bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of methods to determine phytoplankton bloom initiation
Authors: Brody Sarah R. , Lozier M. Susan , Dunne John P.
Published: April 2013

Phytoplankton bloom phenology has important consequences for marine ecosystems and fisheries. Recent studies have used remotely sensed ocean color data to calculate metrics associated with the phenological cycle, such as the phytoplankton bloom initiation date, on regional and global scales. These metrics are often linked to physical or biological forcings. Most studies choose one of several common methods for calculating bloom initiation, leading to questions about whether bloom initiation dates calculated with different methods yield comparable results. Here we compare three methods for finding the date of phytoplankton bloom initiation in the North Atlantic: a biomass‐based threshold method, a rate of change method, and a cumulative biomass‐based threshold method. We use these methods to examine whether the onset of positive ocean‐atmosphere heat fluxes coincides with subpolar bloom initiation. In several coherent locations, we find differences in the patterns of bloom initiation created by each method and differences in the synchrony between bloom initiation and positive heat fluxes, which likely indicate various physical processes at play in the study region. We also assess the effect of missing data on the chosen methods.
Detecting Labrador Sea Water formation from space
Authors: Gelderloos R. , Katsman C. A. , Våge K.
Published: April 2013

In situ monitoring of deep water formation in the Labrador Sea is severely hampered by the harsh winter conditions in this area. Furthermore, the ongoing monitoring programs do not cover the entire Labrador Sea and are often summer observations. The network of satellite altimeters does not suffer from these limitations and could therefore give valuable additional information. Altimeters can in theory detect deep water formation, because the water column becomes denser during convection and therefore the sea surface becomes lower. This signal is small compared to variability in sea surface height induced by other sources, but when properly filtered and appropriately averaged in time and space, all four winters with Labrador Sea Water formation or renewal in the 1994–2009 period (1994, 1995, 2000, and 2008) have a clear large negative anomaly. The magnitude of this anomaly compares favorably with the range predicted by theory and in situ data analysis. Out of 16 winters, only one winter (2006) would be falsely identified as a deep convection winter based on its sea surface height anomaly signal, while the method did not miss a single deep convection winter. For most deep‒water‒formation winters even the spatial structure of the mixed layer depth distribution can be inferred.
Impact of bio‐optical data assimilation on short‐term coupled physical, bio‐optical model predictions
Authors: Shulman Igor , Frolov Sergey , Anderson Stephanie , Penta Brad , Gould Rick , Sakalaukus Peter , Ladner Sherwin
Published: April 2013

Data assimilation experiments with the coupled physical, bio‐optical model of Monterey Bay are presented. The objective of this study is to investigate whether the assimilation of satellite‐derived bio‐optical properties can improve the model predictions (phytoplankton population, chlorophyll) in a coastal ocean on time scales of 1–5 days. The Monterey Bay model consists of a physical model based on the Navy Coastal Ocean Model and a biochemical model which includes three nutrients, two phytoplankton groups (diatoms and small phytoplankton), two groups of zooplankton grazers, and two detrital pools. The Navy Coupled Ocean Data Assimilation system is used for the assimilation of physical observations. For the assimilation of bio‐optical observations, we used reduced‐order Kalman filter with a stationary forecast error covariance. The forecast error covariance is specified in the subspace of the multivariate (bio‐optical, physical) empirical orthogonal functions estimated from a monthlong model run. With the assimilation of satellite‐derived bio‐optical properties (chlorophyll a or absorption due to phytoplankton), the model was able to reproduce intensity and tendencies in subsurface chlorophyll distributions observed at water sample locations in the Monterey Bay, CA. Data assimilation also improved agreement between the observed and model‐predicted ratios between diatoms and small phytoplankton populations. Model runs with or without assimilation of satellite‐derived bio‐optical observations show underestimated values of nitrate as compared to the water sample observations. We found that an instantaneous update of nitrate based on statistical relations between temperature and nitrate corrected the model underestimation of the nitrate fields during the multivariate update.
Variations in the Pacific Decadal Oscillation since 1853 in a coral record from the northern South China Sea
Authors: Deng Wenfeng , Wei Gangjian , Xie Luhua , Ke Ting , Wang Zhibing , Zeng Ti , Liu Ying
Published: April 2013

The Pacific Decadal Oscillation (PDO) has been shown to have significant climatic and environmental impacts across the Pan‐Pacific Basin; however, there are no records of PDO activity from the South China Sea (SCS), the largest marginal sea in the northwest Pacific Ocean. This study suggests that a series of geochemical profiles obtained from a modern coral in the northern SCS records annual PDO activity dating back to 1853. These geochemical data are significantly correlated with the PDO index, and their patterns of variation closely match those of the PDO index over the last century. The relationship between the PDO and coral geochemistry may be related to the influence of the PDO on rainfall on Hainan Island. Rainfall patterns influence the volume of terrestrial runoff, which, in turn, is a primary determinant of δ18O and Δδ18O values in coral; however, coral δ13C values are also influenced by the 13C Suess effect. The results indicate that Sr/Ca ratios in coral are affected by a combination of sea surface temperature and terrestrial runoff.
Variability of the Atlantic off‐equatorial eastward currents during 1993–2010 using a synthetic method
Authors: Goes Marlos , Goni Gustavo , Hormann Verena , Perez Renellys C.
Published: April 2013

We have developed, validated, and applied a synthetic method to monitor the off‐equatorial eastward currents in the central tropical Atlantic. This method combines high‐density expendable bathythermograph (XBT) temperature data along the AX08 transect with altimetric sea level anomalies (SLAs) to estimate dynamic height fields from which the mean properties of the North Equatorial Countercurrent (NECC), the North Equatorial Undercurrent (NEUC) and the South Equatorial Undercurrent (SEUC), and their variability can be estimated on seasonal to interannual timescales. On seasonal to interannual timescales, the synthetic method is well suited for reconstructions of the NECC variability, reproduces the variability of the NEUC with considerable skill, and less efficiently describes variations of the SEUC, which is located in a region of low SLA variability. A positive correlation is found between interannual variations of the NECC transport and two indices based on an interhemispheric sea surface temperature (SST) gradient and southeasterly wind stress in the central tropical Atlantic. The NEUC is correlated on interannual timescales with SSTs and meridional wind stress in the Gulf of Guinea and zonal equatorial wind stress. This study shows that both altimetry and XBT data can be effectively combined for near‐real‐time inference of the dynamic and thermodynamic properties of the tropical Atlantic current system.
Changes in the onset and intensity of wind‐driven upwelling and downwelling along the North American Pacific coast
Authors: Bylhouwer Brian , Ianson Debby , Kohfeld Karen
Published: April 2013

The timing, duration, and intensity of wind‐driven upwelling and downwelling along the North American Pacific coast play an integral role in coastal circulation and basinwide ecosystem composition. It has been suggested that global warming will cause changes in these winds. Here we develop a new set of objective criteria to unambiguously determine the onset, duration, and intensity of upwelling and downwelling seasons due to local wind forcing. We use these criteria to examine and better characterize temporal trends in wind‐driven coastal currents over the previous 60 years and relate them to global warming and large‐scale climate oscillations in the coastal ocean between northern California and Vancouver Island (37°N and 51°N). We find an exceptionally variable onset of upwelling at all locations. Some significant temporal trends are found in summer onset and upwelling intensity time series near the Juan de Fuca Strait and off the coast of Oregon. Positive phases of the Pacific Decadal Oscillation are correlated to later and shorter upwelling seasons with weaker upwelling. Warm phases of the El Niño Southern Oscillation are associated with a later onset of summer upwelling south of Oregon and with more intense downwelling throughout the study area. Our analysis identifies strong interannual to interdecadal variability, and emphasizes the importance of time series length when isolating physical temporal trends influenced by large‐scale oscillatory behavior of the climate.
Barotropic and deep‐referenced baroclinic SSH variability derived from Pressure Inverted Echo Sounders (PIES) south of Africa
Authors: Behnisch M. , Macrander A. , Boebel O. , Wolff J.‐O. , Schröter J.
Published: April 2013

The objective of this paper is to evaluate the baroclinic and barotropic components of sea surface height (SSH) anomalies in the Southern Ocean and investigates the causes for the weak correlation between hydrographically derived and satellite measured SSH anomalies. To this end, data obtained by six Pressure Inverted Echo Sounders (PIES) deployed south of Africa were used to derive baroclinic and barotropic SSH anomalies. Our results show that the barotropic component accounts for 30%–60% of the variability between the jets of the Antarctic Circumpolar Current (ACC). In contrast, the jets associated with the major ACC fronts are predominantly baroclinic. The deep baroclinic component is important in the ACC and accounts for the major part of the baroclinic variability. The comparison with along‐track satellite altimetry (Jason 1/2) shows correlations coefficients of 0.24–0.92, with low values in regions of high‐barotropic variability. The comparison with gridded Aviso satellite altimetry generally shows higher correlation coefficients (0.33–0.92) and lower root mean square (RMS) errors compared to the along‐track product. In conclusion the barotropic SSH anomaly plays a major role in this region and has to be accounted for when assimilating SSH into ocean models. Due to their high‐baroclinic component, the correct representation of the time and space varying ACC fronts seems to be crucial for the right SSH anomaly partitioning used for assimilation purposes. Gridded products, namely Aviso, seems to be more suitable compared to along‐track products (Jason 1/2) in representing the variability of SSH anomalies.
cBathy: A robust algorithm for estimating nearshore bathymetry
Authors: Holman Rob , Plant Nathaniel , Holland Todd
Published: April 2013

A three‐part algorithm is described and tested to provide robust bathymetry maps based solely on long time series observations of surface wave motions. The first phase consists of frequency‐dependent characterization of the wave field in which dominant frequencies are estimated by Fourier transform while corresponding wave numbers are derived from spatial gradients in cross‐spectral phase over analysis tiles that can be small, allowing high‐spatial resolution. Coherent spatial structures at each frequency are extracted by frequency‐dependent empirical orthogonal function (EOF). In phase two, depths are found that best fit weighted sets of frequency‐wave number pairs. These are subsequently smoothed in time in phase 3 using a Kalman filter that fills gaps in coverage and objectively averages new estimates of variable quality with prior estimates. Objective confidence intervals are returned. Tests at Duck, NC, using 16 surveys collected over 2 years showed a bias and root‐mean‐square (RMS) error of 0.19 and 0.51 m, respectively but were largest near the offshore limits of analysis (roughly 500 m from the camera) and near the steep shoreline where analysis tiles mix information from waves, swash and static dry sand. Performance was excellent for small waves but degraded somewhat with increasing wave height. Sand bars and their small‐scale alongshore variability were well resolved. A single ground truth survey from a dissipative, low‐sloping beach (Agate Beach, OR) showed similar errors over a region that extended several kilometers from the camera and reached depths of 14 m. Vector wave number estimates can also be incorporated into data assimilation models of nearshore dynamics.
Changes of ns‐soot mixing states and shapes in an urban area during CalNex
Authors: Adachi Kouji , Buseck Peter R.
Published: April 2013

Aerosol particles from megacities influence the regional and global climate as well as the health of their occupants. We used transmission electron microscopes (TEMs) to study aerosol particles collected from the Los Angeles area during the 2010 CalNex campaign. We detected major amounts of ns‐soot, defined as consisting of carbon nanospheres, sulfate, sea salt, and organic aerosol (OA) and lesser amounts of brochosome particles from leaf hoppers. Ns‐soot‐particle shapes, mixing states, and abundances varied significantly with sampling times and days. Within plumes having high CO2 concentrations, much ns‐soot was compacted and contained a relatively large number of carbon nanospheres. Ns‐soot particles from both CalNex samples and Mexico City, the latter collected in 2006, had a wide range of shapes when mixed with other aerosol particles, but neither sets showed spherical ns‐soot nor the core‐shell configuration that is commonly used in optical calculations. Our TEM observations and light‐absorption calculations of modeled particles indicate that, in contrast to ns‐soot particles that are embedded within other materials or have the hypothesized core‐shell configurations, those attached to other aerosol particles hardly enhance their light absorption. We conclude that the ways in which ns‐soot mixes with other particles explain the observations of smaller light amplification by ns‐soot coatings than model calculations during the CalNex campaign and presumably in other areas.
Multisegment statistical bias correction of daily GCM precipitation output
Authors: Grillakis Manolis G. , Koutroulis Aristeidis G. , Tsanis Ioannis K.
Published: April 2013

An improved bias correction method for daily general circulation model (GCM) precipitation is presented. The method belongs to the widely used family of quantile mapping correction methods. The method uses different instances of gamma function that are fitted on multiple discrete segments on the precipitation cumulative density function (CDF), instead of the common quantile‐quantile approach that uses one theoretical distribution to fit the entire CDF. This imposes to the method the ability to better transfer the observed precipitation statistics to the raw GCM data. The selection of the segment number is performed by an information criterion to poise between complexity and efficiency of the transfer function. The global precipitation output of Institut Pierre Simon Laplace Coupled Model for the period 1960–2000 is bias corrected using the precipitation observations of WATCH Forcing Data. The 1960–1980 period of observations was used to calibrate the bias correction method, while 1981–2000 was used for validation. The proposed method performs well on the validation period, according to two performance estimators.
Heterogeneous ice nucleation ability of crystalline sodium chloride dihydrate particles
Authors: Wagner Robert , Möhler Ottmar
Published: April 2013

The aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of the Karlsruhe Institute of Technology has been used to quantify the deposition mode ice nucleation ability of airborne crystalline sodium chloride dihydrate (NaCl ∙ 2H2O) particles with median diameters between 0.06 and 1.1 µm. For this purpose, expansion cooling experiments with starting temperatures from 235 to 216 K were conducted. Recently, supermicron‐sized NaCl ∙ 2H2O particles deposited onto a surface have been observed to be ice‐active in the deposition mode at temperatures below 238 K, requiring a median threshold ice saturation ratio of only 1.02 in the range from 238 to 221 K. In AIDA, heterogeneous ice nucleation by NaCl ∙ 2H2O was first detected at a temperature of 227.1 K with a concomitant threshold ice saturation ratio of 1.25. Above that temperature, the crystallized salt particles underwent a deliquescence transition to form aqueous NaCl solution droplets upon increasing relative humidity. At nucleation temperatures below 225 K, the inferred threshold ice saturation ratios varied between 1.15 and 1.20. The number concentration of the nucleated ice crystals was related to the surface area of the seed aerosol particles to deduce the ice nucleation active surface site (INAS) density of the aerosol population as a function of the ice supersaturation. Maximum INAS densities of about 6 ⋅ 1010 m−2 at an ice saturation ratio of 1.20 were found for temperatures below 225 K. These INAS densities are similar to those recently derived for deposition mode ice nucleation on mineral dust particles.
New airborne retrieval approach for trade wind cumulus properties under overlying cirrus
Authors: Werner F. , Siebert H. , Pilewskie P. , Schmeissner T. , Shaw R. A. , Wendisch M.
Published: April 2013

A new retrieval method is presented to derive the optical thickness τ and effective droplet radius reff of shallow cumulus in the presence of overlying thin cirrus. This new approach allows for a retrieval without a priori knowledge of the microphysical and optical properties of the overlying cirrus. The retrieval is applied to helicopter‐borne solar spectral reflectivity measurements gathered by the Spectral Modular Airborne Radiation measurements sysTem (SMART‐HELIOS) above trade wind cumuli near Barbados. Collocated microphysical cumulus properties (liquid water content, effective droplet radius, droplet number concentration) were measured by in situ instruments installed on the Airborne Cloud Turbulence Observation System (ACTOS). Cloud inhomogeneities lead to an underestimation of retrieved τ of up to 114%, while reff is biased by up to 27%. Moreover, misrepresentation of the overlying cirrus may cause an overestimation of the classically retrieved cumulus reff of up to 50% and an underestimation of τof up to 6%. The new retrieval, effectively correcting for the influence of overlying cirrus, enables reliable estimates of τ of the cumuli for optically thin, overcast cirrus conditions and reduces the retrieval error for reff of the cumuli by almost 50%. Agreement between in situ measured and retrieved reff is in the range of ±1 μm. The retrieval can also reproduce the wide range of in situ measured mean reff (7–18  μm), which is a result of different aerosol load and cloud top heights on the different flight days. The observed τ ranges between 5 and 36.
Simultaneous observations of convective gravity waves from a ground‐based airglow imager and the AIRS satellite experiment
Authors: Yue Jia , Hoffmann Lars , Joan Alexander M.
Published: April 2013

We report the first joint observations of convectively generated gravity waves (GWs) using an OH airglow imager in Colorado and the Atmospheric Infrared Sounder (AIRS) onboard the Aqua satellite. Convective GWs, appearing as concentric rings, are observed over the western Great Plain regions of North America in the evening of 3 June 2008 in the airglow images. Inspecting both weather radars and AIRS radiances at 8.1 µm, strong convective clouds are found near the center of the concentric rings. The AIRS data at 4.3 µm show semicircular GWs with horizontal wavelengths of 60–80 km at 0900 UT, whereas the airglow imager observed circular GWs with horizontal wavelengths of ~44 km and airglow emission perturbation of ~6% at the same geographic location at 0910 UT. Large‐scale GWs (horizontal wavelengths greater than 100 km) emanating northwestward can be seen in both AIRS data and airglow images at different times. The imager observed small‐scale ripples associated with unstable concentric GWs in the mesopause in the early evening. Given that the brightness temperature perturbation of the GWs in the AIRS data is about 0.16 K and assuming that the GWs propagate without dissipation from the stratosphere to the upper mesosphere, the expected airglow emission perturbation caused by the GWs would be 4%–17%. Ray tracing simulations are performed to demonstrate that the GWs seen in AIRS and in the imager were likely excited by the same convective system.
Dimming and brightening trends in direct solar irradiance from 1909 to 2010 over Davos, Switzerland: Proportions of aerosol and gaseous transmission
Authors: Lachat D. , Wehrli Ch.
Published: April 2013

Apparent atmospheric transmission has been calculated from the longest stationary pyrheliometer record worldwide located at the Physikalisch‐Meteorologisches Observatorium Davos covering the years 1909 to 2010. Periods of early brightening up to 1929, dimming from 1930 to 1973, and brightening from 1974 onward were localized in this transmission record and found to be highly significant. Total transmission was partitioned into factors accounting for water vapor and aerosols. By modeling the apparent transmission of the gaseous components, aerosol transmission was obtained as a residual factor. Compared to the trend results found in total transmission, the dimming and brightening periods are more pronounced by a factor of 2; the early brightening period was found to be insignificant in aerosol transmission. The transmission records of water vapor, pure Rayleigh scattering, and ozone have shown only small trends over these periods, which are considered negligible.
An assessment of the diurnal variation of upper tropospheric humidity in reanalysis data sets
Authors: Chung Eui‐Seok , Soden Brian J. , Sohn B. J. , Schmetz Johannes
Published: April 2013

Diurnal variations of upper tropospheric humidity (UTH) in five different reanalysis data sets are compared over convective land and ocean regions and evaluated using multiple satellite observations as a reference. All reanalysis data sets reproduce the day‐night contrast of upper tropospheric humidity and the land‐ocean contrast in the diurnal amplitude. The infrared satellite measurements indicate a slightly later diurnal minimum over land relative to most reanalyses and the microwave satellite measurements, suggesting that cloud masking of the infrared radiances may introduce a small (∼3 h) bias in the phase. One reanalysis exhibits a substantially different diurnal cycle over land which is inconsistent with both infrared and microwave satellite measurements and other reanalysis products. This product also exhibits a different covariance between vertical velocity, cloud water, and humidity than other reanalyses, suggesting that the phase bias is related to deficiencies in the parameterization of moist convective processes.
Trajectory analysis of Saudi Arabian dust storms
Authors: Notaro Michael , Alkolibi Fahad , Fadda Eyad , Bakhrjy Fawzieh
Published: April 2013

Temporal and spatial characteristics of Saudi Arabian dust storms, with focus on associated air parcel trajectories, are investigated using station and gridded weather observations and remotely‐sensed aerosol optical depth (AOD). For 13 focal stations, an extensive pool of 84‐h backward trajectories is developed for dust storm days, and the trajectories are grouped into 3–5 representative clusters based on the K‐means technique and Silhouette Coefficients. Saudi Arabian dust storms are most prominent during February–June, with a mid‐winter peak along the southern coast of the Red Sea, spring peak across northern Saudi Arabia around the An Nafud Desert, and early summer peak in eastern Saudi Arabia around the Ad Dahna Desert. Based on backward trajectories, the primary local dust source is the Rub Al Khali Desert and the primary remote sources are the Saharan Desert, for western Saudi Arabia, and Iraqi Deserts, for northern and eastern Saudi Arabia. During February–April, the Mediterranean storm track is active, with passing cyclones and associated cold fronts carrying Saharan dust to Saudi Arabian stations along the northern coast of the Red Sea. Across Saudi Arabia, the highest AOD is achieved during dust storms that originate from the Rub Al Khali and Iraqi Deserts. Most stations are dominated by local dust sources (primarily Rub Al Khali), are characterized by three dominant trajectory paths, and achieve AOD values exceeding 1. In contrast, for stations receiving predominantly remote dust (particularly Saharan), 3–5 trajectory paths emerge and AOD values only reach approximately 0.6 as dust is lost during transport.
Sensitivity of typhoon track to asymmetric latent heating/rainfall induced by Taiwan topography: A numerical study of Typhoon Fanapi (2010)
Authors: Wang Chung‐Chieh , Chen Yu‐Han , Kuo Hung‐Chi , Huang Shin‐Yi
Published: April 2013

Typhoon Fanapi (2010) traveled westward across the Central Mountain Range of Taiwan on 19 September and its rainfall shifted from a symmetric to an asymmetric pattern with convection mostly to the south and southeast. Meanwhile, the storm slowed down from 22 to 14 km h−1 for 12 h upon leaving Taiwan, and led to heavy rainfall (>800 mm) and serious flooding over the low‐lying southwestern plains. Through simulation and sensitivity tests using the Cloud‐Resolving Storm Simulator at 3 km grid size, this study shows that the sudden and temporary speed reduction was caused by the asymmetric latent heating (LH), not the environmental flow. Specifically, over a 9 h period, the model storm moved westward at 16 km h−1 in the control run, but increasingly faster and more toward the northwest when the moisture (and thus the LH effect and its asymmetry) is gradually reduced. Steering flow analysis and estimation using model results suggest an eastward motion vector of about 8 km h−1, consistent with the observation, is produced by the asymmetric LH effect, when the effects from the vertical wind shear and beta‐drift are both taken into account. This result is further supported by the diagnosis on storm motion based on potential vorticity tendency. Although important, such feedback to typhoon track from rainfall asymmetry that is induced by the blocking effect of topography have not been reported or studied.
Vertical structure of aerosols, temperature, and moisture associated with an intense African dust event observed over the eastern Caribbean
Authors: Jung Eunsil , Albrecht Bruce , Prospero Joseph M. , Jonsson Haflidi H. , Kreidenweis Sonia M.
Published: April 2013

An unusually intense African dust event affected a large area of the western Atlantic and eastern Caribbean in early April 2010. Measurements made east of Barbados from the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter research aircraft are used to characterize particle size distributions; vertical distributions of aerosols, temperature, and moisture; and processes leading to the observed stratification in the boundary layer. The vertical profiles of various aerosol characterizations were similar on both days and show three layers with distinct aerosol and thermodynamic characteristics: the Saharan Air Layer (SAL; ~2.2 km ± 500 m), a subcloud layer (SCL; surface to ~500 m), and an intermediate layer extending between them. The SAL and SCL display well‐mixed aerosol and thermodynamic characteristics; but the most significant horizontal and vertical variations in aerosols and thermodynamics occur in the intermediate layer. The aerosol variability observed in the intermediate layer is likely associated with modification by shallow cumulus convection occurring sometime in the prior history of the air mass as it is advected across the Atlantic. A comparison of the thermodynamic structure observed in the event from its origin over Africa with that when it reached Barbados indicates that the lower part of the SAL was moistened by surface fluxes as the air mass was advected across the Atlantic. Mixing diagrams using aerosol concentrations and water vapor mixing ratios as conserved parameters provide insight into the vertical transports and mixing processes that may explain the observed aerosol and thermodynamic variability in each layer.
Roughness metrics of prismatic facets of ice
Authors: Neshyba S. P. , Lowen B. , Benning M. , Lawson A. , Rowe P. M.
Published: April 2013

We define a surface normal roughness metric for mesoscopically rough ice facets and present methods for inferring its value from variable pressure scanning electron micrographs. The methods rely on the anisotropic morphology of roughening in the prismatic plane, in which nearly all the variation in surface height occurs in the direction of the main symmetry axis of hexagonal‐habit ice prisms. Because of this symmetry, roughening appears at boundaries between prismatic facets in a way that readily permits quantitative analysis. Prismatic surfaces of four ice crystals grown between −45 and −30°C are found to have mean surface normal roughness values of 0.04–0.1, a range that corresponds to Cox–Munk roughness scale parameters 0.3–0.5. The distribution of tilt angles also suggests a Weibull shape parameter smaller than unity, a result that compares favorably with field observations. Shortwave scattering calculations of hexagonal polyhedra with surface morphologies derived from these observations indicate substantial retention of the well‐known 22° halo, despite a large (4–6%) reduction in the asymmetry parameter compared to smooth‐surface counterparts. We argue that this signature is a generic outcome of the symmetry of the roughening, which in turn originates in the anisotropic surface self‐diffusivity of these facets.
Estimation of the absolute surface air temperature of the Earth
Authors: Jones P. D. , Harpham C.
Published: April 2013

Average temperatures for the hemispheres and the globe are generally expressed as anomalies from a base period. Most users of these data and the underlying constituent gridded datasets do not require the values in absolute degrees, but a number of users might require this additional detail. An example group of users are climate modellers, who want to directly compare their simulations with reality in absolute units. Reanalysis datasets offer opportunities of assessing earlier absolute temperature estimates, but until recently their quality over data‐sparse regions of the world was questionable. Here, we assess the latest Reanalysis (ERA‐Interim) which is available from 1979 to the present against earlier direct estimates. Globally averaged ERA‐Interim and the earlier direct estimates of absolute surface temperatures across the world are about 0.55°C different for the 1981–2010 period, with ERA‐Interim cooler. The difference is only 0.29°C for the Northern Hemisphere, but larger at 0.81°C for the Southern Hemisphere. Spatially, the largest differences come from the Polar Regions, particularly the Antarctic.
Shadow‐band correction for diffuse ultraviolet radiation measurements
Authors: Sánchez G. , Serrano A. , Cancillo M. L.
Published: April 2013

Although the correction of shadow‐band solar total diffuse measurements has been extensively studied, the case of diffuse ultraviolet measurements has not been properly addressed. This study analyzes the correction factor to be applied to experimental measurements performed adapting a shadow‐band to a UV radiometer at a radiometric station in Badajoz (Spain). Three different models, based on approaches widely used for correcting total diffuse measurements, have been revised and adapted for the ultraviolet spectral range. Results reveal that some aspects of the correction proposed for total diffuse radiation are not suitable for ultraviolet diffuse radiation. The mathematical expressions are consequently modified to match the behavior in the ultraviolet range. Thus, three correction models particularized for ultraviolet diffuse measurements are proposed and validated against experimental data. The two models adapted from the original expressions proposed by Battles et al., and Steven show the best performance, with rRMSE of 2.74% and 2.20% and rMBE of 1.53% and 0.46%, respectively.
An observed negative trend in West Antarctic accumulation rates from 1975 to 2010: Evidence from new observed and simulated records
Authors: Burgener Landon , Rupper Summer , Koenig Lora , Forster Rick , Christensen William F. , Williams Jessica , Koutnik Michelle , Miège Clément , Steig Eric J. , Tingey David , Keeler Durban , Riley Laura
Published: April 2013

Observations of snow accumulation rates from five new firn cores show a negative trend that is statistically significant over the past several decades across the central West Antarctic ice sheet (WAIS). A negative temporal trend in accumulation rates is unexpected in light of rising surface temperatures as well as model simulations predicting higher accumulation rates for the region. Both the magnitude of the mean accumulation rates and the range of interannual variability observed in the new records compare favorably to older records collected from a broad area of the WAIS, suggesting that the new data may serve as a regional proxy for recent temporal trends in West Antarctic accumulation rates. The observed negative trend in accumulation is likely the result of a shift in low‐pressure systems over the Amundsen Sea region, dominated by changes in the austral fall season. Regional‐scale climate models and reanalysis data do not capture the negative trend in accumulation rate observed in these firn cores. Nevertheless the models and reanalyses agree well in both accumulation‐rate means and interannual variability, with no single model or dataset standing out as significantly more skilled at capturing the observed magnitude of and trend in accumulation rates in this region of the WAIS.
Improving the seasonal forecast for summertime South China rainfall using statistical downscaling
Authors: Tung Ying Lut , Tam Chi‐Yung , Sohn Soo‐Jin , Chu Jung‐Lien
Published: April 2013

The performance of various seasonal forecast systems in predicting the station‐scale summer rainfall in South China (SC) was assessed and was compared with that based on a statistical downscaling scheme. Hindcast experiments from 11 dynamical models covering the period of 1983–2003 were taken from the Asia‐Pacific Economic Cooperation Climate Center multimodel ensemble. Based on observations, singular value decomposition analysis (SVDA) showed that SC precipitation is strongly related to the broad‐scale sea level pressure (SLP) variation over Southeast Asia, western north Pacific, and part of the Indian Ocean. Analogous covariability was also found between model hindcasts and the observed station precipitation. Based on these results from SVDA, a statistical downscaling scheme for predicting SC station rainfall with model SLP as predictor was constructed. In general, the statistical scheme is superior to the original model prediction in two geographical regions, namely, western SC (near Guangxi) and eastern coastal SC (eastern Guangdong to part of Fujian). Further analysis indicated that dynamical models are able to reproduce the large‐scale circulation patterns associated with the recurrent modes of SC rainfall, but not the local circulation features. This probably leads to erroneous rainfall predictions in some locations. On the other hand, the statistical scheme was able to map the broad‐scale SLP patterns onto the station‐scale rainfall anomalies, thereby correcting some of the model biases. Overall, our results demonstrate how SC summer rainfall predictions can be improved by tapping the source of predictability related to large‐scale circulation signals from dynamical models.
Evaluation of the cloud thermodynamic phase in a climate model using CALIPSO‐GOCCP
Authors: Cesana Grégory , Chepfer Hélène
Published: April 2013

Level 1 measurements, including cross‐polarized backscatter, from the Cloud‐Aerosol Lidar with Orthogonal Polarization lidar, have been used to document the vertical structure of the cloud thermodynamic phase at global scale. We built a cloud phase identification (liquid, ice, or undefined) in the Global Climate Model (GCM)–oriented Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Cloud Product (GOCCP) and analyzed the spatial distribution of liquid and ice clouds in five January, February, March (JFM) seasons of global‐scale observations (2007–2011). We developed a cloud phase diagnosis in the Cloud Feedback Model Intercomparison Program Observation Simulator Package to evaluate the cloud phase description in the LMDZ5B climate model. The diagnosis in the simulator is fully consistent with the CALIPSO‐GOCCP observations to ensure that differences between the observations and the “model + simulator” ensemble outputs can be attributed to model biases. We compared the liquid and ice cloud vertical distributions simulated by the model with and without the simulator to quantify the impact of the simulator. The model does not produce liquid clouds above 3 km and produces ice instead of liquid at low and middle altitudes in polar regions, as well as along the Intertropical Convergence Zone. The model is unable to replicate the observed coexistence of liquid and ice cloud between 0°C and −40°C. Liquid clouds dominate T > −21°C in the observations, T > −12°C in the model + simulator, and T > −7.5°C in the model parameterization. Even if the simulator shifts the model cloud phase parameterization to colder temperature because of the lidar instrument peculiarities, the cloud phase transition remains too warm compared to the observations.
Evaluation of a coupled event‐driven phenology and evapotranspiration model for croplands in the United States northern Great Plains
Authors: Kovalskyy V. , Henebry G. M. , Roy D. P. , Adusei B. , Hansen M. , Senay G. , Mocko D. M.
Published: April 2013

A new model coupling scheme with remote sensing data assimilation was developed for estimation of daily actual evapotranspiration (ET). The scheme consists of the VegET, a model to estimate ET from meteorological and water balance data, and an Event Driven Phenology Model (EDPM), an empirical crop specific model trained on multiple years of flux tower data transformed into six types of environmental forcings that are called “events” to emphasize their temporally discrete character, which has advantages for modeling multiple contingent influences. The EDPM in prognostic mode supplies seasonal trajectories of normalized difference vegetation index (NDVI); whereas in diagnostic mode, it can adjust the NDVI prediction with assimilated remotely sensed observations. The scheme was deployed within the croplands of the Northern Great Plains. The evaluation used 2007–2009 land surface forcing data from the North American Land Data Assimilation System and crop maps derived from remotely sensed data of NASA's Moderate Resolution Imaging Spectroradiometer (MODIS). We compared the NDVI produced by the EDPM with NDVI data derived from the MODIS nadir bidirectional reflectance distribution function adjusted reflectance product. The EDPM performance in prognostic mode yielded a coefficient of determination (r2) of 0.8 ± 0.15and the root mean square error (RMSE) of 0.1 ± 0.035 across the entire study area. Retrospective correction of canopy attributes using assimilated MODIS NDVI values improved EDPM NDVI estimates, bringing the errors down to the average level of 0.1. The ET estimates produced by the coupled scheme were compared with the MODIS evapotranspiration product and with ET from NASA's Mosaic land surface model. The expected r2 = 0.7 ± 0.15 and RMSE = 11.2 ± 4 mm per 8 days achieved in earlier point‐based validations were met in this study by the coupling scheme functioning in both prognostic and retrospective modes. Coupled model performance was diminished at the periphery of the study area where r2 values were about 0.5 and RMSEs up to 15 ± 5 mm per 8 days. This performance degradation can be attributed both to insufficient EDPM training and to spatial heterogeneity in the accuracy of the crop maps. Overall, the experiment provided sufficient evidence of soundness of the EDPM and VegET coupling scheme, assuring its potential for spatially explicit applications.
Evaluation of an improved intermediate complexity snow scheme in the ORCHIDEE land surface model
Authors: Wang Tao , Ottlé Catherine , Boone Aaron , Ciais Philippe , Brun Eric , Morin Samuel , Krinner Gerhard , Piao Shilong , Peng Shushi
Published: April 2013

Snow plays an important role in land surface models (LSM) for climate and model applied over Fran studies, but its current treatment as a single layer of constant density and thermal conductivity in ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystems) induces significant deficiencies. The intermediate complexity snow scheme ISBA‐ES (Interaction between Soil, Biosphere and Atmosphere‐Explicit Snow) that includes key snow processes has been adapted and implemented into ORCHIDEE, referred to here as ORCHIDEE‐ES. In this study, the adapted scheme is evaluated against the observations from the alpine site Col de Porte (CDP) with a continuous 18 year data set and from sites distributed in northern Eurasia. At CDP, the comparisons of snow depth, snow water equivalent, surface temperature, snow albedo, and snowmelt runoff reveal that the improved scheme in ORCHIDEE is capable of simulating the internal snow processes better than the original one. Preliminary sensitivity tests indicate that snow albedo parameterization is the main cause for the large difference in snow‐related variables but not for soil temperature simulated by the two models. The ability of the ORCHIDEE‐ES to better simulate snow thermal conductivity mainly results in differences in soil temperatures. These are confirmed by performing sensitivity analysis of ORCHIDEE‐ES parameters using the Morris method. These features can enable us to more realistically investigate interactions between snow and soil thermal regimes (and related soil carbon decomposition). When the two models are compared over sites located in northern Eurasia from 1979 to 1993, snow‐related variables and 20 cm soil temperature are better reproduced by ORCHIDEE‐ES than ORCHIDEE, revealing a more accurate representation of spatio‐temporal variability.
The role of deep convection and nocturnal low‐level jets for dust emission in summertime West Africa: Estimates from convection‐permitting simulations
Authors: Heinold B. , Knippertz P. , Marsham J. H. , Fiedler S. , Dixon N. S. , Schepanski K. , Laurent B. , Tegen I.
Published: April 2013

Convective cold pools and the breakdown of nocturnal low‐level jets (NLLJs) are key meteorological drivers of dust emission over summertime West Africa, the world's largest dust source. This study is the first to quantify their relative contributions and physical interrelations using objective detection algorithms and an off‐line dust emission model applied to convection‐permitting simulations from the Met Office Unified Model. The study period covers 25 July to 02 September 2006. All estimates may therefore vary on an interannual basis. The main conclusions are as follows: (a) approximately 40% of the dust emissions are from NLLJs, 40% from cold pools, and 20% from unidentified processes (dry convection, land‐sea and mountain circulations); (b) more than half of the cold‐pool emissions are linked to a newly identified mechanism where aged cold pools form a jet above the nocturnal stable layer; (c) 50% of the dust emissions occur from 1500 to 0200 LT with a minimum around sunrise and after midday, and 60% of the morning‐to‐noon emissions occur under clear skies, but only 10% of the afternoon‐to‐nighttime emissions, suggesting large biases in satellite retrievals; (d) considering precipitation and soil moisture effects, cold‐pool emissions are reduced by 15%; and (e) models with parameterized convection show substantially less cold‐pool emissions but have larger NLLJ contributions. The results are much more sensitive to whether convection is parameterized or explicit than to the choice of the land‐surface characterization, which generally is a large source of uncertainty. This study demonstrates the need of realistically representing moist convection and stable nighttime conditions for dust modeling.
Atmospheric water balance over oceanic regions as estimated from satellite, merged, and reanalysis data
Authors: Park Hyo‐Jin , Shin Dong‐Bin , Yoo Jung‐Moon
Published: April 2013

The column integrated atmospheric water balance over the ocean was examined using satellite‐based and merged data sets for the period from 2000 to 2005. The data sets for the components of the atmospheric water balance include evaporation from the HOAPS, GSSTF, and OAFlux and precipitation from the HOAPS, CMAP, and GPCP. The water vapor tendency was derived from water vapor data of HOAPS. The product for water vapor flux convergence estimated using satellite observation data was used. The atmospheric balance components from the MERRA reanalysis data were also examined. Residuals of the atmospheric water balance equation were estimated using nine possible combinations of the data sets over the ocean between 60°N and 60°S. The results showed that there was considerable disagreement in the residual intensities and distributions from the different combinations of the data sets. In particular, the residuals in the estimations of the satellite‐based atmospheric budget appear to be large over the oceanic areas with heavy precipitation such as the intertropical convergence zone, South Pacific convergence zone, and monsoon regions. The lack of closure of the atmospheric water cycle may be attributed to the uncertainties in the data sets and approximations in the atmospheric water balance equation. Meanwhile, the anomalies of the residuals from the nine combinations of the data sets are in good agreement with their variability patterns. These results suggest that significant consideration is needed when applying the data sets of water budget components to quantitative water budget studies, while climate variability analysis based on the residuals may produce similar results.