Showing papers on "Precipitation published in 2009"

Impact of Cloud Microphysics on the Development of Trailing Stratiform Precipitation in a Simulated Squall Line: Comparison of One- and Two-Moment Schemes

Abstract: A new two-moment cloud microphysics scheme predicting the mixing ratios and number concentrations of five species (i.e., cloud droplets, cloud ice, snow, rain, and graupel) has been implemented into the Weather Research and Forecasting model (WRF). This scheme is used to investigate the formation and evolution of trailing stratiform precipitation in an idealized two-dimensional squall line. Results are compared to those using a one-moment version of the scheme that predicts only the mixing ratios of the species, and diagnoses the number concentrations from the specified size distribution intercept parameter and predicted mixing ratio. The overall structure of the storm is similar using either the one- or two-moment schemes, although there are notable differences. The two-moment (2-M) scheme produces a widespread region of trailing stratiform precipitation within several hours of the storm formation. In contrast, there is negligible trailing stratiform precipitation using the one-moment (1-M) scheme. The primary reason for this difference are reduced rain evaporation rates in 2-M compared to 1-M in the trailing stratiform region, leading directly to greater rain mixing ratios and surface rainfall rates. Second, increased rain evaporation rates in 2-M compared to 1-M in the convective region at midlevels result in weaker convective updraft cells and increased midlevel detrainment and flux of positively buoyant air from the convective into the stratiform region. This flux is in turn associated with a stronger mesoscale updraft in the stratiform region and enhanced ice growth rates. The reduced (increased) rates of rain evaporation in the stratiform (convective) regions in 2-M are associated with differences in the predicted rain size distribution intercept parameter (which was specified as a constant in 1-M) between the two regions. This variability is consistent with surface disdrometer measurements in previous studies that show a rapid decrease of the rain intercept parameter during the transition from convective to stratiform rainfall.

The global climatology of an interannually varying air–sea flux data set

Abstract: The air–sea fluxes of momentum, heat, freshwater and their components have been computed globally from 1948 at frequencies ranging from 6-hourly to monthly. All fluxes are computed over the 23 years from 1984 to 2006, but radiation prior to 1984 and precipitation before 1979 are given only as climatological mean annual cycles. The input data are based on NCEP reanalysis only for the near surface vector wind, temperature, specific humidity and density, and on a variety of satellite based radiation, sea surface temperature, sea-ice concentration and precipitation products. Some of these data are adjusted to agree in the mean with a variety of more reliable satellite and in situ measurements, that themselves are either too short a duration, or too regional in coverage. The major adjustments are a general increase in wind speed, decrease in humidity and reduction in tropical solar radiation. The climatological global mean air–sea heat and freshwater fluxes (1984–2006) then become 2 W/m2 and −0.1 mg/m2 per second, respectively, down from 30 W/m2 and 3.4 mg/m2 per second for the unaltered data. However, decadal means vary from 7.3 W/m2 (1977–1986) to −0.3 W/m2 (1997–2006). The spatial distributions of climatological fluxes display all the expected features. A comparison of zonally averaged wind stress components across ocean sub-basins reveals large differences between available products due both to winds and to the stress calculation. Regional comparisons of the heat and freshwater fluxes reveal an alarming range among alternatives; typically 40 W/m2 and 10 mg/m2 per second, respectively. The implied ocean heat transports are within the uncertainty of estimates from ocean observations in both the Atlantic and Indo-Pacific basins. They show about 2.4 PW of tropical heating, of which 80% is transported to the north, mostly in the Atlantic. There is similar good agreement in freshwater transport at many latitudes in both basins, but neither in the South Atlantic, nor at 35°N.

Untangling aerosol effects on clouds and precipitation in a buffered system

Abstract: It is thought that changes in the concentration of cloud-active aerosol can alter the precipitation efficiency of clouds, thereby changing cloud amount and, hence, the radiative forcing of the climate system. Despite decades of research, it has proved frustratingly difficult to establish climatically meaningful relationships among the aerosol, clouds and precipitation. As a result, the climatic effect of the aerosol remains controversial. We propose that the difficulty in untangling relationships among the aerosol, clouds and precipitation reflects the inadequacy of existing tools and methodologies and a failure to account for processes that buffer cloud and precipitation responses to aerosol perturbations.

The physical basis for increases in precipitation extremes in simulations of 21st-century climate change

Abstract: Global warming is expected to lead to a large increase in atmospheric water vapor content and to changes in the hydrological cycle, which include an intensification of precipitation extremes. The intensity of precipitation extremes is widely held to increase proportionately to the increase in atmospheric water vapor content. Here, we show that this is not the case in 21st-century climate change scenarios simulated with climate models. In the tropics, precipitation extremes are not simulated reliably and do not change consistently among climate models; in the extratropics, they consistently increase more slowly than atmospheric water vapor content. We give a physical basis for how precipitation extremes change with climate and show that their changes depend on changes in the moist-adiabatic temperature lapse rate, in the upward velocity, and in the temperature when precipitation extremes occur. For the tropics, the theory suggests that improving the simulation of upward velocities in climate models is essential for improving predictions of precipitation extremes; for the extratropics, agreement with theory and the consistency among climate models increase confidence in the robustness of predictions of precipitation extremes under climate change.

Global pyrogeography: the current and future distribution of wildfire.

Abstract: Climate change is expected to alter the geographic distribution of wildfire, a complex abiotic process that responds to a variety of spatial and environmental gradients. How future climate change may alter global wildfire activity, however, is still largely unknown. As a first step to quantifying potential change in global wildfire, we present a multivariate quantification of environmental drivers for the observed, current distribution of vegetation fires using statistical models of the relationship between fire activity and resources to burn, climate conditions, human influence, and lightning flash rates at a coarse spatiotemporal resolution (100 km, over one decade). We then demonstrate how these statistical models can be used to project future changes in global fire patterns, highlighting regional hotspots of change in fire probabilities under future climate conditions as simulated by a global climate model. Based on current conditions, our results illustrate how the availability of resources to burn and climate conditions conducive to combustion jointly determine why some parts of the world are fire-prone and others are fire-free. In contrast to any expectation that global warming should necessarily result in more fire, we find that regional increases in fire probabilities may be counter-balanced by decreases at other locations, due to the interplay of temperature and precipitation variables. Despite this net balance, our models predict substantial invasion and retreat of fire across large portions of the globe. These changes could have important effects on terrestrial ecosystems since alteration in fire activity may occur quite rapidly, generating ever more complex environmental challenges for species dispersing and adjusting to new climate conditions. Our findings highlight the potential for widespread impacts of climate change on wildfire, suggesting severely altered fire regimes and the need for more explicit inclusion of fire in research on global vegetation-climate change dynamics and conservation planning.

Catchments as simple dynamical systems: catchment characterization, rainfall-runoff modeling, and doing hydrology backward.

Abstract: [1] Water fluxes in catchments are controlled by physical processes and material properties that are complex, heterogeneous, and poorly characterized by direct measurement. As a result, parsimonious theories of catchment hydrology remain elusive. Here I describe how one class of catchments (those in which discharge is determined by the volume of water in storage) can be characterized as simple first-order nonlinear dynamical systems, and I show that the form of their governing equations can be inferred directly from measurements of streamflow fluctuations. I illustrate this approach using data from the headwaters of the Severn and Wye rivers at Plynlimon in mid-Wales. This approach leads to quantitative estimates of catchment dynamic storage, recession time scales, and sensitivity to antecedent moisture, suggesting that it is useful for catchment characterization. It also yields a first-order nonlinear differential equation that can be used to directly simulate the streamflow hydrograph from precipitation and evapotranspiration time series. This single-equation rainfall-runoff model predicts streamflow at Plynlimon as accurately as other models that are much more highly parameterized. It can also be analytically inverted; thus, it can be used to “do hydrology backward,” that is, to infer time series of whole-catchment precipitation directly from fluctuations in streamflow. At Plynlimon, precipitation rates inferred from streamflow fluctuations agree with rain gauge measurements as closely as two rain gauges in each catchment agree with each other. These inferred precipitation rates are not calibrated to precipitation measurements in any way, making them a strong test of the underlying theory. The same approach can be used to estimate whole-catchment evapotranspiration rates during rainless periods. At Plynlimon, evapotranspiration rates inferred from streamflow fluctuations exhibit seasonal and diurnal cycles that agree semiquantitatively with Penman-Monteith estimates. Thus, streamflow hydrographs may be useful for reconstructing precipitation and evapotranspiration records where direct measurements are unavailable, unreliable, or unrepresentative at the scale of the landscape.

Evaluating the “Rich-Get-Richer” Mechanism in Tropical Precipitation Change under Global Warming

Abstract: Examining tropical regional precipitation anomalies under global warming in 10 coupled global climate models, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zones with large climatological precipitation and to decrease in subsidence regions—the rich-get-richer mechanism—has previously been examined in different approximations by Chou and Neelin, and Held and Soden. The effect of increased moisture transported by the mean circulation (the “direct moisture effect” or “thermodynamic component” in respective terminology) is relatively robust, while dynamic feedback is poorly understood and differs among models. The argument outlined states that the thermodynamic component should be a good approximation for large-scale averages; this is confirmed for averages across convection zones and descent regions, respectively. Within the convergence zones, however, dynamic feedback can substantially increase or decrease precipitation anomalies. Regions of negative...

A 44-Year Daily Gridded Precipitation Dataset for Asia Based on a Dense Network of Rain Gauges

Abstract: A daily gridded precipitation dataset for 1961-2004 was created by collecting rain gauge observation data across Asia through the activities of the Asian Precipitation—Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) project. Our number of valid stations was between 5000 and 12,000, representing 2.3 to 4.5 times the data available through the Global Telecommunication System network, which were used for most daily grid precipitation products. APHRODITE’s daily gridded precipitation (APHRO_V0902) is the only long-term (1961 onward) continental-scale daily product that contains a dense network of daily rain gauge data for Asia including the Himalayas and mountainous areas in the Middle East. The product contributes to studies such as the evaluation of Asian water resources, diagnosis of climate change, statistical downscaling, and verification of numerical model simulation and high-resolution precipitation estimates using satellites. We released APHRO_V0902 datasets for Monsoon Asia, Russia and the Middle East (on 0.5° × 0.5° and 0.25° × 0.25° grids) at http://www.chikyu.ac.jp/precip/. Herein, we show the algorithm and input data of APHRO_V0902.

The Andes climate and weather

Abstract: . This paper documents the main features of the weather, climate and climate variability over Andes cordillera in South America on the basis of instrumental observations. We first provide a basic physical understanding of the mean annual cycle of the atmospheric circulation and precipitation and over the Andes and adjacent lowlands. In particular, the diversity of precipitation, temperature and wind patterns is interpreted in terms of the long meridional extent of the Andes and the disruption of the large-scale circulation by this formidable topographic barrier. We also document the impact of the El Nino Southern Oscillation phenomenon on the temperature and precipitation regimes along the Andes.

Future change of temperature and precipitation extremes in South America as derived from the PRECIS regional climate modeling system

Abstract: Using the PRECIS regional climate modeling system this study analyses the distribution of extremes of temperature and precipitation in South America in the recent past (1961–1990) and in a future (2071–2100) climate under the IPCC SRES A2 and B2 emissions scenarios. The results show that for the present climate the model simulates well the spatial distribution of extreme temperature and rainfall events when compared with observations, with temperature the more realistic. The observations over the region are far from comprehensive which compromises the assessment of model quality. In all the future climate scenarios considered all parts of the region would experience significant and often different changes in rainfall and temperature extremes. In the future, the occurrence of warm nights is projected to be more frequent in the entire tropical South America while the occurrence of cold night events is likely to decrease. Significant changes in rainfall extremes and dry spells are also projected. These include increased intensity of extreme precipitation events over most of Southeastern South America and western Amazonia consistent with projected increasing trends in total rainfall in these regions. In Northeast Brazil and eastern Amazonia smaller or no changes are seen in projected rainfall intensity though significant changes are seen in the frequency of consecutive dry days. Copyright © 2009 Royal Meteorological Society

The Response of Northern Hemisphere Snow Cover to a Changing Climate

Abstract: A snowpack model sensitivity study, observed changes of snow cover in the NOAA satellite dataset, and snow cover simulations from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset are used to provide new insights into the climate response of Northern Hemisphere (NH) snow cover. Under conditions of warming and increasing precipitation that characterizes both observed and projected climate change over much of the NH land area with seasonal snow cover, the sensitivity analysis indicated snow cover duration (SCD) was the snow cover variable exhibiting the strongest climate sensitivity, with sensitivity varying with climate regime and elevation. The highest snow cover–climate sensitivity was found in maritime climates with extensive winter snowfall—for example, the coastal mountains of western North America (NA). Analysis of trends in snow cover duration during the 1966–2007 period of NOAA data showed the largest decreases were concentrated in a zone where seasonal mean air ...

Implications of global climate change for snowmelt hydrology in the twenty-first century.

Abstract: For most of the global land area poleward of about 40° latitude, snow plays an important role in the water cycle. The (seasonal) timing of runoff in these areas is especially sensitive to projected losses of snowpack associated with warming trends, whereas projected (annual) runoff volume changes are primarily associated with precipitation changes, and to a lesser extent, with changes in evapotranspiration (ET). Regional studies in the USA (and especially the western USA) suggest that hydrologic adjustments to a warming climate have been ongoing since the mid-twentieth century. We extend the insights extracted from the western USA to the global scale using a physically based hydrologic model to assess the effects of systematic changes in precipitation and temperature on snow-affected portions of the global land area as projected by a suite of global climate models. While annual (and in some cases seasonal) changes in precipitation are a key driver of projected changes in annual runoff, we find, as in the western USA, that projected warming produces strong decreases in winter snow accumulation and spring snowmelt over much of the affected area regardless of precipitation change. Decreased snowpack produces decreases in warm-season runoff in many mid- to high-latitude areas where precipitation changes are either moderately positive or negative in the future projections. Exceptions, however, occur in some high-latitude areas, particular in Eurasia, where changes in projected precipitation are large enough to result in increased, rather than decreased, snow accumulation. Overall, projected changes in snowpack and the timing of snowmelt-derived runoff are largest near the boundaries of the areas that currently experience substantial snowfall, and at least qualitatively, they mirror the character of observed changes in the western USA. Copyright © 2008 John Wiley & Sons, Ltd.

Assessing the impact of climate change on water resources in Iran

Abstract: [1] As water resources become further stressed due to increasing levels of societal demand, understanding the effect of climate change on various components of the water cycle is of strategic importance in management of this essential resource. In this study, we used a hydrologic model of Iran to study the impact of future climate on the country's water resources. The hydrologic model was created using the Soil and Water Assessment Tool (SWAT) model and calibrated for the period from 1980 to 2002 using daily river discharges and annual wheat yield data at a subbasin level. Future climate scenarios for periods of 2010–2040 and 2070–2100 were generated from the Canadian Global Coupled Model (CGCM 3.1) for scenarios A1B, B1, and A2, which were downscaled for 37 climate stations across the country. The hydrologic model was then applied to these periods to analyze the effect of future climate on precipitation, blue water, green water, and yield of wheat across the country. For future scenarios we found that in general, wet regions of the country will receive more rainfall while dry regions will receive less. Analysis of daily rainfall intensities indicated more frequent and larger-intensity floods in the wet regions and more prolonged droughts in the dry regions. When aggregated to provincial levels, the differences in the predictions due to the three future scenarios were smaller than the uncertainty in the hydrologic model. However, at the subbasin level the three climate scenarios produced quite different results in the dry regions of the country, although the results in the wet regions were more or less similar.

Moisture Vertical Structure, Column Water Vapor, and Tropical Deep Convection

Abstract: The vertical structure of the relationship between water vapor and precipitation is analyzed in 5 yr of radiosonde and precipitation gauge data from the Nauru Atmospheric Radiation Measurement (ARM) site. The first vertical principal component of specific humidity is very highly correlated with column water vapor (CWV) and has a maximum of both total and fractional variance captured in the lower free troposphere (around 800 hPa). Moisture profiles conditionally averaged on precipitation show a strong association between rainfall and moisture variability in the free troposphere and little boundary layer variability. A sharp pickup in precipitation occurs near a critical value of CWV, confirming satellite-based studies. A lag–lead analysis suggests it is unlikely that the increase in water vapor is just a result of the falling precipitation. To investigate mechanisms for the CWV–precipitation relationship, entraining plume buoyancy is examined in sonde data and simplified cases. For several differe...

Climate change projections for the tropical Andes using a regional climate model: Temperature and precipitation simulations for the end of the 21st century

Abstract: [1] High-elevation tropical mountain regions may be more strongly affected by future climate change than their surrounding lowlands. In the tropical Andes a significant increase in temperature and changes in precipitation patterns will likely affect size and distribution of glaciers and wetlands, ecosystem integrity, and water availability for human consumption, irrigation, and power production. However, detailed projections of future climate change in the tropical Andes are not yet available. Here we present first results for the end of the 21st century (2071‐2100) using a regional climate model (RCM) based on two different emission scenarios (A2 and B2). The model adequately simulates the spatiotemporal variability of precipitation and temperature but displays a cool and wet bias, in particular along the eastern Andean slope during the wet season, December‐ February. Projections of changes in the 21st century indicate significant warming in the tropical Andes, which is enhanced at higher elevations and further amplified in the middle and upper troposphere. Temperature changes are spatially similar in both scenarios, but the amplitude is significantly higher in RCM-A2. The RCM-A2 scenario also shows a significant increase in interannual temperature variability, while it remains almost unchanged in RCM-B2 when compared to a 20th century control run. Changes in precipitation are spatially much less coherent, with both regions of increased and decreased precipitation across the Andes. These results provide a first attempt at quantifying future climate change in the tropical Andes and could serve as input for impact models to simulate anticipated changes in Andean glaciation, hydrology, and ecosystem integrity.

Projected Changes in Mean and Extreme Precipitation in Africa under Global Warming. Part I: Southern Africa

Abstract: Probable changes in mean and extreme precipitation in East Africa are estimated from general circulation models (GCMs) prepared for the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4). Bayesian statistics are used to derive the relative weights assigned to each member in the multimodel ensemble. There is substantial evidence in support of a positive shift of the whole rainfall distribution in East Africa during the wet seasons. The models give indications for an increase in mean precipitation rates and intensity of high rainfall events but for less severe droughts. Upward precipitation trends are projected from early this (twenty first) century. As in the observations, a statistically significant link between sea surface temperature gradients in the tropical Indian Ocean and short rains (October–December) in East Africa is simulated in the GCMs. Furthermore, most models project a differential warming of the Indian Ocean during boreal autumn. This is favorable for an increa...

Inter-decadal variation of the summer precipitation in China and its association with decreasing Asian summer monsoon Part II: Possible causes

Abstract: The present article is the second part of a study on the inter-decadal variability of the summer precipitation in East China, which mainly addresses the possible cause of this change. Firstly, an updated analysis of the long-term variations of snow cover, snow days and snow depth in the preceding winter and spring over the Tibetan Plateau (TP) was done by using station and satellite data. The abrupt increase in the winter and spring snow over the TP since around 1977 has been well documented. At that time, the inter-decadal variation of the atmospheric heating over the TP in spring and summer had been estimated. It has been revealed that the atmospheric heating fields in subsequent spring and summer over the TP assumed a significant weakening after the late 1970s. This weakening is closely related to the significantly reduced surface sensible heat flux into the atmosphere and subsequent cooling over the TP and its surrounding atmosphere. The latter was produced by the increase of surface albedo and soil hydrological effect of melting snow under the condition of abrupt increase in the preceding winter and spring snow over the TP. On the other hand, three phases of significant inter-decadal warming of the sea surface temperature (SST) in the tropical central and eastern Pacific, which occurred in the mid-1960s, the late 1970s and the early 1990s, respectively, have been found. The above inter-decadal variability of heating fields over the land area in the Asian region and neighbouring oceanic region of the West Pacific has consistently reduced the land–sea thermal contrast in summer in the Asian monsoon region based on the estimate of atmospheric heating fields. This cause is likely to lead to weakening of the Asian summer monsoon. In such case, the northward moisture transport in East Asia is greatly weakened and cannot reach North China, thus causing the condition of less precipitation or droughts. In contrast, the Yangtze River basin and South China receive a large amount of moisture supply and have strong upward motion, creating favourable conditions for frequent occurrence of heavy rainfall. In the process of the southward shift of the high-precipitation zone, two abrupt or rapid regime shifts observed in the late 1970s and the early 1990s were possibly in response to the increase in the winter and spring snow over the TP, and two major rapid warming events of the SST in the tropical central and eastern Pacific in the late 1970s and the early 1990s. Correlative analysis has further confirmed that high TP snow and oceanic forcing factors have a positive correlation with the subsequent summer precipitation in the Yangtze River basin and most of South China, and a negative correlation with the summer precipitation in North China. This correlative relationship implies that if the TP has excessive (deficient) snow in the preceding winter and spring and the tropical central and eastern Pacific anomalously warms up (cools down), North China will have decreasing (increasing) summer precipitation, whereas the Yangtze River basin and South China will have increasing (decreasing) summer precipitation. Copyright © 2009 Royal Meteorological Society

Will Extratropical Storms Intensify in a Warmer Climate

Abstract: Extratropical cyclones and how they may change in a warmer climate have been investigated in detail with a high-resolution version of the ECHAM5 global climate model. A spectral resolution of T213 (63 km) is used for two 32-yr periods at the end of the twentieth and twenty-first centuries and integrated for the Intergovernmental Panel on Climate Change (IPCC) A1B scenario. Extremes of pressure, vorticity, wind, and precipitation associated with the cyclones are investigated and compared with a lower-resolution simulation. Comparison with observations of extreme wind speeds indicates that the model reproduces realistic values. This study also investigates the ability of the model to simulate extratropical cyclones by computing composites of intense storms and contrasting them with the same composites from the 40-yr ECMWF Re-Analysis (ERA-40). Composites of the time evolution of intense cyclones are reproduced with great fidelity; in particular the evolution of central surface pressure is almost ex...

The Soil Moisture–Precipitation Feedback in Simulations with Explicit and Parameterized Convection

Abstract: Moist convection is a key aspect of the extratropical summer climate and strongly affects the delicate balance of processes that determines the surface climate in response to larger-scale forcings. Previous studies using parameterized convection have found that the feedback between soil moisture and precipitation is predominantly positive (more precipitation over wet soils) over Europe. Here this feedback is investigated for one full month (July 2006) over the Alpine region using two different model configurations. The first one employs regional climate simulations performed with the Consortium for Small-Scale Modeling Model in Climate Mode (CCLM) on a grid spacing of 25 km. The second one uses the same model but integrated on a cloud-resolving grid of 2.2 km, allowing an explicit treatment of convection. Each configuration comprises one control and two sensitivity experiments. The latter start from perturbed soil moisture initial conditions. Comparison of the simulated soil moisture–precipitatio...

Contingent productivity responses to more extreme rainfall regimes across a grassland biome.

Abstract: Climate models predict, and empirical evidence confirms, that more extreme precipitation regimes are occurring in tandem with warmer atmospheric temperatures. These more extreme rainfall patterns are characterized by increased event size separated by longer within season drought periods and represent novel climatic conditions whose consequences for different ecosystem types are largely unknown. Here, we present results from an experiment in which more extreme rainfall patterns were imposed in three native grassland sites in the Central Plains Region of North America, USA. Along this 600km precipitation‐productivity gradient, there was strong sensitivity of temperate grasslands to more extreme growing season rainfall regimes, with responses of aboveground net primary productivity (ANPP) contingent on mean soil water levels for different grassland types. At the mesic end of the gradient (tallgrass prairie), longer dry intervals between events led to extended periods of below-average soil water content, increased plant water stress and reduced ANPP by 18%. The opposite response occurred at the dry end (semiarid steppe), where a shift to fewer, but larger, events increased periods of above-average soil water content, reduced seasonal plant water stress and resulted in a 30% increase in ANPP. At an intermediate mixed grass prairie site with high plant species richness, ANPP was most sensitive to more extreme rainfall regimes (70% increase). These results highlight the inherent complexity in predicting how terrestrial ecosystems will respond to forecast novel climate conditions as well as the difficulties in extending inferences from single site experiments across biomes. Even with no change in annual precipitation amount, ANPP responses in a relatively uniform physiographic region differed in both magnitude and direction in response to within season changes in rainfall event size/frequency.

Rainfall retrieval over the ocean with spaceborne W‐band radar

Abstract: [1] A method for retrieving precipitation over the ocean using spaceborne W-band (94 GHz) radar is introduced and applied to the CloudSat Cloud Profiling Radar. The method is most applicable to stratiform-type precipitation. Measurements of radar backscatter from the ocean surface are combined with information about surface wind speed and sea surface temperature to derive the path-integrated attenuation through precipitating cloud systems. The scattering and extinction characteristics of raindrops are modeled using a combination of Mie theory (for raindrops) and the discrete dipole approximation (for ice crystals and melting snow), and a model of the melting layer is implemented to represent the transition between ice and liquid water. Backward Monte Carlo modeling is used to model multiple scattering from precipitating hydrometeors between the radar and ocean surface, which is shown to be significant for precipitation rates exceeding 3–5 mm h−1, particularly when precipitating ice is present. An uncertainty analysis is presented and the algorithm is applied to near-global CloudSat observations and compared with other near-global precipitation sources. In the tropics, CloudSat tends to underestimate the heaviest precipitation. It is found that in the middle latitudes, however, CloudSat observes precipitation more often and with greater resulting accumulation than other spaceborne sensors.

Increasing Australian–Indonesian monsoon rainfall linked to early Holocene sea-level rise

Abstract: The Australian–Indonesian summer monsoon affects rainfall variability across the Indo–Pacific region. Reconstructions of monsoon strength from stalagmites show that precipitation increased from 11,000 to 7,000 years ago, as rising global sea level caused the flooding of the Indonesian continental shelf. The Australian–Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo–Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation1,2,3, but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak4. Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise5,6,7. We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone8. However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago.

Precipitation, Recycling, and Land Memory: An Integrated Analysis

Abstract: A synthesis of several approaches to quantifying land–atmosphere interactions is presented. These approaches use data from observations or atmospheric reanalyses applied to atmospheric tracer models and stand-alone land surface schemes. None of these approaches relies on the results of general circulation model simulations. A high degree of correlation is found among these independent approaches, and constructed here is a composite assessment of global land–atmosphere feedback strength as a function of season. The composite combines the characteristics of persistence of soil moisture anomalies, strong soil moisture regulation of evaporation rates, and reinforcement of water cycle anomalies through recycling. The regions and seasons that have a strong composite signal predominate in both summer and winter monsoon regions in the period after the rainy season wanes. However, there are exceptions to this pattern, most notably over the Great Plains of North America and the Pampas/Pantanal of South Ame...

Orbitally driven east-west antiphasing of South American precipitation

Abstract: The variations of tropical precipitation are antiphased between the hemispheres on orbital timescales. A comparison between a speleothem record of precipitation in northeast Brazil and rainfall reconstructions from the rest of tropical South America shows that a similar antiphasing operated in the same hemisphere during the Holocene. The variations of tropical precipitation are antiphased between the hemispheres on orbital timescales. This antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres, which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the Hadley cells1,2,3,4. Here we compare an oxygen isotopic record recovered from a speleothem from northeast Brazil for the past 26,000 years with existing reconstructions of precipitation in tropical South America5,6,7,8. During the Holocene, we identify a similar, but zonally oriented, antiphasing of precipitation within the same hemisphere: northeast Brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high, whereas the rest of southern tropical South America shows opposite characteristics. Simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation-driven monsoon activity. Our results suggest that convective heating over tropical South America and associated adjustments in large-scale subsidence over northeast Brazil lead to a remote forcing of the South American monsoon, which determines most of the precipitation changes in the region on orbital timescales.

21st century climate change in the Middle East

Abstract: This study examined the performance and future predictions for the Middle East produced by 18 global climate models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. Under the Special Report on Emission Scenarios A2 emissions scenario the models predict an overall temperature increase of ~1.4 K by mid-century, increasing to almost 4 K by late-century for the Middle East. In terms of precipitation the southernmost portion of the domain experiences a small increase in precipitation due to the Northward movement of the Inter-Tropical Convergence Zone. The largest change however is a decrease in precipitation that occurs in an area covering the Eastern Mediterranean, Turkey, Syria, Northern Iraq, Northeastern Iran and the Caucasus caused by a decrease in storm track activity over the Eastern Mediterranean. Other changes likely to impact the region include a decrease of over 170,000 km2 in viable rainfed agriculture land by late-century, increases in the length of the dry season that reduces the length of time that the rangelands can be grazed, and changes in the timing of the maximum precipitation in Northern Iran that will impact the growing season, forcing changes in cropping strategy or even crop types.

Climate, Niche Evolution, and Diversification of the “Bird‐Cage” Evening Primroses (Oenothera, Sections Anogra and Kleinia)

Abstract: We integrate climatic niche models and dated phylogenies to characterize the evolution of climatic niches in Oenothera sections Anogra and Kleinia (Onagraceae), and from that we make inferences on diversification in relation to climate. The evolution of climatic tolerances in Anogra + Kleinia has been heterogeneous, across phylogenetic groups and across different dimensions of climate. All the extant taxa occur in semiarid to arid conditions (annual precipitation of 10.1–49.1 cm and high temperatures in the warmest month of 28.5°–40.1°C), but there is striking variation among taxa in their climatic tolerances, especially temperature (minimum temperatures in the coldest month of −14.0° to 5.3°C) and summer versus winter precipitation (precipitation in the warmest quarter of 0.6–19.4 cm). Climatic disparity is especially pronounced in two subclades (californica, deltoides) that radiated in the southwestern United States and California, apparently including both divergent and convergent evolution o...

Heavy pollution suppresses light rain in China: Observations and modeling

Abstract: Long-term observational data reveal that both the frequency and amount of light rain have decreased in eastern China (EC) for 1956-2005 with high spatial coherency. This is different from the trend of total rainfall observed in EC, which decreases in northern EC and increases in southern EC. To examine the cause of the light rain trends, we analyzed the long-term variability of atmospheric water vapor and its correlation with light rain events. Results show very weak relationships between large-scale moisture transport and light rain in EC. This suggests that light rain trend in EC is not driven by large-scale circulation changes. Because of human activities, pollutant emission has increased dramatically in China for the last few decades, leading to significant reductions in visibility between 1960 and 2000. Cloud-resolving model simulations show that aerosols corresponding to heavily polluted conditions can significantly increase the cloud droplet number concentration (CDNC) and reduce droplet sizes compared to pristine conditions. This can lead to a significant decline in raindrop concentration and delay raindrop formation because smaller cloud droplets are less efficient in the collision and coalescence processes. Together with weaker convection, the precipitation frequency and amount are significantly reduced in the polluted case. Satellite datamore » also reveal higher CDNC and smaller droplet size over polluted land in EC relative to pristine regions, which is consistent with the model results. This evidence suggests that the significantly increased aerosol particles produced by air pollution are at least partly responsible for the decreased light rain events observed in China over the past fifty years.« less

Notes on state-of-the-art investigations of aerosol effects on precipitation: a critical review

Abstract: There is no agreement between the results of different studies as regards quantitative and even qualitative evaluation of aerosol effects on precipitation. While some observational and numerical studies report a decrease in precipitation in polluted areas, in some other observations and numerical studies aerosol-induced precipitation enhancement was reported. This study analyses possible reasons for the discrepancy between the results. The analysis of aerosol effects on precipitation is performed using the mass and heat budgets. The analysis is concentrated on clouds and cloud systems arising in the environment with relatively high freezing level. It is shown that for such clouds aerosols increase both the generation and the loss of the condensate mass. The net effect of aerosols on the precipitation depends on the environment conditions (air humidity, buoyancy, and wind shear) as well as on the cloud type determining whether the increase in the condensate generation or in the condensate loss will dominate with increase in the aerosol concentration. In the case when the loss increases more than the generation, a decrease in precipitation will take place. If the increase in the condensate generation dominates, an increase in precipitation will take place. A classification scheme of aerosol effects on precipitation is proposed and its relation to the observational and numerical results available is analysed. Possible reasons for the uncertainties and discrepancies of the numerical results, as well as between measurements, are analysed. A discussion of unsolved problems is presented in the conclusion.