bluuebunny/crossref_metadata_2025 huggingface dataset:
Cold core eddies and fronts of the Antarctic Circumpolar Current south of New Zealand from in situ and satellite data
Authors: Cotroneo Yuri , Budillon Giorgio , Fusco Giannetta , Spezie Giancarlo
Published: April 2013

The meridional heat flux required to balance the heat lost by ocean to atmosphere at high latitudes must be accomplished by some mechanism other than mean advection and the heat flux by eddies crossing the Antarctic Circumpolar Current (ACC) may be a candidate. In this study, the positions of the main ACC fronts are determined based on 23 expendable bathythermographs (XBT) transects collected from 1994 to 2010 and are compared with those detected through satellite altimetry. Then, cold core anomalies in XBT sections are identified and altimetry is used to follow the spatial‐temporal evolution of these cold, low sea level anomalies. Mean values of main parameters, such as speed (0.35 km/h), lifetime (79 weeks), and diameter (105 km), are estimated. Moreover, estimations of rotational speed (0.9–76.8 cm/s), ocean surface layer heat content along temperature sections and eddy available heat anomaly (mean value −9.74 × 109 Jm−2) give a wider description of the detected eddies. In our study area, the spawning of eddies is found to occur downstream of the Southeast Indian Ridge and in correspondence of the polar front (PF) with regard to the ACC frontal structure. The contribution of eddies to the global heat budget is not only linked to their ability to cross the ACC fronts but also to the capacity of keeping partially unaltered the properties of water inside them. Analysis of the relation between the translation and rotational speeds shows that a typical eddy may effectively be a significant part (0.8%) of the net meridional heat transport across the PF with a mean heat content/anomaly of −7.65 × 1019 J.
Dynamical responses of the west Pacific North Equatorial Countercurrent (NECC) system to El Niño events
Authors: Zhao Jun , Li Yuanlong , Wang Fan
Published: April 2013

Analysis of data sets based on satellite measurements during 1992–2011 reveals pronounced interannual‐to‐decadal variations of the west Pacific North Equatorial Countercurrent (NECC) system, involving the intensity (INT), position (YCM), and path length (LCM) of the surface NECC jet, together with the associated recirculation gyres and mesoscale eddies. During the 1997–1998 and 2009–2010 El Niño events, the NECC jet showed increased INT before, and decreased INT, northerly position, and lengthened path after the mature phase. During the 1993–1995 and 2002–2005 central Pacific warming events, it also showed increased INT but no evident changes in YCM and LCM. The varied responses caused the different natures of individual El Niño events. In 1998 and 2010, reflected upwelling Kelvin waves south of the NECC jet, together with the downwelling Rossby waves north of it induced by the following strong La Niña events, weakened the NECC jet through geostrophy and shifted it northward. This process was however absent during the 1993–1995 and 2002–2005 events. Intensified NECC jet during warm conditions gives rise to anomalously active mesoscale eddies, which also contribute to the unstable states of the NECC system in 1998 and 2010. Hindcast with a linear Rossby wave model reveals a slow change of the western Pacific NECC system during the past 50 years. Since 1990s, low‐frequency variance of the NECC system has been dominated by quasi‐decadal signals and more closely associated with wind forcing in the western Pacific Ocean, which corresponds to the slow changes of the El Niño/Southern Oscillation (ENSO)‐related wind forcing pattern.
Acoustic scattering from a suspension of flocculated sediments
Authors: MacDonald Iain T. , Vincent Christopher E. , Thorne Peter D. , Moate Benjamin D.
Published: April 2013

A series of controlled laboratory experiments have been conducted to investigate the backscatter of high frequency sound (3–5 MHz) from suspensions of fine sediment in its unflocculated (primary) state and at various levels of flocculation. The size and fall‐velocity distributions of the flocs were determined using an optical system and a settling tube, thus allowing floc density to be determined. The measurements have conclusively demonstrated that the acoustic properties of the flocculated particles are not solely controlled by the primary particles; some aspect of the floc structure is influencing the scattering characteristics. The overall trend is for the form function (Ks) to increase as the degree of flocculation increases. This trend was also observed in the total scattering cross section (
) but this result is dependent on the assumption that viscous absorption for flocculated particles is negligible. The measured scattering properties are compared to the predicted values from two theoretical models, the elastic (ES) and fluid sphere (FS) models. While the results show that, in their current form, neither model is capable of adequately representing the scattering characteristics of a suspension of flocculated particles, the two models did provide upper (ES) and lower (FS) bounds to the measurements. In terms of the operational use of acoustics to measure the concentration of flocculated sediments, empirical relationships could be fitted to the observations but, until a better theoretical understanding of how sound interacts with flocculated particles is achieved, the fitting of such empirical relations may be somewhat premature.
Stationary waves in the wintertime mesosphere: Evidence for gravity wave filtering by stratospheric planetary waves
Authors: Lieberman R. S. , Riggin D. M. , Siskind D. E.
Published: April 2013

Quasi‐stationary planetary‐scale waves in the wintertime mesosphere and lower thermosphere (MLT) are thought to be forced in part by drag imparted by gravity waves that have been modulated by underlying stratospheric waves. Although this mechanism has been demonstrated numerically, there have been very few observational studies that examine wave driving as a source of planetary waves in the MLT. This study uses data from EOS Aura and TIMED between 2005 and 2011 to examine the momentum budget of MLT wintertime planetary waves. Monthly averages for January indicate that the dynamics of zonal wave number 1 are determined from a three‐way balance among the Coriolis acceleration, the pressure gradient force, and a momentum residual term that reflects wave drag. The MLT circulations in January 2005, 2006, 2009, and 2011 are qualitatively consistent with a simple model of wave forcing by drag from gravity waves that have been modulated by stratospheric planetary waves. MLT winds during these years are also consistent with analyses from a high‐altitude operational prediction model that includes parameterized nonorographic gravity wave drag. The importance of wave drag for the MLT momentum budget suggests that the gradient wind approximation is inadequate for deriving planetary‐scale winds from global temperature measurements. Our results underscore the need for direct global wind measurements in the MLT.
CALIOP and AERONET aerosol optical depth comparisons: One size fits none
Authors: Omar A. H. , Winker D. M. , Tackett J. L. , Giles D. M. , Kar J. , Liu Z. , Vaughan M. A. , Powell K. A. , Trepte C. R.
Published: April 2013

We compare the aerosol optical depths (AOD) retrieved from backscatter measurements of the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) satellite with coincident Aerosol Robotic Network (AERONET) measurements. Overpass coincidence criteria of ±2 h and within a 40 km radius are satisfied at least once at 149 globally distributed AERONET sites from 2006 to 2010. Most data pairs (>80%) use AERONET measurements acquired ±30 min of the overpass. We examine the differences in AOD estimates between CALIOP and AERONET for various aerosol, environmental, and geographic conditions. Results show CALIOP AOD are lower than AERONET AOD especially at low optical depths as measured by AERONET (500 nm AOD < 0.1). Furthermore, the median relative AOD difference between the two measurements is 25% of the AERONET AOD for AOD > 0.1. Differences in AOD between CALIOP and AERONET are possibly due to cloud contamination, scene inhomogeneity, instrument view angle differences, CALIOP retrieval errors, and detection limits. Comparison of daytime to nighttime number of 5 km × 60 m (60 m in the vertical) features detected by CALIOP show that there are 20% more aerosol features at night. We find that CALIPSO and AERONET do not agree on the cloudiness of scenes. Of the scenes that meet the above coincidence criteria, CALIPSO finds clouds in more than 45% of the coincident atmospheric columns AERONET classifies as clear.
The role of groundwater in the Amazon water cycle: 3. Influence on terrestrial water storage computations and comparison with GRACE
Authors: Pokhrel Yadu N. , Fan Ying , Miguez‐Macho Gonzalo , Yeh Pat J.‐F. , Han Shin‐Chan
Published: April 2013

We explore the mechanisms whereby groundwater influences terrestrial water storage (TWS) in the Amazon using GRACE observations and two contrasting versions of the LEAF‐Hydro‐Flood hydrological model: one with and the other without an interactive groundwater. We find that, first, where the water table is shallow as in northwestern Amazonia and floodplains elsewhere, subsurface stores (vadose zone and groundwater) are nearly saturated year‐round, hence river and flooding dominate TWS variation; where the water table is deep as in southeastern Amazonia, the large subsurface storage capacity holds the infiltrated water longer before releasing it to streams, hence the subsurface storage dominates TWS variation. Second, over the whole Amazon, the subsurface water contribution far exceeds surface water contribution to total TWS variations. Based on LEAF‐Hydro‐Flood simulations, 71% of TWS change is from subsurface water, 24% from flood water, and 5% from water in river channels. Third, the subsurface store includes two competing terms, soil water in the vadose zone and groundwater below the water table. As the water table rises, the length of vadose zone is shortened and hence the change in groundwater store is accompanied by an opposite change in soil water store resulting in their opposite phase and contributions to total TWS. We conclude that the inclusion of a prognostic groundwater store and its interactions with the vadose zone, rivers, and floodplains in hydrological simulations enhances seasonal amplitudes and delays seasonal peaks of TWS anomaly, leading to an improved agreement with GRACE observations.
Validation of the diurnal cycles in atmospheric reanalyses over Antarctic sea ice
Authors: Tastula Esa‐Matti , Vihma Timo , Andreas Edgar L. , Galperin Boris
Published: April 2013

The diurnal cycles of near‐surface meteorological parameters over Antarctic sea ice in six widely used atmospheric reanalyses are validated against observations from Ice Station Weddell. The station drifted from February through May 1992 and provided the most extensive set of meteorological observations ever collected in the Antarctic sea ice zone. For the radiative and turbulent surface fluxes, both the amplitude and shape of the diurnal cycles vary considerably among different reanalyses. Near‐surface temperature, specific humidity, and wind speed in the reanalyses all feature small diurnal ranges, which, in most cases, fall within the uncertainties of the observed cycle. A skill score approach revealed the superiority of the ERA‐Interim reanalysis in reproducing the observed diurnal cycles. An explanation for the shortcomings in the reanalyses is their failure to capture the diurnal cycle in cloud cover fraction, which leads to errors in other quantities as well. Apart from the diurnal cycles, NCEP‐CFSR gave the best error statistics.
The signature of the stratospheric Brewer‒Dobson circulation in tropospheric clouds
Authors: Li Ying , Thompson David W.J.
Published: April 2013

The signature of the stratospheric Brewer‒Dobson circulation (BDC) in tropospheric cloudiness is investigated in CloudSat and CALIPSO data from June 2006 through April 2011. During the Northern Hemisphere winter, periods of enhanced stratospheric wave driving are associated with increased cloud incidence in the tropical tropopause transition layer (TTL) juxtaposed against decreased cloud incidence in the Arctic troposphere. The results are consistent with the physical linkages between (1) the BDC and near‒tropopause temperature and (2) near‒tropopause temperature and upper tropospheric cloud incidence. The key finding of the work is that changes in the stratospheric circulation not only influence cloud amounts in the troposphere but also that they do so in a coupled manner that links climate variability in the Arctic and upper tropical troposphere. The results provide a pathway through which stratospheric processes influence tropospheric climate that is in addition to stratosphere/troposphere dynamical coupling.
Modeling of scattering properties of mineral aerosols using modified beta function
Authors: Tang Hong , Lin Jian‐Zhong
Published: April 2013

In the combined experiment from Amsterdam and modeling study, the spheroid is applied to represent the real mineral aerosols, and the single scattering properties of spheroids are computed using the T matrix combined with the improved geometric optical method in the wider size range and aspect ratio range. After that a modeling method for the scattering matrices of the feldspar, quartz, and red clay is proposed. In this approach, the ensemble‐average scattering matrix is calculated by using the number density distribution deduced from the fit of the measured normalized projected surface area distribution with the modified beta function and the shape distribution obtained by the least squares fit of all the six measured scattering elements. On the other hand, the models of scattering matrices using the lognormal function to fit the measured normalized number distribution and fit the measured normalized projected surface area distribution are also conducted as comparison. Furthermore, the asymmetry parameter is calculated in order to further verify the reliability of the proposed method. Simulation experiments indicate that the simulated scattering matrices with the proposed method are rather close to the measured scattering matrices for the three mineral aerosols, and this proposed numerical model can provide a simple, reliable, and efficient method to reproduce the scattering properties of the mineral aerosols based on the Amsterdam database.
Microphysical implications of cloud‐precipitation covariance derived from satellite remote sensing
Authors: Lebsock Matthew , Morrison Hugh , Gettelman Andrew
Published: April 2013

Covariance between cloud and precipitation water in shallow marine boundary layer clouds is assessed using collocated satellite observations from CloudSat and the moderate resolution imaging spectroradiometer (MODIS) at spatial scales typical of global models. An analytic construct is presented, which suggests that global models that do not take subgrid scale cloud‐precipitation covariance into account in their microphysical parameterizations may significantly underestimate grid mean microphysical process rates in warm clouds. The proposed framework indicates a mean bias in autoconversion rates of 129% when subgrid scale cloud water variability is neglected and bias in accretion rates of 60% when subgrid cloud‐precipitation covariability is neglected at a model grid resolution of 141 km. The bias in accretion rate is dependent on the significant correlation (ρ) found between cloud and precipitation, which in the global mean is found to be ρ = 0.44. The regional distribution of the process rate biases is largely governed by the spatial pattern of cloud water variance. Specific areas of low cloud water variance are found in the subtropical eastern ocean basins and the high latitudes, whereas much of the tropics display relatively larger cloud water variance. These regional distinctions in cloud water variance are associated with commensurate regionality in the process rate biases. The magnitude of the bias has a scale dependence that is governed by the spatial scaling behavior of the cloud and precipitation variances, which follow a power law scaling with exponent of 2/3 at scales below about 10 km and decreasing exponent above this length scale. While the parametric framework reduces biases in the accretion rate estimated from the grid‐mean values of cloud and precipitation water, it is shown that it still undercorrects the accretion rate because it neglects the fact that the precipitation fractional area is less than the cloud fractional area and is preferentially colocated with the highest cloud water concentrations. These results imply that (1) predicting the appropriate balance of autoconversion to accretion in global models requires not only the subgrid scale cloud water variability but also the subgrid scale covariability of cloud and precipitation water and (2) the ability of a global model to calculate the correct regional variation in process rates depends crucially on the fidelity of that model to predict or diagnose the spatial distribution of the variance in cloud water.
Two years of free‐tropospheric aerosol layers observed over Portugal by lidar
Authors: Preißler J. , Wagner F. , Guerrero‐Rascado J. L. , Silva A. M.
Published: April 2013

Multi‐wavelength Raman light detection and ranging (lidar) observations were analyzed, which were performed in Évora, Portugal, during more than 2 years on a regular basis in the framework of the European Aerosol Research Lidar Network (EARLINET). An aerosol characterization in terms of the lidar ratios at 355 and 532 nm and the extinction and backscatter related Ångström exponents is presented. Aerosol layers in the free troposphere were classified according to their origin. Clear differences in the intensive optical properties were found for layers of mineral dust from the Sahara and from Asia, of anthropogenic aerosol from Europe and from North America, as well as of biomass burning smoke from the Iberian Peninsula and from North America, respectively. In general, the mean Ångström exponents of aerosol layers of the same type, but from closer source regions, were smaller than those from aerosol layers transported over a longer distance. This hints at the deposition of large particles along the transportation path, especially for anthropogenic aerosol and mineral dust. Besides, the seasonal behavior of aerosol in the free troposphere over Évora was studied. Seventy‐three percent of the detected layers were observed during spring and summer. On average, the layers were highest in summer with an overall mean layer height of (3.8 ±1.9) km above sea level (asl), and lowest in winter with (2.3 ±0.9) km asl.
Evaluation of satellite and reanalysis products of downward surface solar radiation over East Asia: Spatial and seasonal variations
Authors: Jia Binghao , Xie Zhenghui , Dai Aiguo , Shi Chunxiang , Chen Feng
Published: April 2013

Surface solar radiation plays a crucial role in surface energy and water budgets, and it is also an important forcing for land hydrological models. In this study, the downward surface solar radiation (DSSR) from two satellite products, the Fengyun‐2C satellite (FY‐2C) and the Fast Longwave and Shortwave Radiative Fluxes project (FLASHFlux), and two reanalysis datasets, NCEP‐DOE and ERA‐Interim, was evaluated against ground‐based observations (OBS) from 94 stations over mainland China during July 2006 to June 2009. It is found that the mean DSSR derived from FY‐2C is comparable to OBS, with small positive biases of 3.0 Wm–2 for daily data and 3.5 Wm‐2 for monthly data and moderate RMSEs of 49.3 Wm‐2 (daily) and 31.9 Wm‐2 (monthly). These results are comparable to those for FLASHFlux, which has the lowest RMSEs (43.2 Wm‐2 and 30.5 Wm‐2 for daily and monthly data, respectively) and the strongest correlations with OBS (r = 0.90 and 0.93 for daily and monthly data, respectively) among the four products. The DSSR from the reanalyses has much larger RMSEs and generally lower correlations with OBS than the satellite products, especially for the NCEP‐DOE products. Results also show that daily DSSR values are sensitive to the averaging grid size, while monthly mean DSSR is largely insensitive to the averaging scale. The DSSR from the four datasets over East Asia shows similar spatial patterns with large seasonal variations but differs in magnitude. In summer, high DSSR is observed over western China, while low DSSR is seen primarily over South Asia and the Sichuan Basin associated with extensive cloud cover (CC) and large precipitable water (PW). In winter, the high DSSR center shifts to South Asia due to decreased CC and PW, and the DSSR decreases from the South to the North. Deficiencies in the parameterizations of clouds, aerosols, and water vapor, as well as errors in atmospheric and surface properties for the retrieval algorithms contribute to the lower correlation of the DSSR derived from FY‐2C (r = 0.82 and 0.90 for daily and monthly data) with OBS than those from FLASHFlux product. Further improvements to the representation of clouds and aerosols in the FY‐2C retrieval algorithm are needed.
The role of time in heterogeneous freezing nucleation
Authors: Wright Timothy P. , Petters Markus D.
Published: April 2013

A small fraction of particles in the atmosphere can catalyze ice formation in cloud water drops through heterogeneous freezing nucleation at temperatures warmer than the homogeneous freezing temperature of approximately −38°C. The rate for heterogeneous freezing nucleation is dependent on several factors, including the type and surface area of dust that is immersed inside the drop. Although nucleation is an inherently stochastic process resulting from size fluctuations of the incipient ice germ, there is a growing body of literature that suggests that quasi‐deterministic models of ice nucleation can describe laboratory experiments. Here we present new experiments and simulations that aim to better constrain theoretical models fitted to laboratory data. We collected ice nucleation data for Arizona Test Dust aerosol immersed in water using a droplet freezing assay setup that allows for the cooling rates to be changed between 10 and 0.01 K min−1. Discrete event simulations based on a variant of the multiple‐component stochastic model of heterogeneous freezing nucleation were used to simulate different experimental procedures. The nucleation properties of the dust are specified by four material‐dependent parameters that accurately describe the time dependence of the freezing process. We anticipate that the combination of discrete event simulations and a spectrum of experimental procedures described here can be used to design more meaningful laboratory experiments probing ice nucleation and will aid the development of better parameterizations for use in models.
Influence of monsoon system on α‐HCH fate in Asia: A model study from 1948 to 2008
Authors: Xu Yue , Tian Chongguo , Zhang Gan , Ming Lili , Wang Yan , Chen Yingjun , Tang Jianhui , Li Jun , Luo Chunling
Published: April 2013

The Chinese Gridded Pesticide Emission and Residue Model was applied to simulate long‐term environmental fate of α‐hexachlorocyclohexane (HCH) in Asia spanning 1948 to 2008. The simulation identifies the southern Tibetan Plateau and the margin of the Asian monsoon regime from the northeast area of the Tibetan Plateau to the Northeast China as sink areas. This spatial pattern of sink distribution is closely related to the summer monsoon activities in Asia. Wet deposition associated with the summer monsoon (June to August) is responsible for at least half of the α‐HCH mass entering into the sink areas. After deposition, relatively low temperature and high soil organic matter in the sink areas are favorable for α‐HCH detention. Chinese and Indian sources are the major contributors to α‐HCH contamination over the monsoon margin and the Tibetan Plateau, driven by the East Asian summer monsoon and South Asian summer monsoon, respectively.
Characteristics of global precipitable water in ENSO events revealed by COSMIC measurements
Authors: Teng Wen‐Hsin , Huang Ching‐Yuang , Ho Shu‐Peng , Kuo Ying‐Hwa , Zhou Xin‐Jia
Published: April 2013

Precipitable water (PW) retrievals from FORMOSAT‐3/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Global Positioning System (GPS) radio occultation (RO) measurements were analyzed and compared with those derived from Special Sensor Microwave/Imager (SSM/I) and Advanced Microwave Scanning Radiometer for Earth Observation System (AMSR‐E) during the El Niño‐Southern Oscillation (ENSO) events from 2007 to 2011. For the three ENSO events in 2007–2011, monthly mean binned COSMIC PW results are in a very high correlation (up to 0.98) with those of SSM/I and AMSR‐E over the ocean, generally with root‐mean‐square differences less than 4 mm. PW retrievals from the three satellites are also of similar latitudinal variations. However, the PW is slightly underestimated by GPS RO, in particular, in the tropical regions. This underestimate may be caused partially by the fact that not all RO measurements can reach the surface. Inter‐satellite PW anomaly comparisons for the winter months in the ENSO events, with respect to those during the neutral (non‐ENSO) months, show consistent ENSO signals with major PW anomaly near the central Pacific in the warm event and near the Indonesian region and east of Australia in the two cold events. However, the 2007/2008 La Niña is somewhat less correlated for COSMIC with AMSR‐E and SSM/I. For the stronger 2010/2011 La Niña, their PW anomalies are in higher correlations of about 0.8.
Solar wind dynamic pressure effect on planetary wave propagation and synoptic‐scale Rossby wave breaking
Authors: Lu Hua , Franzke Christian , Martius Olivia , Jarvis Martin J. , Phillips Tony
Published: April 2013

We provide statistical evidence of the effect of the solar wind dynamic pressure (Psw) on the northern winter and spring circulations. We find that the vertical structure of the Northern Annular Mode (NAM), the zonal mean circulation, and Eliassen‐Palm (EP)‐flux anomalies show a dynamically consistent pattern of downward propagation over a period of ~45 days in response to positive Psw anomalies. When the solar irradiance is high, the signature of Psw is marked by a positive NAM anomaly descending from the stratosphere to the surface during winter. When the solar irradiance is low, the Psw signal has the opposite sign, occurs in spring, and is confined to the stratosphere. The negative Psw signal in the NAM under low solar irradiance conditions is primarily governed by enhanced vertical EP‐flux divergence and a warmer polar region. The winter Psw signal under high solar irradiance conditions is associated with positive anomalies of the horizontal EP‐flux divergence at 55°N–75°N and negative anomalies at 25°N–45°N, which corresponds to the positive NAM anomaly. The EP‐flux divergence anomalies occur ~15 days ahead of the mean‐flow changes. A significant equatorward shift of synoptic‐scale Rossby wave breaking (RWB) near the tropopause is detected during January–March, corresponding to increased anticyclonic RWB and a decrease in cyclonic RWB. We suggest that the barotropic instability associated with asymmetric ozone in the upper stratosphere and the baroclinic instability associated with the polar vortex in the middle and lower stratosphere play a critical role for the winter signal and its downward propagation.
The precipitating cloud population of the Madden‐Julian Oscillation over the Indian and west Pacific Oceans
Authors: Barnes Hannah C. , Houze Robert A.
Published: April 2013

The variability of the precipitating cloud population of the Madden‐Julian Oscillation (MJO) is represented by statistics of echo features seen by the Tropical Rainfall Measuring Mission's Precipitation Radar over the central Indian and west Pacific Oceans. Echo features include isolated shallow echoes, deep convective cores, wide convective cores, and broad stratiform regions. Isolated shallow echoes are ever present but most numerous during suppressed MJO phases. Broad stratiform regions dominate variability in areal coverage and maximize during active phases. Deep convective and wide convective cores are more common and variable in number than broad stratiform regions. The magnitude of variability is similar in both regions. In the central Indian Ocean, active MJO phases have synchronous maximization of deep convective entities. In the west Pacific, broad stratiform regions maximize prior to wide convective cores. Reanalysis indicates that isolated shallow echoes are most numerous in dry mid‐tropospheric conditions and strong low‐level (1000–750 hPa) shear. Mid‐tropospheric moisture increases before deeper convective features increase in number, maximizes as deep convective features maximize, and decreases as wide convective cores and broad stratiform regions decline in population. Active‐stage deep and wide convective cores occur preferentially with moist mid‐tropospheric conditions and strong low‐level shear. Acute shear may favor downdraft momentum transport and consequently more robust gust‐front convective triggering. Broad stratiform regions maximize with a moist mid‐troposphere, strong low‐level shear, and moderate upper‐level (750–500 hPa) shear that is not so strong that the stratiform region disconnects from its convective moisture source.
Assimilation of microwave brightness temperature in a land data assimilation system with multi‐observation operators
Authors: Jia Binghao , Tian Xiangjun , Xie Zhenghui , Liu Jianguo , Shi Chunxiang
Published: April 2013

A radiative transfer model (RTM) that provides a link between model states and satellite observations (e.g., brightness temperature) can act as an observation operator in land data assimilation to directly assimilate brightness temperatures. In this study, a microwave Land Data Assimilation System (LDAS) was developed with three RTMs (The radiative transfer model for bare field (QH), land emissivity model (LandEM), and Community Microwave Emission Model (CMEM)) as its multi‐observation operators (LDAS‐MO). Assimilation experiments using the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR‐E) satellite brightness temperature data from July 2005 to December 2008 were then conducted to investigate the impact of the RTMs on the assimilated results over China. It was found that the assimilated volumetric soil‐water content using each of the three observation operators improved the estimation of soil moisture content in the top soil layer (0–10 cm), with reduced root mean square errors (RMSEs), and increased correlation coefficients with field observations (OBS) as compared to a control run with no assimilation for the absence of frozen or snow‐covered conditions. The assimilated soil moisture for the QH model, which was more sensitive to dry soil than the other models, produced closer correlations with OBS in arid and semi‐arid regions while smaller RMSEs were observed for LandEM. CMEM agreed most closely with OBS over the middle and lower reaches of the Yangtze River due to its better simulation of the brightness temperature over densely vegetated areas. To improve assimilation accuracy, a Bayesian model averaging (BMA) scheme for the LDAS‐MO was developed. The BMA scheme was found to significantly enhance assimilation capability producing the soil moisture analysis, showing the lowest RMSEs and highest correlations with OBS over all areas. It was demonstrated that the BMA scheme with LDAS‐MO has the potential to estimate soil moisture with high accuracy.
Characterizing and understanding radiation budget biases in CMIP3/CMIP5 GCMs, contemporary GCM, and reanalysis
Authors: Li J.‐L. F. , Waliser D. E. , Stephens G. , Lee Seungwon , L'Ecuyer T. , Kato Seiji , Loeb Norman , Ma Hsi‐Yen
Published: April 2013

We evaluate the annual mean radiative shortwave flux downward at the surface (RSDS) and reflected shortwave (RSUT) and radiative longwave flux upward at top of atmosphere (RLUT) from the twentieth century Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 3 (CMIP3) simulations as well as from the NASA GEOS5 model and Modern‐Era Retrospective Analysis for Research and Applications analysis. The results show that a majority of the models have significant regional biases in the annual means of RSDS, RLUT, and RSUT, with biases from −30 to 30 W m−2. While the global average CMIP5 ensemble mean biases of RSDS, RLUT, and RSUT are reduced compared to CMIP3 by about 32% (e.g., −6.9 to 2.5 W m−2), 43%, and 56%, respectively. This reduction arises from a more complete cancellation of the pervasive negative biases over ocean and newly larger positive biases over land. In fact, based on these biases in the annual mean, Taylor diagram metrics, and RMSE, there is virtually no progress in the simulation fidelity of RSDS, RLUT, and RSUT fluxes from CMIP3 to CMIP5. A persistent systematic bias in CMIP3 and CMIP5 is the underestimation of RSUT and overestimation of RSDS and RLUT in the convectively active regions of the tropics. The amount of total ice and liquid atmospheric water content in these areas is also underestimated. We hypothesize that at least a part of these persistent biases stem from the common global climate model practice of ignoring the effects of precipitating and/or convective core ice and liquid in their radiation calculations.
Distribution and production of reactive mercury and dissolved gaseous mercury in surface waters and water/air mercury flux in reservoirs on Wujiang River, Southwest China
Authors: Fu Xuewu , Feng Xinbin , Guo Yanna , Meng Bo , Yin Runsheng , Yao Heng
Published: April 2013

Transformation and distribution of mercury (Hg) species play an important role in the biogeochemical cycling of mercury in aquatic systems. Measurements of water/air exchange fluxes of Hg, reactive mercury (RHg), and dissolved gaseous mercury (DGM) concentrations were conducted at 14 sites in five reservoirs on the Wujiang River, Guizhou, Southwest China. Clear spatial and temporal variations in Hg fluxes, RHg, and DGM concentrations were observed in the study area. Hg fluxes and RHg concentrations exhibited a consistent diurnal variation in the study area, with maximum fluxes and concentrations during daytime. A typical diurnal trend of DGM with elevated concentration at night was observed in a eutrophic reservoir with elevated bacteria abundance, suggesting a bacteria‐induced production of DGM in this reservoir. For other reservoirs, a combination of sunlight‐stimulated production and loss via photo‐induced oxidation and evaporation regulated the diurnal trends of DGM. Seasonal variations with elevated Hg fluxes and RHg concentrations in warm season were noticeable in the study area, which highlighted the combined effect of interrelationships between Hg species in water and environmental parameters. Hg fluxes exhibited much more significant correlations with RHg and THg concentrations and air temperature compared to DGM concentrations and solar radiation. The measured fluxes were significantly higher than those simulated using the water/air thin film Hg0 gradient model. Aside from the potential limitations of dynamic flux chamber method, this may also suggest the thin film gas exchange model is not capable of predicting water/air Hg flux under low wind speed conditions. Additionally, it is speculated that DGM concentrations might vary significantly in surface waters with depth, and measurements of DGM at a depth of 2–4 cm below the water surface probably underestimated the DGM concentration that should be taken into account in simulations of water/air flux using the thin film gas exchange model. An empirical model of water/air Hg flux was developed, and the simulated fluxes were compared well with measurements. The model yields a mean annual Hg emission of 3.2 ± 1.0 kg in the study area.
Microphysical and radiative changes in cirrus clouds by geoengineering the stratosphere
Authors: Cirisan A. , Spichtinger P. , Luo B. P. , Weisenstein D. K. , Wernli H. , Lohmann U. , Peter T.
Published: April 2013

In the absence of tangible progress in reducing greenhouse gas emissions, the implementation of solar radiation management has been suggested as measure to stop global warming. Here we investigate the impacts on northern midlatitude cirrus from continuous SO2emissions of 2–10 Mt/a in the tropical stratosphere. Transport of geoengineering aerosols into the troposphere was calculated along trajectories based on ERA Interim reanalyses using ozone concentrations to quantify the degree of mixing of stratospheric and tropospheric air termed “troposphericity”. Modeled size distributions of the geoengineered H2SO4‐H2O droplets have been fed into a cirrus box model with spectral microphysics. The geoengineering is predicted to cause changes in ice number density by up to 50%, depending on troposphericity and cooling rate. We estimate the resulting cloud radiative effects from a radiation transfer model. Complex interplay between the few large stratospheric and many small tropospheric H2SO4‐H2O droplets gives rise to partly counteracting radiative effects: local increases in cloud radiative forcing up to +2 W/m2for low troposphericities and slow cooling rates, and decreases up to −7.5 W/m2for high troposphericities and fast cooling rates. The resulting mean impact on the northern midlatitudes by changes in cirrus is predicted to be low, namely <1% of the intended radiative forcing by the stratospheric aerosols. This suggests that stratospheric sulphate geoengineering is unlikely to have large microphysical effects on the mean cirrus radiative forcing. However, this study disregards feedbacks, such as temperature and humidity changes in the upper troposphere, which must be examined separately.
A link between tropical intraseasonal variability and Arctic stratospheric ozone
Authors: Li King‐Fai , Tian Baijun , Tung Ka‐Kit , Kuai Le , Worden John R. , Yung Yuk L. , Slawski Benjiman L.
Published: April 2013

Previous studies using satellite measurements showed evidence that subtropical upper troposphere/lower stratosphere ozone (O3) can be modulated by tropical intraseasonal variability, the most dominant form of which is the Madden Julian Oscillation (MJO) with a period of 30–60 days. Here we further study the MJO modulation in the upper troposphere/lower stratosphere O3 over the northern extratropics and the Arctic. Significant MJO‐related O3 signals (13–20 Dobson units) are found over the northern extratropics (north of 30°N). The O3 anomalies change their magnitude and patterns depending on the phase of the MJO. Over the Arctic, the MJO‐related O3 anomalies are dominated by a wave number 2 structure and are anticorrelated with the geopotential height (GPH) anomalies at 250 hPa. The latter is similar to the findings in the previous studies over subtropics and indicates that the Arctic upper troposphere/lower stratosphere O3 anomalies are associated with dynamical motions near the tropopause. The teleconnection from the tropics to the Arctic is likely through propagation of planetary waves generated by the equatorial heating that affects the tropopause height and O3 at high latitudes.
Climatology of nocturnal low‐level jets over North Africa and implications for modeling mineral dust emission
Authors: Fiedler S. , Schepanski K. , Heinold B. , Knippertz P. , Tegen I.
Published: April 2013

This study presents the first climatology for the dust emission amount associated with Nocturnal Low‐Level Jets (NLLJs) in North Africa. These wind speed maxima near the top of the nocturnal boundary layer can generate near‐surface peak winds due to shear‐driven turbulence in the course of the night and the NLLJ breakdown during the following morning. The associated increase in the near‐surface wind speed is a driver for mineral dust emission. A new detection algorithm for NLLJs is presented and used for a statistical assessment of NLLJs in 32 years of ERA‐Interim reanalysis from the European Centre for Medium‐Range Weather Forecasts. NLLJs occur in 29% of the nights in the annual and spatial mean. The NLLJ climatology shows a distinct annual cycle with marked regional differences. Maxima of up to 80% NLLJ frequency are found where low‐level baroclinicity and orographic channels cause favorable conditions, e.g., over the Bodélé Depression, Chad, for November–February and along the West Saharan and Mauritanian coast for April–September. Downward mixing of NLLJ momentum to the surface causes 15% of mineral dust emission in the annual and spatial mean and can be associated with up to 60% of the total dust amount in specific areas, e.g., the Bodélé Depression and south of the Hoggar‐Tibesti Channel. The sharp diurnal cycle underlines the importance of using wind speed information with high temporal resolution as driving fields for dust emission models.
Top‐down estimate of China's black carbon emissions using surface observations: Sensitivity to observation representativeness and transport model error
Authors: Wang Xuan , Wang Yuxuan , Hao Jiming , Kondo Yutaka , Irwin Martin , Munger J. William , Zhao Yongjing
Published: April 2013

This study examines the sensitivity of “top‐down” quantification of Chinese black carbon (BC) emissions to the temporal resolution of surface observations and to the transport model error associated with the grid resolution and wet deposition. At two rural sites (Miyun in North China Plain and Chongming in Yangtze River Delta), the model‐inferred emission bias based on hourly BC observations can differ by up to 41% from that based on monthly mean observations. This difference relates to the intrinsic inability of the grid‐based model in simulating high pollution plumes, which often exert a larger influence on the arithmetic mean of observations at monthly time steps. Adopting the variation of BC to carbon monoxide correlation slope with precipitation as a suitable measure to evaluate the model's wet deposition, we found that wet removal of BC in the model was too weak, due in part to the model's underestimation of large precipitation events. After filtering out the observations during high pollution plumes and large precipitation events for which the transport model error should not be translated into the emission error, the inferred emission bias changed from −11% (without filtering) to −2% (with filtering) at the Miyun site, and from −22% to +1% at the Chongming site. Using surface BC observations from three more rural sites (located in Northeast, Central, and Central South China, respectively) as constraints, our top‐down estimate of total BC emissions over China was 1.80 ± 0.65 Tg/yr in 2006, 0.5% lower than the bottom‐up inventory of Zhang et al. (2009) but with smaller uncertainty.
Radiative and energetic constraints on the global annual mean atomic oxygen concentration in the mesopause region
Authors: Mlynczak Martin G. , Hunt Linda H. , Mertens Christopher J. , Marshall B. Thomas , Russell James M. , López Puertas Manuel , Smith Anne K. , Siskind David E. , Mast Jeffrey C. , Thompson R. Earl , Gordley Larry L.
Published: April 2013

We present a new approach to constrain and validate atomic oxygen (O) concentrations in the mesopause region (~ 80 to ~ 100 km). In a prior companion paper [Mlynczak et al., ], we presented O‐atom concentrations in the mesopause region inferred from measurements of day ozone and night hydroxyl emission rates made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. The approach presented here uses the constraint of global, annual mean energy balance to derive atomic oxygen concentrations, consistent with rates of radiative cooling by carbon dioxide (CO2) and solar heating due to molecular oxygen (O2). The mathematical difference between these cooling and heating rates, on a global annual mean basis, effectively constrains the maximum heating rate for the sum of all other processes. The remaining terms, solar heating due to ozone plus a series of exothermic chemical reactions can be expressed as functions of O. This new approach enables a simple mathematical expression that yields the vertical profile of global annual mean “radiatively constrained” atomic oxygen in the mesopause region. The radiatively constrained atomic oxygen depends only on the CO2 cooling rates, O2 solar heating rates, and standard reaction rate coefficients and enthalpies. Radiative cooling and solar heating rates used in these analyses are derived from measurements made by the SABER instrument on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. There is excellent agreement between the SABER radiatively constrained atomic oxygen and that derived from the SABER ozone and OH emission measurements over most of the mesopause region. Radiatively constrained atomic oxygen represents an upper limit on the global average O‐atom concentration in the mesopause region.
Atomic oxygen in the mesosphere and lower thermosphere derived from SABER: Algorithm theoretical basis and measurement uncertainty
Authors: Mlynczak Martin G. , Hunt Linda A. , Mast Jeffrey C. , Thomas Marshall B. , Russell James M. , Smith Anne K. , Siskind David E. , Yee Jeng‐Hwa , Mertens Christopher J. , Javier Martin‐Torres F. , Earl Thompson R. , Drob Douglas P. , Gordley Larry L.
Published: April 2013

Atomic oxygen (O) is a fundamental component in chemical aeronomy of Earth's mesosphere and lower thermosphere region extending from approximately 50 km to over 100 km in altitude. Atomic oxygen is notoriously difficult to measure, especially with remote sensing techniques from orbiting satellite sensors. It is typically inferred from measurements of the ozone concentration in the day or from measurements of the Meinel band emission of the hydroxyl radical (OH) at night. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the NASA Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite measures OH emission and ozone for the purpose of determining the O‐atom concentration. In this paper, we present the algorithms used in the derivation of day and night atomic oxygen from these measurements. We find excellent consistency between the day and night O‐atom concentrations from daily to annual time scales. We also examine in detail the collisional relaxation of the highly vibrationally excited OH molecule at night measured by SABER. Large rate coefficients for collisional removal of vibrationally excited OH molecules by atomic oxygen are consistent with the SABER observations if the deactivation of OH(9) proceeds solely by collisional quenching. An uncertainty analysis of the derived atomic oxygen is also given. Uncertainty in the rate coefficient for recombination of O and molecular oxygen is shown to be the largest source of uncertainty in the derivation of atomic oxygen day or night.