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

Outline Introduction Dynamics Data Assumptions Results Sources of Error Conclusions

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

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

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

Scatterometry Winds

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

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

Data Three rawinsondes depict the boundary layer jet Launch times: 1323 06 Aug 2001 2046 06 Aug 2001 0154 07 Aug 2001 Used down part for plotting and most calculations

Map

Station 1

Station 2

Station 3

Layer Heights

Temperature Profile

Wind Speed Profile

Wind Direction Profile

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

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=8.1667 m/s Station 1: 15.2 m/s Station 2: 17 m/s Station 3: 13.9 m/s Mean Computed: 13.94 m/s; Mean Actual: 15.37 m/s

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=21.8867 m/s From Station 1 – Station 2: V=15.68 m/s From Station 2 – Station 3: V=6.2067 m/s Station 1: 15.2 m/s Station 2: 17 m/s Station 3: 13.9 m/s Mean Computed: 14.59 m/s; Mean Actual: 15.37 m/s

Results – Computing VT Calculated Mean Temperature for each station Station 1: 18.3692 ºC (1011.3 – 688.4 mb) Station 2: 17.5111 ºC (1012.7 – 638.6 mb) Station 3: 17.1427 ºC (1011.5 – 624.8 mb) Using Mean Horizontal Temperature and Mean Horizontal Pressures VT = 74 m/s Actual Calculated VT: Station 1: -21.5 m/s; Station 2: -24.5 m/s; Station 3: -19.9 m/s

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

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

Questions? “We have splash down!”