher altitudes.
Available at update frequencies up to 15 minutes.
Range (km) from western edge of specification domain.
Example of mesoscale atmospheric wind specification for an east-west vertical cross-section of southwestern US, showing variable vertical resolution of sigma coordinate system and terrain-following grid domain.

8. Comparison of Wind Specifications
Wind magnitude shown below over southwest US domain, ~2.5 km altitude.
Global G2S, at 2.5 km altitude
G2S-Mesoscale, at contour level 15, approx. 2.5 km alt.

9. Regional Topography Specifications
Terrain elevation model used to specify the lower boundary of model domain
Height (km) above mean sea level of the first (lowest) layer of a mesoscale atmospheric specification

10. Regional Weather Specification
Meridional Wind (SW US, over White Sands, New Mexico)
Horizontal axis is distance from western edge of specification domain.
March 25, 2006 , 9UT

11. Range Dependence in Atmosphere
Zonal
Local/regional variations may be significant
Shown are 20 zonal wind profiles at 15 km intervals (along a meridian)
Green: from the WRF mesoscale weather analysis.
Red: the WRF profile at the midpoint of the 300 km path.
Blue: profile from global G2S, corresponding to the midpoint.
Terrain elevation at grid cells shown by red dots
Example shown: Midpoint location is I31KZ
WRF
Local
G2S

12. Comparison of Specification Profiles

13. Example Mesoscale Domains

14. Atmosphere over Mesoscale Domain
Static sound velocity (m/s) at a height 10 m above the ground/ocean surface
Vertical Wind Slices (along white line segments) are shown in next slide

15. Examples: Wind Specification
Winds (m/s), from 0-20 km, across slices of SW US domain shown in previous slide
North-south vertical cross-section of the zonal (east-west) wind fields (in m/s)
East-west vertical cross-section of the meridional (north-south) wind fields (in m/s)

16. Terrain Effect in Ray Tracing Model
Hypothetical modeling scenario shown over 450 km path from source (S) to receiver (R)
Topography over great circle path between source and receiver

17. Ray Interaction with Surface
Typical eigenray solution, for stratospheric path
Interaction with ground at first bounce, in detail

18. Propagation Example (1)
Propagation paths from elevated source (left) to ground-based receiver (right)
Propagation paths using global G2S
Propagation paths using G2S-Mesoscale

19. Propagation Example (2)
Propagation paths from elevated source (left) to ground-based receiver (right)
Propagation paths using global G2S
Propagation paths using G2S-Mesoscale

20. Conclusions
High-resolution mesoscale grids enable propagation calculations suitable for regional events.
Improved characterization of the lower regions of the atmosphere
Incorporation of terrain effects on propagation
Specifications of arbitrary regional domains of interest can be generated with the Weather Research and Forecasting (WRF) model using observational inputs from global scale weather models.
Modeling capabilities for mesoscale atmosphere and terrain are in use.
Capability developed to supplement existing global G2S specifications with high-resolution mesoscale-G2S specifications
Significant progress on production of multi-day mesoscale specifications  
New capabilities are being integrated within InfraMAP
Ongoing analysis of ground truth infrasound events
Understanding of mesoscale effects on propagation characteristics
Determination of appropriate spatial length scales in atmosphere and terrain specification for propagation modeling at infrasonic frequencies of interest