Dominant Moisture Research Articles

Where Does the Iberian Peninsula Moisture Come From? An Answer Based on a Lagrangian Approach

Abstract This study investigated the main sources of moisture in the atmosphere over the Iberian Peninsula (IP) at annual and seasonal scales using FLEXPART, a powerful new 3D Lagrangian diagnosis method that identifies the humidity contributions to the moisture budget of a region. This method can identify moisture sources at lower cost and with greater accuracy than standard isotopic content methods. The results are based on back-tracking analysis of all air masses residing over the IP in the 5-yr period from 2000 to 2004. The results show that the two most important moisture source regions affecting the IP are in a tropical–subtropical North Atlantic corridor that extends from the Gulf of Mexico to the IP, and the IP itself and the surrounding Mediterranean. The importance of these two source areas varies throughout the year, and also with respect to different climatic regions inside the IP. The former source region is the dominant moisture source for the entire IP during winter and in western regions throughout the year, whereas the latter source region dominates the moisture supply to the IP in summer and in the eastern Mediterranean region of the IP throughout the year. The results also demonstrate that winter precipitation in the IP is influenced by both atmospheric instability that forces air masses to rise, and the supply of moisture from the tropical–subtropical North Atlantic corridor on a daily scale and a seasonal basis. Thus, a combination of high (low) moisture supply from the North Atlantic corridor and high (low) atmospheric instability appears to be responsible for the most recent wet (dry) winter in the IP.

Oxygen isotope values of precipitation and surface waters in northern Central America (Belize and Guatemala) are dominated by temperature and amount effects

An understanding of the climatic controls on precipitation δ 18O is required to interpret isotopic records of paleoclimate and paleoaltimetry. However, variations in precipitation δ 18O in time and space are only poorly known in northern Central America. To test the hypothesis that precipitation and surface water δ 18O values are dominated by temporal and spatial amount effects, we analyzed δ 18O in surface waters collected from Guatemala and Belize, and in precipitation from the Global Network for Isotopes in Precipitation database for Veracruz, Mexico, and San Salvador, El Salvador. Herein we show that the dominant controls on δ 18O values of precipitation and surface waters are fairly simple. Temporally, the dominant control on precipitation δ 18O values is the amount effect, whereby there is an inverse correlation between rainfall amount and δ 18O. Precipitation δ 18O values decrease by 1.24‰ per 100 mm increase of monthly rainfall. Spatially, only two variables – distance from the coast and mean catchment altitude – explain 84% of the surface water δ 18O variability. Surface water δ 18O values show an altitude effect of − 1.9 to − 2.4‰ km − 1 and a continental effect of 0.69‰ per 100 km once corrected for altitude effects. A decrease in surface water δ 18O by 3 to 4‰ from the Caribbean Sea to the Pacific Ocean is evident as an isotopic rain shadow on the Pacific slope. Our data also show that river waters in this humid tropical environment are good proxies for δ 18O values of precipitation in northern Central America. The Guatemala/Belize surface water line is defined as δD = 8.0 × δ 18O + 8.7, which is similar to the meteoric water line at San Salvador of δD = 8.1 × δ 18O + 10.9. Spatial variability in δ 18O values is interpreted to reflect 1) progressive rainout of Caribbean-sourced air masses upon traverse of Central America, and 2) the temperature-dependent equilibrium fractionation between vapor and condensate related to the altitude effect. The data show that the northeast trade winds are the dominant moisture source to Central America, mixing with Pacific-derived moisture west of the cordilleran divide.

Mass balance of the Prince of Wales Icefield, Ellesmere Island, Nunavut, Canada

This paper estimates the mass balance of the Prince of Wales Icefield, Ellesmere Island, Canada, averaged over four decades, from measurements of surface mass balance (SMB) and iceberg calving. Shallow ice core net accumulation measurements and annual mass balance stake measurements are used in conjunction with a digital elevation model and knowledge of the location of the dominant moisture source for precipitation over the ice cap to interpolate and extrapolate spatial patterns of SMB across the Prince of Wales Icefield. The contribution of iceberg calving to the mass balance is calculated from estimates of (1) the annual volume of ice discharged at the major tidewater glacier termini and (2) the annual volume loss or gain due to terminus fluctuations. Two different approaches to determining the SMB conclude that the SMB of the ice field is approximately in balance (average equals −0.1 ± 0.4 km3 w.e. a−1, where w.e. means water equivalent) largely because of its proximity to the main year‐round moisture source that is the Smith Sound portion of the North Open Water polynya. Iceberg calving is a highly significant component of mass loss (−1.9 ± 0.2 km3 w.e. a−1) and is sufficient to make the overall mass balance of the ice field averaged over the period 1963–2003 clearly negative (−2 ± 0.45 km3 w.e. a−1, equivalent to a mean‐specific mass balance across the ice field of −0.1 m w.e. a−1). The Prince of Wales Icefield contributes ∼0.005 mm a−1 to global eustatic sea level rise.

Common tree growth anomalies over the northeastern Tibetan Plateau during the last six centuries: implications for regional moisture change

AbstractThe world's hydrological cycle is believed to intensify with global warming, yet current climate models have only a limited ability to assess moisture responses at regional scales. Tree‐ring records are a valuable source of information for understanding long‐term, regional‐scale moisture changes, particularly for large regions such as the Tibetan Plateau (TP), where the observational data are short and sparse. Here, we present a new ring‐width chronology developed from Qilian Juniper (Sabina przewalskii) wood at two sites on the northeastern TP. This chronology, combined with others from the same region, demonstrates that tree growth anomalies are linked to regional late spring to early summer moisture availability. Although late monsoon season precipitation in the study area decreased during recent decades, tree growth continued to increase due to persistent moisture availability in the early monsoon season. Comparison with global sea surface temperatures (SSTs) indicates that early (late) monsoon season precipitation is closely related to tropical Pacific (Indian Ocean) SSTs, suggesting a possible seasonal shift in the dominant moisture source area for monsoonal precipitation over the northeastern TP. It is further shown that there is a very high degree of coherency regarding low‐frequency tree growth anomalies over the northeastern TP during the last six centuries. The most prominent drought epoch occurred during ca. 1450–1500, which may have been caused by a significant decrease in the thermal gradient between the Eurasian continent and the tropical oceans. A persistent tree growth increase since the 1880s is coincident with global warming, suggesting an intensified early monsoon season moisture regime in the study area.

Interannual variability of Greenland winter precipitation sources: Lagrangian moisture diagnostic and North Atlantic Oscillation influence

We present a new Lagrangian diagnostic for identifying the sources of water vapor for precipitation. Unlike previous studies, the method allows for a quantitative demarcation of evaporative moisture sources. This is achieved by taking into account the temporal sequence of evaporation into and precipitation from an air parcel during transport, as well as information on its proximity to the boundary layer. The moisture source region diagnostic was applied to trace the origin of water vapor for winter precipitation over the Greenland ice sheet for 30 selected months with pronounced positive, negative, and neutral North Atlantic Oscillation (NAO) index, using the European Centre for Medium‐Range Weather Forecasts' ERA‐40 reanalysis data. The North Atlantic and the Nordic seas proved to be the by far dominant moisture sources for Greenland. The location of the identified moisture sources in the North Atlantic basin strongly varied with the NAO phase. More specifically, the method diagnosed a shift from sources north of Iceland during NAO positive months to a maximum in the southeastern North Atlantic for NAO negative months, qualitatively consistent with changes in the concurrent large‐scale mean flow. More long‐range moisture transport was identified during the NAO negative phase, leading to the advection of moisture from more southerly locations. Different regions of the Greenland ice sheet experience differing changes in the average moisture source locations; variability was largest in the north and west of Greenland. The strong moisture source variability for Greenland winter precipitation with the NAO found here can have a large impact on the stable isotope composition of Greenland precipitation and hence can be important for the interpretation of stable isotope data from ice cores. In a companion paper, the implications of the present results are further explored in that respect.

Importance of rain evaporation and continental convection in the tropical water cycle

Atmospheric moisture cycling is an important aspect of the Earth's climate system, yet the processes determining atmospheric humidity are poorly understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.

Caribbean and Pacific moisture sources on the Isthmus of Panama revealed from stalagmite and surface water δ18O gradients

We test the hypothesis that the Pacific Ocean contributes moisture to the Intertropical Convergence Zone (ITCZ) over southern Central America, by spatial analysis of surface water δ18O values from Panama and Costa Rica. The δ18O values decrease with distance from the Caribbean Sea to the isthmian divide then gradually increase from the divide toward the Pacific slope, which suggests a contribution of both Caribbean and Pacific sourced moisture to the isthmus. We estimated the Pacific moisture contribution for Pacific slope regions of 22% to 64%. The δ18O values from stalagmites from five cave systems demonstrate decreasing δ18O values with distance from the Caribbean, implicating the Atlantic Basin as a dominant moisture source. Constraining modern moisture sources is important for the interpretation of stable isotopic proxy records of past rainfall, because of the combined influence of Pacific and Atlantic ocean‐atmosphere phenomena on ITCZ rainfall over the Isthmus of Panama.

The sensitivity of tropical squall lines to surface fluxes: Three‐dimensional cloud‐resolving model simulations

AbstractTwo tropical oceanic squall lines, from the Tropical Ocean–Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) and the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE), that developed over the west Pacific and east Atlantic, respectively, are simulated using a three‐dimensional cloud‐resolving model to examine the impact of surface fluxes on tropical squall line development and associated precipitation processes. The important question of how convective available potential energy (CAPE) is maintained in clear and cloudy areas in the tropics is investigated. The boundary‐layer structure and evolution in the clear inflow area are also discussed. Although the cloud structure and precipitation intensity are different between the TOGA COARE and GATE squall line cases, the effects of the surface fluxes on the amount of rainfall and on the cloud development processes are quite similar. The area where surface fluxes originated was categorized into clear and cloudy regions according to whether there was cloud in the vertical column. The model results indicated that the surface fluxes from the large clear‐air environment are the dominant moisture source for tropical squall line development, even though the surface fluxes in the cloud region display a large peak. The high‐energy air from the boundary layer in the clear area is what feeds the convection, while the CAPE is removed by the convection. Trajectory and water budget analyses also indicate that most of the moisture (90%) is from the boundary layer of the clear‐air environment. Copyright © 2003 Royal Meteorological Society

Gulf of California Sea Surface Temperatures and the North American Monsoon: Mechanistic Implications from Observations

Abstract Perhaps the most regular and predictable weather pattern in North America is the North American (NA) or Mexican monsoon. Occurring in summer, it delivers about 35% and 45% of Arizona's and New Mexico's annual precipitation, respectively, and about 60% of northern Mexico's. While recent studies have linked strong NA monsoons to summer drought in the U.S. Midwest, the sequence of events that produce the NA monsoon remain unclear. This empirical study builds on the findings of many other studies that implicate the Gulf of California [(GOC) or simply the gulf] as the dominant moisture source for the monsoon. It examines six monsoon seasons in detail, and quantitatively relates GOC sea surface temperatures (SST) to the timing, amount, and regional extent of monsoon rainfall. This six season study is based on satellite measurements of rainfall [using the Special Sensor Microwave Imager (SSM/I)] and GOC SST at high spatial and temporal resolution. Key findings include the following. 1) Monsoon rainfall ...

Canadian Arctic vegetation mapping

During the next few decades the Arctic is expected to experience unprecedented changes in climate and resource development. All of these will potentially affect land use and vegetation cover. There is a need for a comprehensive and consistent circumpolar map of arctic vegetation that will be useful for modelling vegetation change in the circumpolar region. The Canadian arctic vegetation map is part of the Circumpolar Arctic Vegetation Mapping project (CAVM) which was initiated to fulfil this need. The CAVM is an effort by an international group of arctic vegetation scientists to create a map and GIS database of circumpolar vegetation at the 1:7 500 000 scale. The Canadian vegetation map and ultimate circumpolar map will be useful for the study of arctic vegetation, modelling vegetation change at the continental and circumpolar scale, interpreting patterns of wildlife distribution and migration, land management, and educational purposes. The mapping effort combines information on soils, bedrock and surficial geology, hydrology, remotely-sensed vegetation characteristics, previous vegetation studies and regional expertise of mapping scientists. Map units are drawn using photo-interpretation of a 1:4 000 000 scale AHVRR false colour infrared image basemap. Mapped polygons represent homogeneous landscape terrain units (e.g. hills, plains, plateaus, mountains and valleys). A GIS database contains ancillary information for each polygon and vegetation is defined through a series of lookup tables with information on dominant climatic, parent material chemistry and topographic characteristics. We present the mapping methods, a vegetation map of the Canadian Arctic, and ancillary maps developed in the mapping process. Twenty land cover classes are presented on the map, including 17 vegetation classes that are defined by dominant physiognomy (growth form), dominant moisture regime, characteristic plant communities and characteristic degree of vegetation cover. Ancillary data presented include the AVHRR CIR basemap and landscape unit polygons, a maximum NDVI image, bioclimatic and elevational zones, and a map of parent material pH.

An unstructured mesh cell-centered control volume method for simulating heat and mass transfer in porous media: Application to softwood drying—Part II: The anisotropic model

In Part I of this research, a mathematical model that describes the drying process was presented. The model, which was simplified for the case of an isotropie porous material, was then numerically resolved using a cell-centered unstructured mesh control volume technique known as UM-CV. The results presented in Part I indicated that the UM-CV four-node discretization formulas adequately described the physics of the complex heat and mass transport phenomena that occur during the drying process. The numerical algorithm was tested for a variety of different mesh structures constructed from polygon-shaped elements. There were no obvious mesh dependencies reported for the four-node formulation, with only minor differences in the overall drying kinetics exhibited. In Part II, an investigation of the UM-CV method will be undertaken for porous materials that are highly anisotropic. An example of such a material is wood, where it is found that the ratio between longitudinal and transverse permeabilities is of the order of one thousand, and the dominant moisture migration occurs in the longitudinal sense, which is the most permeable direction. This severe anisotropy ratio coupled together with high temperature drying conditions will test the performance of the four-node formulation under extremely harsh numerical conditions. The results of the UM-CV code will be analyzed for three different mesh structures.

Model Climatology of the Mexican Monsoon

The Mexican monsoon is a significant feature in the climate of the southwestern United States and Mexico during the summer months. Rainfall in northwestern Mexico during the months of July through September accounts for 60% to 80% of the total annual rainfall, while rainfall in Arizona for these same months accounts for over 40% of the total annual rainfall. Deep convection during the monsoon season produces frequent damaging surface winds, flash flooding, and hail and is a difficult forecast problem. Past numerical simulations frequently have been unable to reproduce the widespread, heavy rains over Mexico and the southwestern United States associated with the monsoon. The Pennsylvania State University/National Center for Atmospheric Research mesoscale model is used to simulate 32 successive 24-h periods during the monsoon season. Mean fields produced by the model simulations are compared against observations to validate the ability of the model to reproduce many of the observed features, including the large-scale midtropospheric wind field, southerly low-level winds over the Gulf of California, and the heavy rains over western Mexico. Preliminary analysis of the mean model fields also suggest that the Gulf of California is the dominant moisture source for deep convection over Mexico and the southwestern United States, with upslope flow along the Sierra Madre Occidental advecting low-level gulf moisture into western Mexico during the daytime and southerly flow at the northern end of the gulf advecting gulf moisture into Arizona on most days. These results illustrate the usefulness of four-dimensional data assimilation techniques to create proxy datasets containing realistic mesoscale features that can be used for detailed diagnostic studies.

Effects of soil moisture dynamics on slope failure at Hyrum Reservior, Utah

AbstractField observations of shoreline conditions at Hyrum Reservoir, Utah, were conducted during the summers of 1991 to 1993. A process of bluff retreat is described for a multiple‐layered bluff environment of sand and clay layers. Failure is initiated by wetting and drying of clay sediments, which produces horizontal cracks within bluff material. These cracks appear to penetrate to a depth of approximately 100‐150 mm before initiating vertical cracking in the sediments. The vertical cracks are propagated by continued drying of the surface sediment, ultimately leading to failure of the bluff material. The physical dimensions of sediment blocks succumbing to this mechanism range from a few hundred millimetres up to 3 m on a side, with a depth of approximately 100‐150 mm. The mechanism described here appears to operate optimally when the supply of subsurface moisture is abundant and nearly continuous throughout the spring and early summer. Reservoir draw‐down, large capillary fringe effects in the bluff and periodic wetting from upslope undrained hollows are the dominant moisture controls at this site. Moisture delivery to the face is strongly influenced by anisotropy of saturated hydraulic conductivity in the alternating clay and sand layers and related differences in sediment texture.