Gravity Wave Drag (GWD) & Mountain Blocking (MB)

• “Mountain blocking” (Lott & Miller, 1997)

  • Wind flow around subgrid orography
  • Low-level flow is blocked below a dividing streamline (air flows around, not over barrier)

• Gravity wave drag (Alpert et al., 1988, 1996; Kim & Arakawa,1995)

  • Mountain wave stress, pressure drag
  • Vertical distribution of the wave stress, changes winds aloft (momentum deposition)

• Effect/influence of subgrid scale orography controlled by tuning parameter SIGFAC, via:

  • SIGFAC * Standard_deviation_of_terrain
  • Set SIGFAC=0 in 12-km NAM after extensive tuning experiments