Showing papers on "Precipitation published in 1998"

Secular Trends of Precipitation Amount, Frequency, and Intensity in the United States

Abstract: Twentieth century trends of precipitation are examined by a variety of methods to more fully describe how precipitation has changed or varied Since 1910, precipitation has increased by about 10% across the contiguous United States The increase in precipitation is reflected primarily in the heavy and extreme daily precipitation events For example, over half (53%) of the total increase of precipitation is due to positive trends in the upper 10 percentiles of the precipitation distribution These trends are highly significant, both practically and statistically The increase has arisen for two reasons First, an increase in the frequency of days with precipitation ]6 days (100 yr)−1[ has occurred for all categories of precipitation amount Second, for the extremely heavy precipitation events, an increase in the intensity of the events is also significantly contributing (about half) to the precipitation increase As a result, there is a significant trend in much of the United States of the highest

A precipitation climatology of the alps from high-resolution rain-gauge observations

Abstract: A new precipitation climatology covering the European Alps is presented. The analysis covers the entire mountain range including adjacent foreland areas and exhibits a resolution of about 25 km. It is based on observations at one of the densest rain-gauge networks over complex topography world-wide, embracing more than 6600 stations from the high-resolution networks of the Alpine countries. The climatology is determined from daily analyses of bias-uncorrected, quality controlled data for the 20 year period 1971‐1990. The daily precipitation fields were produced with an advanced distance-weighting scheme commonly adopted for the analysis of precipitation on a global scale. The paper describes the baseline seasonal means derived from the daily analysis fields. The results depict the mesoscale distribution of the Alpine precipitation climate, its relations to the topography, and its seasonal cycle. Gridded analysis results are also provided in digital form. The most prominent Alpine effects include the enhancement of precipitation along the Alpine foothills, and the shielding of the inner-Alpine valleys. A detailed analysis along a section across the Alps also demonstrates that a simple precipitation‐height relationship does not exist on the Alpine scale, because much of the topographic signal is associated with slope and shielding rather than height effects. Although systematic biases associated with the rain-gauge measurement and the topographic clustering of the stations are not corrected for, a qualitative validation of the results, using existing national climatologies shows good agreement on the mesoscale. Furthermore a comparison is made between the present climatology and the Alpine sections of the global climatology of Legates and Willmott and the Greater European climatology from the Climate Research Unit (University of East Anglia). Results indicate that the pattern and magnitude of analysed Alpine precipitation critically depend upon the density of available observations and the analysis procedure adopted. © 1998 Royal Meteorological Society.

Stable isotope composition of precipitation over southeast Asia

Abstract: Spatial and temporal variability of the stable isotope composition of precipitation in the southeast Asia and western Pacific region is discussed, with emphasis on the China territory, based on the database of the International Atomic Energy Agency/World Meteorological Organization Global Network Isotopes in Precipitation and the available information on the regional climatology and atmospheric circulation patterns. The meteorological and pluviometric regime of southeast Asia is controlled by five different air masses: (1) polar air mass originating in the Arctic, (2) continental air mass originating over central Asia, (3) tropical-maritime air mass originating in the northern Pacific, (4) equatorial-maritime air mass originating in the western equatorial Pacific, and (5) equatorial-maritime air mass originating in the Indian Ocean. The relative importance of different air masses in the course of a given year is modulated by the monsoon activity and the seasonal displacement of the Intertropical Convergence Zone (ITCZ). Gradual rain-out of moist, oceanic air masses moving inland, associated with monsoon circulation, constitutes a powerful mechanism capable of producing large isotopic depletions in rainfall, often completely overshadowing the dependence of δ 18 O and δ 2 H on temperature. For instance, precipitation at Lhasa station (Tibetan Plateau) during rainy period (June-September) is depleted in 18 O by more than 6 ‰ with respect to winter rainfall, despite of 10°C higher surface air temperature in summer. This characteristic isotopic imprint of monsoon activity is seen over large areas of the region. The oceanic air masses forming the two monsoon systems, Pacific and Indian monsoon, differ in their isotope signatures, as demonstrated by the average δ 18 O of rainfall, which in the south of China (Haikou, Hong Kong) is about 2.5‰ more negative than in the Bay of Bengal (Yangoon). Strong seasonal variations of the deuterium excess values in precipitation observed in some areas of the studied region result from a complete reversal of atmospheric circulation over these areas and changing source of atmospheric moisture. High d-excess values observed at Tokyo and Pohang during winter (15-25‰) result from interaction of dry air masses from the northern Asian continent passing the Sea of Japan and the China Sea and picking up moisture under reduced relative humidity. The isotopic composition of precipitation also provides information about the maximum extent of the ITCZ on the continent during summer.

Atmospheric moisture residence times and cycling: implications for rainfall rates and climate change

Abstract: New estimates of the moistening of the atmosphere through evaporation at the surface and of the drying through precipitation are computed. Overall, the e-folding residence time of atmospheric moisture is just over 8 days. New estimates are also made of how much moisture that precipitates out comes from horizontal transport versus local evaporation, referred to as ‘recycling’. The results depend greatly on the scale of the domain under consideration and global maps of the recycling for annual means are produced for 500 km scales for which global recycling is 9.6%, consisting of 8.9% over land and 9.9% over the oceans. Even for 1000 km scales, less than 20% of the annual precipitation typically comes from evaporation within the domain. While average overall atmospheric moisture depletion and restoration must balance, precipitation falls only a small fraction of the time. Thus precipitation rates are also examined. Over the United States, one hour intervals with 0.1 mm or more are used to show that the frequency of precipitation ranges from over 30% in the Northwest, to about 20% in the Southeast and less than 4% just east of the continental divide in winter, and from less than 2% in California to over 20% in the Southeast in summer. In midlatitudes precipitation typically falls about 10% of the time, and so rainfall rates, conditional on when rain is falling, are much larger than evaporation rates. The mismatches in the rates of rainfall versus evaporation imply that precipitating systems of all kinds feed mostly on the moisture already in the atmosphere. Over North America, much of the precipitation originates from moisture advected from the Gulf of Mexico and subtropical Atlantic or Pacific a day or so earlier. Increases in greenhouse gases in the atmosphere produce global warming through an increase in downwelling infrared radiation, and thus not only increase surface temperatures but also enhance the hydrological cycle, as much of the heating at the surface goes into evaporating surface moisture. Global temperature increases signify that the water-holding capacity of the atmosphere increases and, together with enhanced evaporation, this means that the actual atmospheric moisture should increase. It follows that naturally-occurring droughts are likely to be exacerbated by enhanced potential evapotranspiration. Further, globally there must be an increase in precipitation to balance the enhanced evaporation but the processes by which precipitation is altered locally are not well understood. Observations confirm that atmospheric moisture is increasing in many places, for example at a rate of about 5% per decade over the United States. Based on the above results, we argue that increased moisture content of the atmosphere therefore favors stronger rainfall or snowfall events, thus increasing risk of flooding, which is a pattern observed to be happening in many parts of the world. Moreover, because there is a disparity between the rates of increase of atmospheric moisture and precipitation, there are implied changes in the frequency of precipitation and/or efficiency of precipitation (related to how much moisture is left behind in a storm). However, an analysis of linear trends in the frequency of precipitation events for the United States corresponding to thresholds of 0.1 and 1 mm/h shows that the most notable statistically significant trends are for increases in the southern United States in winter and decreases in the Pacific Northwest from November through January, which may be related to changes in atmospheric circulation and storm tracks associated with El Nino–Southern Oscillation trends. It is suggested that as the physical constraints on precipitation apply only globally, more attention should be paid to rates in both observations and models as well as the frequency of occurrence.

The climate of Patagonia: general patterns and controls on biotic processes

Abstract: In this article we review the main characteristics of the Patagonian climate, the spatial and temporal patterns of the most important climatic variables, and the influence of climate on ecosystem processes. The winter distribution of precipitation determines an asynchrony between the wet and the growing season in Patagonia. The amount of water that can be transferred from the wet season to the growing season depends mainly on the physical characteristics of the soil. In the semiarid steppe of Chubut, drainage accounted for 10% of annual precipitation. Winter distribution of precipitation determines also an asynchronic dynamics of evaporation and transpiration fluxes. The ENSO phenomenon have a significant impact on regional precipitation. In central-west Patagonia, spring precipitation (September to November) was lower than normal during La Nina events and greater than normal during El Nino events. From December to February the opposite pattern can be observed: higher than normal precipitation during La Nina events and lower than normal precipitation during El Nino events. The impact of this phenomenon on the seasonal temperature was not as clear as for precipitation. We did not detect any temporal trends in annual precipitation for the period 1961-1996. The phenology of carbon gains is quite homogeneous in Patagonia. Most of the region showed a peak of production in November, when, simultaneously, water availability and temperature are high. Toward the west, production peaked later (December). Deciduous forests showed the peak in January and February.

Effect of interannual climate variability on carbon storage in Amazonian ecosystems

Abstract: The Amazon Basin contains almost one-half of the world's undisturbed tropical evergreen forest as well as large areas of tropical savanna1,2. The forests account for about 10 per cent of the world's terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems2,3, and short-term field measurements4 suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Nino episodes in recent decades5. Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage6,7,8. Previous model analyses have demonstrated the importance of temperature in controlling carbon storage9,10. Here we use a transient process-based biogeochemical model of terrestrial ecosystems3,11 to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO2 concentration during the period 1980 to 1994. In El Nino years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere (up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecosystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere associated with deforestation inthe Amazon Basin in the early 1990s12. Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.

Precipitation sensitivity to global warming: Comparison of observations with HadCM2 simulations

Abstract: Recent century-long experiments performed with global climate models have simulated observed trends in global-mean temperature quite successfully when both greenhouse gas and aerosol forcing has been included. The performance of these same experiments in simulating observed global-scale changes in precipitation has not previously been examined. Here we use a gridded terrestrial precipitation dataset for the period 1900 to 1996 to examine the extent to which observed global and zonal-mean precipitation sensitivities to global warming have been captured by a series of model simulations recently completed by the UK Hadley Centre. There are signs that the model has been able to reproduce at least some of the observed zonal-mean variations in the precipitation sensitivity to warming. Questions remain both about the quality of the observed precipitation data and about the spatial scale at which anthropogenically-forced global climate models can be expected to reproduce observed variations in precipitation.

Changes in the Extremes of the Climate Simulated by CCC GCM2 under CO2 Doubling

Abstract: Changes due to CO2 doubling in the extremes of the surface climate as simulated by the second-generation circulation model of the Canadian Centre for Climate Modelling and Analysis are studied in two 20-yr equilibrium simulations. Extreme values of screen temperature, precipitation, and near-surface wind in the control climate are compared to those estimated from 17 yr of the NCEP‐NCAR reanalysis data and from some Canadian station data. The extremes of screen temperature are reasonably well reproduced in the control climate. Their changes under CO2 doubling can be connected with other physical changes such as surface albedo changes due to the reduction of snow and sea ice cover as well as a decrease of soil moisture in the warmer world. The signal in the extremes of daily precipitation and near-surface wind speed due to CO 2 doubling is less obvious. The precipitation extremes increase almost everywhere over the globe. The strongest change, over northwest India, is related to the intensification of the summer monsoon in this region in the warmer world. The modest reduction of wind extremes in the Tropics and middle latitudes is consistent with the reduction of the meridional temperature gradient in the 23CO2 climate. The larger wind extremes occur in the areas where sea ice has retreated.

Water isotope module of the ECHAM atmospheric general circulation model: A study on timescales from days to several years

Abstract: Results are presented of a global simulation of the stable water isotopes H218O and HD16O as implemented in the hydrological cycle of the ECHAM atmospheric general circulation model. The ECHAM model was run under present-day climate conditions at two spatial resolutions (T42,T21), and the simulation results are compared with observations. The high-resolution model (T42) more realistically reproduced the observations, thus demonstrating that an improved representation of advection and orography is critical when modeling the global isotopic water cycle. The deuterium excess (d=δD−8*δ18O) in precipitation offers additional information on climate conditions (e.g., relative humidity and temperature) which prevailed at evaporative sites. Globally, the simulated deuterium excess agrees fairly well with observations showing maxima in the interior of Asia and minima in cold marine regions. However, over Greenland the model failed to show the observed seasonality of the excess and its phase relation to δD reflecting either unrealistic source areas modeled for Greenland precipitation or inadequate description of kinetics in the isotope module. When the coarse resolution model (T21) is forced with observed sea surface temperatures from the period 1979 to 1988, it reproduced the observed weak positive correlation between the isotopic signal and the temperature as well as the weak negative anticorrelation between the isotopic signal and the precipitation. This model simulation further demonstrates that the strongest interannual climate anomaly, the El Nino Southern Oscillation, imprints a strong signal on the water isotopes. In the central Pacific the anticorrelation between the anomalous precipitation and the isotope signal reaches a maximum value of−0.8.

Modelling the response of glaciers to climate warming

Abstract: Dynamic ice-flow models for 12 glaciers and ice caps have been forced with various climate change scenarios The volume of this sample spans three orders of magnitude Six climate scenarios were considered: from 1990 onwards linear warming rates of 001, 002 and 004 K a-1, with and without concurrent changes in precipitation The models, calibrated against the historic record of glacier length where possible, were integrated until 2100 The differences in individual glacier responses are very large No straightforward relationship between glacier size and fractional change of ice volume emerges for any given climate scenario The hypsometry of individual glaciers and ice caps plays an important role in their response, thus making it difficult to generalize results For a warming rate of 004 K a-1, without increase in precipitation, results indicate that few glaciers would survive until 2100 On the other hand, if the warming rate were to be limited to 001 K a-1 with an increase in precipitation of 10% per degree warming, we predict that overall loss would be restricted to 10 to 20% of the 1990 volume

The relationships between tropical Pacific and Atlantic SST and northeast Brazil monthly precipitation

Abstract: The monthly patterns of northeast Brazil (NEB) precipitation are analyzed in relation to sea surface temperature (SST) in the tropical Pacific and Atlantic Oceans, using singular value decomposition. It is found that the relationships between precipitation and SST in both basins vary considerably throughout the rainy season (February‐May). In January, equatorial Pacific SST is weakly correlated with precipitation in small areas of southern NEB, but Atlantic SST shows no significant correlation with regional precipitation. In February, Pacific SST is not well related to precipitation, but south equatorial Atlantic SST is positively correlated with precipitation over the northern Nordeste, the latter most likely reflecting an anomalously early (or late) southward migration of the ITCZ precipitation zone. During March, equatorial Pacific SST is negatively correlated with Nordeste precipitation, but no consistent relationship between precipitation and Atlantic SST is found. Atlantic SST‐ precipitation correlations for April and May are the strongest found among all months or either ocean. Precipitation in the Nordeste is positively correlated with SST in the south tropical Atlantic and negatively correlated with SST in the north tropical Atlantic. These relationships are strong enough to determine the structure of the seasonal mean SST‐precipitation correlations, even though the corresponding patterns for the earlier months of the season are quite different. Pacific SST‐precipitation correlations for April and May are similar to those for March. Extreme wet (dry) years for the Nordeste occur when both Pacific and Atlantic SST patterns for April and May occur simultaneously. A separate analysis reinforces previous findings in showing that SST in the tropical Pacific and the northern tropical Atlantic are positively correlated and that tropical Pacific‐south Atlantic correlations are negligible. Time-lagged analyses show the potential for forecasting either seasonal mean or monthly precipitation patterns with some degree of skill. In some instances, individual monthly mean SST versus seasonal mean (February‐ May) precipitation relationships differ considerably from the corresponding monthly SST versus monthly precipitation relationships. It is argued that the seasonal mean relationships result from the relatively strong monthly relationships toward the end of the season, combined with the considerable persistence of SST in both oceans.

The Land Surface Climatology of the NCAR Land Surface Model Coupled to the NCAR Community Climate Model

Abstract: The National Center for Atmospheric Research (NCAR) Land Surface Model (LSM, version 1.0) provides a comprehensive treatment of land surface processes for the NCAR Community Climate Model version 3 (CCM3). It replaces the prescribed surface wetness, prescribed snow cover, surface albedo, and surface flux parameterizations used in the CCM2. A 15-yr simulation of the coupled atmosphere (CCM3) and land (LSM1.0) models using observed sea surface temperatures for the period December 1978–September 1993 is used to document the model’s land surface climate. The model simulates many of the observed geographic and seasonal patterns of surface air temperature, precipitation, and soil water. In general, the transition seasons (spring, autumn) are better simulated than winter and summer. Annual precipitation and runoff are well simulated for some river basins and poorly simulated for others. In general, precipitation is better simulated than runoff. The inclusion of net land–atmosphere CO2 exchange is an imp...

Decadal Variability of Precipitation over Western North America

Abstract: Decadal (>7- yr period) variations of precipitation over western North America account for 20%–50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O(1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few short-period circulation “modes” such as the Pacific–North American pattern. Precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensiv...

Using fossil leaves as paleoprecipitation indicators : an eocene example

Abstract: Estimates of past precipitation are of broad interest for many areas of inquiry, including reconstructions of past environments and topography, climate modeling, and ocean circulation studies. The shapes and sizes of living leaves are highly sensitive to moisture conditions, and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators. Most quantitative estimates of paleoprecipitation have been based on a multivariate data set of morphological leaf characters measured from samples of living vegetation tied to climate stations. However, when tested on extant forests, this method has consistently overestimated precipitation. We present a simpler approach that uses only the mean leaf area of a vegetation sample as a predictor variable but incorporates a broad range of annual precipitation and geographic coverage into the predictor set. The significant relationship that results, in addition to having value for paleoclimatic reconstruction, refines understanding of the long-observed positive relationship between leaf area and precipitation. Seven precipitation estimates for the Eocene of the Western United States are revised as lower than previously published but remain far wetter than the same areas today. Abundant moisture may have been an important factor in maintaining warm, frost-free conditions in the Eocene because of the major role of water vapor in retaining and transporting atmospheric heat.

The Effects of Domain Choice on Summer Precipitation Simulation and Sensitivity in a Regional Climate Model

Abstract: Recent results show disagreement between global and limited-area models as to the role of soil moisture feedback during the summer of 1993 in the central United States. July precipitation totals increase by 50% in the European Centre for Medium-Range Weather Forecasts global model when soil moisture is initialized “wet,” but two separate regional modeling groups [University of Utah Limited Area Model group and National Center for Atmospheric Research Regional Climate Model (RegCM) group] have found very different responses to soil moisture, indicating that drier soil moisture conditions might actually lead to increased precipitation via an increase in convective instability and an enhancement of the low-level jet from the Gulf of Mexico. To further evaluate the sensitivity results of RegCM in this context, a new suite of simulations, driven by analyses of observations for May–July of 1988 and 1993 is performed. The model domain is larger than in the previous experiments and the sensitivity of pre...

Sea-salt aerosol in coastal Antarctic regions

Abstract: Continuous year round records of atmospheric sea-salt concentrations have been recovered at three coastal Antarctic stations (Halley, Dumont D'Urville, and Neumayer) at temporal resolutions typically between 1 day and 2 weeks. The records were evaluated in terms of their spatial and seasonal variability as well as with respect to changes in the relative ion composition of airborn sea-salt particles. Annual mean sea-salt concentrations vary between 1400 ng m−3 at Dumont D'Urville, 850 ng m−3 at Neumayer, and 200 ng m−3 at Halley, respectively. They are thus considerably lower than the mean levels previously observed at the north tip of the Antarctic Peninsula but are, at their lower end, comparable to the level previously reported from Mawson. The representativeness of the atmospheric sea-salt data appears to be weak due to their high temporal variability, strong impacts of site specific aspects (such as site topography) but also due to the nonuniform sampling techniques applied so far. In accordance with the ice core evidence, the seasonal change in the atmospheric sea-salt load is found to be clearly out of phase with the seasonal cycle of the open water fraction offshore from the station as (with the exception of Dumont D'Urville) the lowest concentrations are generally observed during the local summer months. Major ion analyses of bulk aerosol and concurrently sampled fresh snow show a strong, systematic depletion of the SO42− to Na+ (Cl−) ratios with respect to bulk sea water, which appeared to be confined to the local winter half year. During that time, sea-salt SO42− was found to be depleted typically by 60–80% along with a concurrent Na+ deficit, which is in accordance with the precipitation of mirabilite. No significant fractionation of Mg2+, K+, and Ca2+ between seawater and sea-salt particles is observed. Laboratory experiments failed to simulate the SO42− fractionation in airborne seawater droplets or in the skin of seawater bubbles at low air temperatures. They gave, however, SO42− depletion factors, similar to the field observation in air and snow, in the remaining brine of seawater which was partly frozen below −8°C to an artificial sea ice surface. It is suggested therefore that the mobilization of brine from the sea ice surface constitutes an important sea-salt source in winter which may dominate the atmospheric sea-salt load at high latitudes of coastal Antarctica.

Monsoon precipitation in the AMIP runs

Abstract: We present an analysis of the seasonal precipitation associated with the African, Indian and the Australian-Indonesian monsoon and the interannual variation of the Indian monsoon simulated by 30 atmospheric general circulation models undertaken as a special diagnostic subproject of the Atmospheric Model Intercomparison Project (AMIP) The seasonal migration of the major rainbelt observed over the African region, is reasonably well simulated by almost all the models The Asia West Pacific region is more complex because of the presence of warm oceans equatorward of heated continents Whereas some models simulate the observed seasonal migration of the primary rainbelt, in several others this rainbelt remains over the equatorial oceans in all seasons Thus, the models fall into two distinct classes on the basis of the seasonal variation of the major rainbelt over the Asia West Pacific sector, the first (class I) are models with a realistic simulation of the seasonal migration and the major rainbelt over the continent in the boreal summer; and the second (class II) are models with a smaller amplitude of seasonal migration than observed The mean rainfall pattern over the Indian region for July-August (the peak monsoon months) is even more complex because, in addition to the primary rainbelt over the Indian monsoon zone (the monsoon rainbelt) and the secondary one over the equatorial Indian ocean, another zone with significant rainfall occurs over the foothills of Himalayas just north of the monsoon zone Eleven models simulate the monsoon rainbelt reasonably realistically Of these, in the simulations of five belonging to class I, the monsoon rainbelt over India in the summer is a manifestation of the seasonal migration of the planetary scale system However in those belonging to class II it is associated with a more localised system In several models, the oceanic rainbelt dominates the continental one On the whole, the skill in simulation of excess/deficit summer monsoon rainfall over the Indian region is found to be much larger for models of class I than II, particularly for the ENSO associated seasons Thus, the classification based on seasonal mean patterns is found to be useful for interpreting the simulation of interannual variation The mean rainfall pattern of models of class I is closer to the observed and has a higher pattern correlation coefficient than that of class II This supports Sperber and Palmer’s (1996) result of the association of better simulation of interannual variability with better simulation of the mean rainfall pattern The hypothesis, that the skill of simulation of the interannual variation of the all-India monsoon rainfall in association with ENSO depends upon the skill of simulation of the seasonal variation over the Asia West Pacific sector, is supported by a case in which we have two versions of the model where NCEP1 is in class II and NCEP2 is in class I The simulation of the interannual variation of the local response over the central Pacific as well as the all-India monsoon rainfall are good for NCEP2 and poor for NCEP1 Our results suggest that when the model climatology is reasonably close to observations, to achieve a realistic simulation of the interannual variation of all-India monsoon rainfall associated with ENSO, the focus should be on improvement of the simulation of the seasonal variation over the Asia West Pacific sector rather than further improvement of the simulation of the mean rainfall pattern over the Indian region

On the stability of the atmosphere‐vegetation system in the Sahara/Sahel region

Abstract: A conceptual model has been developed for the analysis of atmosphere- vegetation interaction in subtropical deserts. The model can exhibit multiple stable states in the system' a "desert" equilibrium with low precipitation and absent vegetation and a "green" equilibrium with moderate precipitation and permanent vegetation cover. The conceptual model is applied to interpret the results of two climate-vegetation models' a comprehensive coupled atmosphere-biome model and a simple box model. In both applications, two stable states exist for the western Sahara/Sahel region for the present-day climate, and the only green equilibrium is found for the mid-Holocene climate. The latter agrees well with paleoreconstructions of Sahara/Sahel climate and vegetation. It is shown that for present-day climate the green equilibrium is less probable than the desert equilibrium, and this explains the existence of the Sahara desert as it is today. The difference in albedo between the desert and vegetation cover appears to be the main parameter that controls an existence of multiple stable states. The Charney's mechanism of self-stabilization of subtropical deserts is generalized by accounting for atmospheric hydrology, the heat and moisture exchange at the side boundaries, and taking into account the dynamic properties of the surface. The generalized mechanism explains the self-stabilization of both desert and vegetation in the western Sahara/Sahel region. The role of surface roughness in climate-vegetation interaction is shown to be of secondary importance in comparison with albedo. Furthermore, for the high albedo, precipitation increases with increasing roughness while, for the low albedo, the opposite is found.

Principal components-based regionalization of precipitation regimes across the southwest United States and Northern Mexico, with an application to monsoon precipitation variability

Abstract: We determine precipitation regions for the United States-Mexico border region based on seasonality and variability of monthly precipitation at 309 stations for the period 1961 to 1990. Using a correlation matrix of input data to avoid the effect of elevation on precipitation, we apply principal components analysis with oblique rotation to regionalize this large, climatologically complex study area. We examine the applicability of the method, 2 techniques for defining region boundaries, the various defined regions themselves, and the effects of transforming input data and changing obliquity of component rotation. We obtain 9 consistent and largely contiguous regions from each of the analyses, including regions for the North American monsoon, the low deserts, the California Mediterranean regime, and for summer precipitation regimes adjoining the Gulf of Mexico. The derived regions and associated boundaries make physical sense in terms of the driving atmospheric processes, and they are robust to transformed input data and changes in rotation procedures. The central border regions are remarkably consistent across analyses, with small changes to peripheral regions. We also identify 4 monsoon sub-regions, and we illustrate the applicability of the regionalization via an analysis of relationships between monsoon precipitation variability and 500 mb pressure heights. Significantly different 500 mb circulation patterns are associated with wet and dry monsoon seasons in each of the subregions, and it appears that shifts in 500 mb circulation relative to the geographic position of each sub-region influence seasonal precipitation variability, directly or indirectly. There are important differences between some sub-regions, but in general wet monsoons are associated with northward meridional bulging of the subtropical anticyclone over the continental monsoon areas, while dry monsoons are associated with zonal stretching of the subtropical anticyclone over adjacent oceans with slightly higher pressure-heights. Overall, the study provides a clear regionalization of the precipitation climatology for the southwest United States and northern Mexico, and shows its utility for studies of climate variability.

Heavy precipitation processes in a warmer climate

Abstract: Climate simulations have suggested that a greenhouse-gas induced global warming would also lead to a moistening of the atmosphere and an intensification of the mean hydrological cycle. Here we study possible attendant effects upon the frequency of heavy precipitation events. For this purpose simulations with a regional climate model are conducted, driven by observed and modified lateral boundary conditions and sea-surface temperature distributions. The modifications correspond to a uniform 2K temperature increase and an attendant 15% increase of the specific humidity (unchanged relative humidity). This strategy allows to isolate the effects of an increased atmospheric moisture content from changes in the atmospheric circulation. The numerical experiments, carried out over Europe and for the fall season, indicate a substantial shift towards more frequent events of strong precipitation. The magnitude of the response increases with the intensity of the event and reaches several 10s of percent for events exceeding 30 mm per day. These results appear to apply to all precipitation events dominated by sea-to-land moisture transport.

Global Monthly Precipitation Estimates from Satellite-Observed Outgoing Longwave Radiation

Abstract: The relationship between the flux of outgoing longwave radiation (OLR) estimated from satellite observations and precipitation is investigated using monthly OLR data from the NOAA polar-orbiting satellites and the merged analysis of precipitation of Xie and Arkin for the 8-yr period from July 1987 to June 1995. The mean annual cycle of OLR in the Tropics is dominated by changes in cloudiness and exhibits a strong negative correlation with precipitation, while in the extratropics the strongest influence on the annual cycle of OLR is surface temperature and a positive correlation with precipitation is found. However, the anomaly of OLR exhibits a negative correlation with precipitation over most of the globe. The regression coefficient relating the anomaly of precipitation to that of OLR is spatially inhomogeneous and seasonally dependent but can be expressed with high accuracy as a globally uniform linear function of the local mean precipitation. Based on these results, a new technique is develope...

Climate extremes in Loess of China coupled with the strength of deep-water formation in the North Atlantic

Abstract: The loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly 18 . developed S4, S5-1 and S5-3 soils corresponding to the marine d O stages 11, 13, and 15, respectively as evidence of 18 three episodes of great warmth and two coarse-grained loess units L9 and L15, corresponding to the marine d O stages 22, . 23, 24 and 38, respectively which indicate severest glacial conditions. The climatic and geographical significance of these events are still unclear, and their cause remains a puzzle. . Paleopedological, geochemical and magnetic susceptibility data from three loess sections Xifeng, Changwu and Weinan suggest that the S4, S5-1 and S5-3 soils were formed under sub-tropical semi-humid climates with a tentatively estimated . . mean annual temperature MAT of at least 4-68C higher and a mean annual precipitation MAP of 200-300 mm higher than for the present-day, indicating a much strengthened summer monsoon. The annual rainfall was particularly accentuated for the southern-most part of the Loess Plateau, suggesting that the monsoon rain belt the contact of the monsoonal . northward warm-humid air mass with the dry-cold southward one might have stood at the southern part of the Plateau for a relatively long period each year. The loess units L9 and L15 were deposited under semi-desertic environments with a tentatively estimated MAT and MAP of only about 1.5-38C and 150-250 mm, indicating a much strengthened winter monsoon, and that the summer monsoon front could rarely penetrate into the Loess Plateau region. Correlation with marine carbon isotope records suggests that these climate extremes have large regional, even global, significance rather than being local phenomena in China. They match the periods with greatestrsmallest Atlantic-Pacific 13 . d C gradients, respectively, indicating their relationships with the strength of Deep Water NADW production in the North Atlantic. These results suggest that the monsoon climate in the Loess Plateau region was significantly linked with the North Atlantic thermohaline circulation on timescales of 10 4 years. q 1998 Elsevier Science B.V. All rights reserved.

Numerical Simulations of the 1994 Piedmont Flood: Role of Orography and Moist Processes

Abstract: The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM). The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband...

Associations between China monsoon rainfall and tropospheric jets

Abstract: Strong associations, in both the annual cycle and interannual variations, between east China monsoon rainfall and tropospheric jets are established with the use of observations and general-circulation model simulations. Two distinct systems dominate regional rainfall: the east Asian jet stream (EAJ) in the north and the Hadley cell in the south. The EAJ is associated with Mei-Yu and polar fronts as well as vigorous jet-transverse circulations, whereas the Hadley cell is allied to tropical upper-level easterlies and intertropical convergence zone convection. An equatorward EAJ displacement causes precipitation to increase over south-central (south) China during June-August (January-March). Conversely, a poleward shift of the summer (winter) EAJ brings heavier precipitation over north (central-north) China. On the other hand, over the South China Sea the Hadley cell influence prevails and, consequently, increased rainfalls concur with enhanced lower-level westerlies. Furthermore, the EAJ fluctuations are strongly coupled with southern oscillation variations. Their interactions tend to precede (follow) El Nino phenomena during October through May (summer). The EAJ related flow anomalies also have potential skill to predict China rainfall interannual variability. To conclude, a realistic China monsoon simulation requires accurate representation of the EAJ and Hadley cell. Both features link regional rainfall to tropical and extratropical planetary-scale circulations and, in turn, to global surface characteristics.

FIFE Surface Climate and Site-Average Dataset 1987–89

Abstract: This paper analyzes the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment site-average datasets for near-surface meteorology, soil moisture, and temperature; the surface fluxes of radiation, sensible, and latent heat; and the ground heat flux, for the period May 1987‐November 1989. The diurnal and seasonal variation of surface albedo for this grassland site are discussed. The coupling of precipitation, soil moisture, evaporation, pressure height to the lifting condensation level, and equivalent potential temperature (uE) on seasonal and diurnal timescales is also discussed. The 1988 data confirm the authors’ result, shown earlier from the 1987 data that over moist soils increased evapotranspiration lowers afternoon lifting condensation level and increases afternoon uE, suggesting a mechanism for a local positive feedback between soil moisture and precipitation on horizontal scales greater than 200 km. The seasonal cycle of ground heat flux and soil temperature is examined and the authors show that the coupling in the warm months between uE and soil temperature on seasonal scales is similar over land to the coupling found over warm oceans despite very different controls on the surface fluxes. The boundary layer equilibrium over the ocean is contrasted with the diurnal cycle over land, which is soil moisture dependent.

Toward Cloud Resolving Modeling of Large-Scale Tropical Circulations: A Simple Cloud Microphysics Parameterization

Abstract: This paper discusses cloud microphysical processes essential for the large-scale tropical circulations and the tropical climate, as well as the strategy to include them in large-scale models that resolve cloud dynamics. The emphasis is on the ice microphysics, which traditional cloud models consider in a fairly complex manner and where a simplified approach is desirable. An extension of the classical warm rain bulk parameterization is presented. The proposed scheme retains simplicity of the warm rain parameterization (e.g., only two classes of condensed water are considered) but introduces two important modifications for temperatures well below freezing:1) the saturation conditions are prescribed based on saturation with respect to ice, not water; and 2) growth characteristics and terminal velocities of precipitation particles are representative for ice particles, not raindrops. Numerical tests suggest that, despite its simplicity, the parameterization is able to capture essential aspects of the ...