Showing papers on "Water cycle published in 1995"

Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle

Abstract: The sensitivity of the North Atlantic thermohaline circulation to the input of fresh water is studied using a global ocean circulation model coupled to a simplified model atmosphere. Owing to the nonlinearity of the system, moderate changes in freshwater input can induce transitions between different equilibrium states, leading to substantial changes in regional climate. As even local changes in freshwater flux are capable of triggering convective instability, quite small perturbations to the present hydrological cycle may lead to temperature changes of several degrees on timescales of only a few years.

The role of hydrological processes in ocean-atmosphere interactions

Abstract: Earth is unique among the planets of the solar system in possessing a full hydrological cycle. The role of water in the evolution of planetary atmospheres is discussed. As the atmospheres of the planets developed and modified the early climates of the planets, only the climate trajectory of Earth intercepted the water phase transitions near the triplet point of water, thus allowing the full gamut of water forms to coexist. As a result, transitions between the water phases pervade the entire system and probably are responsible for the creation of a unique climate state. The interactions between the components of the climate system are enriched by the nonlinearity of the water phase transitions. The nonlinear character of the phase transitions of water suggests that the climate should be particularly sensitive to hydrological processes, especially in the tropics. Signatures of the nonlinearity are found in both the structures of the oceans and the atmosphere. Models of the ocean and atmospheric and oceanic data and models of the coupled system are used to perform systematic analyses of hydrological processes and their role in system interaction. The analysis is extended to consider the role of hydrological processes in the basic dynamics and thermodynamicsmore » of oceanic and atmospheric systems. The role hydrological processes play in determining the scale of the major atmospheric circulation patterns is investigated. Explanations are offered as to why large-scale convection in the tropical atmosphere is constrained to lie within the 28{degrees}C sea surface temperature contour and how hydrological processes are involved in interannual climate variability. The relative roles of thermal and haline forcing of the oceanic thermohaline circulation are discussed. Hydrological processes are considered in a global context by the development of a conceptual model of a simple planetary system. 94 refs., 38 figs., 5 tabs.« less

The interaction of ground water with prairie pothole wetlands in the Cottonwood Lake area, east-central North Dakota, 1979–1990

Abstract: The interaction of ground water with prairie wetlands in the Cottonwood Lake area has been the focus of research by the U.S. Geological Survey and the U.S. Fish and Wildlife Service since 1977. During this time, climatic conditions at the site ranged from near the driest to near the wettest of the century. Water levels in wetlands and in water-table wells throughout the study area responded to these changing climate conditions in a variety of ways. The topographically highest wetlands recharged ground water whenever they received water from precipitation. The wetland of principal interest, Wetland P1, which is at an intermediate altitude, received ground-water discharge much of the time, but it also had transpiration-induced seepage from it along parts of its perimeter during all but the wettest year. The large fluctuations of the water table in response to recharge and transpiration reflect the ease with which water moves vertically through the fractured till. Lateral movement of ground water is much slower; pore-water moves vertically through the fractured till. Lateral movement of ground water is much slower; pore-water velocities are generally less than 3 m yr−1. The water supply to the wetlands is largely from precipitation during fall, winter, and spring. During these periods, precipitation either falls directly on the wetland, or precipitation that falls on the upland runs over frozen soils or saturated soils into the wetland. The average ratio of stage rise to total overwinter precipitation was 2.59 for the 12-year study period. After plants leaf out, precipitation generally results in much lower rises of the wetland water level. The average ratio of stage rise to over-summer precipitation was less than 1.0.

Simulation of recent global temperature trends

Abstract: Observations show that global average tropospheric temperatures have been rising during the past century, with the most recent portion of record showing a sharp rise since the mid-1970s. This study shows that the most recent portion of the global temperature record (1970 to 1992) can be closely reproduced by atmospheric models forced only with observed ocean surface temperatures. In agreement with a diverse suite of controversial observational evidence from the past 40 years, the upward trend in simulated tropospheric temperatures is caused by an enhancement of the tropical hydrologic cycle driven by increasing tropical ocean temperatures. Although it is possible that the observed behavior is due to natural climate variability, there is disquieting similarity between these model results, observed climate trends in recent decades, and the early expressions of the climatic response to increased atmospheric carbon dioxide in numerical simulations.

Effects of Spatial Variability and Scale on Areally Averaged Evapotranspiration

Abstract: This paper explores the effects of spatial variability and scale on areally averaged evapotranspiration. A spatially distributed water and energy balance model is employed to determine the effect of explicit patterns of land surface characteristics and atmospheric forcing on areally averaged evapotranspiration over a range of increasing spatial scales. The analysis is performed from the local scale to the catchment scale. The study area is King's Creek catchment, an 11.7 km2 watershed located on the native tallgrass prairie of Kansas. It is shown that a threshold scale, or representative elementary area (REA) exists for evapotranspiration modeling. It is shown further that the dominant controls on the scaling behavior of catchment-average evapotranspiration, and thus the size of the REA, depend on the dominant controls on its components (bare-soil evaporation, wet canopy evaporation, and dry canopy transpiration) and whether evapotranspiration is occurring at potential rates or soil- and vegetation-controlled rates. The existence of an REA for evapotranspiration modeling suggests that in catchment areas smaller than this threshold scale, actual patterns of model parameters and inputs may be important factors governing catchment-scale evapotranspiration rates in hydrological models. In models applied at scales greater than the REA scale, spatial patterns of dominant process controls can be represented by their statistical distribution functions. It appears that some of our findings are fairly general and will therefore provide a framework for understanding the scaling behavior of areally averaged evapotranspiration at the catchment and larger scales. Our results may have further implications for representing subgrid-scale land surface heterogeneity in hydrological parameterizations for atmospheric models.

Hillslope and Climatic Controls on Hydrologic Fluxes

Abstract: The position and shape of the hillslope water table has long been considered to be a useful diagnostic both for field analysis of the spatial structure of hydrologic behavior and for initializing hydrologic forecasting models. Water table positions near the surface often indicate areas which contribute to surface runoff generation, yield evaporation at the climate-demanded rate, and produce a net discharge of saturated zone groundwater. Deeper water tables typically indicate drier areas where evaporation is suppressed and infiltration enhanced, therefore promoting net groundwater recharge. The long time integral of these fluxes over an area forms the local climatic hydrologic cycle. Continuity (throughout a hillslope) of modeled long-term mean moisture fluxes at the land-atmosphere interface within the unsaturated and saturated zones provides closure to the climate-hillslope system, and it is used to determine the equilibrium water table position and the corresponding spatial structure of mean recharge, discharge, evaporation, and surface runoff production. This method of analysis provides a tool for the systematic study of how characteristics of the atmospheric forcing (e.g., mean precipitation, evaporative demand, and storm intermittency and intensity), soil properties, and geologic features (e.g., topography) interact to yield observed spatial patterns of distinct hydrologic behavior at various scales. Application of the methodology to simplified and idealized hillslope geometries reveals spatial patterns in good qualitative agreement with field observations. Case studies demonstrate that the wide range of behavior observed in nature is reproducible within an observed range of climatic, geologic, and soil parameters. Partial analysis of the full system yields a few dimensional parameter groups, the relative values of which are indicative of transitions between geologic-, soil-, and climate-controlled conditions and the presence or absence of characteristic hydrologic zones such as seepage faces, recharge areas, hinges, midline zones, discharge areas, and partial areas of surface runoff production. Under conditions of limited lateral transmissivity and slope, the midline region (over which the long-term mean net exchange between the saturated and unsaturated zones is zero) can dominate the hillslope and lead to an invariance of area-averaged surface runoff and evaporation with respect to hillslope length scale.

The ocean component of the global water cycle

Abstract: The distribution of evaporation and precipitation over the ocean (its hydrologic cycle) is one of the least understood elements of the climate system. However, it is now considered one of the most important, especially for ocean circulation changes on decadal to millennial time-scales. The ocean covers 70% of the Earth's surface and contains nearly all (97%) of its free water, thus, it plays a dominant role in the global water cycle. The atmosphere only holds a few centimeters of liquid water, or 0.001% of the total. However, most discussions of the water cycle focus on the rather small component associated with terrestrial processes [Chahine, 1992]. This is understandable, since the water cycle is so vital to agriculture and all of man's activities. Yet current estimates indicate that 86% of global evaporation and 78% of global precipitation occurs over the oceans [Baumgartner and Reichel, 1975]; (Figure 1 ). Since the oceans are the source of most rainwater, it behooves us to work toward a better understanding of the ocean hydrologic cycle; small changes in ocean evaporation and precipitation patterns may have dramatic consequences for the much smaller terrestrial water cycle. For example, if less than 1% of the rain falling on the Atlantic Ocean were to be concentrated in the central US, it would double the discharge of the Mississippi river!

Climate change and hydrology with emphasis on the Indian subcontinent

Abstract: On a regional scale, some of the most profound impacts of climate change due to increases in greenhouse gases would probably be major changes in the hydrological cycle, in water availability, in agricultural production and in the use of energy. This paper gives a brief overview of studies carried out on climate change and possible impacts on hydrology and water resources in India, covering also the agricultural aspect. The need is emphasized for carrying out further studies in this important subject area at the national level, keeping in view the data and computing facilities available.

Interannual variability of regional climate and its change due to the greenhouse effect

Abstract: Interannual variability of regional climate was investigated on a seasonal basis. Observations and two global climate model (GCM) simulations were intercompared to identify model biases and climate change signals due to the enhanced greenhouse effect. Observed record length varies from 40 to 100 years, while the model output comes from two 100-year equilibrium climate simulations corresponding to atmospheric greenhouse gas concentrations at observed 1990 and projected 2050 levels. The GCM includes an atmosphere based on the NCAR CCM1 with the addition of the radiative effects of CH 4 , N 2 O and CFCs, a bulk layer land surface and a mixed-layer ocean with thermodynamic sea-ice and fixed meridional oceanic heat transport. Because comparisons of interannual variability are sensitive to the time period chosen, a climate ensemble technique has been developed. This technique provides comparisons between variance ratios of two time series for all possible contiguous sub-periods of a fixed length. The time autocorrelation is thus preserved within each sub-period. The optimal sub-period length was found to be 30 years, based on which robust statistics of the ensemble were obtained to identify substantial differences in interannual variability that are both physically important and statistically significant. Several aspects of observed interannual variability were reproduced by the GCM. These include: global surface air temperature; Arctic sea-ice extent; and regional variability of surface air temperature, sea level pressure and 500 mb height over about one quarter of the observed data domains. Substantial biases, however, exist over broad regions, where strong seasonality and systematic links between variables were identified. For instance, during summer substantially greater model variability was found for both surface air temperature and sea-level pressure over land areas between 20–50°N, while this tendency was confined to 20–30°N in other seasons. When greenhouse gas concentrations increase, atmospheric moisture variability is substantially larger over areas that experience the greatest surface warming. This corresponds to an intensified hydrologic cycle and, hence, regional increases in precipitation variability. Surface air temperature variability increases where hydrologic processes vary greatly or where mean soil moisture is much reduced. In contrast, temperature variability decreases substantially where sea-ice melts completely. These results indicate that regional changes in interannual variability due to the enhanced greenhouse effect are associated with mechanisms that depend on the variable and season.

Low-frequency variations in the atmospheric branch of the global hydrological cycle

Abstract: The annual variation of the hydrological cycle is illustrated in terms of hemispheric-mean hydrological variables for the Northern and Southern Hemispheres, while the intraseasonal variations of the global hydrological cycle are illustrated with mean values over two hemispheres that form an east-west partition of the globe. This partition is defined by the 60 deg E-120 deg W great circle and was chosen so that the mean precipitation difference and the divergent water vapor transport between the two hemispheres was maximized. Two years (1979-80) of daily precipitation estimates from the Goddard Laboratory for Atmospheres an 14 years (1979-92) of upper-air data generated by the Global Data Assimilation System at the National Meteorological Center are used in making quantitative estimates of the annual and intraseasonal variations in the global hydrological cycle. The annual variations in hemispheric-mean precipitation (P-circumflex) and water vapor flux divergence (del(vector differential operator) dot Q-circumflex) for the Northern and Southern Hemispheres are comparable with amplitudes of about 0.5 approximately 0.7 mm/day. Both (P-circumflex) and (del(vector differential operator) dot Q-circumflex) vary annually in a coherent way in each hemisphere so that water vapor diverges from the winter hemisphere, where (P-circumflex) reaches its minimum, to the summer hemisphere, where (P-circumflex) attains its maximum. In fact, the hemispheric-mean divergence of water vapor flux changes sign during the annual cycle. Intraseasonal variations of hemispheric-mean precipitation mean P-tilde, evaporation mean E-tilde, and water vapor flux divergence mean del (vector differential operator) dot Q-tilde in the two hemisphres in the east-west direction are comparable with amplitudes of about 0.1 approximately 0.2 mm/day, although amplitudes in some cases exceed 0.3 mm/day. Hemispheric-mean precipitation mean P-tilde varies coherently in opposite phase for the two hemispheres, while mean del (vector differential operator) dot Q-tilde varies so that water vapor diverges from the hemisphere of maximum mean P-tilde to the hemisphere to the hemisphere of minimum mean P-tilde. Intraseasonal variations of mean P-tilde, mean E-tilde, and mean del (vector differential oprator) dot Q-tilde are in accord with the eastward propagation of the intraseasonal global divergent circulation.

Hydrograph estimations by flow routing modelling from AGCM output in major basins of the world

Abstract: The river flow is indispensable to the hydrologic cycle. But in the Atmospheric General Circulaton Model, the river flow is always ignored. So in this paper, we make a global river direction file for T21 grid resolution, at first, and then make a river routing model and calculate it. As input, we use runoff from the CCSR/NIES AGCM (T21 resolution). After calculation, we analyze the results of some world major basins by comaparing the results of model with observations. As a result of this paper, we reproduce seasonal changes of discharge better than before.

Arctic and Antarctic precipitation simulations produced by the NCAR community climate models

Abstract: Precipitation predictions from globai-climate models (GCMs) for the ice-covered Arctic Ocean and the ice sheets of Antarctica are among the most important aspects of the inferred response of the polar areas to climate change It is generally recognized that the atmospheric hydrologic cycle, which includes precipitation as a key part, is one of the components of the climate system that GCMs do not handle particularly well The present-day atmospheric-moisture budget poleward of 70° latitude in both hemispheres, as represented by two versions of the NCAR (US National Center for Atmospheric Research) community climate model (CCM1 and CCM2), is compared with observational analyses The quantities examined on the seasonal and annual timescales are precipitation, evaporation/sublimation and atmospheric poleward moisture transport The results are discussed in terms of the physiographic and climatic characteristics of both polar regions and how the particular models handle moisture transport: CCM1 uses the positive-moisture fixer and CCM2 the semi- Lagrangian transport A particularly important test both for models and for observations is the degree to which the independently determined moisture-budget quantities actually balance Deficiencies of both observations and models are discussed

The Hydrologic Cycle of Major Continental Drainage and Ocean Basins: A Simulation of the Modern and Mid-Holocene Conditions and a Comparison with Observations

Abstract: Each continental grid box of the National Center for Atmospheric Research (NCAR) community climate model (CCM1) is assigned to a particular continental drainage basin based on river basin extent and river flow direction. Boundaries are similarly assigned for the Arctic, Atlantic, Indian, and Pacific Oceans. The hydrologic variables from general circulation model simulations representing modern (control) and interglacial-6000 yr before present (6 ka bp)—climates are then summarized for continental drainage basins and oceans in order to examine the regional response of the hydrologic cycle to these climatic extremes. The NCAR CCM1 simulation of the modem climate reproduces the general features of the observed hydrologic cycle. The simulations for the individual oceans are generally in good agreement with the observational estimates; however, the magnitude of the precipitation, evaporation, and runoff for nearly all continental regions is 20% to 30% greater than the observed. Comparisons of the CCM1...

Climate stability as deduced from an idealized coupled atmosphere-ocean model

Abstract: The stability of an idealized climate system is investigated using a simple coupled atmosphere-ocean box model. Motivated by the results from general circulation models, the main physical constraint imposed on the system is that the net radiation at the top of the atmosphere is fixed. The specification of an invariant equatorial atmospheric temperature, consistent with paleoclimatic data, allows the hydrological cycle to be internally determined from the poleward heat transport budget, resulting in a model that has a plausible representation of the hydrological cycle-thermohaline circulation interaction. The model suggests that the stability and variability of the climate system depends fundamentally on the mean climatic state (total heat content of the system). When the total heat content of the climate system is low, a stable present-day equilibrum exists with high-latitude sinking. Conversely, when the total heat content is high, a stable equatorial sinking equilibrium exists. For a range of intermediate values of the total heat content, internal climatic oscillations can occur through a hydrological cycle-thermohaline circulation feedback process. Experiments conducted with the model reveal that under a 100-year 2 × CO2 warming, the thermohaline circulation first collapses but then recovers. Under a 100-year 4 × CO2 warming, the thermohaline circulation collapses and remains collapsed. Recent paleoclimatic data suggest that the climate system may behave very differently for a warmer climate. Our results suggest that this may be attributed to the enhanced poleward freshwater transport, which causes increased instability of the presentday thermohaline circulation.

Intercomparison of hydrologic processes in global climate models

Abstract: In this report, we address the intercomparison of precipitation (P), evaporation (E), and surface hydrologic forcing (P-E) for 23 Atmospheric Model Intercomparison Project (AMIP) general circulation models (GCM's) including relevant observations, over a variety of spatial and temporal scales. The intercomparison includes global and hemispheric means, latitudinal profiles, selected area means for the tropics and extratropics, ocean and land, respectively. In addition, we have computed anomaly pattern correlations among models and observations for different seasons, harmonic analysis for annual and semiannual cycles, and rain-rate frequency distribution. We also compare the joint influence of temperature and precipitation on local climate using the Koeppen climate classification scheme.

Chapman Conference Probes Water Vapor in the Climate System

Abstract: About 125 scientists from 11 countries met in Jekyll Island, Ga., in October to discuss the unique role that water vapor plays in the climate system. Water vapor links the surface and atmospheric branches of the global hydrologic cycle. Its horizontal and vertical fluxes are key to the energy cycle, and its radiative effects are the major factor in the atmospheric greenhouse effect. Theoretical calculations indicate that global climate is highly sensitive to small changes in humidity at all levels in the atmosphere, but observations to test this hypothesis are lacking. Because few high-quality humidity observations exist, especially in the upper troposphere, researchers are uncertain of the nature and strength of climate feedback mechanisms involving water vapor and its distribution and long-term changes. Consequently, water vapor is not well treated in global climate models and requires more attention.

Process controls and similarity in the us continental-scale hydrological cycle from eof analysis of regional climate model simulations

Abstract: The surface hydrological output of a regional climate model is investigated with implications for process controls on the spatial-temporal variability of the water cycle over the continental USA. Principal component analysis was performed on the seasonal and annual hydrological cycles to determine their dominant modes of spatial variability. At both seasonal and annual time-scales, the first principal component is dominated by precipitation, which controls seasonal wetness and evaporation and accounts for only 52 to 58% of the variability in the continental-scale hydrological cycle. The second principal component is related to both snowmelt runoff and the time variability of weather (via its influence on the residence time of soil moisture near the land surface) and explains another 22% to 34% of the variability in the hydrological cycle. Based on these findings, a classification of hydroclimatological similarity is proposed in which two areas are similar in their hydroclimatology if their first and second principal components are similar. The classification scheme differs from classical approaches because it is based on dominant modes of variability rather than specific indices such as vegetation or seasonal wetness.

Chapter 15 The geophysiology of climate

Abstract: This chapter provides a geophysiological perspective on climate change, both in Earth history, and in future. Ozone in the unpolluted atmosphere is affected in the stratosphere by the biological production of chlorine and bromine-containing gases and nitrous oxide, and in the troposphere by the emission of terpenes and other hydrocarbons by vegetation. The response of the atmospheric water cycle to global warming is complex, and no credible prediction of future climate is possible without an account of the redistribution of water in the environment. Biological systems are known to affect the state and abundance of water in the atmosphere. Over land evapo-transpiration affects the water cycle on both local and regional scales. The gaseous greenhouse is a property of the geo-physiological system, not just a part of an inert environment to which organisms merely adapt. Where there is feedback from climate change on the rate of biogenic or biologically mediated removal or production of greenhouse gases, a coupled feedback system exists that inextricably links the evolution of organisms and climate.

Combining hydrological modeling and remote sensing for large scale water and energy balance studies

Abstract: To address the objectives of large scale water and energy studies, such as GCIP and NASA's Earth Observing System IDS project on the Global Hydrological Cycle, requires a combination of water and energy balance modeling in conjunction with remote sensing. Remote sensing plays a critical role in providing the needed distributed data sets. A macroscale, process-based terrestrial water and energy balance model appropriate for large scale water and energy balance studies has been developed previously (by Wood and Lettenmaier). This model has been applied to the Red-Arkansas River basin in the southern Great Plains of the USA. Remote sensing methods for deriving required forcing inputs have been developed based on GOES and AVHRR satellite data and are the focus of this paper. The authors also present some preliminary simulations showing model performance.

Hydrologic cycle parameterization for energy balance climate models

Abstract: The exchange of moisture and heat between the atmosphere and the Earth's surface fundamentally affect the dynamics and thermodynamics of the climate system. In order to trace moisture flow through the climate system and examine its impact on climate, a hydrologic cycle and a land surface energy balance are incorporated into a coupled climate - thermodynamic sea ice (CCSI) model. The expanded CCSI model is tested by comparing its climate simulations with available observations and GCM modeling results. In general, the expanded model does a good job in simulating the large-scale features of the atmospheric circulation and precipitation in both space and time. The expanded model is used to examine the role of the hydrologic cycle in initiating ice sheet growth in response to changes in atmospheric composition. The results indicate that variations in summer ice melt in response to changes in the land ice albedo, and thus in air temperatures, are more important in determining the initiation of ice sheet growth than variations in precipitation.

The role of the atmosphere in the water cycle

Abstract: The hydrological cycle is a consequence of the conservation of water substance in the climatic system in its three phases and must be regarded in toto with the aerial and terrestrial branches.

Evaluating the Terrestrial Water Balance from the Historical Climate Record

Abstract: Water is necessary for life on earth. Spatial patterns of both vegetation and human activities are dictated largely by the availability of water. While seasonal or monthly averaged values of precipitation, evapotranspiration, soil moisture, and runoff are useful in describing a region’s climate, the intra-annual variability of these variables also plays a key role in determining the characteristics of a given climate. To assess both the mean and intra-annual variability for terrestrial regions of the globe, the historical climate record and parameterizations of the land surface must be used. Although limitations exist in both climate data and land surface parameterizations, they can be used effectively to provide assessments of the terrestrial water cycle at both global and regional scales.

Atmospheric water balance and global hydrologic cycle

Abstract: 大気柱の水収支から年水蒸気収束量はほぼ年降水量-年蒸発量に等しくなる. この情報を利用し, 従来の流域水収支と結び付けると流出量や蒸発量, 流域貯留量の変化などを算定することができる. この様な水収支解析手法 (大気水収支法) により地球規模の水循環と水収支を明らかにした. まず, 流量データによって水蒸気収束量の精度を検討した上で, 全球の降水量推定値や河川流量データと組み合わせて蒸発量や流域貯留量の分布を地球規模で明らかにした. また, 海洋間の淡水輸送状況や, 地球規模での水の南北輸送における河川の役割の重要性も示された. この様に大気水収支法は地球規模の水収支と水循環とを算定する有力な手法であり, 今後のデータ整備にともないそれらの変動のモニタリングにも利用できるものと期待される.