Earth's Hydrological Cycle Research Articles

An evaluation and comparison of cloud condensation nucleus activity models: Predicting particle critical saturation from growth at subsaturation

The ability of particles to activate and form cloud droplets influences the functioning of the Earth's hydrological cycle. This work links the particle water uptake at subsaturation to the critical supersaturation ratio needed for particles to become cloud condensation nuclei (CCN). Five models using the particle hygroscopic growth at subsaturation for predicting the critical supersaturation needed for droplet activation were applied to a laboratory data set of inorganic and organic compounds and mixtures of them. The data set consisted of hygroscopicity tandem differential mobility analyzer measurements and CCN counter measurements. No chemical composition information was used when applying the models. All models tested were based on modifications of Köhler theory and gave similar results. The agreement between predicted and measured critical supersaturations was good, considering the relatively simple models used. A trend of overestimating the critical supersaturations was observed, typically by ∼15%. The best performing model gave on average only a 4% offset from experimental values; the model with the largest deviation was offset by 20%. A comparison was made between the number of soluble entities (ions or nondissociating molecules) estimated from the particle hygroscopic growth at 90% relative humidity (RH) and the number estimated from the particle critical supersaturation; a ∼35% increase was observed in the effective number of entities in solution when going from 90% RH to activation. For many types of aerosols, differences in the model approaches tested do not induce large differences in the predicted critical supersaturation. However, it is most important to follow the recommendations published with the respective models and not use them indiscriminately.

Workshop Examines Ways to Channel a Flood of Hydrological Data ; Collaborative Energy and Water Cycle Information Services Workshop; Greenbelt, Maryland, 15–16 June 2010

Blizzards, floods, droughts, and heat waves from January to July 2010 brought into sharp focus a primary expected manifestation of global climate change: significant alteration of Earth's hydrological cycle. Increased numbers of extreme events, and longer‐term and larger‐scale precipitation pattern shifts that will stress both natural ecosystems and human societal norms, are anticipated as consequences of this alteration.For 25 years, NASA has orbited satellite sensors providing data on Earth's interconnected energy and hydrologic systems; new missions for this purpose are scheduled to launch in coming years, including the Global Precipitation Measurement mission in 2013. These missions, and complementary studies, have produced large and steadily increasing volumes of data, either archived at NASA data centers or held by investigators, particularly those in the NASA Energy and Water Cycle Study (NEWS) program. This increasing volume has induced a data management dilemma: Investigators encounter an increasingly complex process to locate, access, and utilize such data, especially when seeking to combine heterogeneous data sets for innovative research.

Absolute gravity values in Norway

Absolute gravity observations yield insight into geophysical phenomena such as postglacial rebound, change in the Earth's hydrological cycle, sea level change, and changes in the Earth's cryosphere. In the article, the first gravity values at 16 Norwegian stations measured by a modern absolute gravimeter of the FG5 type are presented. The gravity observations were corrected for Earth tides, varying atmospheric pressure, polar motion, and ocean tide loading. The ocean tide loading corrections were subject to special attention. A model based on locally observed ocean tides was applied at some of the stations. The authors estimated the total uncertainties of the gravity values to range from 3 to 4 µgal (1 µgal = 10−8 m s−2). These errors are of magnitude one order less than previously presented absolute gravity values from Norway. The final gravity values are time tagged and will change due to postglacial rebound. The maximum effect is expected to be approximately −1 µgal yr−1.

Ocean climate change fingerprints attenuated by salt fingering?

Intensified double diffusive mixing may attenuate changes in ocean temperature and salinity patterns from global‐warming induced increases in the Earth's hydrological cycle. Increasingly fresher Antarctic Intermediate Water and saltier subtropical waters would tend to increase destabilizing vertical salinity stratification compared to the stabilizing temperature stratification. Destabilization would increase salinity (and temperature) fluxes through double‐diffusive salt fingering. These fluxes could in turn act to reduce widely recognized climate change fingerprints, potentially leading to underestimates of ocean changes in climate studies that do not account for double‐diffusive mixing. Data from a subtropical trans‐Indian Ocean survey occupied in 1987, 1995, 2002, and 2009 are used to investigate temperature‐salinity changes and to estimate the variations of double diffusive mixing driven by these changes.

Assessing the impact of climate change on the land hydrology in Taiwan

The gradually increased temperature resulting from the enhanced greenhouse effects has been found to be an important factor of changes to the global climate which in turn might significantly affect the Earth's hydrological cycles. The possible outcomes of warming climate are changes of precipitation, surface runoff, evapotranspiration, and frequency of extreme weather events, such as floods and droughts. However, such changes at the global scale may not reflect the variations on a regional scale, and more so at the local scale. In this study, a physically based water balance model was applied to study the impact of climate change on the land hydrology, focusing on trends of surface runoff, evapotranspiration, and infiltration in Taiwan. Model forcing of composite temperatures and precipitations were generated by a weather generation model in association with nine climate change scenarios, including outputs of equilibrium experiments and special reports on emissions scenarios, from the IPCC. Although discrepancies among different climate change scenarios are significant, the trend of more extreme precipitations and surface runoffs were observed in most scenarios' runs. The increase of evapotranspiration in both wet and dry seasons is persistent among different scenarios throughout the island due to the projected consistently higher temperature. Although the trends of infiltration for wet and dry seasons are opposite in curtain scenarios, a decreased yearly infiltration was found in most cases as the result of increased precipitation intensity and more evapotranspiration. Timely adaption measures for water resources managements and natural hazard mitigations are required to face these changes of land hydrology components under changing climate.

Storms, polar deposits and the methane cycle in Titan's atmosphere

In Titan's atmosphere, the second most abundant constituent, methane, exists as a gas, liquid and solid, and cycles between the atmosphere and the surface. Similar to the Earth's hydrological cycle, Titan sports clouds, rain and lakes. Yet, Titan's cycle differs dramatically from its terrestrial counterpart, and reveals the workings of weather in an atmosphere that is 10 times thicker than the Earth's atmosphere, that is two orders of magnitude less illuminated, and that involves a different condensable. While ongoing measurements by the Cassini-Huygens mission are revealing the intricacies of the moon's weather, circulation, lake coverage and geology, knowledge is still limited by the paucity of observations. This review of Titan's methane cycle therefore focuses on measured characteristics of the lower atmosphere and surface that appear particularly perplexing or alien.

Retrieval of Latent Heating from TRMM Measurements

Rainfall is a fundamental process within the Earth's hydrological cycle because it represents a principal forcing term in surface water budgets, while its energetics corollary, latent heating, is the principal source of atmospheric diabatic heating well into the middle latitudes. Latent heat production itself is a consequence of phase changes between the vapor, liquid, and frozen states of water. The properties of the vertical distribution of latent heat release modulate large-scale meridional and zonal circulations within the Tropics, as well as modify the energetic efficiencies of midlatitude weather systems. This paper highlights the retrieval of latent heating from satellite measurements generated by the Tropical Rainfall Measuring Mission (TRMM) satellite observatory, which was launched in November 1997 as a joint American–Japanese space endeavor. Since then, TRMM measurements have been providing credible four-dimensional accounts of rainfall over the global Tropics and subtropics, information that can be used to estimate the space–time structure of latent heating across the Earth's low latitudes. A set of algorithm methodologies for estimating latent heating based on precipitation-rate profile retrievals obtained from TRMM measurements has been under continuous development since the advent of the mission s research program. These algorithms are briefly described, followed by a discussion of the latent heating products that they generate. The paper then provides an overview of how TRMM-derived latent heating information is currently being used in conjunction with global weather and climate models, concluding with remarks intended to stimulate further research on latent heating retrieval from satellites.

Antarctica unravelled

Antarctica is the world's last great wilderness. Desolate, cold and largely unspoilt by humans, the continent provides a natural habitat for penguins, seals, whales and other wildlife. It is also home to the largest reservoir of fresh water anywhere on Earth, in the form of a vast sheet of ice that covers almost all of its land. Bounded by icy peaks, dry valleys and the southern oceans, this sheet plays a key role in the Earth's hydrological cycle.

Space‐based measurement of river runoff

Observations of river inundation areas, water levels, and flow variability from orbital sensors have the potential to directly measure the runoff component of the Earth's hydrologic cycle [Birkett et al., 2002; Brakenridge et al., 1998; Sippel et al., 1994, 1998; Townsend, 2001]. A remote‐sensing‐based measurement strategy for rivers and streams is emerging: Surface water data can be collected, their accuracy evaluated, and the results disseminated without regard to political boundaries. The results can be used to address a wide variety of applications.In this article, the needs for such measurements, a river reach‐based methodology for their collection, and some sample results are presented. Because the international observational capability is increasing, some future opportunities for improving this strategy are also described.

Assimilation and Modeling of the Atmospheric Hydrological Cycle in the ECMWF Forecasting System

Several new types of satellite instrument will provide improved measurements of Earth's hydrological cycle and the humidity of the atmosphere. In an effort to make the best possible use of these data, the modeling and assimilation of humidity, clouds, and precipitation are currently the subjects of a comprehensive research program at the European Centre for Medium-Range Weather Forecasts (ECMWF). Impacts on weather prediction and climate reanalysis can be expected. The preparations for cloud and rain assimilation within ECMWF's four-dimensional variational data assimilation system include the development of linearized moist physics, the development of fast radiative transfer codes for cloudy and precipitating conditions, and a reformulation of the humidity analysis scheme. Results of model validations against in situ moisture data are presented, indicating generally good agreement—often to within the absolute calibration accuracy of the measurements. Evidence is also presented of shortcomings in ECMWF's humidity analysis, from the operational data assimilation and forecasting system in 2002, and from the recently completed ERA-40 reanalysis project. Examples are shown of biases in the data and in the model that lead to biased humidity analyses. Although these biases are relatively small, they contribute to an overprediction of tropical precipitation and to an overly intense Hadley circulation at the start of the forecast, with rapid adjustments taking place during the first 6–12 h. It is shown that with an improved humidity analysis this long-standing “spindown” problem can be reduced.

Direct detection of variable tropospheric clouds near Titan's south pole.

Atmospheric conditions on Saturn's largest satellite, Titan, allow the possibility that it could possess a methane condensation and precipitation cycle with many similarities to Earth's hydrological cycle. Detailed imaging studies of Titan have hitherto shown no direct evidence for tropospheric condensation clouds, although there has been indirect spectroscopic evidence for transient clouds. Here we report images and spectra of Titan that show clearly transient clouds, concentrated near the south pole, which is currently near the point of maximum solar heating. The discovery of these clouds demonstrates the existence of condensation and localized moist convection in Titan's atmosphere. Their location suggests that methane cloud formation is controlled seasonally by small variations in surface temperature, and that the clouds will move from the south to the north pole on a 15-year timescale.

Hydrology wins one, loses one

Hydrologists are lauding the creation of a program for basic hydrologic research within the National Science Foundation, even while another formerly substantial source of funding is drying up at the U.S. Geological Survey. Both events are part of the proposed federal budget for fiscal year 1993.The good news is NSF's proposal for a $2.5 million program intended to fund “basic research dealing with the Earth's hydrologic cycle and the role of water on or near the continental surfaces of the Earth.” According to NSF and hydrologists outside the agency, the program grew out of recommendations from a National Research Council committee that were published in 1991 as Opportunities in the Hydrologic Sciences. The year before, AGU recommended that “NSF take steps to establish a unified program in hydrologic science that is commensurate with the importance of water in Earth processes at all scales.” In its official position on this issue, AGU described NSF's existing support for hydrologic science as fragmented and unstable, impeding the maturation of this area “as a distinct geoscience.”

A STUDY ON MAPPING HYDROLOGICAL ENVIRONMENT AND ITS PROBLEMS

A great number of hydrological studies have recently been done and presented in various fields. If these achievements can be represented in the form of maps, their use will be of a remarkably wide range. The research group of hydrological map was organized in October, 1976 (replaced by the working group in April, 1979) as one of the commissions in the Association of Japanese Geographers to discuss how to map the hydrological environment. The summary of the commission activities is as follows. The regular meeting for research was held 23 times; the annual report, Researches on Hydrological Map was published twice (No. 1, 1977 and No. 2, 1978). Seven papers were presented for IGU-IHP Commission Symposium, 1980 IGC. The group also set up such subjects for study as “The definition of the map of hydrological environment and methods of mapping hydrological elements”, “Mapping of hydrological environment on a middle scale”, and “Mapping of hydrological environment on a large scale” and has been making researches in concrete work. The results of the study were presented in Studies on Mapping Hydrological Environment (Grant-in-aid for co-operative research (A), The Ministry of Education, Science and Culture, Japan 1980). The purpose of the present study is to attempt to decide methods of mapping hydrological environment. Before entering into the main subject, the author refers to the definition of the map of hydrological environment: the map reflects all kinds of hydrological phenomena around earth's surface presented by complex actions of natural factors and human impacts in relation to the earth's hydrological cycle. It is the most important work in mapping to select carefully proper factors for representing on a map from a large amount of information and to develop a method of representation as rational as possible. In other words, the legible representation of carefully selected elements of hydrological phenomena is the key to mapping hydrological environment. Hydrological phenomena are classified into three groups according to the situation in which the phenomena occur: one in the atmosphere, the second on the earth's surface and the last in the underground. The principal hydrological elements in the first group are precipitation, probable precipitation, rainfall intensity, evapotranspiration, potential evapotrans iration temperature, etc.; in the second, topography, river, lake, soil, vegetation, land use, etc.; in the last, hydro-geology, groundwater, etc. As it is impossible both in quality and in quantity to map the hydrological environment embracing all those phenomena. The description of the elements should be confined to the irreducible minimum and the most proper method of representation should be decided according to the object of the map. However, there still remain some problems such as, which scale to use in drawing base map, how to represent the distribution of groundwater use, and how many ranks to use in showing the different elements on the same map.

World water resources, present and future

The earth's hydrologic cycle sets the hydrosphere in motion and continuously renews freshwater reserves. In compiling maps of different water resources, the six-component method (equation) of water balance, suggested by the author, is used instead of the three-component one. Water resources transformation carried out on a large scale with the help of different methods, assists in preserving these reserves for the future. On the basis of an analysis of the present state of world water resources and alternative forecasts for the year 2000, the author concludes that it is of primary importance to reduce and eventually to cease the dumping of wastewaters into rivers and bodies of water and to isolate polluted water from the natural hydrologic cycle.

Numerical Experiments on the Climatic Sensitivity of an Atmospheric Hydrologic Cycle

It is shown for an intermediate numerical model that fractional cloudiness and relative humidity decrease with increasing temperature. The fractional cloudiness decreases at a rate about 1 per deg K. This occurs in spite of an increase in the evaporation, water transport, condensation, precipitation and cloud water content with increasing temperature. These results are quite similar to those found from models with more highly parameterized clouds, notably the NCAR model. The fractional cloudiness in this model is measured by the fractional coverage of total cloud water and the fractional coverage of positive condensation, in addition to the relative humidity. It is also shown that some of the characteristics of a temperate climate can be simulated in an intermediate numerical model with periodic, antisymmetric and symmetric boundary conditions on an f plane. Intermediate models of this sort may therefore be useful to investigate general questions about the earth's hydrologic cycle on climatic space and time scales