Showing papers on "Precipitation published in 2002"

A high-resolution data set of surface climate over global land areas

Abstract: We describe the construction of a 10' latitude/longitude data set of mean monthly sur- face climate over global land areas, excluding Antarctica The climatology includes 8 climate ele- ments —precipitation, wet-day frequency, temperature, diurnal temperature range, relative humid- ity, sunshine duration, ground frost frequency and windspeed—and was interpolated from a data set of station means for the period centred on 1961 to 1990 Precipitation was first defined in terms of the parameters of the Gamma distribution, enabling the calculation of monthly precipitation at any given return period The data are compared to an earlier data set at 05o latitude/longitude resolution and show added value over most regions The data will have many applications in applied climatology, biogeochemical modelling, hydrology and agricultural meteorology and are available through the International Water Management Institute World Water and Climate Atlas (http://wwwiwmiorg) and the Climatic Research Unit (http://wwwcruueaacuk)

Climate Effects of Black Carbon Aerosols in China and India

Abstract: In recent decades, there has been a tendency toward increased summer floods in south China, increased drought in north China, and moderate cooling in China and India while most of the world has been warming. We used a global climate model to investigate possible aerosol contributions to these trends. We found precipitation and temperature changes in the model that were comparable to those observed if the aerosols included a large proportion of absorbing black carbon ("soot"), similar to observed amounts. Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the large-scale circulation and hydrologic cycle with significant regional climate effects.

Rainfall variability, carbon cycling, and plant species diversity in a mesic grassland

Abstract: Ecosystem responses to increased variability in rainfall, a prediction of general circulation models, were assessed in native grassland by reducing storm frequency and increasing rainfall quantity per storm during a 4-year experiment. More extreme rainfall patterns, without concurrent changes in total rainfall quantity, increased temporal variability in soil moisture and plant species diversity. However, carbon cycling processes such as soil CO2 flux, CO2 uptake by the dominant grasses, and aboveground net primary productivity (ANPP) were reduced, and ANPP was more responsive to soil moisture variability than to mean soil water content. Our results show that projected increases in rainfall variability can rapidly alter key carbon cycling processes and plant community composition, independent of changes in total precipitation.

Tropical tropospheric temperature variations caused by ENSO and their influence on the remote tropical climate

Abstract: The warming of the entire tropical free troposphere in response to El Nino is well established, and suggests a tropical mechanism for the El Nino–Southern Oscillation (ENSO) teleconnection. The potential impact of this warming on remote tropical climates is examined through investigating the adjustment of a single-column model to imposed tropospheric temperature variations, assuming that ENSO controls interannual tropospheric temperature variations at all tropical locations. The column model predicts the impact of these variations in three typical tropical climate states (precipitation > evaporation; precipitation < evaporation; no convection) over a slab mixed layer ocean. Model precipitation and sea surface temperature (SST) respond significantly to the imposed tropospheric forcing in the first two climate states. Their amplitude and phase are sensitive to the imposed mixed layer depth, with the nature of the response depending on how fast the ocean adjusts to imposed tropospheric temperature f...

Inferences of Predictability Associated with Warm Season Precipitation Episodes

Abstract: Herein preliminary findings are reported from a radar-based climatology of warm season precipitation ‘‘episodes.’’ Episodes are defined as time‐space clusters of heavy precipitation that often result from sequences of organized convection such as squall lines, mesoscale convective systems, and mesoscale convective complexes. Episodes exhibit coherent rainfall patterns, characteristic of propagating events, under a broad range of atmospheric conditions. Such rainfall patterns are most frequent under ‘‘weakly forced’’ conditions in midsummer. The longevity of episodes, up to 60 h, suggests an intrinsic predictability of warm season rainfall that significantly exceeds the lifetime of individual convective systems. Episodes are initiated primarily in response to diurnal and semidiurnal forcings. Diurnal forcing is dominant near the Rocky and Appalachian Mountains, whereas semidiurnal forcing is dominant between these cordilleras. A most common longitude of origin is at or near the east slope of the Continental Divide (1058W). These observations are consistent with a condition of continual thermal forcing, widespread hydrodynamic instability, and the existence of other processes that routinely excite, maintain, and regenerate organized convection. The propagation speed of major episodes is often in excess of rates that are easily attributable either to the phase speeds of large-scale forcing or to advection from low- to midlevel ‘‘steering’’ winds. It is speculated that wavelike mechanisms, in the free troposphere and/or the planetary boundary layer, may contribute to the rates of motion observed. Once understood, the representation of such mechanisms in forecast models offers the opportunity for improved predictions of warm season rainfall.

Intercomparison of the climatological variations of Asian summer monsoon precipitation simulated by 10 GCMs

Abstract: We assesses the overall performance of state-of-the-art atmospheric GCMs in simulating the climatological variations of summer monsoon rainfall over the Asian-Western Pacific region and the systematic errors that are common to a group of GCMs. The GCM data utilized are obtained from 10 GCM groups participated in the CLIVAR/Monsoon GCM Intercomparison Project. The model composite shows that the overall spatial pattern of summer monsoon rainfall is similar to the observed, although the western Pacific rainfall is relatively weak. For the simulated precipitation over the western Pacific, the models can be classified into two categories. The first category of models simulates the precipitation more confined to the equatorial region and weaker precipitation in the subtropical western Pacific compared to the observed. The second category of models simulates large precipitation in the subtropical western Pacific but the region is shifted to the north by 5–10°. None of the models realistically reproduce the observed Mei-yu rain band in the region from the East China Sea to the mid Pacific. Most of the models produce a rain band along the continental side of East Asia. The climatological variations of simulated summer rainfall are examined in terms of their amplitude and their principal EOF modes. All models simulate larger amplitudes of the climatological seasonal variation of Indian summer monsoon than the observed, though most models simulate smaller amplitudes in the western Pacific. The ten model composite produces four leading EOF modes over the Asian-western Pacific region, which are remarkably similar to the observed counterparts. The first and second eigenmodes, respectively, represent the smoothed seasonal march of broad-scale monsoon and the onsets of the Indian and East Asian summer monsoon. The third and fourth modes relate to the climatological intraseasonal oscillation (CISO). In contrast to the model composite, several models fail to reproduce the first principal mode, and most models do not reproduce the observed modes higher than the second. The CISO of precipitation is also examined over the Indian monsoon and the East Asia-western Pacific monsoon regions separately.

Tropical Rainfall Trends and the Indirect Aerosol Effect

Abstract: An atmospheric global climate model coupled to a mixed layer ocean model is used to study changes in tropical rainfall due to the indirect effects of anthropogenic sulfate aerosol. The model is run to equilibrium for present-day (PD) and preindustrial (PI) sulfur emission scenarios. As in two other recent studies, the model generally gives a southward shift of tropical rainfall in the PD run relative to the PI run. This is largely due to a hemispheric asymmetry in the reduction of sea surface temperature (SST) induced by the perturbation of cloud albedo and lifetime. Observed precipitation trends over land for the period 1900‐98 show a complex pattern in the Tropics, but when zonally averaged, a southward shift similar to (but weaker than) the modeled shift is clearly evident. The zonally averaged tropical trends are significant at the 5% level in several latitude bands. The modeled presentday hemispheric contrast in cloud droplet effective radius (which affects cloud albedo) is well supported by one long-term satellite retrieval, but not by another. A third satellite retrieval, which only covers an 8-month period, does show a marked hemispheric contrast in effective radius. Both in the modeled changes and the observed trends, a prominent feature is the drying of the Sahel in North Africa. Modeled dynamical changes in this region are similar to observed changes that have been associated with Sahelian drought. Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The present results, combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. More generally, it is concluded that spatially varying aerosol-related forcing (both direct and indirect) can substantially alter low-latitude circulation and rainfall.

Variations of the Dust Storm in China and its Climatic Control

Abstract: In previous studies, limited meteorological observations were used to investigate the temporal–spatial changes of dust storms in China. Here, the authors use the daily 850-hPa geopotential height of NCEP–NCAR reanalysis for 1948–99 to examine the vortex fluctuations, which represent daily cyclone activity in east Asia. They also use the 1000-hPa air temperature data to explain the decadal change of the cyclone activity. In addition, the grid cyclone frequency for 1948–99 and the temperature and precipitation for 1950–98 are used to calculate the correlation with the dust weather frequency (for 1954–98) in China. Results show that the interannual variability and long-term trend among dust storm frequency, dust weather frequency, air temperature, and cyclone frequency exist in northern China. In the eastern part of China, the frequencies of dust storms and dust weather in the 1950s–70s were about twice that after the mid-1980s. The reason for this feature may be due to the warming in Mongolia and c...

The Hydrological Cycle in the Mediterranean Region and Implications for the Water Budget of the Mediterranean Sea

Abstract: The hydrological cycle in the Mediterranean region is analyzed focusing on climatology and interannual to interdecadal variability, in particular long-term changes related to the well-established North Atlantic Oscillation (NAO) teleconnection. Recent atmospheric reanalyses and observational datasets are used: precipitation, evaporation, and moisture flux from 50 yr of NCEP's and 15 yr of ECMWF's reanalyses; precipitation from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) and the East Anglia University Climate Research Unit (CRU) datasets; and evaporation from the University of Wisconsin—Milwaukee (UWM) Comprehensive Ocean–Atmosphere Data Set (COADS). A budget analysis is performed to study contributions to the freshwater flux into the Mediterranean Sea, including atmospheric as well as river discharge inputs. The total river discharge is derived using historical time series from Mediterranean Hydrological Cycle Observing System (MED-HYCOS) and Global Runoff Data Center (G...

Siberian Lena River hydrologic regime and recent change

Abstract: [1] The long-term (1935–1999) monthly records of temperature, precipitation, stream flow, river ice thickness, and active layer depth have been analyzed in this study to examine Lena River hydrologic regime and recent change. Remarkable hydrologic changes have been identified in this study. During the cold season (October–April), significant increases (25–90%) in stream flow and decrease in river ice thickness have been found due to warming in Siberia. In the snowmelt period (May–June), strong warming in spring leads to an advance of snowmelt season into late May and results in a lower daily maximum discharge in June. During summer months (July–September) the changes in stream flow hydrology are less significant in comparison to those for winter and spring seasons. A slight stream flow increase is discovered for both July and August, mainly owing to precipitation increase in May and June. Discharge in September has a slight downward trend due to precipitation decrease and temperature increase in August. The magnitudes of changes in stream flow and river ice thickness identified in this study are large enough to alter the hydrologic regime. Investigation into the hydrologic response of the Lena River to climate change and variation reveals strong linkages of stream flow with temperature and precipitation. We therefore believe that Lena River hydrologic regime changes are mainly the consequence of recent climate warming over Siberia and also closely related to changes in permafrost condition.

Influence of the Global Warming on Tropical Cyclone Climatology: An Experiment with the JMA Global Model

Abstract: The influence of the global warming on tropical cyclones has been examined using a high resolution AGCM. Two ten-year integrations were performed with the JMA global model at T106 horizontal resolution. For the control experiment, the observed SST for the period 1979-1988 is prescribed, while for the doubling CO2 (2 × CO2) experiment, SST anomaly due to the global warming estimated from a coupled model transient CO2 experiment (Tokioka et al. 1995) is added to the SST used in the control experiment. The results of experiments show that a significant reduction in the frequency of tropical cyclones is possible in response to the greenhouse gas-induced global warming. The most significant decrease is indicated over the North Pacific. On the other hand, a considerable increase in tropical cyclone frequency is indicated for the North Atlantic. As for the maximum intensity of tropical cyclones, no significant change has been noted. It has been found that the regional change in tropical cyclone frequency is closely related to the distribution of the SST anomaly, and the change in convective activity associated with it. The results of the experiment indicate that the change in tropical cyclogenesis is strongly controlled by dynamical factors associated with the change in SST distribution, rather than the thermodynamical factors associated with the change in absolute value of local SST. On the other hand, for the decrease in the global total number of tropical cyclones on doubling CO2, a weakening of tropical circulation associated with the stabilization of the atmosphere (the increase in dry static stability), seems to be responsible. It is found that the rate of increase in the tropical precipitation due to the global warming is much less than the rate of increase in the atmospheric moisture. With this little increase in precipitation (convective heating), a considerable increase in the dry static stability of the atmosphere leads to a weakening of the tropical circulation.

Extreme Precipitation Events in Southeastern South America and Large-Scale Convective Patterns in the South Atlantic Convergence Zone

Abstract: The occurrence of daily extreme precipitation events in southeast South America (Sao Paulo, Brazil) and the spatial features of convective activity in the South Atlantic convergence zone (SACZ) are investigated. Precipitation data from surface stations in Sao Paulo state from 1979 to 1996 are used to determine the frequency of occurrence of extremely heavy daily precipitation events. Daily averages of outgoing longwave radiation (OLR) are examined to characterize convective activity in the SACZ. OLR features are identified with factor analysis. Two factors explain ∼65% of the total variance of the convective activity patterns in tropical South America and characterize events according to the intensity and extent of the OLR features over the Atlantic Ocean. The combination of factors indicates that 35% of extreme precipitation events occurred when convective activity in the SACZ was intense over large parts of tropical South America, which includes Sao Paulo, but with less extent toward the Atlant...

A teleconnection pattern in upper-level meridional wind over the North African and Eurasian continent in summer

Abstract: One-point correlation analysis on upper-level meridional wind identified the existence of a teleconnection pattern in July, which emerges from North Africa to East Asia along the westerly jet in the middle latitudes. We examined the spatial and temporal structures of this teleconnection pattern, and found the unique characteristics rather different from the patterns in other elements such as geopotential height, streamfunction and vorticity. We also investigated the relationship between this teleconnection and precipitation, and suggested that the teleconnection is a possible linkage of the EASM to the Indian monsoon, and even to subtropical heating anomalies over Atlantic.

Drought in Central and Southwest Asia: La Niña, the Warm Pool, and Indian Ocean Precipitation.

Abstract: Severe drought over the past three years (1998‐2001), in combination with the effects of protracted sociopolitical disruption, has led to widespread famine affecting over 60 million people in central and southwest (CSW) Asia. Here both a regional and a large-scale mode of climate variability are documented that, together, suggest a possible forcing mechanism for the drought. During the boreal cold season, an inverse relationship exists between precipitation anomalies in the eastern Indian Ocean and CSW Asia. Suppression of precipitation over CSW Asia is consistent with interaction between local synoptic storms and wave energy generated by enhanced tropical rainfall in the eastern Indian Ocean. This regional out-of-phase precipitation relationship is

Effects of climate change on US crop production: simulation results using two different GCM scenarios. Part I: Wheat, potato, maize, and citrus

Abstract: We projected US agricultural production in 2030 and 2090 at 45 representative sites, using 2 scenarios of climate change, developed with the Hadley Centre Model and the Canadian Centre Climate Model, and the DSSAT (Decision Support Systems for Agro-technology Transfer) dynamic crop-growth models. These simulation results have previously been aggregated nationally with the aid of economic models to show an increase in overall US agricultural output under climate change. In this work, we analyzed the regional distribution of the simulated yields, showing that positive results largely depend on the precipitation increases projected by the climate scenarios. In contrast, in some important rainfed production areas where precipitation was projected to decrease, such as the Kansas and Oklahoma Bread Basket regions under the Canadian Centre Climate Model scenario, climate change resulted in significant reductions of grain yield (−30 to −40%), accompanied by increased year-to-year variability. We also discussed the response to additional factors affecting the simulated US crop production under climate change, such as higher temperature and elevated CO2.

The Hydrologic Cycle of the La Plata Basin in South America

Abstract: The main components of the hydrologic cycle of the La Plata basin in southeastern South America are investigated using a combination of observations, satellite products, and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) global reanalyses. La Plata basin is second only to the Amazon basin in South America in river discharge and size and plays a critical role in the economies of the region. It is a primary factor in energy production, water resources, transportation, agriculture, and livestock. Of particular interest was the evaluation of the annual cycle of the hydrologic cycle components. The La Plata annual-mean river discharge is about 21 000 m3 s−1, and the amplitude of its mean annual cycle is small: it is slightly larger during late summer, but continues with large volumes even during winter. The reason for this is that different precipitation regimes over different locations contribute to the total river discharge. One regime is found t...

Secular trends in daily precipitation characteristics: greenhouse gas simulation with a coupled AOGCM

Abstract: Secular trends of daily precipitation characteristics are considered in the transient climate change experiment with a coupled atmosphere-ocean general circulation model ECHAM4/OPYC3 for 1900-2099. The climate forcing is due to increasing concentrations of the greenhouse gases in the atmosphere. Mean daily precipitation, precipitation intensity, probability of wet days and parameters of the gamma distribution are analyzed. Particular attention is paid to the changes of heavy precipitation, Analysis of the annual mean precipitation trends for 1900-1999 revealed general agreement with observations with significant positive trends in mean precipitation over continental areas. In the 2000-2099 period precipitation trend patterns followed the tendency obtained for 1900-1999 but with significantly increased magnitudes. Unlike the annual mean precipitation trends for which negative values were found for some continental areas, the mean precipitation intensity and scale parameter of the fitted gamma distribution increased over all land territories . Negative trends in the number of wet days were found over most of the land areas except high latitudes in the Northern Hemisphere. The shape parameter of the gamma distribution in general revealed a slight negative trend in the areas of the precipitation increase. Investigation of daily precipitation revealed an unproportional increase of heavy precipitation events for the land areas including local maxima in Europe and the eastern United States.

Macroscale water fluxes 2. Water and energy supply control of their interannual variability

Abstract: [1] Controls on interannual variations in water and energy balances of large river basins (10,000 km 2 and greater) are evaluated in the framework of the semiempirical relation E=P ¼½ 1 þð R=PÞ � n � � 1/n in which and E, P, and R are basin mean values of annual evaporation, precipitation, and surface net radiation, respectively, expressed as equivalent evaporative water flux, overbars denote long-term means, and n is a parameter. Precipitation is interpolated from gauges; evaporation is taken as the difference between precipitation and runoff, with the latter determined from basin discharge measurements and a simple storage-delay model; and radiation is based on a recent analysis in which 8 years of satellite observations were assimilated into radiative transfer models. Objective estimates of precipitation errors are considered; results suggest that past estimates of n may have been biased by systematic errors in estimates of precipitation. Under the assumption that the semiempirical relation applies also to annual values, long-term mean observations are sufficient to predict the sensitivity of annual runoff to fluctuations in precipitation or net radiation. Additionally, an apparent sensitivity of runoff to precipitation can be inferred from the observations by linear regression. This apparent sensitivity is generally in good agreement with the predicted sensitivity. In particular, the apparent sensitivity increases with decreasing basin R/P; however, slightly excessive apparent sensitivity (relative to the prediction) is found in humid basins of the middle latitudes. This finding suggests a negative correlation between precipitation and net radiation: the increase in runoff caused by a positive precipitation anomaly is amplified by an accompanying decrease in surface net radiation, possibly induced by increased cloud cover. The inferred sensitivity of radiation (water flux equivalent) to precipitation is on the order of � 0.1. Such a value is supported by independent direct analysis of annual precipitation and radiation data. The fraction of interannual variance in runoff explained by the annual precipitation anomaly (including any correlative influence of net radiation) varies systematically with climatic aridity, approaching unity in humid basins and falling to 40–80% in very arid basins. We conclude that the influence of seasonality of the precipitation anomaly on annual runoff is negligible under humid conditions, though it may be significant under arid conditions. INDEX TERMS: 1812 Hydrology: Drought; 1818 Hydrology: Evapotranspiration; 1854 Hydrology: Precipitation (3354); 1878 Hydrology: Water/energy interactions; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: water balance, interannual variability, runoff, radiation

Patterns and controls of primary production in the patagonian steppe: a remote sensing approach†

Abstract: We took advantage of regional gradients to study the spatial relationships between aboveground net primary production (ANPP) and climate in the Patagonian steppe of South America. We explored the same relationships through time, considering the natural variations of ANPP and climate for 11 yr. Based on NOAA/AVHRR satellite normalized difference vegetation index (NDVI) data, we evaluated the effects of climate on annual and seasonal ANPP across regional gradients of precipitation (100-500 mm/yr) and tem- perature (218-98C of annual mean). We studied ANPP climatic controls through time at four sites using NDVI and meteorological data. We used annual NDVI integral as a surrogate of annual ANPP. Annual NDVI integral increased linearly along regional gradients of precipitation, and its annual variability de- creased exponentially. Annual NDVI integral was, in most cases, unrelated to precipitation through time. We described the seasonality of ANPP using four variables derived from seasonal NDVI curves: the dates of growing season start and end, the date of maximum NDVI, and the length of the growing season. The growing season started later toward the cold extreme of the regional temperature gradients and, within a given site through time, during the coldest years. The dates of maximum NDVI and end of the growing season occurred later toward the humid or cold extremes of the regional gradients, whereas the length of the growing season was positively affected by precipitation and temperature along these gradients. These variables were not associated with climate through time. The response of the start of the growing season to temperature was greater in time, following the natural climatic fluctuations, than in space, accompanying regional temperature gradients. This difference probably resulted because the time required for shifts in community composition and plant adaptation is longer than one year. Climatic determinants of ANPP shifted from precipitation alone to precipitation plus temperature when the temporal scale of analysis changed from annual to seasonal. Our results indicate the feasibility of forecasting forage availability a few months prior to the beginning of the growing season, but not during the whole year. Longer term data sets and manipulative experiments are required to forecast annual ANPP and predict its response to climate change.

Preliminary Study on Signal, Impact and Foreground of Climatic Shift from Warm-Dry to Warm-Humid in Northwest China

Abstract: A rapid, high amplitude global climatic warming would speed global water cycle and strengthen rainfall and evaporation. Climatic warming and drying course were dominated in past about 100 years since the end of Little Ice Age in Northwest China. The strong signals of climatic shift to warm humid pattern have been appearing in the western part of Chinese Tianshan Mountains and neighborhood regions including Northern Xinjiang since 1987. Precipitation, meltwater of glacier and runoff of rivers increase continuously , and results in lake level rising, flood damaged magnification and intensified, vegetation coverage extending and dust storm weaken in western parts of Northwest China. In the other areas of Xinjiang, and the middle and western section of Qilian Mountains, the precipitations and runoff of rivers have also an increasing tendency. How is the foreground of climatic shift to warn humid conditions, it is just restricted to decadal fluctuation, or possibility to century scale shift trend, and is just limited to western Tianshan or could extend and/or wholly Northwest China down to North China?Using the results of regional climatic model simulation from IPCC and China Assessment Report published, predicting river runoff regime and similar paleoclimate scenarios for the Northwest China are analysed and discussed, and the trend of shifting warm humid climate can be fixed in recent future. However, uncertain of projected results remain, and rating and magnitude of climatic shift extending in temporal and spatial scales can not at present be projected in detail and in being exactitude.

Evaluation of the diurnal cycle of precipitation, surface thermodynamics, and surface fluxes in the ECMWF model using LBA data

Abstract: [1] The mean diurnal cycle of precipitation, near-surface thermodynamics and surface fluxes from short-term forecasts of the ECMWF model are compared with corresponding observations from the Large-Scale Biosphere-Atmosphere Experiment in the Amazonia wet season campaign in 1999 in Rondonia. Precipitation starts about 2 hours after sunrise in the model, several hours earlier than observed, because the model does not simulate well the morning growth of the nonprecipitating convective boundary layer. However, the mean daily precipitation during the wet season compares well with observed rainfall. On most days, maximum early afternoon temperature and cloud base height are lower in the model than observed. Maximum equivalent potential temperature is close to that observed. The model surface evaporative fraction is higher than observed and rises to near unity in the late afternoon. Work is in progress to evaluate and integrate the parameterizations for shallow and deep convection.

Simulation of hydrologic changes associated with global warming

Abstract: [1] Using the results obtained from a coupled ocean-atmosphere-land model with medium computational resolution, we investigated how the hydrology of the continents changes in response to the combined increases of greenhouse gases and sulfate aerosols in the atmosphere, which are determined based upon the IS92a scenario. In order to extract the forced response from natural, internal variability, the difference between the mean of an eight-member ensemble of numerical experiments and a control experiment are used for the present analysis. The global mean surface air temperature of the coupled model increases by about 2.3°C above the preindustrial level by the middle of the 21st century. Accompanying the warming, the global mean rates of both precipitation and evaporation increase by 5.2%, yielding the average increase in the rate of runoff by approximately 7.3%. The increase in the rate of runoff simulated by the model is particularly large in high northern latitudes, where the runoff from some rivers such as the Mackenzie and Ob′ may increase by as much as 20%. Runoff from many European rivers increases by more than 20%. Runoff also increases substantially in some tropical rivers such as the Amazon and Ganges. However, the percentage changes in simulated runoff from many other tropical rivers and middle latitude rivers are smaller with both positive and negative signs. In middle and high latitudes in the Northern Hemisphere, soil moisture tends to decrease in summer, whereas it increases in winter. However, in many semi-arid regions in subtropical and middle latitudes, soil moisture is reduced during most of a year. These semi-arid regions include the southwestern part of North America, the northeastern part of China in the Northern Hemisphere, and the region in the vicinity of the Kalahari Desert and southern part of Australia in the Southern Hemisphere. Since a semi-arid region usually surrounds a desert, the reduction of soil moisture in such a region often results in the expansion of the desert. Soil moisture is also reduced during the dry season in many semi-arid regions. For example, it is reduced in the savannahs of Africa and South America during winter and early spring in the Southern Hemisphere. In the Northern Hemisphere, it is reduced at the Mediterranean coast of Europe in summer.

Simulating wind fields and snow redistribution using terrain-based parameters to model snow accumulation and melt over a semi-arid mountain catchment

Abstract: In mountainous regions, wind plays a prominent role in determining snow accumulation patterns and turbulent heat exchanges, strongly affecting the timing and magnitude of snowmelt runoff. In this study, digital terrain analysis was employed to quantify aspects of the upwind topography related to wind shelter and exposure, to efficiently develop a distributed time-series of snow accumulation rates and wind speeds to force a distributed snow model. Parameters are presented that determined each grid cell's topographic exposure and potential for drift development relative to observed winds. Using meteorological data taken from both an exposed and a sheltered site in the Reynolds Mountain East watershed (0·38 km2) in southwestern Idaho, the terrain parameters were used to distribute rates of snow accumulation and wind speeds at an hourly time step for input to ISNOBAL, an energy and mass balance snow model. Model runs were initiated prior to the development of the seasonal snow cover and continued through complete meltout for the 1986 (precipitation 128% of average), 1987 (66%), and 1989 (108%) water years. A comprehensive dataset consisting of a time series of aerial photographs taken during meltout, measured runoff, and snow data from the sheltered meteorological site were used to validate the simulations. ISNOBAL forced with accumulation rates and wind fields generated from the applied terrain parameterizations accurately modelled the observed snow distribution (including the formation of drifts and scoured wind-exposed ridges) and snowmelt runoff for all three years of study. By contrast, ISNOBAL forced with spatially constant accumulation rates and wind speeds taken from the sheltered meteorological site, a typical snow-monitoring site, overestimated peak snowmelt inputs and tended to underestimate snowmelt inputs prior to the runoff peak. Published in 2002 by John Wiley & Sons, Ltd.

Deconstructing Atlantic Intertropical Convergence Zone variability: Influence of the local cross-equatorial sea surface temperature gradient and remote forcing from the eastern equatorial Pacific

Abstract: (1) We investigate causes of interannual variability in Atlantic Intertropical Convergence Zone (ITCZ) convection using a monthly mean global precipitation data set spanning 1979-1999. Starting from the hypothesis of two dominant influences on the ITCZ, namely, the cross-equatorial gradient in tropical Atlantic sea surface temperature (SST) and the anomalous Walker circulation due to the rearrangement of tropical Pacific convection associated with the El Nino-Southern Oscillation, we analyze anomaly composites over the 1979-1999 period that best isolate the effects of each mechanism. Our results suggest that to first order, a strong anomalous Walker circulation suppresses precipitation over the tropical Atlantic, whereas an anomalous warm north/cool south SST gradient shifts the meridional location of maximum ITCZ convection anomalously north. We examined the processes underlying each of the two mechanisms. For the anomalous Walker circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial Pacific. For the SST gradient mechanism our results confirm previous studies that link convection to cross-equatorial winds forced by meridional SST gradients. We find that positive surface flux feedback brought about through the cross-equatorial winds is weak and confined to the deep tropics. On the basis of the results of this and other studies we propose an expanded physical picture that explains key features of Atlantic ITCZ variability, including its seasonal preference, its sensitivity to small anomalous SST gradients, and its role in the context of tropical Atlantic SST gradient variability. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309), 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504), 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854), 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology, 4522 Oceanography: Physical: Ocean Optics; KEYWORDS: Tropical Atlantic, precipitation, climate variability, El Nino-Southern Oscillation, Ocean-atmosphere interaction

A glaciation indirect aerosol effect caused by soot aerosols

Abstract: [1] Anthropogenic aerosols can influence the climate indirectly by changing the optical properties and precipitation formation of water clouds An indirect effect that has not been considered involves the subset of anthropogenic aerosols that act as ice nuclei and thereby determines the lifetime of ice and mixed-phase clouds If, in addition to mineral dust, a fraction of the hydrophilic soot aerosol particles is assumed to act as contact ice nuclei as evident from recent laboratory studies, then increases in aerosol concentration from pre-industrial times to present-day pose a new indirect effect, a “glaciation indirect effect”, on clouds Here increases in contact ice nuclei in the present-day climate result in more frequent glaciation of clouds and increase the amount of precipitation via the ice phase This effect can at least partly offset the solar indirect aerosol effect on water clouds