1. Evaporation Duct Profile Comparisons Using Kites and Bulk Methods
LT Deborah L. Mabey
March 20, 2001

2. Acknowledgements Professor Davidson Professor Guest Paul Frederickson
everyone on Leg II, especially Jennifer Hauser

3. Pictures
                
                          

4. Purpose Calculate M directly using a rawinsonde suspended from a kite
Calculate M profiles using bulk methods from
ship’s sensors
kite data
Compare profiles and associated duct heights
Determine the effect of any differences on radar (using AREPS for visualization)

5. Background
vertical gradient of M (modified refractivity) determines how far the horizon is
dM/dz < 0 greatly increases distance to the horizon
M = f( p, T, e )
dM/dz < 0 near the surface due to strong negative vapor pressure gradient caused by evaporation (hence the name evaporation duct)

6. Evaporation Duct 15 10 5 Z0
Z (m) M
RH (%)

7. More background M can be calculated directly:
M profiles can be obtained using bulk methods (which require T, p, e, u, z and an iterative method to determine the scaling parameters)
See Air/Sea Interaction (MR/OC 4413) & EMEO (MR 4416) course notes for painful details

8. Effects of Ducts ) (Hz x 10 Significantly affected by duct MHZ6 10 4
(Hz x 10
Significantly affected
by duct 10 3
MHZ 10 2
Not Significantly
affected by duct
Frequency in 10
/ /120
600/180
800/ /300
Duct Thickness (ft/m)

9. Measurements Rawinsonde suspended from kite
time, height, pressure, air temperature, relative humidity, dew point temperature (recorded every 2 seconds while kite was flying)
R/V Point Sur’s SAIL system
time, pressure, air temperature, relative humidity, port true wind speed, boom temperature (recorded every 52 to 56 seconds)
NPS Meteorology Department instruments
time, pressure, air temperature, relative humidity, true wind speed, infrared detected sea surface temperature (recorded every 5 seconds and integrated over 2 minute intervals)
Hand-held infrared sensor
sea surface temperature (recorded hourly)

10. Problems
Flying a kite with a rawinsonde attached is not as easy as it sounds
Surface pressure was not constant while the kite was being flown
NPS Meteorology Department IR sea surface temperature sensor did not work while in San Francisco Bay

11. SST and pressure problems

12. Solutions (assumptions)
Although the kite was flown on three separate occasions, only the last flight is presented (assuming that skill of flying kite away from ship increased with practice)
Negative heights (due to varying pressure) corrected by assuming minimum rawinsonde height was always 1 meter
Hand-held infrared sea surface temperatures were substituted for unreliable NPS infrared detected sea surface temperatures for use in bulk profiles

13. Analysis
calculated M for each data point for three (2 up, 1 down) kite profiles (blue x’s)
averaged M at each z level (blue o’s) and used a best-fit cubic (blue line)
calculated bulk M profile using ship’s data (red line)
calculated bulk M profile using NPS data (with hand-held IR SST) (green line)
Calculated bulk M profiles using kite data at approximately ¼ (cyan line), ½ (magenta line) and ¾ (black line) of the maximum height of the kite (with hand-held IR SST and NPS wind speed)

14. Results
All graphs shown are for experiment 17 but are representative of all results shown in the table

15. Pressure

16. Temperature

17. Relative Humidity

18. Modified Refractivity

19. Evaporation Duct Height (meters)
Experiment
17 (up)
141 data points
18 (down)
261 data points
19 (up)
Kite
14.4
16.9
13.1
SAIL
6.7
6.2
5.9
NPS
6.4
6.8
7.6
7.3
7.1
7.0
7.5

21. AREPS GENERIC Radar

22. Any Questions? ?