HR: 1340h
AN: B13B-1189    [Abstracts]
TI: Seasonal Patterns of Precipitation Isotopic Composition in the Luquillo Mountains, Puerto Rico
AU: * Scholl, M A
EM: mascholl@usgs.gov
AF: U.S. Geological Survey, 431 National Center, Reston, VA 20192, United States
AU: Shanley, J B
EM: jshanley@usgs.gov
AF: U.S. Geological Survey, 87 State St., Rm. 324, Montpelier, VT 05602, United States
AU: Zegarra, J P
EM: jpzool@yahoo.com
AF: University of Puerto Rico, Dept. of Biology, Mayaguez, PR 00681, United States
AB: Precipitation isotopic signatures can help determine the relative importance of different climate patterns to the hydrologic balance and water supply of a region. A set of rain isotope collectors spanning the Luquillo Mountains in Puerto Rico was sampled monthly for two years and precipitation was analyzed for δ¹⁸O and δ²H. Rain isotopic composition varied with elevation and with season. In Puerto Rico there is little seasonal variation in temperature, but a seasonal cycle in rainfall isotopic composition was apparent. Precipitation enriched in ¹⁸O and ²H occurred during the period December-May, transitioning to precipitation depleted in ¹⁸O and ²H during the period June-November. To better understand the relationship between isotopic composition of rainfall and weather patterns, detailed analyses of rain events were matched with the isotopic compositions for the sample periods. The isotopically enriched precipitation is associated with a weather pattern of trade wind showers and orographic uplift. The isotopically depleted precipitation during summer and fall is correlated with convective rainfall associated with tropical waves embedded in the prevailing easterly airflow. Isotopic values that are distinctly depleted in ¹⁸O and ²H compared to both the aforementioned weather patterns occurred during months that the area received rainfall from large low pressure systems. An estimate of cloud height and rain condensation temperature for the different weather patterns was obtained using NEXRAD radar echo tops, which are a measure of the maximum altitude of rainfall within the clouds. Echo tops were analyzed for the four largest rain events during each monthly sample period to determine the average over the study area during each event. Average echo top altitude was correlated with the rain isotope record, even for the limited number of events analyzed in the monthly sample. A preliminary analysis indicates that 27% of rain input to the Luquillo Mountains was associated with the trade wind orographic rainfall weather pattern and 66% of rainfall occurred during the tropical wave season. The remaining 7% of rainfall was from low pressure systems with distinctly depleted isotopic signatures.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1836 Hydrological cycles and budgets (1218, 1655)
DE: 1854 Precipitation (3354)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting