1. Assessing Precipitation Isotope Variability During
Phase Changes in the Hydrologic Cycle
Ross Harris
Dr. Hans Christian Steen-Larsen
Dr. Andrew Peters

2. Stable Water Isotopes
• Most Used: 18O, D
• Also used: D-excess (D - 18O *8)
1H
16O
1H 2H
16O
1H 1H
18O
1H
Regular water D 18O

3. Stable Water Isotope Behavior
• It takes more energy to evaporate heavy isotopes
from liquid water
• Light isotopes are preferentially evaporated
More Heavy
Isotopes
Less Heavy
Isotopes
LIQUID VAPOR

4. Stable isotopes are key tracers in the
hydrologic cycle.

5. What controls Isotopic Variability in
Precipitation?
• The “amount effect” in the tropics
• The “temperature effect” in mid-latitudes, polar
regions
• Most recently theorized, proportion of stratiform
vs. convective precipitation.
• These effects found in monthly rain samples

6. A B
C
A
B
C
Rainfall samples (n = 314) from BIOS had their isotopic
composition analyzed with a cavity ring down spectroscopy (CRDS)
Meteorological data from L.F. Wade Int. Airport identified
the amount of rain that had fallen for each BIOS sample
Meteorological and isotopic data was collected at
Tudor Hill to identify water vapor isotopologues,
rainfall rate, and other meteorological
parameters during rainfall events

7. Absolute Humidity
Temperature
18O Value of
Water Vapor
Rainfall Rate
Wind Direction
(Degrees)
Water Vapor
D-excess value

8. Results
• The average rain event caused a drop in
temperature, absolute humidity and water vapor
18O
• Also caused a rise in D-excess
• This behavior is explained by moist, rain-cooled
“downdrafts” during rain events

9. What are downdrafts?
• Downdrafts are
evaporation-
cooled, moist air
masses that are
dragged down
by precipitation

10. Sudden rise in water vapor
d-excess values
Sudden drops in
absolute humidity,
temperature, and
water vapor 18O
values
Sudden wind shift
Intense rainfall event

11. Average dip in water vapor 18O for each rainfall event

12. Accompanying the 18O dip is a decrease in
temperature

13. Also accompanying the 18O dip is an increase in water vapor D-
excess. Increases in D-excess in an air mass are caused by
evaporation occurring within the air mass.

14. Wind shifts greater than 45 within a rain event cause a dip
in water vapor 18O to dip as well. This is independent of
any other measured parameter.

15. Despite evaporation during a rain event, absolute
humidity values decline during rain events.

16. The minimum 18O value during each rainfall event
was recorded and compared to the 18O values
measured at BIOS.

17. A significant relationship exists between the minimum water vapor 18O value
measured at Tudor Hill, and the precipitation 18O values measured at BIOS.
Remarkable when these stations are ~25km away!

18. The relationship becomes stronger when a wind shift occurs during a rain event.

19. Conclusions
• The isotopic content of the water vapor of rain-
cooled downdrafts control the isotopic content
of single precipitation events
• This relationship is strongest when a wind shift
at the surface confirms a downdraft has mixed to
the surface.

20. Future Work
• Analyzing the extent evaporation of precipitation
impacts 18O content
• Determining the location of precipitation genesis
in clouds during rain events
• Finding the source location of water vapor that
eventually produces the analyzed rainfall

21. Questions?
• This research was supported by the NSF-REU grant (OCE-
1460686) awarded to the Bermuda Institute of Ocean Sciences
• This research would not be possible without the exceptional
guidance received from Dr. Hans Christian Steen-Larsen, Dr.
Andrew Peters, and the rest of the BIOS faculty, and the
support from my fellow REU students.