Showing papers on "Precipitation published in 1990"

A scheme for predicting layer clouds and their water content in a general circulation model

Abstract: A cloud water variable has been included in the Meteorological Office atmospheric general circulation model. the cloud amount and water content are generated by a scheme which assumes a distribution of thermodynamic and water content variables about their grid-box-mean values. A parametrization of the rate of depletion of cloud water by precipitation is included. the model's boundary layer mixing is reformulated in terms of variables conserved during changes of state of water, allowing a more realistic calculation of atmospheric stability in cloudy regions. The scheme produces realistic cloud distributions and the predicted cloud water content verifies well against microwave radiometer data in all regions except the mid-latitudes of the summer hemisphere. the parametrization of cloud optical properties in terms of the cloud water content will be an important future development of the model.

Climatic Control of Vegetation Distribution: The Role of the Water Balance

Abstract: The water balance describes climate as it is sensed by plants, as the interaction of energy and water in the environment. Discriminant analysis showed that the distribution of North American plant formations was more highly correlated with the water balance (actual evapotranspiration and deficit) than with the more traditional measures of climate (such as temperature and precipitation) used in several studies, including those used in the well-known works of Thornthwaite, Holdridge, and Whittaker. Much of the improved correlation could be attributed to the ability of the water balance to distinguish between climates similar in mean annual energy and water supplies but different in the seasonal timing of the two. Consideration of the water balance aided in the interpretation of possible mechanisms controlling the distribution of plant formations. For example, coniferous forest occurred at low actual evapotranspiration (low simultaneous availability of energy and water), consistent with the suggestion that c...

Hydrologic sensitivities of the Sacramento-San Joaquin River Basin, California, to global warming

Abstract: The hydrologic sensitivities of four medium-sized mountainous catchments in the Sacramento and San Joaquin River basins to long-term global warming were analyzed. The hydrologic response of these catchments, all of which are dominated by spring snowmelt runoff, were simulated by the coupling of the snowmelt and the soil moisture accounting models of the U.S. National Weather Service River Forecast System. In all four catchments the global warming pattern, which was indexed to CO{sub 2} doubling scenarios simulated by three (global) general circulation models, produced a major seasonal shift in the snow accumulation pattern. Under the alternative climate scenarios more winter precipitation fell as rain instead of snow, and winter runoff increased while spring snowmelt runoff decreased. In addition, large increases in the annual flood maxima were simulated, primarily due to an increase in rain-on-snow events, with the time of occurrence of many large floods shifting from spring to winter.

Mesoscale Organization of Springtime Rainstorms in Oklahoma

Abstract: Radar reflectivity and raingage data obtained during six springtimes indicate the types of mesoscale organization that occur in association with major rain events in Oklahoma (at least 25 mm of rain in 24 h over an area exceeding 12 500 km2). In these storms the primary rain area is found to be a contiguous region of precipitation 10s to 100s of km in scale that consists partly of deep convection and partly of stratiform rain. The patterns of rain formed by the convective and stratiform areas comprise a continuous spectrum of mesoscale structures. About two-thirds of the cases examined exhibited variations on the type of organization in which convective cells arranged in a moving line are followed by a region of stratiform rain. Storm organization was graded according to the degree to which it matched an idealized model of this “leading-line/trailing-stratiform” structure. The precipitation pattern was further graded according to whether its structure was relatively symmetric with respect to an a...

Precipitation Measurement and Hydrology

Abstract: The most important advantage of using radar for precipitation measurements is the coverage of a large area with high spatial and temporal resolution from a single point and in real time. Furthermore, the three-dimensional picture of the weather situation can be extended over a very large area by compositing data from several radars. However, we have not been able until recently to make measurements over a large area with an accuracy acceptable for hydrological applications.

Organization of clouds and precipitation in extratropical cyclones

Abstract: A number of models accounting for the distribution of cloud and precipitation in extratropical cyclones were proposed during the 19th and early 20th centuries A history of these has been recounted by Bergeron (1959, abridged version in Bergeron 1981) and also by Ludlam (1966) in his inaugural lecture as professor of meteorology These models culminated in the classical Norwegian polar-front cyclone model of the Bergen school Bjerknes and Solberg 1922) in which the patterns of cloud and precipitation were related to vertical air motions resulting from the relative movement of different air masses along inclined frontal surfaces This model is still widely used today During the past quarter-century, however, the availability of imagery from satellites and radars has revolutionized the capability to observe cloud and precipitation The imagery has drawn attention to many synoptic-scale and mesoscale features not explained by the classical model, as discussed by Reed in Sec(tion 333 It is now clear that the Norwegian model, despite its popularity, is a broad-brush model that fails to explain many important variations in structure among cyclones

Empirical Data on Contemporary Global Climate Changes (Temperature and Precipitation)

Abstract: New data are presented on the changes of mean global surface air temperature and annual precipitation over extratropical continents of the Northern Hemisphere. Global warming occurred during the last century with a mean trend of 0.5°C/100 years. It is shown that for the same period the annual precipitation over the land in the 35°–70°N zone increased by 6%. The observed variations of precipitation coincide with the results of general circulation modeling of doubled CO2 equilibrium climate change by sign but contradict by scale.

Precipitation chemistry in central Amazonia

Abstract: Rain samples from three sites in central Amazonia were collected over a period of 6 weeks during the 1987 wet season and analyzed for ionic species and dissolved organic carbon. A continuous record of precipitation chemistry and amount was obtained at two of these sites, which were free from local or regional pollution, for a time period of over 1 month. The volume-weighted mean concentrations of most species were found to be about a factor of 5 lower during the wet season compared with previous results from the dry season. Only sodium, potassium, and chloride showed similar concentrations in both seasons. When the seasonal difference in rainfall amount is taken into consideration, the deposition fluxes are only slightly lower for most species during the wet season than during the dry season, again with the exception of chloride, potassium, and sodium. Sodium and chloride are present in the same ratio as in sea salt; rapid advection of air masses of marine origin to the central Amazon Basin during the wet season may be responsible for the observed higher deposition flux of these species. Statistical analysis suggests that sulfate is, to a large extent, of marine (sea salt and biogenic) origin, but that long-range transport of combustion-derived aerosols also makes a significant contribution to sulfate and nitrate levels in Amazonian rain. Organic acid concentrations in rain were responsible for a large fraction of the observed precipitation acidity; their concentration was strongly influenced by gas/liquid interactions.

Convection and rainfall over Mexico and their modulation by the Southern Oscillation

Abstract: The role of the Southern Oscillation (SO) in the interannual variability of convection and rainfall over Mexico is studied by correlation analysis of long-term precipitation records, a satellite-derived set of highly reflective clouds (HRC), upper-air soundings, surface ship observations, and an SO index defined as high for anomalously high low pressure at Tahiti/Darwin. Highly reflective clouds and rainfall have a similar annual cycle and interannual variability, except in certain regions and seasons with prevalence of stratiform cloudiness. During the boreal winter (November-April) dry season, the low SO phase (or El NinMo) is, in much of the country, characterized by increased precipitation associated with the enhanced influence of mid-latitude westerlies. However, in the Isthmus of Tehuantepec, most exposed to Norte invasions, precipitation is larger in the high SO phase. During the boreal summer (May-October) rainy season, precipitation is more abundant in the high SO phase, when the northward retraction of the North Atlantic High, weaker trades, and a northward displaced near-equatorial trough are conducive to enhanced ascending motion over the area.

Sensitivity of regional climates to localized precipitation in global models

Abstract: ATMOSPHERIC general circulation climate models are valuable tools for investigating the effects that large-scale perturbations (for example, increasing greenhouse gases1, volcanic dust in the atmosphere2, deforestation3,4 and desertification5) might have on climate. Designed for global-scale climate simulations and having coarse spatial resolutions (from ∼250 km (ref. 6) to 800 km (ref. 7)) such models are now being used to evaluate climatological and hydrological quantities at or near the land surface and at sub-continental scales8–10. They are also being used to provide input data for high-resolution simulations, predicting, for example, the effects of increasing atmospheric CO2 at regional scales11. Here we show that continental surface climatologies and climate change predictions for tropical forest ecotypes derived from general circulation models may be very sensitive to slight modifications in the land-surface/atmosphere coupling. Specifically our results indicate that improving the realism of the areal distribution of precipitation alters the balance between runoff and evaporation. In the case of a tropical forest, we find that by modifying the area over which rainfall is distributed, the surface climatology is changed from an evaporation-dominated regime to one dominated by runoff. Climate studies using near-surface output from such simulations may therefore be misleading.

Rainfall interception in two tropical montane rain forests, Colombia

Abstract: Rainfall interception was studied during one complete year in two montane rain forests in the Colombian Andes at altitudes of 2550 and 3370 m. Additional measurements were made in a subsequent period at 3370 m, comparing two plots with different tree genera. Special attention was paid to the possible consequences of the abundant epiphytic vegetation, representing a high but spatially variable storage capacity. On an annual basis, interception amounted to 262 mm (12.4 per cent) of 2115 mm incident precipitation at 2550 m, and to 265 mm (18.3 per cent) of 1453 mm incident precipitation at 3370 m. There was no evidence for fog precipitation. The fact that, in spite of lower annual rainfall and lower evaporation rate, absolute quantity of interception at 3370 m was not lower than at 2550 m is probably related to differences in rainfall distribution and canopy storage capacities. The different responses of the two tree genera (characterized by distinct epiphyte cover) and the lower interception after prolonged wet periods support the idea that epiphytes play a significant role in the interception process. A realistic description of the water balance of such ecosystems will require the adaptation of existing interception models.

Estimation of Rain and Hail Rates in Mixed-Phase Precipitation

Abstract: Precipitation comprising rain and hail is studied. Specifically, techniques to identify and quantify such precipitation in terms of rain and hail fall rates using dual polarized radar data, are presented. Included for consideration are ZH, the reflectivity factor for horizontal polarization, ZDR, the differential reflectivity, and KDP, the differential propagation constant. A variety of simple models of mixed-phase precipitation are first examined. Electromagnetic scattering computations are performed to simulate and study the behavior of ZH, ZDR, and KDP. It is shown that it is possible to distinguish the mixed-phase precipitation from either rain or hail by using ZH, KDP pair and also to infer the thermodynamic phase and orientation from ZH, ZDR pair. On the basis of physical principles, it is shown that KDP senses primarily liquid water in the form of raindrops even when these are mixed with hailstones. The self-consistency Of ZH, ZDR, and KDP is then exploited to estimate both the rain and ha...

Nutrient fluxes in bulk precipitation and throughfall in two montane tropical rain forests, Colombia.

Abstract: (1) The composition of bulk precipitation (rainfall plus dry deposition as collected by funnels) and throughfall water was studied during one year on a weekly basis in two epiphyte-rich Andean rain forests (Cordillera Central, Colombia, 2550 and 3370 m a.s.l.) (2) Nutrient input to the forest by bulk precipitation was higher at 2550 m because of larger precipitation volumes; rainfall was 2115 mm at 2550 m and 1453 mm at 3370 m. Losses of nutrients from the canopy, both total amounts and amounts per unit of precipitation, were also higher at 2550 m. (3) Net fluxes (throughfall flux minus bulk precipitation flux) and deposition ratios (ratio of throughfall flux to bulk prec;pitation flux) were generally within the range reported for other montane tropical rain forests. Differences between forests appear to be related to climate (mainly precipitation amounts), geographical situation (e.g. maritime influences) and the availability of nutrients in the soil. The patterns described in the present study are in accordance with other studies that show reduced cycling rates and limited availability of nitrogen and phosphorus in high-altitude rain forests. (4) Volcanic activity in the proximity of the forests (continuous SO2 emission and incidental ash-falls) was shown to affect precipitation chemistry. Inputs of S04-S were high (26 2 and 16 8 kg ha-' year' at 2550 and 3370 m, respectively) and correlated with rainfall acidity. As a consequence of an ash-fall, concentrations of Ca, Mg and S04-S in bulk precipitation were far above average on one occasion. Throughfall concentrations fell to normal within two to three weeks in spite of low rainfall.

CO2 climate sensitivity and snow-sea-ice albedo parameterization in an atmospheric GCM coupled to a mixed-layer ocean model

Abstract: The snow-sea-ice albedo parameterization in an atmospheric general circulation model (GCM), coupled to a simple mixed-layer ocean and run with an annual cycle of solar forcing, is altered from a version of the same model described by Washington and Meehl (1984). The model with the revised formulation is run to equilibrium for 1 × CO2 and 2 × CO2 experiments. The 1 ×CO2 (control) simulation produces a global mean climate about 1° warmer than the original version, and sea-ice extent is reduced. The model with the altered parameterization displays heightened sensitivity in the global means, but the geographical patterns of climate change due to increased carbon dioxide (CO2) are qualitatively similar. The magnitude of the climate change is affected, not only in areas directly influenced by snow and ice changes but also in other regions of the globe, including the tropics where sea-surface temperature, evaporation, and precipitation over the oceans are greater. With the less-sensitive formulation, the global mean surface air temperature increase is 3.5 °C, and the increase of global mean precipitation is 7.12%. The revised formulation produces a globally averaged surface air temperature increase of 4.04 °C and a precipitation increase of 7.25%, as well as greater warming of the upper tropical troposphere. Sensitivity of surface hydrology is qualitatively similar between the two cases with the larger-magnitude changes in the revised snow and ice-albedo scheme experiment. Variability of surface air temperature in the model is comparable to observations in most areas except at high latitudes during winter. In those regions, temporal variation of the sea-ice margin and fluctuations of snow cover dependent on the snow-ice-albedo formulation contribute to larger-than-observed temperature variability. This study highlights an uncertainty associated with results from current climate GCMs that use highly parameterized snow-sea-ice albedo schemes with simple mixed-layer ocean models.

Trace elements in the North Atlantic troposphere: Shipboard results of precipitation and aerosols

Abstract: Samples for trace metals in precipitation (rain, snow, and fog) and total aerosols were collected at sea as part of the GCE/CASE/WATOX expedition during August and September 1988 in the greater North Atlantic between Bermuda, Iceland, the Azores, and Barbados. The samples were collected using ultra-clean equipment and methods for the analysis of the trace elements Al, Fe, Mn, Zn, Cu, Ni, Cd, and Pb (including stable isotopes). The samples were analyzed by graphite furnace atomic absorption spectrometry and thermal ionization mass spectrometry. Based on stable lead isotopes and retrospective isentropic air mass analyses, four air mass sources were evidenced including (1) temperate U.S. and (2) boreal Canadian westerlies, and (3) boreal and northern Europe easterlies associated with (4) Mediterranean and Saharan easterlies. Lead isotopic signatures clearly distinguished emission source regions over the North Atlantic with 206Pb/207Pb ratios of 1.19–1.22 for North American westerlies, 1.10–1.12 for European boreal easterlies, and 1.15–1.16 for Saharan trade easterlies. Also, the contrasting ratios between precipitation and aerosol demonstrate a vertical mixing of chemical sources in precipitation from contrasting air masses of different origin. The air mass trajectories were independently calculated and are in general agreement with the lead isotopic tracers. The isotopic tracers allowed more confident conclusions on air mass sources, particularly after a few days of oceanic transport. Also, the trajectories appeared to be very useful in explaining the strong variability of the trace metal concentrations during restricted periods of sampling time aboard a moving ship. To a lesser extent, the atmospheric emission sources were characterized by trace element concentrations. The occurrence of large dust loadings in the Saharan trade easterlies contributed abundant crustal elements and were also associated with enriched anthropogenic elements from southern Europe. Scavenging ratios were calculated for lead and cadmium during select periods when aerosol and precipitation were collected under the same meteorological regimes. The cadmium ratio is much greater than that of lead, indicating that cadmium and lead are each transported or scavenged on distinct populations of aerosols. For lead, the ratio is less in the North Atlantic than in the Pacific, suggesting an aerosol concentration dependence. Like the Pacific, the ratios increase for the lead, dust, and sea salt. One may thus conclude that marine scavenging ratios cannot yet be used effectively until factors such as concentration dependency and local conditions of tropospheric processing are better defined.

Atmospheric response to Ice Age conditions: Climatology near the Earth's surface

Abstract: We present a 6-year simulation of the ice age atmosphere using the T21 Atmospheric General Circulation Model (AGCM) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The lower boundary conditions (18 kyr B.P.) were taken from CLIMAP Project Members (1981). The analysis is restricted to the surface climatology for two reasons: The surface fields are the test data derived from the geological record on land, and they define the upper boundary conditions for simulating the glacial ocean. Model results are shown for the mean annual surface fields of temperature, wind, and precipitation. In the global average the surface temperature was 4.7°C cooler compared to the present temperature. The wind strength increased in mid-latitudes and decreased in tropical trade wind regions. Precipitation did not change significantly in the global average; however, precipitation decreased over land and increased over the ocean. Most of the difference patterns between the present conditions and the ice age climate were statistically significant. The simulated surface climatology is roughly consistent with the paleogeological evidence and with numerical AGCM simulations of other authors. This suggests that presently available AGCMs, including the ECMWF model (T21), are able to describe climates far away from the present, although internal parameterizations were tuned to present data sets.

Seasonality of litter invertebrate populations in an Australian upland tropical rain forest.

Abstract: Monthly samples, taken over 19 months, examined the composition, density, and seasonality of invertebrate litter faunas in an upland tropical rain forest. Arthropods represented 96.0 percent of the total sample number. Numbers of invertebrates increased with an increase in precipitation and there were significantly more individuals in samples during the wetter months. Numbers declined when litter became dry and/or sparse or oversaturated. Fluctuations in invertebrate litter populations appeared to relate to rainfall, litter moisture content, leaf fall, and rates of decomposition. Comparisons were made with similar studies carried out elsewhere in tropical rain forests. The present investigation is part of a study of the breeding and foraging ecology seasonality of Australian upland tropical forest birds. THIS STUDY REPRESENTS some of the first available data on tropical rain forest litter invertebrates in

Prediction of Nine Explosive Cyclones over the Western Atlantic Ocean with a Regional Model

Abstract: A series of 14 numerical experiments were conducted using the Pennsylvania State University/National Center for Atmospheric Research mesoscale model on nine cases of explosive marine cyclogenesis. The main objective was to identify key factors that are important to short-range prediction of explosive cyclones. We found that the intensity and structure of the simulated cyclones were sensitive to the details of precipitation parameterization. The grid-resolvable-scale precipitation associated with the mesoscale slantwise ascent in the vicinity of the warm front was crucial for rapid development. The upright convective precipitation played a relatively unimportant role. Surface energy fluxes had little effect on the development during the 24-h period of rapid cyclogenesis. The pattern of upward and downward fluxes while the storms were in progress was not favorable for storm intensification. Small case-to-case variation was found among the nine-case ensemble in the resulting deepening due to changes...

Patterns of the isotopic composition of precipitation in time and space: data from the Israeli storm water collection program

Abstract: The data from a precipitation sampling program in Israel show the synoptic scale history of the air masses to be the predominant factor which controls the isotopic data. Within a single storm, the passage of a front is always associated with values most depleted in the heavy isotopes. Local factors and especially the enrichment of the heavy isotopic species resulting from partial evaporation of rain in low lying stations, are superimposed on these large-scale effects. DOI: 10.1034/j.1600-0889.1990.t01-2-00005.x

The Impact of Sea Surface Temperature Anomalies on the Rainfall over Northeast Brazil

Abstract: The response of the tropical atmosphere to the sea surface temperature (SST) anomalies in the Northern Hemisphere spring of 1984 is investigated. The methodology for investigation consists of comparing simulations with and without the global distribution of SST anomalies in the boundary conditions of the UCLA General Circulation Model (GCM). At low levels, the response includes weaker southeast trade winds over the Atlantic, increased precipitation off the northeast coast of Brazil, and reduced precipitation west of this region. The increased precipitation is due to enhanced convergence of moisture advected by the southeast trade winds, although the trades themselves are weaker. The results for the western equatorial Atlantic am in apparent agreement with the observed anomalous southern migration of the ITCZ in years with warm SST anomalies in the southern tropical Atlantic. There are strong anomalous trade winds over the Pacific extending east of the date line and weak wind anomalies over the ma...

Precipitation Fluctuation over Semiarid Region in Northern China and the Relationship with El Niño/Southern Oscillation

Abstract: In recent years the semiarid region of northern China, which has total annual precipitation between 200 and 500 mm, has shown signs of severe desertification. Intensive theoretical and observational studies are currently underway to examine the climate changes and other contributing factors. In this study, we used the 1951–86 monthly precipitation measurements in this region to study their fluctuations and relationship with the El Nino/Southern Oscillation. Three main features are identified: 1) a 2–3 year quasi-periodic fluctuation, 2) a tendency for rainfall deficiency for the whole region during ENSO years, and 3) a significant correlation between the precipitation fluctuation in the southern part of this region and Southern Oscillation index, with the former lagging the latter by 2–5 months. These features are also evident from analysis of the proxy data during the last hundred years. Discussions on the possible link between the precipitation fluctuation, the summer monsoon, the western Pacif...

Possible Changes in Natural Vegetation Patterns due to Global Warming

Abstract: The distribution of global vegetation patterns is strongly related to climate patterns Local differences in soil characteristics and water availability, which are in turn also partly determined by climate, force the global vegetation patterns to become even more pronounced and the local patterns more diverse The predicted global climate change, due to the increased levels of greenhouse gases, will thus have large impacts on broad-scale vegetation patterns The Holdridge life zone classification is based on biotemperature (a heat sum), annual mean precipitation and a potential evapotranspiration ratio; it was used in the current paper to create an objective global vegetation (life zone) map The map was created by interpolation of a large data base of weather stations to a grid of land cells with a resolution of 05" The climate scenario for future climate under double CO2 concentrations is based on results of the UK Meteorological Office general circulation model Large changes in vegetation patterns are observed between the life zone classifications of present and future climate The polar regions decrease most in their extent, and all boundaries between the different latitudinal regions move toward their poles Most sensitive to climatic warming are the borders between the latitudinal regions Changes are most clearly observable at the southern and northern edges of the boreal forest region Limitations of the Holdridge classification to study impacts of climate change are discussed and improved approaches are proposed

The Isotopic Composition of Precipitation from Two Extratropical Cyclones

Abstract: Precipitation samples were collected at stations in the Eastern United States for two extratropical cyclones during the Genesis of Atlantic Lows Experiment (GALE) of 1986 and analyzed for their δ18O values. They represent the first synoptic-scale datasets of isotopic values. Measured isotope ratios are explained in terms of physical principles and meteorological processes. They are shown to be related to vertical profiles of ω cloud-top temperatures, evaporation beneath cloud base, isotope equilibration, and water vapor sources for the precipitation. Measured isotope ratios are then compared to values obtained from simple models of convective and stratiform precipitation. Both storms are shown to exhibit a consistent pattern of isotope ratios, with lowest δ18O values occurring in the stratiform precipitation well within the cold air, and highest values associated with the convective precipitation of the warm sector. A pronounced-amount effect, in which δ18O values decrease as rainfall totals incr...