bluuebunny/crossref_metadata_2025 huggingface dataset:
Improved landslide‐tsunami prediction: Effects of block model parameters and slide model
Authors: Heller Valentin , Spinneken Johannes
Published: April 2013

Subaerial landslide‐tsunamis and impulse waves are caused by mass movements impacting into a water body, and the hazards they pose have to be reliably assessed. Empirical equations developed with physical Froude model studies can be an efficient method for such predictions. The present study improves this methodology and addresses two significant shortcomings in detail for the first time: these are the effect of three commonly ignored block model parameters and whether the slide is represented by a rigid block or a deformable granular material. A total of 144 block slide tests were conducted in a wave flume under systematic variation of three important block model parameters, the slide Froude number, the relative slide thickness, and the relative slide mass. Empirical equations for the maximum wave amplitude, height, and period as well as their evolution with propagation distance are derived. For most wave parameters, remarkably small data scatter is achieved. The combined influence of the three block model parameters affects the wave amplitude and wave height by up to a factor of two. The newly derived equations for block slides are then related to published equations for granular slides. This comparison reveals that block slides do not necessarily generate larger waves than granular slides, as often argued in the technical literature. In fact, it is shown that they may also generate significant smaller waves. The new findings can readily be integrated in existing hazard assessment methodologies, and they explain a large part of the discrepancy between previously published data.
Analyzing the footprints of near‐surface aqueous turbulence: An image processing‐based approach
Authors: Schnieders J. , Garbe C. S. , Peirson W. L. , Smith G. B. , Zappa C. J.
Published: April 2013

In this contribution, a detailed investigation of surface thermal patterns on the water surface is presented, with wind speeds ranging from 1 to 7 m s  − 1 and various surface conditions. Distinct structures can be observed on the surface—small‐scale short‐lived structures termed fish scales and larger‐scale cold streaks that are consistent with the footprints of Langmuir circulations. The structure of the surface heat pattern depends strongly on wind‐induced stress. Consistent behavior regarding the spacing of cold streaks can be observed in a range of laboratory facilities when expressed as a function of water‐sided friction velocity, u * . This behavior systematically decreased until a point of saturation at u *  = 0.7 cm/s. We present a new image processing‐based approach to the analysis of the spacing of cold streaks based on a machine learning approach to classify the thermal footprints of near‐surface turbulence. Comparison is made with studies of Langmuir circulation and the following key points are found. Results suggest a saturation in the tangential stress, anticipating that similar behavior will be observed in the open ocean. A relation to Langmuir numbers shows that thermal footprints in infrared images are consistent with Langmuir circulations and depend strongly on wind wave conditions.
Increased CO2 outgassing in February‐May 2010 in the tropical Atlantic following the 2009 Pacific El Niño
Authors: Lefèvre Nathalie , Caniaux Guy , Janicot Serge , Gueye Abdou Karim
Published: April 2013

The fugacity of CO2 (fCO2) has been measured underway during the voyages of a merchant ship sailing from France to Brazil since 2008. High fCO2 values are observed in 2010, between approximately 8°S and 8°N, and are particularly pronounced in boreal spring. These high values are explained by the anomalous situation occurring in the tropical Atlantic in 2010 after the 2009 El Niño in the Pacific. The weakening of the trade winds during the El Niño event contributes to an increase of sea surface temperatures in the tropical Atlantic and a northward shift of the intertropical convergence zone. The anomalous position of the intertropical convergence zone is accompanied by reduced precipitation in boreal spring and hence a higher than usual sea surface salinity. The year 2010 is also characterized by a strong positive Atlantic multidecadal oscillation index and a negative North Atlantic oscillation index that contribute to the tropical Atlantic warming. Positive anomalies of both surface temperature and salinity cause an increase of surface CO2, leading to a stronger outgassing of CO2 in 2010 compared with 2009 and 2011. The main factor responsible for the CO2 anomalies is the Pacific El Niño teleconnection. The Atlantic multidecadal oscillation index also contributes in the northern hemisphere, and the role of the North Atlantic oscillation is negligible.
Role of advection in Arctic Ocean lower trophic dynamics: A modeling perspective
Authors: Popova E. E. , Yool A. , Aksenov Y. , Coward A. C.
Published: April 2013

The Arctic Ocean (AO) is an oligotrophic system with a pronounced subsurface Chl‐a maximum dominating productivity over the majority of the basin. Strong haline stratification of the AO and substantial ice cover suppress vertical mixing and restrict the vertical supply of nutrients to the photic zone. In such a vertically stratified oligotrophic system, the horizontal supply of nutrients by advection plays an important role in sustaining primary production. In this paper, we attempt to characterize the role of nutrient advection in the maintenance of the subsurface Chl‐a maximum, using timescales to determine the connectivity between the photic zone of the deep AO, nutrient‐rich Pacific and Atlantic inflow waters, and bottom waters of the wide continental shelves of the AO. Our study uses output from a general circulation model, Nucleus for European Modeling of the Ocean, coupled to a model of ocean biogeochemistry, Model of Ecosystem Dynamics, carbon Utilization, Sequestration, and Acidification. A Lagrangian particle tracking approach is used to back‐track water from where it forms subsurface Chl‐a maxima to the points of entry into the AO and to analyze nutrient transformation along the route. Our experiments show that advective timescales linking subsurface layers of the central AO with the nutrient‐rich Pacific and Atlantic waters do not exceed 15–20 years and that the advective supply of shelf nutrients to the deep AO occurs on the timescale of about 5 years. We show substantial role of the continental shelf pump in sustaining up to 20% of total AO primary production.
Seasonal to decadal variability of Arctic Ocean heat content: A model‐based analysis and implications for autonomous observing systems
Authors: Lique Camille , Steele Michael
Published: April 2013

A high‐resolution global ocean/sea ice model is used to investigate the modes of Arctic Ocean heat content variability for the period 1968–2007. A rotated empirical orthogonal function analysis is performed on the monthly mean vertically integrated heat content to investigate the mechanisms governing its spatiotemporal variations. In the model, 28% of the heat content variability is driven by the seasonal and interannual fluctuations of the atmospheric heat flux in the seasonally ice free regions. The heat flux variability associated with Atlantic Water advected through Fram Strait drives 31% of the heat content variability. Changes of temperature and circulation drive Fram Strait heat transport variability, and these two effects project on different modes and thus drive heat content variations in different parts of the Eurasian Basin. A second branch of Atlantic Water is modified in the Barents Sea and the variations of the heat flux associated with the Barents Sea water branch penetrating the deep Arctic yield heat content variations in the Eurasian Basin. The effect of the Bering Strait heat flux variations remains limited to the Chukchi Sea. Autonomous observing system may be able to capture the Arctic heat content variability. Sea surface temperature satellite observations combined with temperature profiles of the top 800 m in the deep Arctic covered by sea ice are sufficient to capture most of the variability signal. The results emphasize the crucial need for measurements in the Eurasian Basin.
Vertical structure and temporal evolution of an anticyclonic eddy in the Balearic Sea (western Mediterranean)
Authors: Amores Angel , Monserrat Sebastià , Marcos Marta
Published: April 2013

An anticyclonic eddy in the Balearic Sea (western Mediterranean) was described using data from a mooring line deployed at the northern slope of Mallorca Island at about 900 m deep. Its surface signature was investigated using sea surface height and sea surface temperature images. The eddy, which lasted around 1 month, modified the thermohaline characteristics and the currents of the entire water column. Levantine Intermediate Waters, usually resident in the region, were displaced by colder and fresher Western Mediterranean Intermediate Waters associated with the eddy. Along‐slope main currents (toward NE) were completely reversed at 500 m and significantly deviated at 900 m. Interestingly, near‐bottom velocities were found to be systematically larger than those at intermediate depths. Furthermore, during the eddy, velocities reached values up to 26 cm/s at the bottom, 5 times larger than the bottom average speed. The recurrence of the phenomenon was explored with an eddy detection tool applied to satellite observations. Results indicated that anticyclonic eddies are common structures in the Balearic Current.
(Sub‐)inertial wave boundary turbulence in the Gulf of Valencia
Authors: van Haren Hans , Ribó Marta , Puig Pere
Published: April 2013

The bottom boundary layer above sloping topography can be highly turbulent, even in deep seas. This is demonstrated here using high‐resolution 1‐Hz sampling temperature sensors that were moored for 5 months every 0.5 m between 6.5 and 61 m above a 572 m deep seafloor promontory on the continental slope off Valencia, Spain. Using these data, turbulence parameters have been estimated. With time and in the vertical, values vary over four orders of magnitude. They have a dominant local inertial period which is modulated by an about 11 day periodicity associated with variations in a baroclinic unstable boundary current. When this current is strong and Eastward, the upslope phase of inertial wave generates convective turbulence which reaches closest to the bottom and therefore can effect sediment dispersal. In late winter, equally strong shear‐induced turbulence in 50 m high Kelvin‐Helmholtz (K‐H) overturns is forced by 0.2 m s−1 off/downslope motions, which are preceded by periods of warming of a few 0.01°C before the cooler near‐bottom water is suddenly flushed over the promontory into the basin. Such anomalously large K‐H overturns occurred 6 times in the investigated winter period.
Temperature inversions and their influence on the mixed layer heat budget during the winters of 2006–2007 and 2007–2008 in the Bay of Bengal
Authors: Girishkumar M. S. , Ravichandran M. , McPhaden M. J.
Published: April 2013

Time series measurements of temperature, salinity and surface meteorological parameters recorded at 8°N, 90°E in the southern central Bay of Bengal (BoB) from a Research Moored Array for African‐Asian‐Australian Monsoon Analysis and predication (RAMA) buoy are used to document temperature inversions and their influence on the mixed layer heat budget during the winters, defined as October to March, of 2006–2007 (W67) and 2007–2008 (W78). There is a marked difference in the frequency and amplitude of temperature inversion between these two winters, with variations much stronger in W78 compared to W67. The formation of temperature inversions is favored by the existence of thick barrier layers, which are also more prominent in W78 compared to W67. Inversions occur when heating in the barrier layer below the mixed layer by penetrative shortwave radiation is greater than heating of the mixed layer by net surface heat flux and horizontal advection. Our analysis further demonstrates that intraseasonal and year‐to‐year variability in the frequency and magnitude of temperature inversions during winter have substantial influence on mixed layer temperature through the modulation of vertical heat flux at the base of mixed layer.
Reduction of near‐inertial energy through the dependence of wind stress on the ocean‐surface velocity
Authors: Rath Willi , Greatbatch Richard J. , Zhai Xiaoming
Published: April 2013

A realistic primitive‐equation model of the Southern Ocean at eddying spatial resolution is used to examine the effect of ocean‐surface‐velocity dependence of the wind stress on the strength of near‐inertial oscillations. Accounting for the ocean‐surface‐velocity dependence of the wind stress leads to a large reduction of wind‐induced near‐inertial energy of approximately 40% and of wind power input into the near‐inertial frequency band of approximately 20%. A large part of this reduction can be explained by the leading‐order modification to the wind stress if the ocean‐surface velocity is included. The strength of the reduction is shown to be modulated by the inverse of the ocean‐surface‐mixed‐layer depth. We conclude that the effect of surface‐velocity dependence of the wind stress should be taken into account when estimating the wind‐power input into the near‐inertial frequency band and when estimating near‐inertial energy levels in the ocean due to wind forcing.
Springtime boundary layer O3 and GEM depletion at Toolik Lake, Alaska
Authors: Van Dam Brie , Helmig Detlev , Burkhart John F. , Obrist Daniel , Oltmans Samuel J.
Published: April 2013

Atmospheric surface ozone (O3) was measured at Toolik Lake, Alaska, from September 2010 to August 2011, along with winter and springtime (January–May) measurements of gaseous elemental mercury (GEM). Concurrent episodic springtime depletion of both O3 and GEM was observed between April and May 2011. To investigate these depletion events, surface O3 measurements from Toolik Lake were compared with similar measurements from Barrow, the only Alaskan site available to use as an indicator of conditions at the Arctic Ocean coastline. Barrow is located approximately 400 km to the northwest of Toolik Lake. In all cases where O3 and GEM depletion was observed at Toolik Lake, similar O3 depletion events (ODEs) were observed at Barrow. Conversely, in over half of the instances where ODEs were observed at Barrow, no such events were recorded at Toolik Lake. The Lagrangian particle dispersion model FLEXPART was utilized to interpret the transport conditions and potential influence of oceanic air masses on depletion events observed at Toolik Lake. These analyses indicate that ODEs observed at Toolik Lake are correlated with depletion events observed at Barrow when the prevailing airflow is from the north and the air mass has spent substantial time over the Arctic Ocean or coast within the previous week. These observations suggest that coastal O3 and GEM‐depleted air masses are likely transported from the Arctic Ocean up to ~200 km inland and to at least an altitude of 700 m above sea level. These observations underscore that O3 and GEM depletion events impact not just the Arctic coastal zone but also extensive inland areas with possible implications for the cycling and bioavailability of mercury (Hg) to inland Arctic ecosystems.
Decadal time series of tropospheric abundance of N2O isotopomers and isotopologues in the Northern Hemisphere obtained by the long‐term observation at Hateruma Island, Japan
Authors: Toyoda Sake , Kuroki Natsuko , Yoshida Naohiro , Ishijima Kentaro , Tohjima Yasunori , Machida Toshinobu
Published: April 2013

Decadal time series and short‐term temporal variations in mixing ratio of atmospheric nitrous oxide (N2O) and abundance of its isotopomers (14N15N16O and 15N14N16O) and isotopologue (14N14N18O) relative to 14N14N16O have been observed for the first time in the Northern Hemisphere at Hateruma Island (HAT), Japan during 1999–2010 by monthly air sampling. Results show that the bulk nitrogen isotope ratio δ15Nbulk decreased at the rate of −0.023 ± 0.006‰ yr−1, although the N2O mixing ratio increased at the rate of about 0.7 nmol mol−1 yr−1 (ppb yr−1) during the period. Isotope budget calculation with the δ15Nbulk trend supports the earlier estimates showing that the isotopically light sources such as agriculture and industry contribute to the increase of atmospheric N2O. However, the rate of decrease of δ15Nbulk is slightly smaller in magnitude than the rates obtained virtually for the 20th century from firn air in polar regions and surface air in the Southern Hemisphere (Tasmania and Antarctica), which suggests greater contribution of 15 N‐enriched N2O sources in recent years or in the extra‐polar Northern Hemisphere. In contrast, the oxygen isotope ratio (δ18O) and intramolecular 15N site preference (SP, difference between isotope ratios at central and terminal nitrogen atoms) of N2O showed no significant trends, contrary to previous reports. Results show that no significant seasonal variation exists in δ15Nbulk, δ18O, and SP of N2O at HAT in the past decade within the limits of our sampling frequency and analytical precision.
Investigation of the complex dynamics and structure of the 2010 Eyjafjallajökull volcanic ash cloud using multispectral images and numerical simulations
Authors: Spinetti C. , Barsotti S. , Neri A. , Buongiorno M. F. , Doumaz F. , Nannipieri L.
Published: April 2013

We investigated the structure and evolution of the 2010 Eyjafjallajökull volcanic cloud and its dispersal over Iceland and Europe integrating satellite multispectral images and numerical simulations. Data acquired by Medium Resolution Imaging Spectrometer (MERIS) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) have been analyzed to quantify the cloud extent and composition. The VOL‐CALPUFF dispersal code was applied to reconstruct the transient and 3‐D evolution of the cloud. Source parameters estimated on the base of available a posteriori volcanological data sets have been used. Quantitative comparisons between satellite retrievals and modeling results were performed for two selected instants of time during the first and third eruptive phases on a regional scale. Sensitivity of the model to initial volcanological conditions has been analyzed at continental scale. Several complex non intuitive features of cloud dynamics have been highlighted and strengths and limitations of the adopted methods identified. The main findings are: the level of quantitative agreement between satellite observations and numerical results depends on ash cloud composition (particle sizes and concentration) with better agreement for smaller particles and higher concentrations; the agreement between observations and modeling outcomes also depends on the temporal stability of volcanological conditions and the complexity of the meteorological wind field; the irregular dispersion of ash, as reconstructed from satellite data and numerical modeling, can be well explained by the different response of particle sizes to strong vertical wind‐shear, and by resuspension processes acting at ground level; eruptive source conditions are the main source of uncertainty in modeling, especially during an ongoing crisis and at long‐range scales.
Impact of aerosol and thin cirrus on retrieving and validating XCO2 from GOSAT shortwave infrared measurements
Authors: Guerlet S. , Butz A. , Schepers D. , Basu S. , Hasekamp O. P. , Kuze A. , Yokota T. , Blavier J.‐F. , Deutscher N. M. , Griffith D. W. T. , Hase F. , Kyro E. , Morino I. , Sherlock V. , Sussmann R. , Galli A. , Aben I.
Published: April 2013

Inadequate treatment of aerosol scattering can be a significant source of error when retrieving column‐averaged dry‐air mole fractions of CO2 (XCO2) from space‐based measurements of backscattered solar shortwave radiation. We have developed a retrieval algorithm, RemoTeC, that retrieves three aerosol parameters (amount, size, and height) simultaneously with XCO2. Here we evaluate the ability of RemoTeC to account for light path modifications by clouds, subvisual cirrus, and aerosols when retrieving XCO2 from Greenhouse Gases Observing Satellite (GOSAT) Thermal and Near‐infrared Sensor for carbon Observation (TANSO)‐Fourier Transform Spectrometer (FTS) measurements. We first evaluate a cloud filter based on measurements from the Cloud and Aerosol Imager and a cirrus filter that uses radiances measured by TANSO‐FTS in the 2 micron spectral region, with strong water absorption. For the cloud‐screened scenes, we then evaluate errors due to aerosols. We find that RemoTeC is well capable of accounting for scattering by aerosols for values of aerosol optical thickness at 750 nm up to 0.25. While no significant correlation of errors is found with albedo, correlations are found with retrieved aerosol parameters. To further improve the XCO2 accuracy, we propose and evaluate a bias correction scheme.

Measurements from 12 ground‐based stations of the Total Carbon Column Observing Network (TCCON) are used as a reference in this study. We show that spatial colocation criteria may be relaxed using additional constraints based on modeled XCO2 gradients, to increase the size and diversity of validation data and provide a more robust evaluation of GOSAT retrievals. Global‐scale validation of satellite data remains challenging and would be improved by increasing TCCON coverage.
A multimodel comparison of stratospheric ozone data assimilation based on an ensemble Kalman filter approach
Authors: Nakamura T. , Akiyoshi H. , Deushi M. , Miyazaki K. , Kobayashi C. , Shibata K. , Iwasaki T.
Published: April 2013

For future development of a high‐performance ozone analysis system, we investigated the impact of model performance on stratospheric ozone analysis by using four different models with a common data assimilation framework. For assimilation of ozone and meteorological field variables, we used a local ensemble transform Kalman filter with the CCSR/NIES chemistry‐climate model (CCM), the MIROC3.2 CCM, the MRI CCM, and the CHASER chemical transport model. We examined the effects of model biases on forecast/analysis of ozone based on multimodel comparisons of assimilation results. We assimilated ozone profiles provided by Aura/Microwave Limb Sounder (MLS) and total ozone provided by the Ozone Monitoring Instrument (OMI)‐Total Ozone Mapping Spectrometer (TOMS). For all models, meteorological fields obtained from a global reanalysis dataset (JMA Climate Data Assimilation System) were also assimilated to provide a common framework without any spatiotemporal dependence of data observation quality. Ozone profiles obtained from assimilation of MLS observations showed good agreement with independent ozonesonde observations, with a mean bias of less than 5% in the stratosphere. We found that model bias originating from ozone chemistry degraded the assimilation performance of not only ozone but also temperature in the stratosphere. Assimilation of OMI‐TOMS total ozone data agreed with the independent SCIAMACHY total ozone with a bias of less than 3%. However, a model bias in the tropospheric ozone concentration deteriorated the stratospheric ozone analysis. Finally, the use of both stratospheric ozone profile data and total ozone data greatly improved the overall performance of the ozone analysis, regardless of the model biases.
From pointwise testing to a regional vision: An integrated statistical approach to detect nonstationarity in extreme daily rainfall. Application to the Sahelian region
Authors: Panthou Gérémy , Vischel Théo , Lebel Thierry , Quantin Guillaume , Pugin Anne‐Catherine Favre , Blanchet Juliette , Ali Abdou
Published: April 2013

Global warming is expected to intensify the hydrologic cycle. Documenting whether significant changes in the extreme precipitation regimes have already happened is consequently one of the challenging topics in climatic research. The high natural variability of extreme precipitation often prevents from obtaining significant results when testing changes in the empirical distribution of extreme rainfall at regional scale. A regional integrated approach is proposed here as one possible answer to this complex methodological problem. Three methods are combined in order to detect regionally significant trends and/or breakpoints in series of annual maximum daily rainfall: (1) individual stationarity tests applied to the raw point series of maxima, (2) a maximum likelihood testing of time‐dependent generalized extreme value (GEV) distributions fitted to these series, and (3) a heuristic testing of a regional time‐dependent GEV distribution. This approach is applied to a set of 126 daily rain gauges covering the Sahel over the period 1950–1990. It is found that only a few stations are tested as nonstationary when applying classical tests on the raw series, while the two GEV‐based models converge to show that the extreme rainfall series indeed underwent a negative breakpoint around 1970. The study evidences the limits of the widely used classical stationarity tests to detect trends in noisy series affected by sampling uncertainties, while using a parametric space and time‐dependent GEV efficiently reduces this effect. Showing that the great Sahelian drought was accompanied by a significant decrease of extreme rainfall events is the other main result of this study.
Atmospheric oxidation chemistry and ozone production: Results from SHARP 2009 in Houston, Texas
Authors: Ren Xinrong , van Duin Diana , Cazorla Maria , Chen Shuang , Mao Jingqiu , Zhang Li , Brune William H. , Flynn James H. , Grossberg Nicole , Lefer Barry L. , Rappenglück Bernhard , Wong Kam W. , Tsai Catalina , Stutz Jochen , Dibb Jack E. , Thomas Jobson B. , Luke Winston T. , Kelley Paul
Published: April 2013

Ozone (O3) and secondary fine particles come from the atmospheric oxidation chemistry that involves the hydroxyl radical (OH) and hydroperoxyl radical (HO2), which are together called HOx. Radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) significantly affect the HOx budget in urban environments. These chemical processes connect surface anthropogenic and natural emissions to local and regional air pollution. Using the data collected during the Study of Houston Atmospheric Radical Precursors (SHARP) in spring 2009, we examine atmospheric oxidation chemistry and O3 production in this polluted urban environment. A numerical box model with five different chemical mechanisms was used to simulate the oxidation processes and thus OH and HO2 in this study. In general, the model reproduced the measured OH and HO2 with all five chemical mechanisms producing similar levels of OH and HO2, although midday OH was overpredicted and nighttime OH and HO2 were underpredicted. The calculated HOx production was dominated by HONO photolysis in the early morning and by the photolysis of O3 and oxygenated volatile organic compounds (OVOCs) in the midday. On average, the daily HOx production rate was 24.6 ppbv d−1, of which 30% was from O3 photolysis, 22% from HONO photolysis, 15% from the photolysis of OVOCs (other than HCHO), 14% from HCHO photolysis, and 13% from O3 reactions with alkenes. The O3 production was sensitive to volatile organic compounds (VOCs) in the early morning but was sensitive to NOx for most of afternoon. This is similar to the behavior observed in two previous summertime studies in Houston: the Texas Air Quality Study in 2000 (TexAQS 2000) and the TexAQS II Radical and Aerosol Measurement Project in 2006 (TRAMP 2006). Ozone production in SHARP exhibits a longer NOx‐sensitive period than TexAQS 2000 and TRAMP 2006, indicating that NOx control may be an efficient approach for the O3 control in springtime for Houston. Results from this study provide additional support for regulatory actions to reduce NOx and reactive VOCs in Houston in order to reduce O3 and other secondary pollutants.
A scPDSI‐based global data set of dry and wet spells for 1901–2009
Authors: van der Schrier G. , Barichivich J. , Briffa K. R. , Jones P. D.
Published: April 2013

Global maps of monthly self‐calibrating Palmer Drought Severity Index (scPDSI) have been calculated for the period 1901–2009 based on the CRU TS 3.10.01 data sets. This work addresses some concerns with regard to monitoring of global drought conditions using the traditional Palmer Drought Severity Index. First, the scPDSI has a similar range of variability in diverse climates making it a more suitable metric for comparing the relative availability of moisture in different regions. Second, the more physically based Penman‐Monteith parameterization for potential evapotranspiration is used, calculated using the actual vegetation cover rather than a reference crop. Third, seasonal snowpack dynamics are considered in the water balance model. The leading mode of variability in the new data set represents a trend towards drying conditions in some parts of the globe between 1950 and 1985 but accounts for less than 9% of the total variability. Increasing temperature and potential evapotranspiration explain part of the drying trend. However, local trends in most of the drying regions are not significant. Previously published evidence of unusually strong or widespread drying is not supported by the evidence in this work. A fundamental aspect of the calculation of scPDSI is the selection of a calibration period. When this period does not include the most recent part of the record, trends towards more extreme conditions are amplified. It is shown that this is the principal reason for different published interpretations of the scale of recent global drying and not, as recently claimed, the use of simplified forcing data.
Assessment of intercalibration methods for satellite microwave humidity sounders
Authors: John Viju O. , Allan Richard P. , Bell William , Buehler Stefan A. , Kottayil Ajil
Published: April 2013

Three methods for intercalibrating humidity sounding channels are compared to assess their merits and demerits. The methods use the following: (1) natural targets (Antarctica and tropical oceans), (2) zonal average brightness temperatures, and (3) simultaneous nadir overpasses (SNOs). Advanced Microwave Sounding Unit‐B instruments onboard the polar‐orbiting NOAA 15 and NOAA 16 satellites are used as examples. Antarctica is shown to be useful for identifying some of the instrument problems but less promising for intercalibrating humidity sounders due to the large diurnal variations there. Owing to smaller diurnal cycles over tropical oceans, these are found to be a good target for estimating intersatellite biases. Estimated biases are more resistant to diurnal differences when data from ascending and descending passes are combined. Biases estimated from zonal‐averaged brightness temperatures show large seasonal and latitude dependence which could have resulted from diurnal cycle aliasing and scene‐radiance dependence of the biases. This method may not be the best for channels with significant surface contributions. We have also tested the impact of clouds on the estimated biases and found that it is not significant, at least for tropical ocean estimates. Biases estimated from SNOs are the least influenced by diurnal cycle aliasing and cloud impacts. However, SNOs cover only relatively small part of the dynamic range of observed brightness temperatures.
The contribution of African easterly waves to monsoon precipitation in the CMIP3 ensemble
Authors: Skinner Christopher B. , Diffenbaugh Noah S.
Published: April 2013

African easterly waves (AEWs) are a major source of synoptic‐scale rainfall variability throughout West Africa. Given the persistent uncertainty in the response of precipitation over West Africa to enhanced greenhouse forcing, we analyze the contribution of AEWs to the simulation of precipitation over West Africa in the Coupled Model Intercomparison Project Phase 3 (CMIP3) ensemble of general circulation models. Nearly all models that simulate weaker‐than‐observed AEW activity exhibit a lack of coupling between AEWs and precipitation. Only those models that employ a convective parameterization with a moisture convergence‐based closure or trigger, or a prognostic closure with a convective suppression scheme, demonstrate a consistent connection between AEWs and precipitation. The strength of simulated AEW activity is largely dependent on the simulated magnitude and positioning of the African easterly jet and low‐level westerlies. We find a strong positive relationship between the magnitude of simulated AEW activity and seasonal‐ and synoptic‐scale precipitation. Models with weaker‐than‐observed AEW activity exhibit a disproportionately low percentage of precipitation variability at AEW timescales. Conversely, in those models that simulate stronger‐than‐observed AEW activity, dynamical forcing associated with the wave tends to produce overly intense individual precipitation events. Our results suggest that the simulation of AEWs contributes substantially to the spread in mean seasonal precipitation across the CMIP3 ensemble during the 20th century, and that improved understanding of the effect of enhanced greenhouse forcing on the relationship between AEWs and precipitation could help to narrow the uncertainty in the response of West African rainfall to continued global warming.
Hourly global irradiance from satellite data in Badajoz, Spain: Spatial and temporal dependence
Authors: Nunez M. , Serrano A. , Cancillo M. L.
Published: April 2013

Satellite estimates of solar radiation at the hourly scale depend on the spatial and temporal variability of solar radiation within a region. To examine this effect, a field program was established near Badajoz, Spain (38.88°N, 7.01°W) consisting in deployment of seven pyranometers at or adjoining the Meteosat pixel for the area. A simple semiempirical retrieval approach based on the satellite reflectance was developed using data from one pyranometer station at the University campus and subsequently tested with an independent data set for the same station. The accuracy of the satellite estimate is a strong function of the averaging period and the frequency of satellite scans used. At the hourly scale, best estimates of solar irradiance are obtained with satellite data taken every 5 min, giving a coefficient of determination (R2) of 0.883. Within‐pixel spatial variability of measured irradiance is substantial but only for averaging periods less than 1 h. Comparison of surface point measurements with the satellite retrieval algorithm at the 5 min scale are associated with a relative RMS difference of 20.2% out of which 19.5% is due to model‐induced uncertainties and 5.2% is due to instrumentation uncertainties involved in the retrieval process. Within‐pixel point sampling will lower both the instrument uncertainty and the uncertainty in the retrieval algorithm for averaging periods lower than 1 h. Beyond this time, a single pyranometer is well representative of the overhead cloud structure, reaching root mean square difference values of 14% at the hourly scale.
Toward improved corrections for radiation‐induced biases in radiosonde temperature observations
Authors: Sun Bomin , Reale Anthony , Schroeder Steven , Seidel Dian J. , Ballish Bradley
Published: April 2013

Radiation‐induced biases in global operational radiosonde temperature data from May 2008 to August 2011 are examined by using spatially and temporally collocated Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) data as estimates of the truth. The data on average from most radiosonde types show a nighttime cold bias and a daytime warm bias relative to COSMIC. Most daytime biases increase with altitude and solar elevation angle (SEA). The global average biases in the 15–70 hPa layer are −0.05 ± 1.89 K standard deviation (~52,000 profiles) at night and 0.39 ± 1.80 K standard deviation (~64,500 profiles) in daytime (SEA > 7.5°). Daytime warm biases associated with clouds are smaller than those under clear conditions. Newer sondes (post‐2000) have smaller biases and appear to be less sensitive to effects of clouds. Biases at night show greater seasonal and zonal variations than those for daytime. In general, warm night biases are associated with warm climate regimes and less warm or cold night biases with cold climate regimes. Bias characteristics for 13 major radiosonde types are provided, as a basis for updating radiosonde corrections used in numerical weather predictions, for validating satellite retrievals, and for adjusting archived radiosonde data to create consistent climate records.
A comparison of tropospheric temperature changes over China revealed by multiple data sets
Authors: Zhang Lixia , Zhou Tianjun
Published: April 2013

Based on four radiosonde data sets and three reanalysis data sets, the long‐term tropospheric temperature changes over China for the period 1958–2001, and the uncertainties are analyzed. The results from all data sets, except for HadAT2 and National Centers for Environmental Protection 20th Century Reanalysis data (NCEP‐20CR), show a significant warming trend in the lower troposphere temperature averaged over China during 1958–2001, but this trend decreases with height and is replaced by a cooling tendency at 500 hPa, reaching maximum cooling at 300 hPa. The year‐by‐year changes of temperature over China are largely in agreement among the radiosonde and the reanalysis data sets. The uncertainties of upper troposphere temperature changes are larger compared with those of the lower troposphere. The uncertainty was relatively small during 1970–1990 and large during 1958–1970 and from 1990 to the present. The trend uncertainty is large in the Northwest for the lower troposphere and in south China for the upper troposphere, with the largest trend uncertainty over Northwest China at 850 hPa and east to 100°E at 300 hPa. For the average temperature trend over China, the largest uncertainty peaks at 300 hPa for 1958–2001. The tropospheric temperature of China at 300 hPa, derived from HadAT2, warms up; this differs greatly from the other data sets. The warming trend is produced mainly by the stations over south China and is due to the choice of neighbor stations during construction. The temperature trend at 300 hPa in NCEP‐National Center for Atmospheric Research is cooler than the other data sets due to its abrupt cooling around the early 1990s. NCEP‐20CR can partly capture the cooling tendency at 300 hPa over North China but with weaker magnitude.
Links between mesopause temperatures and ground‐based VLF narrowband radio signals
Authors: Silber Israel , Price Colin , Rodger Craig J. , Haldoupis Christos
Published: April 2013

The Upper Mesosphere–Lower Thermosphere (UMLT) region of the atmosphere is known to vary on many temporal and spatial scales. However, this region of the atmosphere is very difficult to measure and monitor continuously. In this paper, we demonstrate an intriguing connection between mesopause temperatures and the intensity of very low frequencies (VLF) narrowband (NB) signals reflected off the lower ionosphere. The temperature data used are from the SABER instrument onboard the TIMED satellite, while the VLF data are obtained from various ground‐based receiving systems. The results of the analysis show a high anticorrelation between temperature and VLF amplitude. It is shown that the variability of the UMLT temperatures and VLF amplitudes can be explained by global seasonal solar irradiance changes (~72% of the variability), while the remaining variability has its origins from other sources (~28%). High‐resolution mesopause temperature estimates might be achieved in the future by combining VLF NB observations and calculated solar irradiance variability (as a function of hour, day, and location, i.e., latitude).
What is the effect of unresolved internal climate variability on climate sensitivity estimates?
Authors: Olson R. , Sriver R. , Chang W. , Haran M. , Urban N. M. , Keller K.
Published: April 2013

Many studies have attempted to estimate the equilibrium climate sensitivity (CS) to the doubling of CO2concentrations. One common methodology is to compare versions of Earth models of intermediate complexity (EMICs) to spatially and/or temporally averaged historical observations. Despite the persistent efforts, CS remains uncertain. It is, thus far, unclear what is driving this uncertainty. Moreover, the effects of the internal climate variability on the CS estimates obtained using this method have not received thorough attention in the literature. Using a statistical approximator (“emulator”) of an EMIC, we show in an observation system simulation study that unresolved internal climate variability appears to be a key driver of CS uncertainty (as measured by the 68% credible interval). We first simulate many realizations of pseudo‒observations from an emulator at a “true” prescribed CS, and then reestimate the CS using the pseudo‒observations and an inverse parameter estimation method. We demonstrate that a single realization of the internal variability can result in a sizable discrepancy between the best CS estimate and the truth. Specifically, the average discrepancy is 0.84°C, with the feasible range up to several °C. The results open the possibility that recent climate sensitivity estimates from global observations and EMICs are systematically considerably lower or higher than the truth, since they are typically based on the same realization of climate variability. This possibility should be investigated in future work. We also find that estimation uncertainties increase at higher climate sensitivities, suggesting that a high CS might be difficult to detect.
Long‐range memory in Earth's surface temperature on time scales from months to centuries
Authors: Rypdal K. , Østvand L. , Rypdal M.
Published: April 2013

The paper explores the hypothesis that the temporal global temperature response can be modeled as a long‐range memory (LRM) stochastic process characterized by a Hurst exponent 0.5<H≲1.0 on time scales from months to decades. The LRM is a mathematical representation of the multitude of response times associated with the various subsystems. By analysis of instrumental and reconstructed temperature records, we verify LRM on time scales from months to centuries. We employ well‐known detrending methods to demonstrate that LRM increases when one goes from local and regional (H≈0.65) to global (H≈0.75) land temperature records, and LRM is highest in records strongly influenced by the ocean (H≈1.0). The increasing trend through the last century cannot be explained as an unforced LRM fluctuation, but the amplitude of the observed 60 year oscillation can be reconciled with the LRM process. We investigate statistical bias and error of the analysis methods employed, and conclude that, for these short record lengths, the error in estimated H is ±0.07 for the instrumental records. Analysis of a northern‐hemisphere reconstruction confirms that the LRM‐scaling prevails up to at least 250 years with H=0.9±0.1. We show that, if this reconstruction is correct, the temperature difference between the Medieval Warm Period and the Little Ice Age cannot be explained as an LRM fluctuation.
Precipitation intensity and variation during MC3E: A numerical modeling study
Authors: Tao Wei‐Kuo , Wu Di , Matsui Toshihisa , Peters‐Lidard Christa , Lang Stephen , Hou Arthur , Rienecker Michele , Petersen Walter , Jensen Michael
Published: April 2013

Previous observational studies have identified three different types of diurnal precipitation variation over the conterminous U.S.: localized afternoon rainfall maxima over the Mississippi and Ohio valleys, propagating mesoscale convective systems (MCSs) from the Rocky Mountain region, and propagating MCSs over the Appalachian Mountains. This study focuses on the second type, which involves nocturnal rainfall maxima from eastward‐propagating MCSs on the lee side of the Rocky Mountains. This study evaluates model simulations with regard to rainfall using observations and assesses the impact of microphysics, surface fluxes, radiation, and terrain on the simulated diurnal rainfall variation. A regional high‐resolution model was used to conduct a series of real‐time forecasts during the Midlatitude Continental Convective Clouds Experiment (MC3E) in 2011 over the Southern Great Plains. The model ably captured most heavy precipitation events. When all forecast days are composited, the mean forecast depicts accurate, propagating precipitation features and thus the overall diurnal variation. However, individual forecasts tend to overestimate the rainfall for light precipitation events, have location errors, and misrepresent convection in some cases. A post mission case study is performed on one multi‐cell, eastward‐propagating MCS event; the results suggest that cold‐pool dynamics were an important physical process. Model results also indicate that terrain effects are important during the initial stages of MCS development. By increasing the terrain height by 10%, the simulated rainfall is increased and in better agreement with observations. On the other hand, surface fluxes, and radiation processes only have a secondary effect for short‐term simulations.
Evaluation of volcanic aerosol impacts on atmospheric water vapor using CMIP3 and CMIP5 simulations
Authors: Li Jingwan , Sharma Ashish
Published: April 2013

The uncertainty in estimates of volcanic forcing and in the way general circulation models (GCMs) implement volcanic forcing affects the accuracy of the simulated atmospheric water vapor response to volcanic eruptions, which in turn influences the design of hydrologic infrastructure. This paper aims to investigate whether and to what extent the models can simulate the volcanic impacts on atmospheric water vapor. While a Kolmogorov‐Smirnov (K‐S) test on atmospheric water vapor from GCMs, reanalysis products, and observations all imply that volcanic aerosols have a statistically significant impact on atmospheric water vapor, and the areal extent of global atmospheric water vapor affected by volcanic eruptions is oversimulated by GCMs. Additionally, the spatial and temporal patterns of the volcano‐related water vapor variability are not well simulated by GCMs. Furthermore, the strong negative correlation between aerosol optical depth and water vapor residual is observed only in the equatorial Atlantic and Indian Oceans, and parts of the western Pacific Ocean rather than across the entire globe as simulated by volcanic forcing included Coupled Model Intercomparison Project Phase 3 (CMIP3) multimodel average (or over nearly the entire tropics by volcanic forcing included the BCC‐CSM1.1). Although the uncompleted removal of El Niño–Southern Oscillation signal may compromise the results, it is still safe, after performing the control run of K‐S test, to draw the conclusion that while volcanic forcing does exert a significant impact on atmospheric water vapor, the underlying relationship is considerably more complex than that evident in GCM simulations.