1. Examination of a Boundary Layer Jet
LCDR Marc E. Touchton
19 Sept 2001

2. Outline Introduction Dynamics Data Assumptions Results
Sources of Error
Conclusions

3. Before
Port San Luis
“This won’t be too bad”

4. Introduction
R/V Pt Sur experienced strong winds and rough seas August 06-07, 2001
Rawinsondes indicated the existence of a boundary layer jet offshore
Conditions were noticeably improved close-in to shore

5. After
Big Sur Coast
“Please, can I keep something down now?”

6. Scatterometry Winds

7. Dynamics
U= -g/f H/y
A sloping surface in the positive y-direction will produce a U>0(out of the board) y H

8. Dynamics(cont) Thermal wind is a measure of the vertical shear
VT = Vg(p1) – Vg(p0)
VT = R/f (<T>/x) ln(p0 /p1)
R=287 J kg-1 K-1 ; f=10-4 s-1 y
Vg1
Vg0 VT

9. Data Three rawinsondes depict the boundary layer jet Launch times:
Aug 2001
Aug 2001
Aug 2001
Used down part for plotting and most calculations

10. Map

11. Station 1

12. Station 2

13. Station 3

14. Layer Heights

15. Temperature Profile

16. Wind Speed Profile

17. Wind Direction Profile

18. Assumptions Ignored temporal variation
Assumed bearing between stations was in positive x-direction
Assumed no u-component for thermal wind

19. Results – Computing V
Boundary Layer (1-3: 64 m; 1-2: 39m; 2-3: 25 m; dx = 30 km)
From Station 1 – Station 3: V=20.9 m/s
From Station 1 – Station 2: V=12.74 m/s
From Station 2 – Station 3: V= m/s
Station 1: 15.2 m/s
Station 2: 17 m/s
Station 3: 13.9 m/s
Mean Computed: m/s; Mean Actual: m/s

20. Results – Computing V
Top of Inversion (1-3: 67 m; 1-2: 48m; 2-3: 19 m; dx = 30 km)
From Station 1 – Station 3: V= m/s
From Station 1 – Station 2: V=15.68 m/s
From Station 2 – Station 3: V= m/s
Station 1: 15.2 m/s
Station 2: 17 m/s
Station 3: 13.9 m/s
Mean Computed: m/s; Mean Actual: m/s

21. Results – Computing VT Calculated Mean Temperature for each station
Station 1: ºC ( – mb)
Station 2: ºC ( – mb)
Station 3: ºC ( – mb)
Using Mean Horizontal Temperature and Mean Horizontal Pressures VT = 74 m/s
Actual Calculated VT:
Station 1: m/s; Station 2: m/s; Station 3: m/s

22. Sources of Error Rawinsonde wind averaging Temporal variations
Boundary layer determination
Bearing for calculations not perpendicular to wind
Averaging technique

23. Conclusions Boundary Layer Jet does exist
Larger set of data needed for more exact determination of actual intensity, extent and location of jet
Ideal deployment of rawinsondes would be perpendicular to coast with little temporal variation

24. Questions?
“We have splash down!”