Showing papers on "Precipitation published in 2010"

Tropical cyclones and climate change

Abstract: Whether the characteristics of tropical cyclones have altered, or will alter, in a changing climate has been subject of considerable debate. An overview of recent research indicates that greenhouse warming will cause stronger storms, on average, but a decrease in the frequency of tropical cyclones. Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Abstract: Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than -36 °C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼103 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m-2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.

Toward a complete Himalayan hydrological budget: Spatiotemporal distribution of snowmelt and rainfall and their impact on river discharge

Abstract: [1] The hydrological budget of Himalayan rivers is dominated by monsoonal rainfall and snowmelt, but their relative impact is not well established because this remote region lacks a dense gauge network. Here, we use a combination of validated remotely-sensed climate parameters to characterize the spatiotemporal distribution of rainfall, snowfall, and evapotranspiration in order to quantify their relative contribution to mean river discharge. Rainfall amounts are calculated from calibrated, orbital, high-resolution Tropical Rainfall Measurement Mission data, and snow-water equivalents are computed from a snowmelt model based on satellite-derived snow cover, surface temperature, and solar radiation. Our data allow us to identify three key aspects of the spatiotemporal precipitation pattern. First, we observe a strong decoupling between the rainfall on the Himalayan foreland versus that in the mountains: a pronounced sixfold, east-west rainfall gradient in the Ganges plains exists only at elevations <500 m asl. Mountainous regions (500 to 5000 m asl) receive nearly equal rainfall amounts along strike. Second, whereas the Indian summer monsoon is responsible for more than 80% of annual rainfall in the central Himalaya and Tibetan Plateau, the eastern and western syntaxes receive only ∼50% of their annual rainfall during the summer season. Third, snowmelt contributions to discharge differ widely along the range. As a fraction of the total annual discharge, snowmelt constitutes up to 50% in the far western (Indus area) catchments, ∼25% in far eastern (Tsangpo) catchments, and <20% elsewhere. Despite these along-strike variations, snowmelt in the pre- and early-monsoon season (April to June) is significant and important in all catchments, although most pronounced in the western catchments. Thus, changes in the timing or amount of snowmelt due to increasing temperatures or decreasing winter precipitation may have far-reaching societal consequences. These new data on precipitation and runoff set the stage for far more detailed investigations than have previously been possible of climate-erosion interactions in the Himalaya.

Development of an Effective Double-Moment Cloud Microphysics Scheme with Prognostic Cloud Condensation Nuclei (CCN) for Weather and Climate Models

Abstract: A new double-moment bulk cloud microphysics scheme, the Weather Research and Forecasting (WRF) Double-Moment 6-class (WDM6) Microphysics scheme, which is based on the WRF Single-Moment 6-class (WSM6) Microphysics scheme, has been developed. In addition to the prediction for the mixing ratios of six water species (water vapor, cloud droplets, cloud ice, snow, rain, and graupel) in the WSM6 scheme, the number concentrations for cloud and rainwater are also predicted in the WDM6 scheme, together with a prognostic variable of cloud condensation nuclei (CCN) number concentration. The new scheme was evaluated on an idealized 2D thunderstorm test bed. Compared to the simulations from the WSM6 scheme, there are greater differences in the droplet concentration between the convective core and stratiform region in WDM6. The reduction of light precipitation and the increase of moderate precipitation accompanying a marked radar bright band near the freezing level from the WDM6 simulation tend to alleviate existing systematic biases in the case of the WSM6 scheme. The strength of this new microphysics scheme is its ability toallowflexibilityinvariableraindropsizedistributionbypredictingthenumberconcentrationsofcloudsand rain, coupled with the explicit CCN distribution, at a reasonable computational cost.

Dominant control of the South Asian monsoon by orographic insulation versus plateau heating

Abstract: Heat emitted from the Tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the South Asian summer monsoon, but new work suggests that in fact it is the neighbouring mountains that are the major influence. William Boos and Zhiming Kuang use an atmospheric model to show that flattening the Tibetan plateau has little effect on the monsoon, so long as the Himalayas and surrounding mountain ranges remain. The plateau does boost rainfall locally along its southern edge, but it is the build-up of hot, moist air over India, insulated from colder, drier air by the Himalayas, that drives large-scale monsoon circulation. The elevation of the Tibetan plateau is thought to cause its surface to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales. Here, however, an atmospheric model is used to show that flattening of the Tibetan plateau has little effect on the monsoon, provided that the narrow orography of the Himalayas and adjacent mountain ranges is preserved. The Tibetan plateau, like any landmass, emits energy into the atmosphere in the form of dry heat and water vapour, but its mean surface elevation is more than 5 km above sea level. This elevation is widely held to cause the plateau to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales1,2,3,4,5. Observations of the present climate, however, do not clearly establish the Tibetan plateau as the dominant thermal forcing in the region: peak upper-tropospheric temperatures during boreal summer are located over continental India, south of the plateau. Here we show that, although Tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model, the large-scale South Asian summer monsoon circulation is otherwise unaffected by removal of the plateau, provided that the narrow orography of the Himalayas and adjacent mountain ranges is preserved. Additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm, moist air over continental India from the cold and dry extratropics. These results call for both a reinterpretation of how South Asian climate may have responded to orographic uplift, and a re-evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades.

Ecohydrologic separation of water between trees and streams in a Mediterranean climate

Abstract: Water movement in upland humid watersheds from the soil surface to the stream is often described using the concept of translatory flow, which assumes that water at any soil depth is well mixed. A study of water isotopes in an Oregon watershed instead suggests that trees and streams tap into separate water reservoirs.

Dreary state of precipitation in global models

Abstract: [1] New, definitive measures of precipitation frequency provided by CloudSat are used to assess the realism of global model precipitation. The character of liquid precipitation (defined as a combination of accumulation, frequency, and intensity) over the global oceans is significantly different from the character of liquid precipitation produced by global weather and climate models. Five different models are used in this comparison representing state-of-the-art weather prediction models, state-of-the-art climate models, and the emerging high-resolution global cloud “resolving” models. The differences between observed and modeled precipitation are larger than can be explained by observational retrieval errors or by the inherent sampling differences between observations and models. We show that the time integrated accumulations of precipitation produced by models closely match observations when globally composited. However, these models produce precipitation approximately twice as often as that observed and make rainfall far too lightly. This finding reinforces similar findings from other studies based on surface accumulated rainfall measurements. The implications of this dreary state of model depiction of the real world are discussed.

Origin and fate of atmospheric moisture over continents

Abstract: There has been a long debate on the extent to which precipitation relies on terrestrial evaporation (moisture recycling). In the past, most research focused on moisture recycling within a certain region only. This study makes use of new definitions of moisture recycling to study the complete process of continental moisture feedback. Global maps are presented identifying regions that rely heavily on recycled moisture as well as those that are supplying the moisture. An accounting procedure based on ERA‐Interim reanalysis data is used to calculate moisture recycling ratios. It is computed that, on average, 40% of the terrestrial precipitation originates from land evaporation and that 57% of all terrestrial evaporation returns as precipitation over land. Moisture evaporating from the Eurasian continent is responsible for 80% of China’s water resources. In South America, the Rio de la Plata basin depends on evaporation from the Amazon forest for 70% of its water resources. The main source of rainfall in the Congo basin is moisture evaporated over East Africa, particularly the Great Lakes region. The Congo basin in its turn is a major source of moisture for rainfall in the Sahel. Furthermore, it is demonstrated that due to the local orography, local moisture recycling is a key process near the Andes and the Tibetan Plateau. Overall, this paper demonstrates the important role of global wind patterns, topography and land cover in continental moisture recycling patterns and the distribution of global water resources.

Rainforest Aerosols as Biogenic Nuclei of Clouds and Precipitation in the Amazon

Abstract: The Amazon is one of the few continental regions where atmospheric aerosol particles and their effects on climate are not dominated by anthropogenic sources. During the wet season, the ambient conditions approach those of the pristine pre-industrial era. We show that the fine submicrometer particles accounting for most cloud condensation nuclei are predominantly composed of secondary organic material formed by oxidation of gaseous biogenic precursors. Supermicrometer particles, which are relevant as ice nuclei, consist mostly of primary biological material directly released from rainforest biota. The Amazon Basin appears to be a biogeochemical reactor, in which the biosphere and atmospheric photochemistry produce nuclei for clouds and precipitation sustaining the hydrological cycle. The prevailing regime of aerosol-cloud interactions in this natural environment is distinctly different from polluted regions.

Low‐frequency variations in surface atmospheric humidity, temperature, and precipitation: Inferences from reanalyses and monthly gridded observational data sets

Abstract: [1] Evidence is presented of a reduction in relative humidity over low-latitude and midlatitude land areas over a period of about 10 years leading up to 2008, based on monthly anomalies in surface air temperature and humidity from comprehensive European Centre for Medium-Range Weather Forecasts reanalyses (ERA-40 and ERA-Interim) and from Climatic Research Unit and Hadley Centre analyses of monthly station temperature data (CRUTEM3) and synoptic humidity observations (HadCRUH). The data sets agree well for both temperature and humidity variations for periods and places of overlap, although the average warming over land is larger for the fully sampled ERA data than for the spatially and temporally incomplete CRUTEM3 data. Near-surface specific humidity varies similarly over land and sea, suggesting that the recent reduction in relative humidity over land may be due to limited moisture supply from the oceans, where evaporation has been limited by sea surface temperatures that have not risen in concert with temperatures over land. Continental precipitation from the reanalyses is compared with a new gauge-based Global Precipitation Climatology Centre (GPCC) data set, with the combined gauge and satellite products of the Global Precipitation Climatology Project (GPCP) and the Climate Prediction Center (CPC), Merged Analysis of Precipitation (CMAP), and with CPC's independent gauge analysis of precipitation over land (PREC/L). The reanalyses agree best with the new GPCC and latest GPCP data sets, with ERA-Interim significantly better than ERA-40 at capturing monthly variability. Shifts over time in the differences among the precipitation data sets make it difficult to assess their longer-term variations and any link with longer-term variations in humidity.

Implications of 21st century climate change for the hydrology of Washington State

Abstract: Pacific Northwest (PNW) hydrology is particularly sensitive to changes in climate because snowmelt dominates seasonal runoff, and temperature changes impact the rain/snow balance. Based on results from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4), we updated previous studies of implications of climate change on PNW hydrology. PNW 21st century hydrology was simulated using 20 Global Climate Models (GCMs) and 2 greenhouse gas emissions scenarios over Washington and the greater Columbia River watershed, with additional focus on the Yakima River watershed and the Puget Sound which are particularly sensitive to climate change. We evaluated projected changes in snow water equivalent (SWE), soil moisture, runoff, and streamflow for A1B and B1 emissions scenarios for the 2020s, 2040s, and 2080s. April 1 SWE is projected to decrease by approximately 38–46% by the 2040s (compared with the mean over water years 1917–2006), based on composite scenarios of B1 and A1B, respectively, which represent average effects of all climate models. In three relatively warm transient watersheds west of the Cascade crest, April 1 SWE is projected to almost completely disappear by the 2080s. By the 2080s, seasonal streamflow timing will shift significantly in both snowmelt dominant and rain–snow mixed watersheds. Annual runoff across the State is projected to increase by 2–3% by the 2040s; these changes are mainly driven by projected increases in winter precipitation.

Contribution of Semi-Arid Forests to the Climate System

Abstract: Semi-arid forests cover close to 18% of Earth's land surface. If climate change were to stimulate carbon accumulation in these areas, resulting changes in the forests could both promote climate cooling and warming: On one hand, forest growth would draw CO2 from the atmosphere, providing a cooling effect on climate; on the other, as forests grew and became more dense, their albedo would decrease, which would warm climate. Rotenberg and Yakir (p. [451][1]; see the Perspective by Schimel ) now report that a shift in peak photosynthetic activities from summer to early spring would, indeed, cause carbon accumulation by the forests, but that a suppression of reflected longwave radiation effect would complement the better-known (shortwave) albedo effect, doubling the amount of potential warming. Several decades of carbon accumulation would thus be necessary to counteract these radiative changes. [1]: /lookup/doi/10.1126/science.1179998

Climate change and the precipitation variations in the northwestern Himalaya: 1866–2006

Abstract: Using available instrumental records, this paper examines the variation of precipitation from 1866 to 2006 in the northwestern Himalaya (NWH). The study indicates no trend in the winter precipitation but significant decreasing trend in the monsoon precipitation during the study period. Periodicities on a multi-decadal scale (29–34 years and 58–64 years) obtained in power spectrum analyses point towards epochal behaviour in the precipitation series. Analyses of the temperature data show significant increasing trends in annual temperature in all three stations in the NWH during the data period. Warming effect is particularly noteworthy during the winter season. Negative relationships between mean winter air temperature and snowfall amounts recorded at different meteorological stations in this period reveal strong effect of rising temperatures on the decreasing snowfall component in total winter precipitation, reducing effective duration of winter on the windward side of the Pir Panjal Himalayan Range. The study also shows influence of global teleconnections [North-Atlantic Oscillation (NAO) during winter months and Southern Oscillation Index (SOI) during the monsoon months] on precipitation fluctuations in the NWH. The teleconnections that appear to exist between the precipitation and the temperature until the late 1960s seem to have weakened considerably in the last three decades. This may be ascribed to the diminishing effect of the natural factors such as quasi-biennial oscillations (QBO), El Nino Southern Oscillations (ENSO), double sunspot cycles (Hale), etc., in this period. Role of increasing concentration of greenhouse gases in the atmosphere cannot be ruled out. Copyright © 2009 Royal Meteorological Society

Performance of high‐resolution satellite precipitation products over China

Abstract: [1] A gauge-based analysis of hourly precipitation is constructed on a 0.25° latitude/longitude grid over China for a 3 year period from 2005 to 2007 by interpolating gauge reports from ∼2000 stations collected and quality controlled by the National Meteorological Information Center of the China Meteorological Administration. Gauge-based precipitation analysis is applied to examine the performance of six high-resolution satellite precipitation estimates, including Joyce et al.'s (2004) Climate Prediction Center Morphing Technique (CMORPH) and the arithmetic mean of the microwave estimates used in CMORPH; Huffman et al.'s (2007) Tropical Rainfall Measuring Mission (TRMM) precipitation product 3B42 and its real-time version 3B42RT; Turk et al.'s (2004) Naval Research Laboratory blended product; and Hsu et al.'s (1997) Precipitation Estimation From Remotely Sensed Information Using Artificial Neural Network (PERSIANN). Our results showed the following: (1) all six satellite products are capable of capturing the overall spatial distribution and temporal variations of precipitation reasonably well; (2) performance of the satellite products varies for different regions and different precipitation regimes, with better comparison statistics observed over wet regions and for warm seasons; (3) products based solely on satellite observations present regionally and seasonally varying biases, while the gauge-adjustment procedures applied in TRMM 3B42 remove the large-scale bias almost completely; (4) CMORPH exhibits the best performance in depicting the spatial pattern and temporal variations of precipitation; and (5) both the relative magnitude and the phase of the warm season precipitation over China are estimated quite well, but the early morning peak associated with the Mei-Yu rainfall over central eastern China is substantially under-estimated by all satellite products.

Observed relationships between extreme sub‐daily precipitation, surface temperature, and relative humidity

Abstract: [1] Expected changes to future extreme precipitation remain a key uncertainty associated with anthropogenic climate change. Recently, extreme precipitation has been proposed to scale with the precipitable water content in the atmosphere, which assuming relative humidity stays constant, will increase at a rate of ~6.8%/°C as indicated by the Clausius-Clapeyron (C-C) relationship. We examine this scaling empirically using data from 137 long-record pluviograph and temperature gauges across Australia. We find that scaling rates are consistent with the C-C relationship for surface temperatures up to between 20°C and 26°C and for precipitation durations up to 30 minutes, implying that such scaling applies only for individual storm systems. At greater temperatures negative scaling is observed. Consideration of relative humidity data shows a pronounced decrease in the maximum relative humidity for land surface temperatures greater than 26°C, indicating that moisture availability becomes the dominant driver of how extreme precipitation scales at higher temperatures.

Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records

Abstract: We present simulations of water-stable isotopes from the LMDZ general circulation model (the LMDZ-iso GCM) and evaluate them at different time scales (synoptic to interannual). LMDZ-iso reproduces reasonably well the spatial and seasonal variations of both delta O-18 and deuterium excess. When nudged with reanalyses, LMDZ-iso is able to capture the synoptic variability of isotopes in winter at a midlatitude station, and the interannual variability in mid and high latitudes is strongly improved. The degree of equilibration between the vapor and the precipitation is strongly sensitive to kinetic effects during rain reevaporation, calling for more synchronous vapor and precipitation measurements. We then evaluate the simulations of two past climates: Last Glacial Maximum (21 ka) and Mid-Holocene (6 ka). A particularity of LMDZ-iso compared to other isotopic GCMs is that it simulates a lower d excess during the LGM over most high-latitude regions, consistent with observations. Finally, we use LMDZ-iso to explore the relationship between precipitation and delta O-18 in the tropics, and we discuss its paleoclimatic implications. We show that the imprint of uniform temperature changes on tropical delta O-18 is weak. Large regional changes in delta O-18 can, however, be associated with dynamical changes of precipitation. Using LMDZ as a test bed for reconstructing past precipitation changes through local delta O-18 records, we show that past tropical precipitation changes can be well reconstructed qualitatively but not quantitatively. Over continents, nonlocal effects make the local reconstruction even less accurate.

Variable winter moisture in the southwestern United States linked to rapid glacial climate shifts

Abstract: The last glacial period was characterized by large, rapid climate fluctuations. An analysis of a speleothem from New Mexico shows that the coldest conditions over Greenland coincide with increased winter precipitation in the southwestern United States, which can be attributed to a southward displacement of the polar jet stream and the North American storm track. During the last glacial period, the climate of the Northern Hemisphere was characterized by rapid, large-amplitude temperature fluctuations through cycles lasting a few thousand years1,2,3. These fluctuations are apparent in Greenland temperature reconstructions2,3, and corresponding temperature and hydrological variations have been documented throughout the Northern Hemisphere4,5. Here we present a record of precipitation in the southwestern United States from 56,000 to 11,000 yr ago, on the basis of δ18O measurements of speleothem calcite from New Mexico. Our record shows that increased winter precipitation in the southwestern United States is associated with Northern Hemisphere cooling, which we attribute to a southward shift in the polar jet stream, which modulated the position of the winter storm track over North America. On the western side of the Pacific Ocean basin, decreases in summer monsoon precipitation are associated with Northern Hemisphere cooling, due to southward displacement of the intertropical convergence zone4. We conclude that cooling and warming excursions in the Northern Hemisphere lead to concurrent latitudinal displacement of both the intertropical convergence zone and the polar jet stream over the Pacific Ocean. Our data are consistent with modern evidence for a northward shift of the polar jet stream in response to global warming6,7,8, which could lead to increasingly arid conditions in southwestern North America in the future.

Effects of irrigation on global climate during the 20th century

Abstract: Various studies have documented the effects of modern ]day irrigation on regional and global climate, but none, to date, have considered the time ]varying impact of steadily increasing irrigation rates on climate during the 20th century. We investigate the impacts of observed irrigation changes over this century with two ensemble simulations using an atmosphere general circulation model. Both ensembles are forced with transient climate forcings and observed sea surface temperatures from 1902 to 2000; one ensemble includes irrigation specified by a time ]varying data set of irrigation water withdrawals. Early in the century, irrigation is primarily localized over southern and eastern Asia, leading to significant cooling in boreal summer (June.August) over these regions. This cooling spreads and intensifies by century fs end, following the rapid expansion of irrigation over North America, Europe, and Asia. Irrigation also leads to boreal winter (December.February) warming over parts of North America and Asia in the latter part of the century, due to enhanced downward longwave fluxes from increased near ]surface humidity. Precipitation increases occur primarily downwind of the major irrigation areas, although precipitation in parts of India decreases due to a weaker summer monsoon. Irrigation begins to significantly reduce temperatures and temperature trends during boreal summer over the Northern Hemisphere midlatitudes and tropics beginning around 1950; significant increases in precipitation occur in these same latitude bands. These trends reveal the varying importance of irrigation ]climate interactions and suggest that future climate studies should account for irrigation, especially in regions with unsustainable irrigation resources.

Black carbon aerosols and the third polar ice cap

Abstract: . Recent thinning of glaciers over the Himalayas (sometimes referred to as the third polar region) have raised concern on future water supplies since these glaciers supply water to large river systems that support millions of people inhabiting the surrounding areas. Black carbon (BC) aerosols, released from incomplete combustion, have been increasingly implicated as causing large changes in the hydrology and radiative forcing over Asia and its deposition on snow is thought to increase snow melt. In India BC emissions from biofuel combustion is highly prevalent and compared to other regions, BC aerosol amounts are high. Here, we quantify the impact of BC aerosols on snow cover and precipitation from 1990 to 2010 over the Indian subcontinental region using two different BC emission inventories. New estimates indicate that Indian BC emissions from coal and biofuel are large and transport is expected to expand rapidly in coming years. We show that over the Himalayas, from 1990 to 2000, simulated snow/ice cover decreases by ~0.9% due to aerosols. The contribution of the enhanced Indian BC to this decline is ~36%, similar to that simulated for 2000 to 2010. Spatial patterns of modeled changes in snow cover and precipitation are similar to observations (from 1990 to 2000), and are mainly obtained with the newer BC estimates.

Extreme snowfall events linked to atmospheric rivers and surface air temperature via satellite measurements

Abstract: [1] Narrow bands of strong atmospheric water vapor transport, referred to as “atmospheric rivers” (ARs), are responsible for the majority of wintertime extreme precipitation events with important contributions to the seasonal water balance We investigate relationships between snow water equivalent (SWE), precipitation, and surface air temperature (SAT) across the Sierra Nevada for 45 wintertime AR events Analysis of assimilated and in situ data for water years 2004–2010 indicates that ARs on average generate ∼4 times daily SWE accumulation of non-AR storms In addition, AR events contributed ∼30–40% of total seasonal SWE accumulation in most years, with the contribution dominated by just 1–2 extreme events in some cases In situ and remotely sensed observations show that SWE changes associated with ARs are closely related to SAT These results reveal the previously unexplored significance of ARs with regard to the snowpack and associated sensitivities of AR precipitation to SAT

Urbanization signature in the observed heavy rainfall climatology over India.

Abstract: We assess the urbanization impacts on the heavy rainfall climatology during the Indian summer monsoon. While a number of studies have identified the impact of urbanization on local precipitation, a large-scale assessment has been lacking. This relation between urbanization and Indian monsoon rainfall changes is investigated by analyzing in situ and satellite-based precipitation and population datasets. Using a long-term daily rainfall dataset and high-resolution gridded analysis of human population, this study showed a significantly increasing trend in the frequency of heavy rainfall climatology over urban regions of India during the monsoon season. Urban regions experience less occurrences of light rainfall and significantly higher occurrences of intense precipitation compared to nonurban regions. Very heavy and extreme rainfall events showed increased trends over both urban and rural areas, but the trends over urban areas were larger and statistically more significant. Our analysis suggests that there is adequate statistical basis to conclude that the observed increasing trend in the frequency of heavy rainfall events over Indian monsoon region is more likely to be over regions where the pace of urbanization is faster. Moreover, rainfall measurements from satellites also indicate that urban areas are more (less) likely to experience heavier (lighter) precipitation rates compared to those in nonurban areas. While the mechanisms causing this enhancement in rainfall remain to be studied, the results provide the evidence that the increase in the heavy rainfall climatology over the Indian monsoon region is a signature of urban-induced rainfall anomaly. Copyright © 2009 Royal Meteorological Society

Evaluation of High-Resolution Satellite Precipitation Products over Very Complex Terrain in Ethiopia

Abstract: This study focuses on the evaluation of 3-hourly, 0.25° × 0.25°, satellite-based precipitation products: the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) 3B42RT, the NOAA/Climate Prediction Center morphing technique (CMORPH), and Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN). CMORPH is primarily microwave based, 3B42RT is primarily microwave based when microwave data are available and infrared based when microwave data are not available, and PERSIANN is primarily infrared based. The results show that 1) 3B42RT and CMORPH give similar rainfall fields (in terms of bias, spatial structure, elevation-dependent trend, and distribution function), which are different from PERSIANN rainfall fields; 2) PERSIANN does not show the elevation-dependent trend observed in rain gauge values, 3B42RT, and CMORPH; and 3) PERSIANN considerably underestimates rainfall in high-elevation areas.

Atmospheric Brown Clouds in the Himalayas: first two years of continuous observations at the Nepal Climate Observatory-Pyramid (5079 m)

Abstract: . This paper provides a detailed description of the atmospheric conditions characterizing the high Himalayas, thanks to continuous observations begun in March 2006 at the Nepal Climate Observatory-Pyramid (NCO-P) located at 5079 m a.s.l. on the southern foothills of Mt. Everest, in the framework of ABC-UNEP and SHARE-Ev-K2-CNR projects. The work presents a characterization of meteorological conditions and air-mass circulation at NCO-P during the first two years of activity. The mean values of atmospheric pressure, temperature and wind speed recorded at the site were: 551 hPa, −3.0 °C, 4.7 m s−1, respectively. The highest seasonal values of temperature (1.7 °C) and relative humidity (94%) were registered during the monsoon season, which was also characterized by thick clouds, present in about 80% of the afternoon hours, and by a frequency of cloud-free sky of less than 10%. The lowest temperature and relative humidity seasonal values were registered during winter, −6.3 °C and 22%, respectively, the season being characterised by mainly cloud-free sky conditions and rare thick clouds. The summer monsoon influenced rain precipitation (seasonal mean: 237 mm), while wind was dominated by flows from the bottom of the valley (S–SW) and upper mountain (N–NE). The atmospheric composition at NCO-P has been studied thanks to measurements of black carbon (BC), aerosol scattering coefficient, PM1, coarse particles and ozone. The annual behaviour of the measured parameters shows the highest seasonal values during the pre-monsoon (BC: 316.9 ng m−3, PM1: 3.9 μg m−3, scattering coefficient: 11.9 Mm−1, coarse particles: 0.37 cm−3 and O3: 60.9 ppbv), while the lowest concentrations occurred during the monsoon (BC: 49.6 ng m−3, PM1: 0.6 μg m−3, scattering coefficient: 2.2 Mm−1, and O3: 38.9 ppbv) and, for coarse particles, during the post-monsoon (0.07 cm−3. At NCO-P, the synoptic-scale circulation regimes present three principal contributions: Westerly, South-Westerly and Regional, as shown by the analysis of in-situ meteorological parameters and 5-day LAGRANTO back-trajectories. The influence of the brown cloud (AOD>0.4) extending over Indo–Gangetic Plains up to the Himalayan foothills has been evaluated by analysing the in-situ concentrations of the ABC constituents. This analysis revealed that brown cloud hot spots mainly influence the South Himalayas during the pre-monsoon, in the presence of very high levels of atmospheric compounds (BC: 1974.1 ng m−3, PM1: 23.5 μg m−3, scattering coefficient: 57.7 Mm−1, coarse particles: 0.64 cm−3, O3: 69.2 ppbv, respectively). During this season 20% of the days were characterised by a strong brown cloud influence during the afternoon, leading to a 5-fold increased in the BC and PM1 values, in comparison with seasonal means. Our investigations provide clear evidence that, especially during the pre-monsoon, the southern side of the high Himalayan valleys represent a "direct channel" able to transport brown cloud pollutants up to 5000 m a.s.l., where the pristine atmospheric composition can be strongly influenced.

Evidence of Enhanced Precipitation Due to Irrigation over the Great Plains of the United States

Abstract: [1] At the end of World War II, there was a rapid increase in irrigation over the Ogallala Aquifer in the Great Plains of the United States via groundwater withdrawal, and we hypothesize that this disruption of the local hydrological cycle has enhanced the regional precipitation. We examined station and gridded precipitation observations for the warm season months over and downwind of the Ogallala over the 20th century. Increases in precipitation of 15–30% were detected during July from the easternmost part of the aquifer to as far downwind as Indiana. The timing (1940s, July) and spatial pattern of the precipitation increase are consistent with the history of Ogallala irrigation and mechanisms by which increases in evapotranspiration can affect convection. Additionally, we conducted a vapor tracking analysis and found that evapotranspiration over the Ogallala Aquifer contributes to downwind precipitation and that the contribution is greater when the evapotranspiration is higher. This makes it hydrologically possible that the irrigation development was associated with the observed precipitation increases. Finally, there is no clear evidence that atmospheric circulation changes or modes of internal climate variability increased the July precipitation. Further analysis of the influence of Ogallala irrigation on precipitation will include the controlled analysis of climate model simulations that explicitly include irrigation.

Statistical downscaling of daily mean temperature, pan evaporation and precipitation for climate change scenarios in Haihe River, China

Abstract: A statistical downscaling method (SDSM) was evaluated by simultaneously downscaling air temperature, evaporation, and precipitation in Haihe River basin, China. The data used for evaluation were large-scale atmospheric data encompassing daily NCEP/NCAR reanalysis data and the daily mean climate model results for scenarios A2 and B2 of the HadCM3 model. Selected as climate variables for downscaling were measured daily mean air temperature, pan evaporation, and precipitation data (1961–2000) from 11 weather stations in the Haihe River basin. The results obtained from SDSM showed that: (1) the pattern of change in and numerical values of the climate variables can be reasonably simulated, with the coefficients of determination between observed and downscaled mean temperature, pan evaporation, and precipitation being 99%, 93%, and 73%, respectively; (2) systematic errors existed in simulating extreme events, but the results were acceptable for practical applications; and (3) the mean air temperature would increase by about 0.7°C during 2011~2040; the total annual precipitation would decrease by about 7% in A2 scenario but increase by about 4% in B2 scenario; and there were no apparent changes in pan evaporation. It was concluded that in the next 30 years, climate would be warmer and drier, extreme events could be more intense, and autumn might be the most distinct season among all the changes.

Regional, Seasonal, and Diurnal Variations of Extreme Convection in the South Asian Region

Abstract: Temporal and spatial variations of convection in South Asia are analyzed using eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data and NCEP reanalysis fields. To identify the most extreme convective features, three types of radar echo structures are defined: deep convective cores (contiguous 3D convective echo ≥40 dBZ extending ≥10 km in height) represent the most vertically penetrative convection, wide convective cores (contiguous convective ≥40 dBZ echo over a horizontal area ≥1000 km2) indicate wide regions of intense multicellular convection, and broad stratiform regions (stratiform echo contiguous over an area ≥50 000 km2) mark the mesoscale convective systems that have developed the most robust stratiform regions. The preferred locations of deep convective cores change markedly from India’s east coast in the premonsoon to the western Himalayan foothills in the monsoon. They form preferentially in the evening and over land as near-surface moist flow is cap...