Showing papers on "Precipitation published in 1979"

Global climatic interpretation of the deuterium‐oxygen 18 relationship for precipitation

Abstract: A theoretical model is derived to account for the deuterium-oxygen 18 relationship measured in meteoric waters. A steady state regime is assumed for the evaporation of water at the ocean surface and the subsequent formation of precipitation. The calculations show that the deuterium and oxygen 18 content in precipitation can be taken as linearly related. From the slope and the intercept (known as the deuterium excess) of the δD-δ18O linear relationship for precipitation we compute the mean values on a global scale of the evaporating ocean surface temperature and the relative humidity of the air masses overlying the oceans. The deuterium excess is primarly dependent on the mean relative humidity of the air masses formed above the ocean surface. Paleoclimatic data may be obtained by this isotopic method from the analysis of old water and ice samples. A moisture deficit of the air over the ocean, equal to only 10%, in comparison to 20% for modern conditions, is deduced from the deuterium-oxygen 18 distribution measured in groundwater samples older than 20,000 years.

Frontal Influences on Northeast Brazil

Abstract: Surface observations for the 10-year period 1961–70 are analyzed for occurrences of fronts in North-east Brazil. Cold fronts, or their remains, are found to enter the Northeast throughout the year. The southern part of the region (interior Bahia) receives much of its precipitation in the period November-February associated with frontal systems. Coastal areas. which receive maximum rainfall during the months of May–July, experience a marked increase in rainfall associated with the approach and passage of cold fronts. Even precipitation events in the extreme northern part of the region (Ceara) sometimes appear to be associated with systems which are cold-frontal in origin. Pressure fluctuations are felt nearly simultaneously throughout the region with greatest amplitudes occurring in the south. These fluctuations are also shown to occur over a fairly broad range of longitude within tropical Brazil and may have a significant effect on the position of the equatorial trough zone over the Atlantic.

Geochemical studies of the Weddell sea

Abstract: The origin of Antarctic Bottom Water is discussed in terms of the distributions of a number of geochemical tracers in the major water masses of the Weddell Sea. Oxygen, phosphate, nitrate and total inorgabic carbon are shown to be negligibly altered by consumption or production during sub-surface circulation and mixing. All the conservative properties of Weddell Sea Bottom Water and Antarctic Bottom Water are explained as simple mixtures of Winter Water, Warm Deep Water, and Western Shelf Water. Silica is markedly non-conservative, showing enrichments up to ≈35 μM kg−1 which are due mainly to interactions with the bottom. The deuterium and oxygen-18 stable isotope data show that the high salinity of Western Shelf Water is principally the result of freezing rather than evaporation. The isotopic composition of Western Shelf Water also requires a significant admixture of melt-water from the base of the Filchner Ice Shelf, and mass-balance calculations linking the rate of shelf ice melting to the formation of bottom water give a production rate of about 5 × 106 m3 s−1 for Weddell Sea Bottom Water with potentiatial −0.9°C, or about 8 × 106 m3 s−1 in terms of classical Antarctic Bottom Water with a potential temperature of −0.4°C. Concentrations of the radioisotopes carbon-14 and tritium are exceptionally low in the Weddell Sea despite the fact that the CO2 atmospheric exchange rate is comparable to the global average. The low values are seen to be the result of a short surface residence time, a large sub-surface mixing component, and exchange inhibition by sea ice especially during the winter. Unlike the average world ocean, where molecular exchange is about three times more important than precipitation in transporting atmospheric tritium to surface waters, about four times more tritium is added to the Weddell Sea by precipitation than by molecular exchange. The flux of new Weddell Sea Bottom Water based on the tritium data is estimated very roughly at about 3 × 106 m3 s−1, or about 4.5 × 106 m3 s−1 expressed in terms of classical Antarctic Bottom Water.

The Structure and Evolution of Convection in a Tropical Cloud Cluster

Abstract: A large cloud cluster which occurred over the data network of the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) on 5 September 1974 is examined. Data from four quantitative shipboard weather radars show that virtually all of the precipitation in the tropical cloud cluster was associated with six mesoscale precipitation features. A prototype for the structure and life cycle of these features is presented which is sufficiently general to describe all six precipitation features, one of which was a tropical squall-line system. These mesoscale features appear to he the primary entitles within which deep tropical convection occurs. In their formative stage, mesoscale precipitation features consist of a line of isolated cumulonimbus cells oriented perpendicular to the low-level wind flow. In the intensifying stage, the rain areas of the individual cells merge when new convective cells develop between and ahead of the existing cells, where the outflow from convective-scale dow...

Melting and Evaporation of Hydrometeors in Precipitation from the Anvil Clouds of Deep Tropical Convection

Abstract: Five cases of horizontally uniform precipitation associated with anvil clouds were investigated using weather radar, rawinsonde, satellite and raindrop size data collected during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE). The area of horizontally uniform precipitation was in each case characterized by rainfall rates of 1–10 mm h−1 in contrast to the 10–100 mm h−1 observed in convective cells. Concentrations of precipitation-sized ice particles above the melting layer and liquid water below the melting layer, together with observed particle spectra, suggest that aggregation occurs above the melting layer, and that riming occurs in sufficient amounts to produce graupel within the anvil cloud. All five cases exhibited distinct radar bright bands in the melting layer. Cooling rates associated with the melting in this 1 km thick layer near the base of the anvil cloud were 1-7 K h−1. These cooling rates were comparable to the 0.2–6 K h−1 cooling rates due to evaporat...

On the influence of the spatial distribution of rainfall on storm runoff

Abstract: This study is an assessment of the importance of precipitation accuracy on the rainfall-runoff modeling of a small catchment Two mathematical models were used in the investigation: a deterministic rainfall-runoff model based on the kinematic wave approximation and a nonstationary time-varying multidimensional rainfall generation model It is implicitly assumed that this rainfall generation model is an appropriate mathematical representation of the natural phenomenon of rainfall The deterministic rainfall-runoff model is used to represent the 265-mi2 catchment of the Rio Fajardo in northeastern Puerto Rico The rainfall model generates synthetic rainfall which serves as the input to this runoff model The influence of the spatial distribution of the rainfall input on the discharge is analyzed by using 1 rain gage or 20 rain gages to record the synthetic storms The isohyetal maps and hyetographs of the synthetic storms, together with the storm hydrographs produced by the runoff model, are analyzed, with specific attention given to the volume of storm runoff, time-to-peak runoff, and peak runoff The experiments show that the spatial distribution of rain and the accuracy of the precipitation input have a marked influence on the outflow hydrograph from a small catchment

Mesoanalysis of the Big Thompson Storm

Abstract: Mesoscale analyses and descriptions of meteorological conditions that produced the devastating dash flood in the Big Thompson Canyon on 31 July 1976 are presented. The storm developed when strong low-level easterly winds to the rear of a polar front pushed a moist, conditionally unstable air mass upslope into the Front Range of the Rocky Mountains. The main thrust of the moisture flux focused initially into the Big Thompson area. Orographic uplift released the convective instability, and light south-southeasterly winds at steering levels allowed the storm complex to remain nearly stationary over the foothills. Heavy rains fell within the storm along a narrow corridor only 5 km wide oriented north-northeast by south-southwest. Minimal entrainment of relatively moist air at middle and upper levels, very low cloud bases and a slightly tilted, updraft structure contributed to a high precipitation efficiency. A deep warm layer of convective cloud fostered precipitation growth through warm cloud proces...

Rainfall changes in summer caused by st. Louis.

Abstract: Precipitation in and around St. Louis was investigated to study urban influences on summer precipitation conditions. Prerain winds were used to define the "downwind area" where influences would be greatest, and wind-sorted rains were combined into monthly and summer totals. Seventy-five percent of the 16 rain patterns revealed a rainfall maximization downwind of the city, and the rainfall in the downwind area was 22.7 percent more than the rainfall upwind of St. Louis where no urban influences existed. Various statistical tests of the summer rainfall distribution reveal the downwind rainfall to be significantly greater than elsewhere and supportive of other findings that St. Louis increases rainfall.

Mesoscale Unsaturated Downdrafts Driven by Rainfall Evaporation: A Numerical Study

Abstract: It has been proposed that evaporation of precipitation failing from widespread thick masses of nimbostratus derived from cumulonimbus can account for the mesoscale unsaturated downdrafts inferred to occur within certain tropical mesosystems. This paper discusses experiments made with a numerical model suitable for testing this idea. Dynamics of the explicitly described (large-scale) flow are governed by the hydrostatic unfiltered equations specialized to two dimensions. The value of f is for 10°N. Cumulus convection is parameterized through a one-dimensional plume model which allows for vertical transport of water substance in vapor and liquid form. The water budget for the large scale includes vapor and both cloud and precipitation and allows for transformations between these categories. Computations are sensitive to the assigned value of β, the ratio of mass flux upward through the bases of convective clouds to the large-scale upward mass flux through 900 mb. For β=1.0, the initial wave disturb...

Global Climatic Interpretation of the Deuterium-Oxygen 18 Relationship

Abstract: A theoretical model is derived to account for the deuterium-oxygen 18 relationship measured in meteoric waters. A steady state regime is assumed for the evaporation of water at the ocean surface and the subsequent formation of precipitation. The calculations show that the deuterium and oxygen 18 content in precipitation can be taken as linearly related. From the slope and the intercept (known as the deuterium excess) of the bD-b80 linear relationship for precipitation we compute the mean values on a global scale of the evaporating ocean surface temperature and the relative humidity of the air masses overlying the oceans. The deuterium excess is primarly dependent on the mean relative humidity of the air masses formed above the ocean surface. Paleoclimatic data may be obtained by this isotopic method from the analysis of old water and ice samples. A moisture deficit of the air over the ocean, equal to only 10%, in comparison to 20% for modern conditions, is deduced from the deuterium-oxygen 18 distribution measured in groundwater samples older than 20,000 years.

Precipitation over the Pacific Ocean, 30°S to 60°N

Abstract: By using present weather observations taken by ships and relating them to a given amount of precipitation, new estimates of oceanic rainfall for the Pacific Ocean between 30°S and 60°N have been derived. Satellite microwave measurements and Taylor's (1973) island analysis support our findings. Annual and quarterly rainfall maps, drawn from our estimates, agree with other modem, land-derived values, but provide greater detail. Between the equator and 60°N, the annual depth and volume rainfall totals are 1282 mm and 1.16×105 km3, respectively. Maps of amplitude and phase show that most of the rainfall north of 28°N occurs in winter, while maximum rainfall occurs in July and August in the tropics. Diurnal rainfall, studied at selected locations, is at a minimum at noon in all but the western pan of the North Pacific. Here there is no distinct minimum.

Areal Distribution of Deuterium in Eastern California Precipitation, 1968–1969

Abstract: Rain and snow were collected during major storms at 26 stations in California and Nevada during the exceptionally wet 1968–69 season. The deuterium/hydrogen ratios (δD, in per mil, VSMOW) of the samples varied according to the elevation of the sample site and its position relative to the Sierra Nevada. Eighteen storms were sampled at one or more stations. Eleven of them were sampled at almost all of 13 stations that lie east and southeast of the Sierra Nevada which allowed a storm-by-storm comparison of the influence of the topographic setting of the station and the synoptic meteorology of the storm on the δD characteristics of the precipitation. A model is presented that illustrates possible changes in δD values as a storm moves inland. It is calculated using probable starting values for moist air masses arriving from the Pacific and follows the meteorological and isotopic changes that should occur during adiabatic uplift, cooling and condensation. The model agrees satisfactorily with data from ...

On the Effect of Precipitation and Runoff on Coastal Circulation in the Gulf of Alaska

Abstract: Surface waters in the Gulf of Alaska undergo a net dilution throughout most of the. year since the regional precipitation exceeds evaporation. Recent hydrographic data give evidence that seasonal dynamic height fluctuations in the upper layers ( 130 cm year−1), runoff, longshore accumulation of fresh water around the gyre, and the low water temperatures. The coastal sea level is in phase and has nearly the same amplitude as the local dynamic height, though not in phase wit...

Analysis of the Response of a Grassland Ecosystem to Stress

Abstract: As indicated in Chapter 1, the shortgrass prairie of the piedmont of north-central Colorado is a semiarid grassland with an average annual precipitation of about 312 mm (Hyder et al 1975). More than 70% of the precipitation occurs as rain from late April through early August, with the largest quantities occurring in May and June. Year-to-year fluctuations in quantity and seasonal distribution of rainfall are great. In a recent 31-year period, annual precipitation was as low as 110 mm and as high as 582 mm. Average monthly temperatures range from -4°C in January to 21°C in July.

Communications Preliminary Study of Rain Effects on Radar Scattering from Water Surfaces

Abstract: The effect of rain striking the water surface on radar scatterometer measurements has not been investigated previously. When precipitation is present, this effect could be of great importance in achieving accurate estimates of the ocean wind vector using radar backscatter measurements from the SeaSat Active Scatterometer System (SeaSat SASS) [ 1 1. The percentage of the ocean covered by precipitation is small at any one time; also, since the precipitation is far from uniformly distributed, deciding in regions of major precipitation which data should be eliminated presents a problem. Improper interpretation of the errors present in these conditions can result either in leaving large gaps in the data or in erroneous estimates in the ocean wind vector over large regions. The wind sensitivity of the radar backscatter is dependent upon the resonance with the capillary waves of wavelength 1.7 cm [ 21. Rain droplets can create waves of a similar or larger size and disrupt the patterns of the winddriven waves. Preliminary results have been obtained through a controlled wave-tank experiment. Using an FM radar [3], backscatter measurements at 14.0 GHz were obtained for several different wind speeds at an incidence angle of 40’ with and without artificial rain. Measurements of the wind speeds were made using a pitot-tube anemometer. These measurements were extrapolated to obtain the wind speed that would be present

A Model for Orographic Precipitation in the North-Central Great Basin

Abstract: The orographic effect, measured as the ratio of mountain to valley precipitation at 72 pairs of stations in the north-central Great Basin, was tested against 29 relief, location and synoptic parameters. Parameters found significant were then subjected to stepwise multiple regression to develop equations explaining the spatial variation of precipitation for each calendar month, two synoptic seasons and the year. The monthly and seasonal equations explained 52–83% of spatial variance and the annual equation 65%. They showed that the ratio increased with station elevation difference and in winter with proximity to the Pacific Ocean; significant relationships also occurred with station pair orientation, and in summer with ocean direction and Pacific storm rainfall. Best results were obtained either for pairs with long-term records or those with both stations topographically close together. An equation using mean monthly Great Basin wind speed and Pacific storm rainfall explained 88% of the annual var...

Mean Rainfall Patterns for the Three Phases of GATE

Abstract: As a part of the GARP Atlantic Tropical Experiment (GATE), quantitative precipitation measurements were made during the summer of 1974 with four C-band digital radars complemented by shipboard raingages. Isohyetal maps covering a 125 000 km2 array centered at 8°30′N, 23°30′W are presented for each of three, approximately 20-day observational phases of GATE. Large mean rain rates exist for all three phases, with the largest ones corresponding to accumulations exceeding 500 mm for some of the maximum isohyets during Phase I. The mean rainfall rate averaged over the B-scale array for all three phases, 11.3 mm day−1, is apparently not significantly different from pre-GATE rainfall climatology. Another striking characteristic of the phase-mean precipitation patterns is the large spatial gradients; e.g., gradients as large as 200 mm in 16 km are observed. Latitude shifts in the zone of maximum confluence (intertropical convergence zone) and in the tracks of the synoptic disturbances are reflected by in...

Atmospheric contributions to stream water chemistry in the North Cascade Range, Washington

Abstract: Bulk precipitation contributes significant quantities of cations and trace metals to a subalpine catchment located in the North Cascade Range of Washington. Precipitation loadings account for 30% of the major and more than 50% of the minor element export from the catchment. Preliminary data from larger basins in the North Cascades suggest similar values. Calcium and potassium measured in bulk precipitation are largely derived from continental dusts, while sodium and magnesium originate as marine aerosols. Lead, copper, and arsenic, products of anthropogenic activity in the Puget Lowland, are deposited downwind in the Cascade Mountains; mean precipitation pH at the study site was 4.85 during 1974 and 1975. Elements contributed by precipitation become part of complex upland biogeochemical cycles. With the exception of highly mobile ions like sodium, elemental levels in streamflow reflect biologic and pedogenic cycling processes rather than direct precipitation influence.

A Method for Modeling the Deposition of Sulfur by Precipitation over Regional Scales

Abstract: Radioactive fallout data suggest that the concentration of pollutants in rainfall, while highly variable, might be described on the average by about an inverse half-power dependence on the amount of precipitation. Recent measurements of sulfur concentrations in summer rainfall collected at Argonne National Laboratory tend to support this contention, as do preliminary results derived from operations of the DOE precipitation chemistry network. The concept is extended to develop a bulk removal rate for airborne total sulfur by precipitation for use in regional dispersion modeling.

Climatological estimate of the average monthly energy and water budgets of Lake Tahoe California‐Nevada

Abstract: An estimate of the average monthly energy and water budgets of Lake Tahoe, California-Nevada is made from commonly available meteorological, hydrological, and limnological data. The water budget indicates that precipitation, discharge, runoff, and evaporation dominate the water balance in seasonal succession, each associated with an appropriate minimum or maximum in the lake water storage. The annual energy budget is dominated by the net radiation and evaporation terms with 93% of the radiation input used to evaporate water. The seasonal energy budget indicates that (1) energy storage is in phase with the radiation input and is a dominant term in the periods November–January and May–July, (2) evaporation reaches a maximum value in the fall, 3 months after the radiation maximum, (3) upward transfer of sensible heat from the lake surface reaches a maximum another 3 months later, in the winter, and (4) the sensible heat flux is downward in late spring and early summer, indicating stable stratification in the atmospheric surface layer over the lake during this period, a result of possible significance to air quality in the Tahoe basin. The energy storage and sensible heat transfer terms show large fluctuations in the fall which may be associated with large-scale meteorological events during a season in which energy is trapped in the surface waters by stable stratification in the thermocline layer.

δ18O variations in snow on the Devon Island ice cap, Northwest Territories, Canada

Abstract: A study of δ18O variations of snow samples taken on traverses across the Devon Island ice cap in June 1971, 1972, and 1973 has shown a difference between the accumulation conditions on the southeast and northwest sides of the ice cap. On the southeast side there is an increasing depletion of 18O in the snow with increasing elevation. This pattern is attributed to the effect of orographic uplift of air masses moving over the ice cap from the southeast, which promotes condensation and precipitation due to adiabatic cooling. On the northwest side of the ice cap there is no evidence of any further depletion of 18O in snow, neither with increasing distance from the possible moisture source in Baffin Bay to the southeast nor with increasing elevation if the air mass comes from the northwest. In this case condensation is due to isobaric cooling so that precipitation is generally from level cloud bases. The changes inferred for the isotopic composition of the water vapour as it rises up the southeast slope are fo...

New precipitation maps for the North Atlantic and North Pacific oceans

Abstract: Oceanic precipitation intensity values derived in a previous study were combined with precipitation frequency data in the recently revised marine climatic altases to prepare new annual and seasonal precipitation maps for the North Atlantic and North Pacific oceans. The greatest precipitation over both oceans occurs in the eastern and central tropical regions. A distinct minimum is present across both oceans in the subtropical regions, being most marked along the eastern margins. Secondary maxima are present over the northern regions, where considerable snow falls in winter. The principal seasonal changes are an increase in size and northward migration of the subtropical dry zone from winter to summer as well as reduced magnitude and gradients of precipitation to the north in summer. The tropical Pacific shows a remarkably stable distribution during the year, but the zone of maximum rainfall in the tropical Atlantic undergoes considerable variation in location. The new maps show generally less precipitation in extratropical regions than earlier ones, although in the tropics they give values between those of the widely varying previous maps.

Increasing variability in california precipitation

Abstract: Most of California experienced the severest dry spell on record in 1976–77; 1978 was one of its wettest rainy seasons. The economic and environmental impacts of the drought were significant. Comparisons of nonoverlapping, twenty-year periods between 1921 and 1977 produced few statistically significant differences in either means or variances. The probability of occurrence of extreme seasonal precipitation increased in the northern two-thirds of the state in the period 1961–77 relative to the period 1921–60, but decreased in the south. The chance of getting a total seasonal precipitation less than forty-three percent of the mean in any one year in the Sacramento Valley and the North and Central Coast regions since 1961 has increased to 1 in 15 from 1 in 20 in the period 1921–40 and 1 in 21 or 25 over the whole period of record. The period 1961–77 is statistically more similar to 1860–80 than to either 1921–40 or 1941–60. No relationships could be established between hemispheric warming and cooling...

Relationships between Large-Scale Motion and Convective Precipitation During GATE

Abstract: Upper air and surface data from the GARP Atlantic Tropical Experiment (GATE) are used to examine the interrelationships between convective-scale precipitation and the larger scale wind field. The upper air winds from the inner (B) and outer (A/B) hexagonal observational arrays are fit with second-order polynomials to provide smooth estimates of the vorticity, divergence and vertical motion in the observational array. In these analyses we examined archived validated data from all three phases of the experiment and we formed averages based on the radar-estimated precipitation rates. Mean profiles for 19-day periods during each of the three observational phases establish the basic similarity of the kinematics during each phase. Strong boundary-layer convergence balanced, for the most part, by upper tropospheric divergence, is common to all three phases. Radar-estimated precipitation rates are used to define suppressed (precipitation rates 0.5 m...

Comparison of Remote Measurements by Single- and Dual-Wavelength Meteorological Radars

Abstract: Dual-wavelength radar was used routinely to observe and record, in digital form, severe convective storms in northeastern Colorado in 1972-1974. By postprocessing, the precipitation-caused attenuation of the attenuating wavelength of the dual-wavelength pair is determined while the other wavelength remains essentially unattenuated. Precipitation rate computed from this attenuation is more accurate than that computed from the radar reflectivity factor Z, the quantity popularly associated with meteorological measurements of storms. The attenuation-derived precipitation over an artificial catchment shows the greatest improvement over that derived from Z at the highest precipitation rates. The attenuation technique has particular strength in that it discounts regions of hail that occur in the heaviest storms which greatly perturb the simple radar reflectivity estimates. The hail signal H is the ratio (expressed in decibels) of the S-band and X-band normalized echo powers from a point in a storm less the attenuation. Using the assumption of spherical hail with exponential size spectra randomly truncated at the upper limit, the equation D0 = 0.31 + 0.12H is a reasonable relationship between this hail signal and the median volume diameter, in centimeters, of the hail. Theoretical relationships relating the hail mass deposition rate and the vertical hail energy flux density are derived and are shown to be correlated with hail totals over the artificial catchment with correlation coefficients exceeding 0.8 for mass and 0.65 for energy.