P RINCIPLES of STABLE ISOTOPE METEOROLOGY

1. PRINCIPLES of STABLE ISOTOPE METEOROLOGY ABSTRACT The heavy isotopes of water (HDO and H218O) act as tracers of atmospheric processes and motions. Three field experiments (Puerto Escondido, Mexico, KWAJEX, and CAMEX 4) show that 1: Rain systems with deep, extensive clouds and organized, closed circulation (e. g., hurricanes) produce rain and with heavy isotopes in concentrations that are markedly lower than in the surrounding atmosphere. 2: Low-level outflow from a storm limits the decrease of heavy isotope concentrations in the vapor and rain, but transports isotopically depleted vapor to the surrounding atmosphere.

2. H218O Concentrations and d Terminology Oxygen-18 is a rare species in water. [H218O:H2O = .002] -60 Ice Crystals: Jet Stream Snow: South Pole First Snow: Blizzard Rain: Hurricane Eyewall Rain: Hurricane Mean Rain: Summer and Tropics Ocean Water d terminology gives H218O concentration of sample (RSAMPLE) relative to that of standard mean ocean water (RSMOW) in ppt. Most natural waters have negative d’s (lower H218O concentration than sea water) d18O (ppt) 0

3. Fractionation H2O During condensation heavy isotopes, HDO and H218O concentrate in precipitation and are preferentially removed from atmosphere. Result: Heavy isotope concentrations of rain, snow, and vapor decrease with height H218O Rising Air PRECIP VAPOR Rayleigh distillation is the benchmark for fractionation processes. It assumes that all water and ice are removed from the atmosphere immediately upon condensation.

4. Diffusive Equilibration Early Rain Later Rain Falling rain acquires heavy isotopes from the ambient vapor by diffusion and removes them from the air. Result: Heavy isotope concentrations of rain and vapor decrease as rain accumulates and downwind. Vapor Vapor H218O H218O Height H218O H218O

5. d18O and STORM STRUCTURE Because heavy isotope concentrations decrease with altitude, with accumulated rain, and downwind inside the rain shield, patterns of d18O reflect major features of storm structure such as 1: storm size and duration, 2: cloud thickness, 3: mean condensation height, 4: low-level convergence and outflow. Back trajectories show how much precipitation air has passed through. They therefore indicate the expected degree of depression of d18O values below a Rayleigh distillation model. Typical spatial distributions of d18O values of rain in hurricanes and extratropical cyclones are shown below.

6. Extratropical Storm with Overrunning d18O decreases poleward due to overrunning until near polar edge where rain acquires 18O from ambient vapor. Warm (-5) (-20) (-10) Cold Hurricane d18O decreases inward until near eyewall where sea spray in high wind restores 18O. (-2) (-15) (-8) Eye (-8) (-15) (-2)

7. d18O values of Vapor in Hurricanes are depressed below Rayleigh distillation curve

8. d18O of Vapor from Three Field Experiments Key West Clear Mexico Clear d18O vs specific humidity during three field experiments. In placid weather far from organized storms, d18O values increase as specific humidity increases. In or downwind from organized storms, d18O values are much lower since 18O has been removed Kwajalein Clear Key West TS Kwajalein ITCZ Mexico TS Highest d18O values occurred in CAMEX 4 with almost no organized storm activity. Intermediate values occurred during KWAJEX with loosely organized disturbances on ITCZ. Lowest values occurred as tropical depressions and storms brushed Puerto Escondido, Mexico

9. HURRICANE FLOYD 14-17 September 1999 1. First rains at any location had high 18O concentration due to evaporation and acquisition from unprocessed vapor. 2. 18O concentration of rain rapidly decreased to first minimum in association with elevated frontal surface. 3. 18O concentration increased as frontal surface lowered. 4 . A final period of extremely low 18O concentration occurred west of storm center due to two factors A: modest lift of frontal surface, B: removal of heavy isotopes by rain upwind.

10. Floyd asymmetric so 18O decreases west of center. Floyd symmetric so 18O increases west of center.

11. Tracing d18O of VaporWith Back Trajectories To the west of the hurricane center, the wind and trajectories (red arrow in the radar image below) came from the north. Since they passed through a large area of steady rain before reaching New Jersey, d18O values were lowered. To the east of the hurricane center, the wind and trajectories (white arrow in image below) came from south. Since they only passed through spotty convection, d18O remained high. Trajectories shown below and cross sections above were produced by running the Meteorological Community’s Mesoscale model MM5 to simulate Hurricane Floyd.

12. Radar Image 2140 UTC 16 Sept 1999 6 9

13. PUERTO ESCONDIDO, MEXICO 10 - 31 July 1998 Puerto Escondido, Mexico is located in a region of active tropical cyclogenesis. During period 10-31 July 1998, two tropical storms and one tropical depression brushed the Pacific coast of Mexico leaving wakes of vapor with anomalously low concentrations of heavy isotopes. Between stormy periods isotope ratios increased to normal tropical values. Regime changed from stormy period with generally low d18O values from 10-18 July to quiescent period with high d18O values from 18 to 31 July. This change of regime was marked by major change in air trajectories.

14. Tropical Cyclogenesis and 18O -10 -15 -25 -30 0 2 4 6 8 Organized Tropical Depressions Produce Vapor and Rain with low 18O Leave Vapor Wake with low 18O 18O (ppt) Area of Cloud Tops <= -50oC (%) 18O of Vapor Puerto Escondido July 1998 TMI Images 10 15 20 25 30 Day in July 1998 Organized System 10 July Quiescent Period Incipient System 21 July 28 July

15. Tracing d18O of VaporWith Back Trajectories Back trajectories and infrared satellite images below are shown for two different regimes 1. Low d18O regime 15 - 16 July 1998. Tropical cyclogenesis with organized precipitation. Back trajectory passed through the stormy region before reaching Puerto Escondido. 2. High d18O regime. 22 - 23 July 1998. Widely scattered convection. Back trajectory passed through mostly clear air before reaching Puerto Escondido

16. 24 hour back trajectory to 00 UTC 16 July 1998 with organized convection 18 UTC 15 July 1998 24 hour back trajectory to 00 UTC 23 July 1998 with disorganized convection 18 UTC 22 July 1998

17. KWAJALEIN EXPERIMENT 09August - 09 September 1999 KWAJEX was conducted at the northern edge of the Inter Tropical Convergence Zone (ITCZ). Several mesoscale wavelike disturbances developed during this time. Organized, closed circulation did not develop in any of these systems. Instead, much vapor was flushed by outflow at low levels. Consequently, while d18O values were depressed, they never attained the extremely low values associated with tropical storms and hurricanes. The slides below show the poorly organized structure of the systems and the low level outflow as indicated by back trajectories.

18. d18O During KWAJEX:Tracer of Organized Convection • Vapor with low d18O • Created in Rain Region • Advected Downwind • Persists After Rain Ends 190 T (K) 280 * * * * * * 224.75 225.75 226.75 227.75 228.75 229.75 Before Event Event Forms Event Upwind In Event In Wake After Event

19. Tracing d18O of VaporWith Back Trajectories • Trajectory Ending • 14 Aug 18 UTC • Remained in Clear Air • Has High d18O = -14‰ • Trajectory Ending • 16 Aug 18 UTC • Passed Under Rain Area • Has Low d18O = -19‰ 1 2 3 3 2 1 224.75 225.75 226.75 227.75 228.75 224.75 1 225.75 227.75 2 3 228.75 226.75 2 228.75 1 3 190 T (K) 280

20. REFERENCES Gedzelman, S. D., and J. R. Lawrence, 1982: The isotopic composition of cyclonic precipitation. J. Appl. Meteor., 21, 1385-1404. Gedzelman, S. D. and Arnold R. 1994: Modeling the isotopic composition of precipitation. J. Geophys.Res.99, 10455-10571. Gedzelman, S. D, .et. al., 2003: Probing hurricanes with stable isotopes of rain and water vapor. Mon. Wea. Rev., accepted. Lawrence J. R., and Gedzelman S. D. 1996. Low stable isotope ratios of tropical cyclone rains. Geophys. Res Lett. 23, 527-553. Lawrence, J. R., Gedzelman, S. D., Zhang, Z. and Arnold, R. 1998. Stable isotope ratios of rain and vapor in 1995 hurricanes. J. Geophys. Res. 103, D10, 11381-11400. Lawrence, J. R., S. D. Gedzelman, J. Gamche, and M. Black: 2002: Stable isotope ratios: Hurricane Olivia. J. Atmos. Chem., 41, 67-82. Lawrence, J. R., and S. D. Gedzelman: 2003: Tropical ice core isotopes: Do they reflect changes in storm activity? Geophys. Res Lett. 30, in press.