Showing papers on "Precipitation published in 2000"

Reduction of Tropical Cloudiness by Soot

Abstract: Measurements and models show that enhanced aerosol concentrations can augment cloud albedo not only by increasing total droplet cross-sectional area, but also by reducing precipitation and thereby increasing cloud water content and cloud coverage. Aerosol pollution is expected to exert a net cooling influence on the global climate through these conventional mechanisms. Here, we demonstrate an opposite mechanism through which aerosols can reduce cloud cover and thus significantly offset aerosol-induced radiative cooling at the top of the atmosphere on a regional scale. In model simulations, the daytime clearing of trade cumulus is hastened and intensified by solar heating in dark haze (as found over much of the northern Indian Ocean during the northeast monsoon).

Temperature and precipitation trends in Canada during the 20th century

Abstract: Trends in Canadian temperature and precipitation during the 20th century are analyzed using recently updated and adjusted station data. Six elements, maximum, minimum and mean temperatures along with diurnal temperature range (DTR), precipitation totals and ratio of snowfall to total precipitation are investigated. Anomalies from the 1961–1990 reference period were first obtained at individual stations, and were then used to generate gridded datasets for subsequent trend analyses. Trends were computed for 1900–1998 for southern Canada (south of 60°N), and separately for 1950–1998 for the entire country, due to insufficient data in the high arctic prior to the 1950s. From 1900–1998, the annual mean temperature has increased between 0.5 and 1.5°C in the south. The warming is greater in minimum temperature than in maximum temperature in the first half of the century, resulting in a decrease of DTR. The greatest warming occurred in the west, with statistically significant increases mostly seen during...

Simulation of regional‐scale water and energy budgets: Representation of subgrid cloud and precipitation processes within RegCM

Abstract: A new large-scale cloud and precipitation scheme, which accounts for the sub- grid-scale variability of clouds, is coupled to NCAR's Regional Climate Model (RegCM). This scheme partitions each grid cell into a cloudy and noncloudy fraction related to the average grid cell relative humidity. Precipitation occurs, according to a specified autoconversion rate, when a cloud water threshold is exceeded. The specification of this threshold is based on empirical in-cloud observations of cloud liquid water amounts. Included in the scheme are simple formulations for raindrop accretion and evaporation. The results from RegCM using the new scheme, tested over North America, show significant improvements when compared to the old version. The outgoing longwave radiation, albedo, cloud water path, incident surface shortwave radiation, net surface radiation, and surface temperature fields display reasonable agreement with the observations from satellite and surface station data. Furthermore, the new model is able to better represent extreme precipitation events such as the Midwest flooding observed in the summer of 1993. Overall, RegCM with the new scheme provides for a more accurate representation of atmospheric and surface energy and water balances, including both the mean conditions and the variability at daily to interannual scales. The latter suggests that the new scheme improves the model's sensitivity, which is critical for both climate change and process studies.

Deuterium and oxygen-18 isotope composition of precipitation and atmospheric moisture

Abstract: The stable isotopes of oxygen and hydrogen incorporated in the water molecule ( 18 O and 2 H) have become an important tool not only in Isotope Hydrology, routinely applied to study the origin and dynamics of surface and groundwaters, but also in studies related to atmospheric circulation and palaeoclimatic investigations. A proper understanding of the behaviour of these tracers in the water cycle is required for a meaningful use of these tools in any of these disciplines. Our knowledge of the vertical distribution and the factors controlling the stable isotope ratios of oxygen and hydrogen in atmospheric moisture derives from a limited number of observations and vertical profiles in the atmosphere. An international programme jointly operated by the International Atomic Energy Agency (IAEA) and the World Meteorological Organization (WMO), and operational since 1961. has resulted in the development of a dedicated database to monitor isotope ratios in precipitation in more than 500 meteorological stations world-wide. The main features of the spatial and temporal variations of stable isotope ratios of oxygen and hydrogen in precipitation and atmospheric moisture at the global scale are presented based on the analysis of limited data on water vapour, data obtained by the Global Network for Isotopes in Precipitation IGNIP) and the few observations at high latitudes.

Global patterns of ENSO‐induced precipitation

Abstract: Although there have been many analyses of El Nino/Southern Oscillation (ENSO) induced precipitation anomalies, global patterns from these analyses remain incomplete. Here we combine recent satellite estimates of oceanic precipitation and historical rain-gauge records to derive a global climatology of ENSO-induced precipitation anomalies using empirical orthogonal function (EOF) analyses. The patterns suggest that the re-arrangement of convection centers of the Walker circulation during ENSO events induces large precipitation anomalies in the tropics, while associated changes in the monsoon systems (through the Hadley cell) over the Pacific, Indian and Atlantic Oceans, and their interactions with midlatitude westerlies generate coherent anomaly patterns over the extratropics. Our results can be used to evaluate climate models and forecast ENSO-induced precipitation anomalies.

The weather and climate of southern Africa

Abstract: Climatology and the modern science composition and structure of the atmosphere pressure, temperature and density relationships the adiabatic process moisture and precipitation the tephigram atmospheric heat transfer horizontal motion and winds vertical motion and cumulus convection large-scale weather-producing processes and systems the general circulation of the southern hemisphere atmospheric circulation and weather over southern Africa ocean-atmosphere interactions boundary-layer phenomena the transport of aerosols and trace gases climatic change and variability the prediction of future conditions.

Changes in the Extremes in an Ensemble of Transient Climate Simulations with a Coupled Atmosphere–Ocean GCM

Abstract: The extremes of surface temperature, precipitation, and wind speed and their changes under projected changes in radiative forcing are examined in an ensemble of three transient climate change simulations for the years 1900–2100 conducted with the global coupled model of the Canadian Centre for Climate Modelling and Analysis. The evolution of the greenhouse gases and aerosols in these simulations is consistent with the Intergovernmental Panel on Climate Change 1992 scenario A. The extremes are analyzed in three 21-yr time periods centered at years 1985, 2050, and 2090. The model simulates reasonably well the extremes of the contemporary near-surface climate. Changes in extremes of daily maximum and daily minimum temperature are distinctively different and are related to changes in the mean screen temperature, soil moisture, and snow and sea-ice cover. Extreme precipitation increases almost everywhere on the globe. Relative change in extreme precipitation is larger than change in total precipitatio...

Variance and Predictability of Precipitation at Seasonal-to-Interannual Timescales

Abstract: A series of atmospheric general circulation model simulations, spanning a total of several thousand years, is used to assess the impact of land surface and ocean boundary conditions on the seasonal-to-interannual variability and predictability of precipitation in a coupled modeling system. In the first half of the analysis, which focuses on precipitation variance, the contributions of ocean, atmosphere, and land processes to this variance are characterized, to first order, with a simple linear model. The resulting clean separation of the contributions leads to two results: 1) land and ocean processes have essentially different domains of influence, that is, the amplification of precipitation variance by land–atmosphere feedback is most important outside of the regions (mainly in the Tropics) that are most affected by sea surface temperatures; and 2) the strength of land–atmosphere feedback in a given region is controlled largely by the relative availability of energy and water there. In the secon...

Circulation weather types and their influence on the precipitation regime in Portugal.

Abstract: An objective classification scheme of the atmospheric circulation affecting Portugal, between 1946 and 1990, is presented, where daily circulation is characterized through the use of a set of indices associated with the direction and vorticity of the geostrophic flow. The synoptic characteristics and the frequency of ten basic circulation weather types (CWTs) are discussed, as well as the amount of precipitation associated with each type between 1957 and 1986. It is shown that the anticyclonic (A) type, although being the most frequent class in winter (37%), gives a rather small (less then 16%) contribution to the winter precipitation amount, observed on a daily basis. On the other hand, the three wettest CWTs, namely the cyclonic (C), southwesterly (SW) and westerly (W) types, together representing only 32% of all winter days, account for more than 62% of the observed daily precipitation. Results obtained highlight the existence of strong links between the interannual variability of monthly precipitation and interannual variability of CWTs. Multiple regression models, developed for 18 stations, show the ability of modelling monthly winter precipitation through the exclusive use, as predictors, of the wet CWTs (i.e. C, SW and W). The observed decreasing trend of March precipitation is also analysed and shown to be especially associated with the decrease of the three wet weather types. The anomalous low (high) frequency of wet CWTs during the hydrological year is shown to be strongly related with the occurrence of extreme dry (wet) years in Portugal, which had important impacts on Portuguese agriculture. Overall, the results suggest that the precipitation regime over Portugal, including interannual variability, trends and extremes, may be adequately explained in terms of variability of a fairly small number of circulation weather patterns. On the other hand, observed contrasts in the spatial distribution of correlations between frequency of wet CWTs and rainfall amounts suggest that precipitation regimes are of a different nature in northern and southern regions of Portugal; the former possessing an orographic origin and the latter being associated to cyclogenetic activity. Copyright © 2000 Royal Meteorological Society.

A Census of Precipitation Features in the Tropics Using TRMM: Radar, Ice Scattering, and Lightning Observations

Abstract: An algorithm has been developed to identify precipitation features ($75 km2 in size) in two land and two ocean regions during August, September, and October 1998. It uses data from two instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite: near-surface precipitation radar (PR) reflectivities, and TRMM Microwave Imager (TMI) 85.5-GHz polarization corrected temperatures (PCTs). These features were classified by size and intensity criteria to identify mesoscale convective systems (MCSs), precipitation with PCTs below 250 K, and other features without PCTs below 250 K. By using this technique, several hypotheses about the convective intensity and rainfall distributions of tropical precipitation systems can be evaluated. It was shown that features over land were much more intense than similar oceanic features as measured by their minimum PCTs, maximum heights of the 30-dBZ contour, and 6-km reflectivities. The diurnal cycle of precipitation features showed a strong afternoon maximum over land and a rather flat distribution over the ocean, quite similar to those found by others using infrared satellite techniques. Precipitation features with MCSs over the ocean contained significantly more rain outside the 250-K PCT isotherm than land systems, and in general, a significant portion (10%‐15%) of rainfall in the Tropics falls in systems containing no PCTs less than 250 K. Volumetric rainfall and lightning characteristics (as observed by the Lightning Imaging Sensor aboard TRMM) from the systems were classified by feature intensity; similar rain amounts but highly differing lightning flash rates were found among the regions. Oceanic storms have a bimodal contribution of rainfall from two types of systems: very weak systems with little ice scattering and moderately strong systems that do not produce high lightning flash rates. Continental systems that produce the bulk of the rainfall (as sampled) are likely to have higher lightning flash rates, which are shown to be linked to stronger radar and ice-scattering intensities.

Precipitation fluctuations in the Nepal Himalaya and its vicinity and relationship with some large scale climatological parameters.

Abstract: Precipitation records from 78 stations distributed across Nepal were analysed and all-Nepal (1948–1994) and subregional records (1959–1994) were developed. The all-Nepal and regional precipitation series showed significant variability on annual and decadal time scales. Distinct long-term trends were not found in these precipitation records. The all-Nepal record agrees well with the precipitation records from northern India, while it does not compare well with the all-India precipitation record. The all-Nepal monsoon record is highly correlated with the Southern Oscillation Index (SOI) series. The exceptionally dry year of 1992 recorded in Nepal coincides with the elongated El Nino of 1992–1993 and the Mount Pinatubo eruption. A remarkable cooling in the region covering the Tibetan Plateau also occurred in 1992, suggesting that Pinatubo aerosol played a major role in the drought of that particular year in Nepal. In other years, the correlation between the precipitation record from Nepal and the temperature of the Tibetan Plateau is not significant, while a stronger correlation with temperature over the Indian Ocean and southern India exists. This provides further support for the strong relationship between the El Nino–Southern Oscillation (ENSO) and precipitation fluctuation in Nepal. The correlation is stronger between all-Nepal monsoon precipitation and SOI averaged over seasons following the monsoon compared with seasons preceding the monsoon. Copyright © 2000 Royal Meteorological Society

Interannual climate variability in the Central Andes and its relation to tropical Pacific and Atlantic forcing

Abstract: The main spatiotemporal modes of interannual temperature and austral summer (DJF) precipitation variability in the Central Andes are identified based on a two-way principal compo- nent analysis (PCA) of 30-year (1961-1990) monthly station data and related to contemporaneous tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs). In addition, various me- teorological fields, based on National Centers for Environmental Prediction / National Center for Atmospheric Research CNCEP/NCAR) reanalysis, NOAA-Outgoing Longwave Radiation (OLR) and station data, are analyzed during periods of strong positive and negative SSTA and the respec- tive composites tested for local significance using a Student's t-test approach. Temperature vari- ability in the Central Andes is primarily related to E1Nifio - Southern Oscillation (ENSO) and closely follows SSTA in the central equatorial Pacific with a lag of 1-2 months. In the southern Altiplano, temperatures have significantly increased since the late 1970s. DJF precipitation is also primarily related to ENSO, featuring below (above) average precipitation during E1 Nifio (La Nifia). Precipitation over the dry western part of the Altiplano shows the closest relationship with ENSO, due to ENSO-induced atmospheric circulation anomalies. Precipitation variability over the western Altiplano features a decadal-scale oscillation, related to a similar climatic shift in the tropical Pacific domain in the late 1970s. Over the northern Altiplano the precipitation signal is re- versed in the austral summer following the peak phase of ENSO, presumably due to the temporal evolution of tropical Pacific SSTA, rapidly switching from one state to the other. No evidence for a tropical Atlantic influence on DJF precipitation was found. SSTAs in the tropical NE Atlantic, however, presumably are influenced by heating and convection over the Altiplano through an up- per air monsoon return flow, altering the strength of the NE trades that emanate from the Sahara High.

Evaluating uncertainties in the prediction of regional climate change

Abstract: Uncertainties in regional climate change simulations for the 21st century by five coupled atmosphere-ocean General Circulation Models (AOGCMs) (two of them including ensembles of simulations), for different anthropogenic forcing scenarios and 23 regions in the World, are examined. Seasonally and regionally averaged precipitation and surface air temperature for the future period of [2070–2099] as compared to the period of [1961–1990] are considered. The dominant source of uncertainty in the simulation of average regional climate change is due to inter-model variability with inter-scenario and internal model variability playing secondary roles. The range of predicted climate changes by different realizations of the same ensemble is small, and simulated changes exhibit a high level of coherency among different forcing scenarios. Uncertainties in regional changes are 3 K or greater for temperature and 25% of present day values or greater for precipitation. The model biases in reproducing present day climate are ≤1 K to over 5 K for temperature and ≤10% to over 100% for precipitation.

Intra- and interspecific variation for summer precipitation use in pinyon-juniper woodlands

Abstract: In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer ''monsoon'' precipitation generally wets only surface layers. Use of these spatially separated water sources was determined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Arizona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this summer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation input. This prediction was largely supported by our observations of tree water source use over a two-year period and from irrigation ex- periments. Hydrogen and oxygen stable isotope ratios ( dD and d 18 O) of tree xylem water were compared to that of precipitation, groundwater, and deep and shallow soil water to distin- guish among possible tree water sources. dD-d 18 O relationships and seasonal xylem water potential changes revealed that trees of this ecosystem used a mixture of soil water and recent precipitation, but not groundwater. During the monsoon period, a large proportion of xylem water in Pinus edulis and Juniperus osteospermawas from monsoon precipitation, but use of this precipitation declined sharply with decreasing summer rain input at sites near the regional monsoon boundary in Utah. Quercus gambelii at most sites along the gradient used only deep soil water even following substantial inputs of summer rain. Pop- ulations of Quercus at sites with the highest average summer precipitation input, however, predominantly used water in upper soil layers from recent summer rain events. Soil tem- perature correlated with patterns of summer precipitation use across the gradient; high soil temperatures north of the monsoon boundary may have inhibited surface root activity for some or all of the three tree species. Irrigation experiments with deuterium-labeled water revealed that Quercus gambeliiin northern Arizona and southern Utah did not use water from surface layers. In contrast, Juniperus osteosperma at these sites responded significantly to the irrigations: between 37% and 41% of xylem water originated from irrigations that wetted only the top 30 cm of soil. Responses by Pinus edulis to these irrigations were variable; uptake of labeled water by this species was greater in September at the end of the summer than during the hot midsummer period. Inactivity of Pinus roots in midsummer supports the hypothesis that root activity in this species is sensitive to soil temperature. Seasonal patterns of leaf gas exchange and plant water potential corresponded to the seasonality of rainfall at different sites. However, no correlation between a species' ability to use summer rainfall and its tolerance to water deficits at the leaf level was found. Midday stomatal conductance (gs) for Pinus needles approached zero at predawn water potentials near 22 MPa, whereas gs in Quercus and Juniperus declined to zero at 22.8 and 23.7 MPa, respectively. The relationship between photosynthesis (A) and gs was similar among the three species, although Quercus maintained higher overall rates of gas exchange and tended to operate higher on the A/gs curve than the two conifers. At sites in eastern Arizona where Quercus fully used moisture from summer rains, leaf gas exchange characteristics were similar to those of Pinus and Juniperus.

Winter rainfall interception by two mature open‐grown trees in Davis, California

Abstract: A rainfall interception measuring system was developed and tested for open-grown trees. The system includes direct measurements of gross precipitation, throughfall and stemflow, as well as continuous collection of micrometeorological data. The data were sampled every second and collected at 30-s time steps using pressure transducers monitoring water depth in collection containers coupled to Campbell CR10 dataloggers. The system was tested on a 9-year-old broadleaf deciduous tree (pear, Pyrus calleryana ‘Bradford’) and an 8-year-old broadleaf evergreen tree (cork oak, Quercus suber) representing trees having divergent canopy distributions of foliage and stems. Partitioning of gross precipitation into throughfall, stemflow and canopy interception is presented for these two mature open-grown trees during the 1996–1998 rainy seasons. Interception losses accounted for about 15% of gross precipitation for the pear tree and 27% for the oak tree. The fraction of gross precipitation reaching the ground included 8% by stemflow and 77% by throughfall for the pear tree, as compared with 15% and 58%, respectively, for the oak tree. The analysis of temporal patterns in interception indicates that it was greatest at the beginning of each rainfall event. Rainfall frequency is more significant than rainfall rate and duration in determining interception losses. Both stemflow and throughfall varied with rainfall intensity and wind speed. Increasing precipitation rates and wind speed increased stemflow but reduced throughfall. Analysis of rainfall interception processes at different time-scales indicates that canopy interception varied from 100% at the beginning of the rain event to about 3% at the maximum rain intensity for the oak tree. These values reflected the canopy surface water storage changes during the rain event. The winter domain precipitation at our study site in the Central Valley of California limited our opportunities to collect interception data during non-winter seasons. This precipitation pattern makes the results more specific to the Mediterranean climate region. Copyright © 2000 John Wiley & Sons, Ltd.

Vegetation–Climate Feedbacks in the Conversion of Tropical Savanna to Grassland

Abstract: Tropical savannas have been heavily impacted by human activity, with large expanses transformed from a mixture of trees and grasses to open grassland and agriculture. The National Center for Atmospheric Research (NCAR) CCM3 general circulation model, coupled with the NCAR Land Surface Model, was used to simulate the effects of this conversion on regional climate. Conversion of savanna to grassland reduced precipitation by approximately 10% in four of the five savanna regions under study; only the northern African savannas showed no significant decline. Associated with this decline was an increase in the frequency of dry periods within the wet season, a change that could be particularly damaging to shallow-rooted crops. The overall decline in precipitation is almost equally attributable to changes in albedo and roughness length. Conversion to grassland increased mean surface air temperature of all the regions by 0.58C, primarily because of reductions in surface roughness length. Rooting depth, which decreases dramatically with the conversion of savanna to grassland, contributed little to the overall effect of savanna conversion, but deeper rooting had a small positive effect on latent heat flux with a corresponding reduction in sensible heat flux. The authors propose that the interdependence of climate and vegetation in these regions is manifested as a positive feedback loop in which anthropogenic impacts on savanna vegetation are exacerbated by declines in precipitation.

On the existence of Lloró (the rainiest locality on earth): enhanced ocean-land-atmosphere interaction by a low-level jet

Abstract: The department of Choco, on the Colombian Pacific coast experiences 8,000 to 13,000 mm of average annual precipitation. Lloro (5°30′N, 76°32′W, 120m) has received above 12,700 mm (1952–1960). Using the NCEP/NCAR Reanalysis data, we show that the ocean-land-atmosphere interaction over the easternmost fringe of the tropical Pacific, enhanced by the dynamics of a low-level westerly jet (“CHOCO”), contributes to explain the existence of such record-breaking hydrological region. Deep convection develops from low-level moisture convergence by the CHOCO jet, combined with high-level easterly trade winds, orographic lifting on the western Andes, low surface pressures and warm air. Precipitation is organized in mesoscale convective complexes, in turn dynamically linked to the jet. The strength of the CHOCO jet (centered at 5°N) is associated with the gradient of surface air temperatures between western Colombia and the Nino 1+2 region, thereby exhibiting strong annual and interannual variability, which contributes to explaining Colombia's hydro-climatology and its anomalies during ENSO.

Sensitivity of Climate to Changes in NDVI

Abstract: The sensitivity of global and regional climate to changes in vegetation density is investigated using a coupled biosphere-atmosphere model. The magnitude of the vegetation changes and their spatial distribution are based on natural decadal variability of the normalized difference vegetation index (ndvi). Different scenarios using maximum and minimum vegetation cover were derived from satellite records spanning the period 1982-1990. Albedo decreased in the northern latitudes and increased in the tropics with increased ndvi. The increase in vegetation density revealed that the vegetation's physiological response was constrained by the limits of the available water resources. The difference between the maximum and minimum vegetation scenarios resulted in a 46% increase in absorbed visible solar radiation and a similar increase in gross photosynthetic C02 uptake on a global annual basis. This caused the canopy transpiration and interception fluxes to increase, and reduced those from the soil. The redistribution of the surface energy fluxes substantially reduced the Bowen ratio during the growing season, resulting in cooler and moister near-surface climate, except when soil moisture was limiting. Important effects of increased vegetation on climate are : (1) A cooling of about 1.8 K in the northern latitudes during the growing season and a slight warming during the winter, which is primarily due to the masking of high albedo of snow by a denser canopy. and (2) A year round cooling of 0.8 K in the tropics. These results suggest that increases in vegetation density could partially compensate for parallel increases in greenhouse warming . Increasing vegetation density globally caused both evapotranspiration and precipitation to increase. Evapotranspiration, however increased more than precipitation resulting in a global soil-water deficit of about 15 %. A spectral analysis on the simulated results showed that changes in the state of vegetation could affect the low-frequency modes of the precipitation distribution and might reduce its low frequency variability in the tropics while increasing it in northern latitudes.

A study of the 1999 monsoon rainfall in a mountainous region in central Nepal using TRMM products and rain gauge observations

Abstract: Raingauge data from the 1999 monsoon were compared with precipitation derived from the precipitation radar (PR) and the microwave imager instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The raingauges are part of a new hydrometeorological network installed in the Marsyandi river basin, which extends from the edge of the Tibetan Plateau to the Gangetic basin. TRMM-derived precipitation showed better detection of rain at low altitude stations as compared with high elevation stations, with good scores for the PR product for rain rates >0.5 mm/hr. The 3D PR rain rates suggest strong interaction between mesoscale convective systems and steep terrain at elevations of 1–2 km, which is consistent with the very high rainfall measured at those locations. Analysis of the raingauge data shows that even at altitudes as high as 4,000 m the cumulative monsoon rainfall is comparable to the highest amount recorded in the Indian subcontinent.

The Relationship between Precipitation and Lightning in Tropical Island Convection: A C-Band Polarimetric Radar Study

Abstract: One of the primary scientific objectives of the Maritime Continent Thunderstorm Experiment was to study cloud electrification processes in tropical island convection, in particular, the coupling between ice phase precipitation and lightning production. To accomplish this goal, a C-band polarimetric radar was deployed in the Tropics (11.68S, 130.88E) for the first time, accompanied by a suite of lightning measurements. Using observations of the propagation-corrected horizontal reflectivity and differential reflectivity, along with specific differential phase, rain and ice masses were estimated during the entire life cycle of an electrically active tropical convective complex (known locally as Hector) over the Tiwi Islands on 28 November 1995. Hector’s precipitation structure as inferred from these raw and derived radar fields was then compared in time and space to the measured surface electric field, cloud-to-ground (CG) and total lightning flash rates, and ground strike locations. During Hector’s developing stage, precipitating convective cells along island sea breezes were dominated by warm rain processes. No significant electric fields or lightning were associated with this stage of Hector, despite substantial rainfall rates. Aided by gust front forcing, a cumulus merger process resulted in larger, taller, and more intense convective complexes that were dominated by mixed-phase precipitation processes. During the mature phase of Hector, lightning and the surface electric field were strongly correlated to the mixed phase ice mass and rainfall. Merged convective complexes produced 97% of the rainfall and mixed-phase ice mass and 100% of the CG lightning. As Hector dissipated, lightning activity rapidly ceased. As evidenced from the multiparameter radar observations, the multicell nature of Hector resulted in the continuous lofting of supercooled drops to temperatures between 2108 and 2208C in discrete updraft cores during both the early and mature phases. The freezing of these drops provided instantaneous precipitation-sized ice particles that may have subsequently rimed and participated in thunderstorm electrification via the noninductive charging mechanism.

Isotopic (δ18O) characteristics of weekly precipitation collected across the USA : an initial analysis with application to water source studies

Abstract: A portion of the precipitation samples collected and stored by the National Atmospheric Deposition Program (NADP) are shown to be useful for analysis of isotopes in precipitation. The potential problems with evaporation are small based on deuterium excess analyses and comparisons with the Global Meteroic Water Line. Presented here are the δ 18 O values of precipitation collected from nine NADP sites during 1989, 1990 and 1991. The trends in the isotopic (δ 18 O) characteristics of recent precipitation are in agreement with findings from previous International Atomic Energy Agency (IAEA) sites in the USA. The findings are also in agreement with several major isotope-environment relationships, further supporting the use of these samples for a modern global data base on the isotopes in precipitation being developed by IAEA, called GNIP (Global Network for Isotopes in Precipitation) and for use by research groups in the hydrological modelling, palaeoclimate and ecological communities. As expected, the average δ 18 O values of precipitation that is derived from the Gulf of Mexico (-3‰) and from the Pacific North-west are isotopically distinct (-7‰). In addition, using the NADP network, isotopic depletion in the δ 18 O values of precipitation in the range of 8‰ was observed from coastal to inland locations either in the Pacific North-west or along the east side of the Rocky Mountains, from Texas to Eastern Montana. In central USA, especially at high elevation, there is a strong seasonal variation in the δ 18 O values of precipitation, differing by almost 25‰ between January and August, whereas at coastal locations the seasonal variation in the δ 18 O values of precipitation was minimal. Comparisons between the average δ 18 O values of precipitation from several NADP sites and those of the IAEA collected in the 1960s and 1970s are exceptionally comparable, differing in some cases by only 0.1‰, further substantiating the quality and utility of precipitation from this network. The δ 18 O-temperature relationships for two of our sites had slopes of 0.55 (Colorado) and 0-46 (Nebraska) that are within the range of those reported earlier (0.29-0.7), although they are slightly lower, possibly owing to the large temperature variation between winter and summer in central USA. Further, the seasonal variation in the δ 18 O values of precipitation are used in a pair of ecological studies to articulate water sources of different plant life forms in riparian and prairie ecosystems, with grasses typically using summer rain to a much greater extent than woody species, such as willow or saltbush.

Using Self-Organizing Maps to Investigate Extreme Climate Events: An Application to Wintertime Precipitation in the Balkans

Abstract: This paper examines some of the physical mechanisms and remote linkages associated with extreme wintertime precipitation in the Balkans. The analysis is assessed on daily timescales to determine the role of the circulation and atmospheric moisture on extreme events, and also at intraseasonal and interannual timescales to find possible linkages with the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) patterns. A nonlinear classification known as the self-organizing map (SOM) is employed to obtain the climate modes and anomalies that dominated during the 1980–93 period. An artificial neural network (ANN) is also used to derive daily precipitation at gridpoint scale and at local scale in Bucharest, Romania. Of the predictors used, 500–1000-hPa thickness, 700-hPa geopotential heights, and 700-hPa moisture are the most important controls of daily precipitation. These results are substantiated with the climate states from the SOM classification, which show strong meridional flow over c...

Extreme Precipitation Events in the Western United States Related to Tropical Forcing

Abstract: Three-day accumulations of precipitation for 2.5° long × 2.0° lat areas along the west coast of the United States are used to rank precipitation events. Extreme precipitation events (those above the 90th percentile) occur at all phases of the El Nino–Southern Oscillation (ENSO) cycle, but the largest fraction of these events (for the West Coast as a whole) occur during neutral winters just prior to the onset of El Nino. In the tropical Pacific these winters are characterized by enhanced activity on intraseasonal (roughly 20–60 day) timescales and by relatively small sea surface temperature anomalies compared to ENSO winters. For these winters, lagged composites are used to document a coherent relationship between the location of extreme precipitation events along the West Coast and the location of enhanced tropical convection on intraseasonal timescales. The evolution of the atmospheric circulation patterns associated with the extreme precipitation events is described and a physical mechanism rel...

A green planet versus a desert world: estimating the maximum effect of vegetation on the land surface climate.

Abstract: We quantify the maximum possible influence of vegetation on the global climate by conducting two extreme climate model simulations: in a first simulation (‘desert world’), values representative of a desert are used for the land surface parameters for all non glaciated land regions. At the other extreme, a second simulation is performed (‘green planet’) in which values are used which are most beneficial for the biosphere's productivity. Land surface evapotranspiration more than triples in the presence of the ‘green planet’, land precipitation doubles (as a second order effect) and near surface temperatures are lower by as much as 8 K in the seasonal mean resulting from the increase in latent heat flux. The differences can be understood in terms of more absorbed radiation at the surface and increased recycling of water. Most of the increase in net surface radiation originates from less thermal radiative loss and not from increases in solar radiation which would be expected from the albedo change. To illustrate the differences in climatic character and what it would imply for the vegetation type, we use the Koppen climate classification. Both cases lead to similar classifications in the extra tropics and South America indicating that the character of the climate is not substantially altered in these regions. Fundamental changes occur over Africa, South Asia and Australia, where large regions are classified as arid (grassland/desert) climate in the ‘desert world’ simulation while classified as a forest climate in the ‘green planet’ simulation as a result of the strong influence of maximum vegetation on the climate. This implies that these regions are especially sensitive to biosphere-atmosphere interaction.

A hidden Markov model for downscaling synoptic atmospheric patterns to precipitation amounts

Abstract: Nonhomogeneous hidden Markov models (NHMMs) provide a relatively simple frame- work for simulating precipitation at multiple rain gauge stations conditional on synoptic atmospheric patterns. Building on existing NHMMs for precipitation occurrences, we propose an extension to in- clude precipitation amounts. The model we describe assumes the existence of unobserved (or hidden) weather patterns, the weather states, which follow a Markov chain. The weather states depend on observable synoptic information and therefore serve as a link between the synoptic-scale atmospheric patterns and the local-scale precipitation. The presence of the hidden states simplifies the spatio-tem- poral structure of the precipitation process. We assume the temporal dependence of precipitation is completely accounted for by the Markov evolution of the weather state. The spatial dependence of pre- cipitation can also be partially or completely accounted for by the existence of a common weather state. In the proposed model, occurrences are assumed to be conditionally spatially independent given the current weather state and, conditional on occurrences, precipitation amounts are modeled indepen- dently at each rain gauge as gamma deviates with gauge-specific parameters. We apply these methods to model precipitation at a network of 24 rain gauge stations in Washington state over the course of 17 winters. The first 12 yr are used for model fitting purposes, while the last 5 serve to evaluate the model performance. The analysis of the model results for the reserved years suggests that the characteristics of the data are captured fairly well and points to possible directions for future improvements.