Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor (original) (raw)
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Proportions of convective and stratiform precipitation revealed in water isotope ratios
Nature Geoscience, 2016
Tropical and midlatitude precipitation is fundamentally of two types, spatially limited and high-intensity convective or widespread and lower-intensity stratiform, owing to di erences in vertical air motions and microphysical processes governing rain formation. These processes are di cult to observe or model and precipitation partitioning into rain types is critical for understanding how the water cycle responds to changes in climate. Here, we combine two independent data setsconvective and stratiform precipitation fractions, derived from the Tropical Rainfall Measuring Mission satellite or synoptic cloud observations, and stable isotope and tritium compositions of surface precipitation, derived from a global network-to show that isotope ratios reflect rain type proportions and are negatively correlated with stratiform fractions. Condensation and riming associated with boundary layer moisture produces higher isotope ratios in convective rain, along with higher tritium when riming in deep convection occurs with entrained air at higher altitudes. On the basis of our data, stable isotope ratios can be used to monitor changes in the character of precipitation in response to periodic variability or changes in climate. Our results also provide observational constraints for an improved simulation of convection in climate models and a better understanding of isotope variations in proxy archives, such as speleothems and tropical ice.
Journal of Geophysical Research, 2007
1] We have incorporated the cycling of water isotopes into the NCAR atmospheric general circulation model, CAM2. Isotope dynamics mostly follow those of previous isotope GCMs, with fractionation being produced by evaporation at the surface and by cloud processes. A new feature that we have added is the direct estimation of the degree of isotopic equilibration between vapor and raindrops as a function of temperature and rain rate. The model yields a reasonable global pattern of water isotopes in precipitation, but detailed comparison with observations is limited by known inaccuracies in precipitation and temperatures yielded by CAM2. We use the results to evaluate the fundamental controls on water isotopic composition in precipitation. We emphasize that, over much of the surface of the Earth, the concept of Rayleigh distillation is inadequate to understand the large-scale geographic distribution of water isotopes in precipitation, because the effects of surface fluxes are more important than those of distillation, in particular at low and midlatitudes and over the oceans. In oceanic regions the balance between precipitation and evaporation (P À E), which reflects the large-scale atmospheric circulation, is the primary determinant of the isotopic composition of precipitation and vapor. Variations of P À E at low latitudes over the oceans produce about 7% variation of precipitation d 18 O that is independent of temperature variation. Where P > E, the convergence of atmospheric vapor derived from various sources leads to low values and a particularly wide range in d ppt . In the tropical and subtropical troposphere the vertical decrease of d 18 O in vapor is different from the values expected from Rayleigh distillation because of entrainment, convective mixing, detrainment, evaporation of detrained water, and subsidence of low-d 18 O highaltitude air. The low d 18 O of atmospheric vapor over the oceans at high (ca. 55°) latitudes produces, as a result of kinetic effects associated with evaporation, a zone of heavy (high-d 18 O) evaporation from the oceans. This effect may account for the low d 18 O of some high-latitude ocean surface waters and also helps attenuate the effects of global temperature changes on the isotopic composition of polar precipitation.
Extreme short-term stable isotope variability revealed by continuous rainwater analysis
The continuous real-time analysis, at 30-s intervals, of precipitation at an Australian tropical location revealed extreme and rapidly changing d 18 O and dD values related to variations in moisture source areas, transport paths and precipitation histories. The range of d 18 O (À19.6% to +2.6%) and dD (À140% to +13%) values from 5948 measurements of nine rain events over 15 days during an 8-month period at a single location was comparable with the range measured in 1532 monthly samples from all seven Australian Global Network of Isotopes in Precipitation stations from 1962 to 2002. Extreme variations in d 18 O (À8.7% to À19.6%) and dD (À54% to À140%) were recorded within a single 4-h period. Real-time stable isotope monitoring of precipitation at a high temporal resolution enables new and powerful tracer applications in climatology, hydrology, ecophysiology and palaeoclimatology.
Atmospheric Environment, 2018
Continuous measurements were made of isotopic compositions in atmospheric water vapor (δ v) and in atmospheric precipitation (δ p) near the surface in the subtropical monsoon region (Changsha, China) from October 2014 to March 2017 to investigate the change characteristics of vapor isotopes and their interactions with precipitation isotopes. On a diurnal time scale, the δ 18 O v values were more negative in the daytime and more positive at night, and the change in d v was reversed during non-rainy days; however, the δ 18 O v values were more positive in the daytime and more negative at night, and the variation in d v was not significant during rainy days. On a seasonal time scale, affected by distinctive moisture sources, the vapor isotopes had obvious seasonal variations, with higher δ 18 O v and d v values occurring in cold seasons than in warm seasons. There were generally consistent and significant negative linear relationships of d v with temperature and absolute humidity, but the linear relationships between δ 18 O v and meteorological factors were variable on the different time scales, which is closely associated with the local weather conditions on the short time scales from diurnal to intra-seasonal and movements of air masses with distinct thermodynamic properties on seasonal and sometimes intra-seasonal time scales. Although the change trend of daily δ 18 O v closely approximated that of δ 18 O p , δ 18 O p experienced significant relative enrichment compared to δ 18 O v , with an average enrichment of 9.18 ‰. Overall, the isotopic compositions of atmospheric water vapor were in equilibrium with those of precipitation while rainfall events occurred, except for those in the cold season in 2014, and when tracing short-duration weather events, δ 18 O v provided more details and information related to weather processes than δ 18 O p .
Annual, monthly and daily analyses of stable isotopes in precipitation are commonly made worldwide, yet only a few studies have explored the variations occurring on short timescales within individual precipitation events, particularly at mid-latitude locations. This study examines hydrogen isotope data from sequential, intra-event samples from sixteen precipitation events during different seasons and a range of synoptic conditions over an 18-month period in Birmingham, UK. Precipitation events were observed simultaneously using a vertically-pointing micro rain radar (MRR), which, for the first time at a mid-latitude location, allowed high resolution examination of the microphysical characteristics (e.g. rain rate, fall velocity, drop size distributions) that may influence the local isotopic composition of rainwater. The range in δD from 242 samples from 16 events was -87.0‰ to +9.2‰, whilst the largest variation observed in a single event was 55.4‰. In contrast to previous work, the results indicate that some mid-latitude precipitation events do indeed show significant intra-event trends that are strongly influenced by precipitation processes and parameters such as rain rate, melting level height and droplet sizes. Inverse relationships between rain rate and isotopic composition are observed, representing an example of a local type of ‘amount effect’, a still poorly-understood process occurring at different scales. For these particular events the mean δ value may therefore not provide all the relevant information. This work has significance for the testing and development of isotope-enabled cloud resolving models and land surface models at higher resolutions, and provides improved insights into a range of environmental processes that are influenced by sub-sampled precipitation events.
Journal of Geophysical Research, 2009
1] A unified approach of observation and modeling was applied to the investigation of three circulation types that typically bring rain to southeastern Australia. Observations from the Melbourne University Network of Isotopes in Precipitation of high-resolution variations in the ratios of 18 O and 2 H were collected for (1) mixed frontal, (2) convective, and (3) stratiform precipitation events. Isotopic content of precipitation varied over both high and low frequencies because of influences from local variations in rain intensity and rainout by large-scale precipitation. Deuterium excess showed a weak relationship with rainfall amount on intraevent time scales but was stronger under convective rainfall conditions. As a supplement to the observations, a version of the National Center for Atmospheric Research Community Atmosphere Model running an isotope hydrology scheme simulated the mixed frontal and stratiform events by nudging with reanalyses. The simulations represented well the evolution of vapor profiles of 18 O and deuterium excess. Trajectories for the mixed frontal case illustrated the structure of the vapor profiles, revealing a convergence of air masses from different source regions. Deuterium excess in precipitation was represented less accurately by the model, indicating a possible shortcoming in the parameterization of postcondensation processes in the general circulation model. By combining observations and modeling in this way, detail of the structure and history of the events was provided that would be unavailable from the sampling of precipitation alone.
Stable water isotopologues, mainly 1 H 2 O, 1 H 2 HO (HDO), and 1 H 2 18 O, are useful tracers for processes in the global hydrological cycle. The incorporation of water isotopes into Atmospheric General Circulation Models (AGCMs) since 1984 has helped scientists gain substantial new insights into our present and past climate. In recent years, there have been several significant advances in water isotopes modeling in AGCMs. This paper reviews and synthesizes key advances accomplished in modeling (1) surface evaporation, (2) condensation, (3) supersaturation, (4) postcondensation processes, (5) vertical distribution of water isotopes, and (6) spatial 18 O-temperature slope and utilizing (1) spectral nudging technique, (2) higher model resolutions, and (3) coupled atmosphereocean models. It also reviews model validation through comparisons of model outputs and ground-based and spaceborne measurements. In the end, it identifies knowledge gaps and discusses future prospects of modeling and model validation.
Water isotopes in precipitation
Quaternary Science Reviews, 2000
O concentrations are observed in precipitation both on a geographical and on a temporal basis. These variations, resulting from successive isotopic fractionation processes at each phase change of water during its atmospheric cycle, are well documented through the IAEA/WMO network. Isotope concentrations are, in middle and high latitudes, linearly related to the annual mean temperature at the precipitation site. Paleoclimatologists have used this relationship to infer paleotemperatures from isotope paleodata extractable from ice cores, deep groundwater and other such sources. For this application to be valid, however, the spatial relationship must also hold in time at a given location as the location undergoes a series of climatic changes. Progress in water isotope modeling aimed at examining and evaluating this assumption has been recently reviewed with a focus on polar regions and, more speci"cally, on Greenland. We extend this review in comparing the results of two di!erent isotopic AGCMs (NASA/GISS and ECHAM) and in examining, with a more global perspective, the validity of the above assumption, i.e. the equivalence of the spatial and temporal isotope}temperature relationships. These results con"rm the dominating role of local temperature changes on the paleo isotope signal in most regions. However, the exact calibration of this valuable paleothermometer is biased by, for example, the seasonality of precipitation and other factors. We forced the two models by the climatic boundary conditions of the mild-holocene at 6 kyr BP which only slightly di!ers from today's climate. The isotope response on this weak forcing is quite heterogeneous. The only robust common response is the intensi"cation of the hydrological cycle in low latitudes and, therefore, isotopically more depleted precipitation in the tropics and subtropics. We also examine recent progress made in modeling: the relationship between the conditions prevailing in moisture source regions for precipitation and the deuterium excess of that precipitation.
Stable isotope compositions of waters in the Great Basin, United States 2. Modern precipitation
Journal of Geophysical Research, 2002
1] Precipitation was collected between 1991 and 1997 at 41 locations within and adjacent to parts of the Great Basin lying in California, Oregon, Nevada, and Utah. These samples were analyzed for their deuterium (dD) and oxygen-18 (d 18 O) contents. Separate collections were made of summer and winter season precipitation at stations ranging in elevation from -65 m to 3246 m. The dD per mil values of stations that were closely spaced but at different elevations showed an average dD decrease of approximately 10%/km rise in elevation. Data for all samples representing winter precipitation, when plotted on a dD versus d 18 O plot, fall close to the Meteoric Water Line (dD = 8 d 18 O + 10); samples representing summer precipitation define a line of slightly lower slope due to evaporation of the raindrops during their passage from cloud to ground. Comparison of our 1991-1997 dD data with those from the same three stations reported by an earlier study in the southeastern California shows seasonal differences ranging from 0 per mil to 19% (average: 15) and annual differences ranging from 0 to 13 per mil (average: 2), illustrating the degree of annual and seasonal variability in this region. When contoured, the dD values display gradients indicating a north to northwest decrease in deuterium, with values ranging from À60 to À125% in winter precipitation and from À40 to À110% in summer precipitation. These gradient trends can be explained by the predominance of air mass trajectories originating in the tropical Pacific, the Gulf of California, and (in summer) the Gulf of Mexico.
Bulletin of the American Meteorological Society, 2016
The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions. To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemis...