1. Evaporation Duct Profiles
Lt. Charlotte A. Welsch
OC 3570
19 MAR 02

2. Evaporation Duct Profiles
Comparisons using Kites and Bulk Methods
Review Evaporation Duct
Results
Conclusions

3. An over the water surface duct which increases distances of radar transmissions is likely to form when there is significant temperature and humidity gradients.
Three parameters determine duct height: Pressure, Temperature, and water vapor.
Relative humidity has greatest impact on evaporation ducts

4. Evaporation Duct Effect
Effect Factor
Tair = 19 C
EDH = 7.6 m
Wind speed = 10 kts
RH = 80%
SST = 20 C
Tair = 21 C
EDH = 14.8 m 40 30 20 10
SPS-12
1.5 GHz
at 44 m
Duct Height (m)
SPS-10
5 GHz
at 39 m
Radar Detection Range (nm)
Detection range for a destroyer size
target vs duct height

6. 29 Jan 1800
29 Jan 2200
One average period fits bulk method better than smaller periods
2 Feb 2200

7. 29 Jan 1800 One average period Bin averages: 2m 4m 8 m
1m 2m m
29 Jan 1800
Showing bin averages really don’t change the result when one average period 3

8. Edited/1st ave 29 Jan 2200 Edited/2nd ave One average Edited/3rd ave
Each kite profile has many up/downs. When the kite is brought down it comes closer to the ship than any other time. When editing out the very bottom of the down segment,
It is possible to improve the profiles from the overall average as seen here.

9. One must note it is difficult to cut out the bottom data and does not give a consistant improved profile as seen here for 29 Jan 1800

10. 29 Jan 1800
29 Jan 1800
29 Jan 2200
29 Jan 2200
It appears that the M profile is closer to the bulk method when the relative humidity is higher in the time series.
This could be because the relative humidity increase is indicating the kite is actually close enough to the surface
To record the rh and temp gradients needed to have an evap duct
2 Feb 2200
2 Feb 2200

11. One up down was examined where rh was hi and a better profile was the result

12. Conclusions: Subjective Error Lower than 3-5 meters unreliable
swell, kite dipping ht, sonde response
Contaminated averages/uncertainty
Bulk Method showed a higher duct height than kite data
Shorter periods did not represent bulk method/Atmosphere variability important