Gravity wave breaking over the central Gravity wave breaking over the central Alps: Role of complex topographyAlps: Role of complex topography

Qingfang Jiang, UCAR/NRL, Monterey

James D. Doyle, NRL, Monterey, CA

Acknowledgements: MAP scientists and staff.

COAMPS Grid 1

1200 UTC (6 h) October 21, 1999

Terrain (gray scale)

Wind vectors (500mb)

Geopotential Height Contours

(500mb)

Large Scale Conditions

Terrain

Electra flight track

Turbulence

Upstream Sounding

SABL + Vertical Displacement

0

(x) (w / U)dx

GPS Dropsonde Trajectories1 2 3 4 5 7 6

 

100km

20km

309

311

307

1 2 3 4 5 7 6

 

Manual Isentropic Analysis of GPS Dropsonde Data

309

311

307

1 2 3 4 5 7 6

 

Manual Isentropic Analysis of GPS Dropsonde Data

Along Flight Track Wind Component

21

5

-2

5

20

Vipiteno Soundings (0600, 0900 UTC)

V (m/s)

W (m/s)

Potential temp. (K)

Terrain (m)

Flight Level Data Examples

TKE, Leg1

TKE, Leg2

Buoyancy Production Rates

Terrain

Turbulent Kinetic Energy

COAMPS, 4th mesh (~1 km)

Potential temperature

(solid contours)

Along flight wind component

(in grayscale)

Turbulent kinetic energy

(dashed lines)

COAMPS 2D Idealized Simulations

Smooth terrain Smaller-scale terrain superposed on the lee-slope

h=hw + hm*[1-cos(2πkx/a)]

Drag vs. Wave Number of Small Scale Terrain

h=hw+hm*[1-cos(2πkx/a)]

Where,

hw: is the large-scale terrain height,

a: is the large-scale terrain width,

k: is the small scale terrain wave number

Conclusions

• The observed wave-breaking event was associated with the presence of a critical level, backward wind shear, and small Richardson number.

• GPS dropsondes observed strong flow descent associated with severe down-slope winds, and local convective instability in breaking regions. The structure of the wave-breaking section resembles a hydraulic jump.

• The underlying terrain and observed waves show multiscale features.

• Idealized simulations indicate that small-scale terrain superposed on larger scale terrain promotes wave breaking and enhances downslope winds, turbulence, and drag.