Showing papers on "Precipitation published in 1981"

Radar and raingauge observations of orographic rain over south Wales

Abstract: Eight detailed case studies are summarized to clarify the structure and mechanism of orographically enhanced frontal rain over hills of modest height. the observations were obtained as part of a field project in south Wales in which data from a 3-dimensionally scanning radar were combined with autographic raingauge data. the results show that the generation of orographic rain is consistent with Bergeron's seeder-feeder mechanism, according to which raindrops from upper-level (seeder) clouds wash out small droplets within low-level (feeder) clouds formed over the hills. It is demonstrated that the orographic enhancement is strongly influenced by the low-level wind speed. the largest enhancement of rainfall occurred in association with strong winds, and also high relative humidity, below 2km. the radar showed that over 80% of the enhancement occurred in the lowest 1.5 km above the hills. It also showed that the periods of enhanced rainfall were associated with the passage of pre-existing areas of precipitation. the precise value of the upwind rainfall rate was rather unimportant in influencing the orographic increment provided the rainfall rate upwind exceeded about 0.5mmh−1, These findings are compared with the results of theoretical calculations based upon the washout model of Bader and Roach.

Structures of atmospheric precipitation systems: A global survey

Abstract: A survey of atmospheric precipitation systems, ranging from mid-latitude cyclones and thunderstorms to tropical cloud clusters, hurricanes, and monsoons, shows that all these systems are well described in terms of the rather traditional concepts of stratiform and convective precipitation. In stratiform precipitation, ice particles grow as they drift downward from high levels and pass through a well-defined melting layer. In convective precipitation, particles begin growing at low levels and are carried upward by strong updrafts and fall out in intense vertically oriented showers. Modern observations show that all the major types of precipitation observed over the globe can be and often are combinations of these two basic types of precipitation. Extratropical cyclonic precipitation is basically stratiform. However, it is typically intensified in regions called rainbands. Some rainbands are highly convective features which move through the basic stratiform precipitation. In other rainbands, shallow convective cells occur aloft and help to enhance the basic stratiform precipitation. Mid-latitude thunderstorms and tropical precipitation systems are basically convective. However, stratiform precipitation can develop in the middle to late stages of development. This type of stratiform precipitation, which can become quite extensive in both tropical and mid-latitude systems, apparently arises as groups or successions of active convective cells leave ice particles aloft to settle downward gradually after the cells' updrafts die out.

On a Statistical Relationship between Autumn Rainfall in the Central Equatorial Pacific and Subsequent Winter Precipitation in Florida

Abstract: A statistical relationship is found to exist between autumn rainfall in the central equatorial Pacific and subsequent winter precipitation in the southern United States In a case study, it was found that wet winters in south-central Florida are generally associated with warm water events along the equator, while dry conditions are more commonly associated with cold water events in the eastern tropical Pacific Some suggestions are offered concerning the physical mechanisms responsible for this distant teleconnection

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. III: Air Motions and Precipitation Growth in a Warm-Frontal Rainband

Abstract: Doppler radar data and airborne cloud microphysical measurements obtained in the CYCLES PROJECT indicate that a warm-frontal rainband in an extratropical cyclone was characterized by a precipitation process in which clouds at low levels were enhanced by a mesoscale updraft. Ice particles, apparently formed in shallow convective cells aloft and then drifted downward, undergoing aggregation just above the melting layer. This study demonstrates the crucial role of the low-level mesoscale updraft in condensing a sufficient amount of cloud water for particles to accrete as they fell through the lower portion of the frontal cloud.

Areal distribution of snow water equivalent evaluated by snow cover monitoring

Abstract: A method is proposed to determine the areal distribution of the maximum seasonal water equivalent of snow in mountain basins. Because a sufficient density of direct measurements is not available in remote, inaccessible areas, the accumulation of snow at the start of the melting season is reconstituted. The disappearance of snow in grid units is monitored using Landsat data, the number of degree days necessary to melt the snow is totalized, and the water equivalent of the snow melted is calculated. The reconstituted water equivalent values can be used to correct precipitation measurements in winter. Together with limited point measurements these new areal data can improve the evaluation of snow reserves for seasonal discharge forecasts.

Precipitation over the Atlantic Ocean, 30°S to 70°N

Abstract: New estimates of rainfall over the Atlantic Ocean between 30°S to 70°N have been constructed based an a technique that uses the present weather observations taken by ships. Annual and quarterly rainfall maps are presented. Between the equator and 60°N, the average annual rainfall depth is 1034 mm and the annual volume is 3.93 × 104km3. Compared to the Pacific, the Atlantic is significantly drier and has less extreme values. Maps of amplitude and phase show that most of the North Atlantic cast of 60°W experiences a inter peak rainfall. The South Atlantic experiences its peak rainfall in the Southern Hemisphere summer.

210 Pb in surface air at Enewetak and the Asian dust flux to the Pacific

Abstract: As part of the SEAREX programme, an air filter system and a precipitation collector were deployed at Enewetak (11°20′ N, 162°20′ E). A description of the air filter system and initial results on the chemical composition of the collected aerosols1 and on the organic components2 have been reported elsewhere. As part of the same programme we have measured 210Pb (and 210Po) in air filter and integrated monthly precipitation samples collected during 1979 to estimate the 210Pb flux (0.15±0.02 d.p.m. cm−2 yr−1) and the Asian dust flux (38±20 µg cm−2 yr−1) at this location in the Pacific.

Influence of the Tibetian Plateau on cumulative and diurnal changes of weather and climate in summer

Abstract: An investigation of the dynamic and thermodynamic influences of the Tibetian Plateau on the monsoon circulation in southeastern Asia and China is reported. A five-day primitive equation numerical model was used to simulate the influences of the high mountains in the 0-180 deg E and 25-55 deg N region on a typical July day. The equations predicted horizontal motion, heat, the water vapor mixing ratio, and surface pressure. Consideration was given to solar and longwave radiation and the release of latent heat by deep cumulus clouds. A net warming over the Plateau as opposed to the surrounding areas was found to lead to vertical air ascent and the establishment of strong summer monsoon circulation to the south. It is noted that the grid was too coarse to adequately describe the distribution of precipitation on the steep mountains.

Urban Precipitation Processes

Abstract: Precipitation processes in convective clouds in and around St. Louis were studied by radar, numerical modeling and surface rainfall data.

Temperature and precipitation record in southern Chile extended to ∼ 43,000 yr ago

Abstract: Southern Chile (41–56 °S), in the belt of westerly winds, receives mostly heavy precipitation, reaching 8,500 mm yr−1 near 50 °S and decreasing northwards and southwards from this latitude to ≤1,500 mm; mean January (summer) temperatures near sea level along the latitudinal gradient are 8–16 °C (refs 1, 2). Dense rain forest covers much of the region to 48 °S; south to Cape Horn, magellanic moorland prevails and rain forest becomes limited. Modern pollen measured in surface samples reflects the distribution of plant species in the vegetation. We have now applied regression equations relating present-day pollen to temperature and precipitation to fossil pollen data at Taiquemo to assess climatic conditions during the Quaternary. The results extend our previous record of the past 16,000 yr at Alerce3 beyond the lateglacial to ∼43,000 yr BP. For the 27,000-yr interval, they show mean January temperatures of 10–12 °C and mean annual precipitation centred around 1,000 mm except during the time span of ∼31,000–43,000 yr BP when amounts increased to 4,000 mm. In general fluctuations correspond to the isotopic climatic reconstruction in Antarctica4 and to changes inferred from pollen data at comparable latitudes in Tasmania5–8 and New Zealand9,10.

Acidic precipitation: considerations for an air-quality standard

Abstract: Acidic precipitation, wet or frozen precipitation with a H+ concentration greater than 2.5 μeq l−1, is a significant air pollution problem in the United States. The chief anions accounting for the H+ in rainfall are nitrate and sulfate. Agricultural systems may derive greater net nutritional benefits from increasing inputs of acidic rain than do forest systems when soils alone are considered. Agricultural soils may benefit because of the high N and S requirements of agricultural plants. Detrimental effects to forest soils may result if atmospheric H+ inputs significantly add to or exceed H+ production by soils. Acidification of fresh waters of southern Scandinavia, southwestern Scotland, southeastern Canada, and northeastern United States is caused by acid deposition. Areas of these regions in which this acidification occurs have in common, highly acidic precipitation with volume weighted mean annual H+ concentrations of 25 μeq l−1 or higher and slow weathering of granitic or precambrian bedrock with thin soils deficient in minerals which would provide buffer capacity. Biological effects of acidification of fresh waters are detectable below pH 6.0. As lake and stream pH levels decrease below pH 6.0, many species of plants, invertebrates, and vertebrates are progressively eliminated. Generally, fisheries are severely impacted below pH 5.0 and are completely destroyed below pH 4.8. At the present time studies documenting effects of acidic precipitation on terrestrial vegetation are insufficient to establish an air quality standard. It must be demonstrated that current levels of precipitation acidity alone significantly injure terrestrial vegetation. For aquatic ecosystems, current research indicates that establishing a maximum permissible value for the volume weighted annual H+ concentration of precipitation at 25 μeq l−1 may protect the most sensitive areas from permanent lake acidification. Such a standard would probably protect other systems as well.

A simple method for estimating convective rain volume over an area

Abstract: Previous authors have reported significant correlations between the horizontal extent of convective showers or storms and the volume of rain they produce. This paper employs that idea to develop a simplified method for estimating convective rainfall by considering only the horizontal extent and duration of the precipitation. The present analysis is based on rain gage and radar data from an area in western North Dakota. A synoptic adjustment is applied to the radar rain volume estimates. A quantity called the integrated rainfall coverage can be calculated from either gage or radar data and is found to be well correlated with the rain volumes. The maximum echo area during any one scan in one hour seems to be the hourly radar measurement best correlated with the rain volumes. This limited study suggests that the accuracy of the simplified method approaches that of methods using radar reflectivity data and may have operational value in some special situations.

Accuracy of Snowmelt Runoff Simulation

Abstract: Results of runoff simulations from various basins using a snowmelt runoff model were analyzed in order to predict the accuracy of simulations in future applications of the model. It was found that the model can be applied to nearly any mountainous basin where snowmelt runoff is an important factor if input data on temperature, precipitation, and snow cover are available. The simulation accuracy will depend on the quality of the input data as well as on the density of observations, size of the basin, care in determination of the recession coefficient, and amount of precipitation during snowmelt. Most accurate simulations will result when: 1) temperature and precipitation are recorded at the basin mean elevation; 2) snow cover observations are available once per week; 3) several climatic stations are available for large basins; and 4) a few years of runoff records exist for determination of the recession coefficient. Decreases in simulation accuracy will be expected as these optimum conditions are compromised, however, acceptable simulations will result with the following minimum conditions: 1) temperature and precipitation data are available in the general vicinity of the basin; and 2) snow cover observations are available 2-3 times during the snowmelt season. The availability of satellite observations of snow cover extent has permitted successful application of the model to large basins.

Overland Flow from Time-Distributed Rainfall

Abstract: The influence of time-varying rainfall on overland flow is investigated. The kinematic wave equations for turbulent flow (across a plane, impermeable surface) are solved using a time-varying rainfall appropriate for thunderstorms. The peak discharge is shown to be a function of surface length, total precipitation, storm duration, and time to equilibrium for rainfall of constant intensity. For rainfall durations equal or less than the time-of-concentration, there is little difference between peak discharges estimated using time-varying rainfall and rainfall of constant intensity. For rainfall of long relative duration, the thunderstorm distribution gives much higher peak discharges.

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. IV: Vertical Air Motions and Microphysical Structures of Prefrontal Surge Clouds and Cold-Frontal Clouds

Abstract: The vertical air motions and microphysical structures of the clouds associated with two mesoscale precipitation systems in the Pacific Northwest are examined using rawinsonde, aircraft and vertically pointing Doppler radar data. A rainband associated with a prefrontal surge of cold air aloft was found to consist of deep (3–4 km) convective cells. Natural seeding by cirrostratus cloud spread ice crystals throughout the rainband. All of the precipitation growth observed took place through ice-phase processes. Much of the moisture necessary for precipitation growth entered the rainband at low levels in the form of vapor and condensate associated with widespread stratiform cloud. A weakly organized cold-frontal precipitation area was found to consist of snow trails originating from shallow (1–2 km) convective cells in a “seeder” zone above 5.5 km altitude. Below this level the trails swept through a “feeder” zone which consisted of stratiform cloud. Downdrafts observed in one region of the feeder zon...

Secular changes in annual and seasonal Great Lakes precipitation, 1854–1979, and their implications for Great Lakes water resource studies

Abstract: An analysis of annual precipitation over the Great Lakes Basin from 1854 to 1979 indicates two distinct precipitation regimes. The first, a relatively dry regime, lasted from the mid-1880's until the late 1930's. This was followed by a relatively wet regime, which continues to the present. The analysis also indicates that the annual precipitation regime prior to the mid-1880's was similar to the present regime. The change in precipitation appears to be the result of increased precipitation during spring and summer. A continuation of the present wet regime will present many challenges for water resource managers and planners concerned with lake regulation, water supply forecasting, coastal zone management, and water allocation and uses in the Great Lake Basin.

The effect of a geographical cloud distribution on climate: A numerical experiment with an atmospheric general circulation model

Abstract: This study is an attempt to estimate the effect of a geographical distribution of clouds on climate. A method of determination of a three-dimensional cloud distribution is proposed. It is based on the solution of the inverse problem for the radiative transfer equation. By using climatic data on total cloud amount, temperature, mixing ratio of water vapor, and satellite data on outgoing longwave radiation, the global distributions of high, middle, and low clouds were computed for July. The derived vertical cloud extension is in fair agreement with available data on the frequency distribution of stratiform and cumulus clouds. Two numerical experiments are carried out with an atmospheric general circulation model in which zonal and geographical cloud distributions are prescribed. The integrations are performed for 60 days with a GFDL model, and the last 30 days are analyzed. The geographical cloud distribution causes the increase of surface temperature over the continents by 2°–4° and leads to a decrease of surface pressure there and an increase over the oceans. The largest changes in the surface pressure, up to ±12 mbar, occur in the middle latitudes of both hemispheres. The largest differences in precipitation are observed in the tropics and over some coastal regions of North and South America. Arid areas in the subtropical belt become more pronounced in case of the geographical distribution of clouds. Estimates of the level of significance for precipitation and surface pressure changes reveal that they are statistically significant in some areas of the globe.

Palaeo-Climatic Studies on Ice Cores

Abstract: The most fascinating aspect of ice cores from the polar ice sheets is that they contain all kinds of fall-out from the atmosphere, i.e. precipitation, volcanic debris, airborne biological material, radioactive isotopes, cosmic particles etc. In addition, the air bubbles entrapped in the ice are samples of past atmospheres, so they contain information about the palaeo-atmosphere composition, including CO2 content. In the Greenland ice sheet the annual snow and ice layers lie in a continuous sequence reaching as far back in time as to the deposition of the first snow layer that remained cold enough to avoid melting. In fact, any wide spread snowfall that has occurred in the last several hundred thousand years is still represented in the ice — nothing melts away. And yet, less and less is left per cm2 of a given annual layer the older it gets, because it is continuously being streched, thus getting thinner and thinner as it approaches the bottom (Figure 1).

Characterization of acidic precipitation in the Adirondack region

Abstract: Atmospheric inputs into three lake watersheds within a 30 km radius of each other in the Adirondack Park region of New York State were quantified for the period May 1978-August 1979. This was accomplished with a wet/dry precipitation network which collected samples on an event basis. Rain and snow amounts were measured and samples were analyzed for pH, conductivity, SO/sub 4/, NO/sub 3/, Cl, NH/sub 4/, Ca, Mg, K, and Na. Precipitation quantity and quality for the three watersheds was found to be similar, on a monthly or yearly basis, but individual events would occasionally show wide fluctuation in both manners across the network. Dry deposition made significant contributions (10 to 30%) to the total atmospheric loading to these basins.

There is More to ``Acid Rain'' Than Rain

Abstract: Significant amounts of acid and other pollutants reach the ground without the aid of precipitation, but most of it goes unrecorded.

Large-Scale Effects of Deep Convection on the GATE Tropical Boundary Layer

Abstract: The large-scale response of the atmospheric boundary layer to the passage of tropical wave disturbances is investigated. Observations from GATE indicate that during periods of extensive deep convective activity rather shallow mixed layers frequently are found, primarily in association with mesoscale precipitation systems. Convective-scale precipitation downdrafts accompanying such systems contribute to the formation of large trailing “wakes” wherein there exist considerably enhanced fluxes of sensible heat from the mean surface. Observations in the convectively active trough portion of tropical waves indicate that a considerable fraction of the total area is covered by wakes. However, there remain regions between rain systems in the wave trough covering ∼15–30% of the total area, depending on how these regions are defined, which have mixed layers more characteristic of those observed in the undisturbed ridge portion of the wave than those in wakes. A simple model is developed to study the mainten...