bluuebunny/crossref_metadata_2025 huggingface dataset:
The Hyperdense Posterior Cerebral Artery Sign
Authors: Krings Timo , Noelchen Dagmar , Mull Michael , Willmes Klaus , Meister Ingo G. , Reinacher Peter , Toepper Rudolf , Thron Armin K.
Published: January 2006

Background and Purpose—

In the anterior circulation, the hyperdense middle cerebral artery (MCA) sign is a well-established marker for early ischemia. Similarly, the hyperdense basilar artery sign or the MCA “dot” sign may be a diagnostic clue for basilar artery or distal MCA branch thrombosis. The purpose of this study was to define the hyperdense posterior cerebral artery (PCA) sign and determine its incidence, diagnostic value, and reliability as a marker for ischemia in the territory of the PCA.

Methods—

Cranial computed tomographies (CCTs) of 48 patients with proven acute ischemia (<12 hours) in the PCA territory were compared by 3 independent and blinded readers to the CCTs of 86 age-matched patients without PCA infarction. Using follow-up imaging, the correlation of the hyperdense PCA (HPCA) with infarct size, thalamic infarction, and bleeding were investigated.

Results—

An HPCA was found in 35.4% of all patients with PCA infarction, typically within the ambient cistern, with a specificity of 95.4%. The thalamus was affected significantly more often (
P
=0.009) and the size of the infarct was significantly more often large than medium (
P
=0.018) or small (
P
<0.001) when an HPCA was present. Hemorrhagic transformation tended to occur more often when the HPCA was present.

Conclusions—

An HPCA was detected in more than one third of all patients with PCA ischemia, suiting the incidence of the hyperdense MCA. Based on our results, this sign may not only be helpful in the early diagnosis of PCA infarction but might also act as a prognostic marker in acute PCA territory ischemic stroke.
The Lifetime Risk of Stroke
Authors: Seshadri Sudha , Beiser Alexa , Kelly-Hayes Margaret , Kase Carlos S. , Au Rhoda , Kannel William B. , Wolf Philip A.
Published: January 2006

Background and Purpose—

The lifetime risk (LTR) of stroke has not been reported for the United States population; such data would assist public education and health planning.

Methods—

Framingham Original cohort participants (n=4897) who were stroke- and dementia-free at 55 years of age were followed biennially for up to 51 years (115 146 person years). We estimated the sex-specific 10-, 20-, and 30-year risks and LTR of developing a stroke by baseline age and blood pressure (BP) and compared it with the risk of developing Alzheimer disease (AD).

Results—

A total of 875 participants (522 women) developed a first-ever stroke; 749 (448 women) had an ischemic stroke. LTR of stroke was high and remained similar at ages 55, 65, and 75 years, approximating 1 in 5 for women and 1 in 6 for men. Participants with a normal BP (<120/80 mm Hg) had approximately half the LTR of stroke compared with those with high BP (≥140/90 mm Hg). The LTR of AD at age 65 (292 participants; 211 women) approximated 1 in 5 for women and 1 in 10 for men. The LTR of developing either stroke or dementia approximated 1 in 3 in both sexes.

Conclusion—

The LTR of stroke in middle-aged adults is 1 in 6 or more, which is equal to or greater than the LTR of AD. Women had a higher risk because of longer life expectancy. BP is a significant determinant of the LTR of stroke, and promotion of normal BP levels in the community might be expected to substantially reduce this risk.
Methylenetetrahydrofolate Reductase Polymorphisms and Homocysteine-Lowering Effect of Vitamin Therapy in Singaporean Stroke Patients
Authors: Ho Grace Y.-H. , Eikelboom John W. , Hankey Graeme J. , Wong Chen-Ru , Tan Siew-Li , Chan Jesscia B.-C. , Chen Christopher P.L.-H.
Published: January 2006

Background and Purpose—

Increased plasma total homocysteine (tHcy) levels are a risk factor for stroke and can be reduced with vitamin therapy. However, data on the tHcy-lowering effects of vitamins are limited largely to white populations. Thus, we aimed to determine in Singaporean patients with recent stroke: (1) the efficacy of vitamin therapy (folic acid, vitamin B
12
, and B
6
) on lowering tHcy, and (2) whether efficacy is modified by Methylenetetrahydrofolate reductase (
MTHFR
) gene polymorphism(s).

Methods—

A total of 443 eligible patients were recruited after presenting with ischemic stroke within the past 7 months. Patients were randomized to receive either placebo or vitamins. Fasting blood samples collected at baseline and at 1 year were assayed for levels of plasma tHcy. Patients were genotyped for
MTHFR C677T
and
A1298C
polymorphisms.

Results—

Mean baseline tHcy was similar in the 2 groups (placebo 13.7 μmol/L; vitamins 14.0 μmol/L;
P
=0.70). At 1 year, mean tHcy was 14.5 μmol/L in the placebo group compared with 10.7 μmol/L in the vitamin group (difference 3.8 μmol/L; 95% CI, 2.8 to 4.8 μmol/L;
P
<0.0001).
MTHFR C677T
genotype was an independent determinant of tHcy levels at baseline (
P
=0.005), but
A1298C
was not (
P=
0.08). Neither polymorphism significantly influenced the effect of vitamin therapy on tHcy at 1 year. The magnitude of the reduction in tHcy levels at 1 year with vitamin therapy was similar, irrespective of
MTHFR
genotypes.

Conclusions—

Vitamin therapy reduces mean tHcy levels by 3.8 μmol/L in the Singaporean stroke population studied.
MTHFR C677T
but not
A1298C
is independently associated with tHcy levels at baseline, and neither impacts the tHcy-lowering effect of vitamins used in this study.
Cerebral Microhemorrhage
Authors: Viswanathan Anand , Chabriat Hugues
Published: January 2006

Background and Purpose—

With the advent of modern MRI imaging techniques, cerebral microhemorrhages have been increasingly recognized on gradient-echo (GE) or T2*-weighted MRI sequences in different populations. However, in clinical practice, their diagnostic value, associated risk, and prognostic significance are often unclear. This review summarizes the pathophysiology, differential diagnosis, epidemiology, and clinical significance of cerebral microhemorrhages.

Summary of Review—

Focal areas of signal loss on GE MRI imaging pathologically represent focal hemosiderin deposition associated with previous hemorrhagic events. Cerebral microhemorrhages have been noted in healthy elderly, ischemic cerebrovascular disease, intracerebral hemorrhage (ICH), cerebral amyloid angiopathy (CAA), and in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. Microhemorrhages have been associated with older age, hypertension, smoking, white matter disease, lacunar infarcts, previous ischemic stroke, or ICH. In CAA, microhemorrhages predict both the risk of recurrent lobar ICH and future clinical decline. In patients with ischemic cerebrovascular disease, microhemorrhage number and location may be associated with executive dysfunction and may predict the occurrence of ICH and lacunar infarction.

Conclusions—

When cerebral microhemorrhages are diagnosed on MRI, conclusions regarding their significance and associated risks should be made based on the population examined. Further studies to characterize the associated risks of cerebral microhemorrhages in different stroke populations are needed to use this new imaging marker in therapeutic decisions.
Factors Associated With In-Hospital Mortality After Administration of Thrombolysis in Acute Ischemic Stroke Patients
Authors: Bateman Brian T. , Schumacher H. Christian , Boden-Albala Bernadette , Berman Mitchell F. , Mohr J.P. , Sacco Ralph L. , Pile-Spellman John
Published: January 2006

Background and Purpose—

The prospective trials evaluating the safety and efficacy of intravenous tissue plasminogen activator have generally been conducted at academic medical centers and community hospitals with an institutional commitment to stroke care. Relatively little is known about the safety of this therapy as it is used in the community. We therefore examined outcomes in acute stroke patients treated with thrombolysis using the largest discharge database available in the United States for the years 1999 to 2002.

Methods—

Data were derived from the Nationwide Inpatient Sample for the years 1999 to 2002. Using the appropriate
International Classification of Disease—Clinical Modification, 9th revision
, codes, patients admitted through the emergency room with a primary diagnosis of acute ischemic stroke were selected for analysis. From these patients, those coded as receiving thrombolysis were identified. Multivariate logistic regression was performed on the thrombolysis and nonthrombolysis cohorts to identify independent predictors of in-hospital mortality from among those clinical elements available in the database.

Results—

We identified 2594 patients treated with thrombolysis from a group of 248 964 patients admitted through the emergency room with a primary diagnosis of acute ischemic stroke. The thrombolysis cohort had a higher in-hospital mortality rate compared with the nonthrombolysis patients (11.4% versus 6.8%). The rate of intracerebral hemorrhage was 4.4% for the thrombolysis cohort and 0.4% for nonthrombolysis patients. Multivariate logistic regression showed advanced age, Asian/Pacific Islander race, congestive heart failure, and atrial fibrillation/flutter to be independent predictors of in-hospital mortality after thrombolysis. Thrombolysis volume, overall ischemic stroke volume, and teaching status were not significant predictors of in-hospital mortality after thrombolysis.

Conclusions—

Thrombolysis, as it is used in the community, has a safety profile that is similar to that observed in the large, prospective clinical trials.
New Cardioprotective Agent K201 Is Natriuretic and Glomerular Filtration Rate Enhancing
Authors: Lisy Ondrej , Burnett John C.
Published: January 2006

Background—

K201 (JTV519) is a newly developed 1,4-benzothiazepine drug with antiarrhythmic and cardioprotective properties. It functions via stabilization of the ryanodine receptor–calcium release channel in the heart (RyR2). This receptor has been identified in the kidney, and in vitro studies suggest a role in the control of renal hemodynamics. To date, the in vivo function of this receptor is undefined. We hypothesized that this new drug, which is being developed for the treatment of heart failure for its myocardial actions, also possesses renal hemodynamic enhancing and excretory properties. We also used immunohistochemistry to identify RyR2 in the normal canine kidney.

Methods and Results—

We investigated the renal actions of K201 during intrarenal infusion in normal anesthetized dogs. K201 was infused after baseline measurements at 2 doses (0.1 and 0.5 mg · kg
−1
· min
−1
). Immunohistochemistry was used to identify RyR2 presence in the kidney not exposed to K201. K201 was potently natriuretic and diuretic, with glomerular filtration rate– and renal blood flow–enhancing actions. The excretory responses to K201 administration were associated with decreases in distal tubular reabsorption of sodium despite a mild decrease in mean arterial pressure, which returned to baseline levels after K201 discontinuation. Immunohistochemistry of the normal canine kidney revealed the presence of RyR2 in the medullary collecting duct cells.

Conclusions—

We report for the first time that the newly developed cardioprotective drug K201 possesses natriuretic, diuretic, glomerular filtration rate–enhancing, and vasodilating properties that go beyond myocardial actions and may support its therapeutic role in treatment of heart failure.
Reference Station Networks for Monitoring Climatic Change in the Conterminous United States
Authors: Vose Russell S.
Published: January 2006

Set cover models are used to develop two reference station networks that can serve as near-term substitutes (as well as long-term backups) for the recently established Climate Reference Network (CRN) in the United States. The first network contains 135 stations distributed in a relatively uniform fashion in order to match the recommended spatial density for CRN. The second network contains 157 well-distributed stations that are generally not in urban areas in order to minimize the impact of future changes in land use. Both networks accurately reproduce the historical temperature and precipitation variations of the twentieth century.
Evaluation and Comparison of MODIS and IMS Snow-Cover Estimates for the Continental United States Using Station Data
Authors: Brubaker K. L. , Pinker R. T. , Deviatova E.
Published: January 2006

Satellite-derived information on fractional snow cover is essential to resource monitoring, hydrologic modeling, and climate change assessment. Evaluating the accuracy of remotely sensed snow-cover products is important but difficult, largely because point-scale surface observations are spatially sparse and generally nonrepresentative of the remote sensor footprint.

In this study, two remotely sensed snow-cover products [the Interactive Multisensor Snow and Ice Mapping System (IMS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) Climate Modeling Grid (CMG), v.3] are evaluated against ground observations from the Cooperative Observing Network and SNOTEL on a daily basis over the continental United States for calendar year 2000. Ground observations are treated as points in space and time; no physical modeling or statistical interpolation is applied. Hypothesis tests based on discrete and continuous distributions are developed to assess agreement between ground observations and the remotely sensed snow-cover products at 0.25° resolution. (The MODIS CMG product was degraded from 0.05° for this study, thus its potential is not fully evaluated.)

As overall snow extent increases in the course of the season, both MODIS and IMS improve in identifying snow-covered areas (fewer errors of omission), but deteriorate in identifying snow-free areas (more errors of commission). The detection of scattered areas of snow is generally better during ablation than during accumulation. Weaknesses of the statistical methods and assumptions are discussed.

This work will help to identify areas for improvement in snow-cover detection algorithms and provides a framework to assess the accuracy of remotely sensed snow cover used as model input and/or confirmation.
Comparing Aircraft-Based Remotely Sensed Energy Balance Fluxes with Eddy Covariance Tower Data Using Heat Flux Source Area Functions
Authors: Chávez JoséL. , Neale Christopher M. U. , Hipps Lawrence E. , Prueger John H. , Kustas William P.
Published: January 2006

In an effort to better evaluate distributed airborne remotely sensed sensible and latent heat flux estimates, two heat flux source area (footprint) models were applied to the imagery, and their pixel weighting/integrating functionality was investigated through statistical analysis. Soil heat flux and sensible heat flux models were calibrated. The latent heat flux was determined as a residual from the energy balance equation. The resulting raster images were integrated using the 2D footprints and were compared to eddy covariance energy balance flux measurements. The results show latent heat flux estimates (adjusted for closure) with errors of (mean ± std dev) −9.2 ± 39.4 W m−2, sensible heat flux estimate errors of 9.4 ± 28.3 W m−2, net radiation error of −4.8 ± 20.7 W m−2, and soil heat flux error of −0.5 ± 24.5 W m−2. This good agreement with measured values indicates that the adopted methodology for estimating the energy balance components, using high-resolution airborne multispectral imagery, is appropriate for modeling latent heat fluxes. The method worked well for the unstable atmospheric conditions of the study. The footprint weighting/integration models tested indicate that they perform better than simple pixel averages upwind from the flux stations. In particular the flux source area model (footprint) seemed to better integrate the resulting heat flux image pixels. It is suggested that future studies test the methodology for heterogeneous surfaces under stable atmospheric conditions.
Ocean Data Assimilation and Initialization Procedure for the Coupled GFDL/URI Hurricane Prediction System
Authors: Falkovich Aleksandr , Ginis Isaac , Lord Stephen
Published: January 2006

A new ocean data assimilation and initialization procedure is presented. It was developed to obtain more realistic initial ocean conditions, including the position and structure of the Gulf Stream (GS) and Loop Current (LC), in the Geophysical Fluid Dynamics Laboratory/University of Rhode Island (GFDL/URI) coupled hurricane prediction system used operationally at the National Centers for Environmental Prediction. This procedure is based on a feature-modeling approach that allows a realistic simulation of the cross-frontal temperature, salinity, and velocity of oceanic fronts. While previous feature models used analytical formulas to represent frontal structures, the new procedure uses the innovative method of cross-frontal “sharpening” of the background temperature and salinity fields. The sharpening is guided by observed cross sections obtained in specialized field experiments in the GS. The ocean currents are spun up by integrating the ocean model for 2 days, which was sufficient for the velocity fields to adjust to the strong gradients of temperature and salinity in the main thermocline in the GS and LC. A new feature-modeling approach was also developed for the initialization of a multicurrent system in the Caribbean Sea, which provides the LC source. The initialization procedure is demonstrated for coupled model forecasts of Hurricane Isidore (2002).
The Impact of Altimeter Sampling Patterns on Estimates of Background Errors in a Global Wave Model
Authors: Greenslade Diana J. M. , Young Ian R.
Published: January 2006

One of the main limitations to current wave data assimilation systems is the lack of an accurate representation of the structure of the background errors. One method that may be used to determine background errors is the observational method of Hollingsworth and Lönnberg. The observational method considers correlations of the differences between observations and the background. For the case of significant wave height (SWH), potential observations come from satellite altimeters. In this work, the effect of the irregular sampling pattern of the satellite on estimates of background errors is examined. This is achieved by using anomalies from a 3-month mean as a proxy for model errors. A set of anomaly correlations is constructed from modeled wave fields. The isotropic length scales of the anomaly correlations are found to vary considerably over the globe. In addition, the anomaly correlations are found to be significantly anisotropic. The modeled wave fields are then sampled at simulated altimeter observation locations, and the anomaly correlations are recalculated from the simulated altimeter data. The results are compared to the original anomaly correlations. It is found that, in general, the simulated altimeter data can capture most of the geographic and seasonal variability in the isotropic anomaly correlation length scale. The best estimates of the isotropic length scales come from a method in which correlations are calculated between pairs of observations from prior and subsequent ground tracks, in addition to along-track pairs of observations. This method was found to underestimate the isotropic anomaly correlation length scale by approximately 10%. The simulated altimeter data were not so successful in producing realistic anisotropic correlation functions. This is because of the lack of information in the zonal direction in the simulated altimeter data. However, examination of correlations along ascending and descending ground tracks separately can provide some indication of the areas on the globe for which the anomaly correlations are more anisotropic than others.
Performance Evaluation of Honeywell Silicon Piezoresistive Pressure Transducers for Oceanographic and Limnological Measurements*
Authors: Kumar Vijay , Joseph Antony , Prabhudesai R. G. , Prabhudesai S. , Nagvekar Surekha , Damodaran Vimala
Published: January 2006

Simultaneous calibrations of three temperature-compensated piezoresistive ruggedized precision “absolute” pressure transducers (Honeywell model PPTR0040AP5VB-BD), which have been designed specially for long-term coastal oceanographic and limnological measurements, have been carried out at four differing temperatures (10°, 20°, 30°, and 40°C) to evaluate their suitability for such applications. The full-scale pressure range of these shallow water absolute pressure sensors is ≈ 2800 hPa (equivalent to water depth of ≈ 18 m). Measurement results have been used to examine the transducers’ performance indicators, such as zero-point offset, accuracy, linearity, hysteresis, temperature sensitivity, and slope. Differing piezoresistive ruggedized precision absolute pressure transducers (PPTRs) exhibited differing zero-point offset values, ranging from 2 to −79 hPa. Temperature sensitivity of zero-point offset was ≈0.3 hPa over the temperature range 10°–40°C. The mean hysteresis over the full-scale absolute pressure range (≈2800 hPa) varied from approximately 2 to 8 hPa over the temperature range 10°–40°C. The slope of the least squares–fitted linear graph (taking the mean of ascending and descending pressures) was close to the ideal value of unity (deviation from 1 over the temperature range 10°–40°C was in the range of −0.001 to +0.005). Linearity was excellent, its mean over the entire pressure range being between ≈ −0.006% and 0.008% of full-scale (FS) over the above temperature range. The worst performance was exhibited at input pressures below ≈1500 hPa. Zero-point offset has played a significant role in deteriorating the accuracy of the PPTR, the mean accuracy (within ≈0.1% and −5%) having been exhibited by those transducers having offsets of 2 and −79 hPa, respectively. The mean accuracy exhibited temperature sensitivity of ≈1% in the range 10°–20°C and negligible sensitivity beyond 20°C. Use of a calibration equation significantly improved the mean static accuracy obtainable from the PPTR, to between −0.04% and 0.01% of FS. Evaluation results have indicated that a suitably calibrated temperature-compensated Honeywell PPTR provides an alternate cost-effective means for pressure measurements for coastal oceanographic and limnological studies.
An Estimate of the Vertical Ozone Profile Discrepancy between the Australian Brewer–Mast and Electrochemical Concentration Cell Ozonesondes
Authors: Lehmann Paul
Published: January 2006

An analysis is described that provides an additive correction for referencing the vertical ozone profiles of the Australian Brewer–Mast (BM; October 1984–December 1990) ozonesonde to those of the electrochemical concentration cell (ECC; January 1991–December 1999) using approximately coincident Stratospheric Aerosol and Gas Experiment (SAGE) II satellite ozone profile data. Because Australian BM ozonesondes may have been prepared differently from BM ozonesondes elsewhere, other results of intercomparisons between the two different ozonesonde types cannot be used. However, the present results are consistent with previously reported intercomparison studies between these two ozonesonde types, where the lowest altitude tropospheric ozone levels measured by the BM ozonesonde were approximately 25% lower than the values measured by the ECC ozonesonde. The magnitudes of the BM corrections were found to be generally less than about 0.5 mPa in partial pressure up to an altitude of approximately 28 km. Without direct intercomparison measurements, the corrections given here provide the only means of removing the discontinuity in the Melbourne ozonesonde dataset that occurred when ECC ozonesondes replaced the BM ozonesondes in 1991.
Detection of the MJO Signal from QuikSCAT
Authors: Arguez Anthony , Bourassa Mark A. , O’Brien James J.
Published: January 2006

Wind data from the SeaWinds instrument on NASA’s Quick Scatterometer (QuikSCAT) satellite are investigated to ascertain how well the surface manifestation of the Madden–Julian oscillation (MJO) can be resolved. The MJO signal is detected in nonfiltered gridded data using extended EOF analysis of the zonal wind field, overshadowed by annual, semiannual, and monsoon-related modes. After bandpass filtering with Lanczos weights, MJO signals are clearly detected in several kinematic quantities, including the zonal wind speed, the zonal pseudostress, and the velocity potential. Extraction of the MJO using QuikSCAT winds compares favorably with extraction using NCEP Reanalysis 2, except that the QuikSCAT signal appears to be more robust.

In addition, an alternative bandpass-filtering technique using variable filter weights near time series endpoints is presented. The method uses least squares minimization to match newly created frequency response functions in edge zones as closely as possible to the predetermined frequency response function of interior points. This method stands in contrast to the common practice of simply discarding those endpoints where a convolution cannot be computed.
Estimation of Ultraviolet-A Irradiance from Measurements of 368-nm Spectral Irradiance
Authors: Grant R. H. , Slusser J. R.
Published: January 2006

The estimation of ultraviolet-A (UV-A) radiation across the earth’s surface is needed to model plant productivity and future impacts of ultraviolet-B radiation on plant productivity. We have developed two models to estimate the UV-A irradiance from measurements of the diffuse and global spectral irradiance at 368 nm. The models were developed from 30-min-interval measurements made throughout 2000 at three locations across the United States and evaluated from 30-min measurements made throughout 2000 at three additional locations and throughout 2001 and 2002 at seven locations. UV-A irradiance was best estimated from measured global 368-nm irradiance and empirical functions defining the UV-A and 368-nm irradiance values estimated from a theoretical pseudospherical two-stream discrete-ordinates radiative transfer model. The radiative transfer model provided baseline irradiance relationships between UV-A irradiance and 368-nm spectral irradiance. The semiempirical model estimated the UV-A irradiance at seven locations across the United States with a mean bias error of 0.5 W m−2 and a root-mean-square error of 2 W m−2, corresponding to approximately ±4% of a clear-sky irradiance of 50 W m−2 for a solar zenith angle of 30°. This model error was comparable to the combined effect of previously estimated UV-A and 368-nm irradiance measurement errors.
Sensitivity of the Sandblasting Flux Calculations to the Soil Size Distribution Accuracy
Authors: Menut Laurent , Schmechtig Catherine , Marticorena Béatrice
Published: January 2006

Atmospheric dust concentrations are very sensitive to the dust surface emissions that are mainly controlled by saltation and sandblasting processes. Thus, a correct modeling of concentrations directly depends on mass flux parameterization accuracy. In 2001, Alfaro and Gomes proposed a whole set of parameterizations linking the dust flux to surface wind speed and soil characteristics. Their formulation is based on the integration of elementary fluxes, discretized along a soil size distribution. But, because the sandblasting is a threshold process, this discretization must be as fine as possible when the threshold acts. And because this threshold depends on dynamic parameters, it is necessary to always integrate fluxes with a high resolution. This leads to large numerical simulations. In this paper it is shown that it is possible to estimate fluxes with a good accuracy by adding equations that are dedicated to better describing the sensitive parts of the emission scheme.
Modulation of Daily Precipitation over Southwest Asia by the Madden–Julian Oscillation
Authors: Barlow Mathew , Wheeler Matthew , Lyon Bradfield , Cullen Heidi
Published: January 2006

Analysis of daily observations shows that wintertime (November–April) precipitation over Southwest Asia is modulated by Madden–Julian oscillation (MJO) activity in the eastern Indian Ocean, with strength comparable to the interannual variability. Daily outgoing longwave radiation (OLR) for 1979–2001 is used to provide a long and consistent, but indirect, estimate of precipitation, and daily records from 13 stations in Afghanistan reporting at least 50% of the time for 1979–85 are used to provide direct, but shorter and irregularly reported, precipitation data. In the station data, for the average of all available stations, there is a 23% increase in daily precipitation relative to the mean when the phase of the MJO is negative (suppressed tropical convection in the eastern Indian Ocean), and a corresponding decrease when the MJO is positive. The distribution of extremes is also affected such that the 10 wettest days all occur during the negative MJO phase. The longer record of OLR data indicates that the effect of the MJO is quite consistent from year to year, with the anomalies averaged over Southwest Asia more negative (indicating more rain) for the negative phase of the MJO for each of the 22 yr in the record. Additionally, in 9 of the 22 yr the average influence of the MJO is larger than the interannual variability (e.g., the relationship results in anomalously wet periods even in dry years and vice versa).

Examination of NCEP–NCAR reanalysis data shows that the MJO modifies both the local jet structure and, through changes to the thermodynamic balance, the vertical motion field over Southwest Asia, consistent with the observed modulation of the associated synoptic precipitation. A simple persistence scheme for forecasting the sign of the MJO suggests that the modulation of Southwest Asia precipitation may be predictable for 3-week periods. Finally, analysis of changes in storm evolution in Southwest Asia due to the influence of the MJO shows a large difference in strength as the storms move over Afghanistan, with apparent relevance for the flooding event of 12–13 April 2002.
Diurnal Circulation of the Bolivian Altiplano. Part II: Theoretical and Model Investigations
Authors: Zängl Günther , Egger Joseph
Published: January 2006

The first step in the evolution of the diurnal circulation of grand plateaus is the matutinal buildup of inflow through mountain passes connecting the lowlands with the plateau proper. Maximum inward transport is attained in the afternoon. Corresponding observations at the Bolivian Altiplano are described in the first part of this paper. Here, both a linear model and the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used to better understand the dynamics of this process. First discussed is the solution to the linear sea-breeze problem where a heated fluid layer is restricted to the half plane x ≤ 0. Rapidly moving barotropic modes lead to surface pressure fall in the heated half plane and to pressure rise for x ≥ 0. Slow baroclinic modes describe the horizontally expanding inflow toward the heated layer near the ground with return flow aloft. This basic structure of the response carries over to more complicated topographies where a heated plateau with vertical sidewalls is separated from the lowlands by a barrier and a pass. The baroclinic modes are fanning out from the pass into the plateau’s interior in linear numerical calculations. These flow patterns hardly change when additional slopes connect the plains and the plateau.

The restrictions imposed by the linear approach are removed step by step in simulations with MM5 in which an idealized plateau with an optional pass is prescribed. There is good agreement with respect to the basic flow pattern, but the linear theory is found to overestimate inflow velocities because of its neglect of momentum and perturbation temperature advection. Moreover, a front moves from the pass into the plateau’s interior where the stratification is neutral or even unstable, a situation that is beyond the scope of the linear theory. The upslope winds evolving at the slope connecting the plateau with the lowlands are unimportant for the thermal circulation of the plateau, a result also suggested by the linear theory. Finally, simulations of the diurnal cycle are performed for the real topography of the Altiplano. The presentation of results concentrates on the observation sites. It is demonstrated that the idealized calculations help to better understand the resulting flows as well as the observations reported in Part I. The total inflow to the Altiplano is discussed as well.
The Dependence of Numerically Simulated Cyclic Mesocyclogenesis upon Environmental Vertical Wind Shear
Authors: Adlerman Edwin J. , Droegemeier Kelvin K.
Published: January 2006

Building upon the authors’ previous work that examined the dynamics of numerically simulated cyclic mesocyclogenesis and its dependence upon model physical and computational parameters, this study likewise uses idealized numerical simulations to investigate associated dependencies upon ambient vertical wind shear. Specifically, the authors examine variations in hodograph shape, shear magnitude, and shear distribution, leading to storms with behavior ranging from steady state to varying degrees of aperiodic occluding cyclic mesocyclogenesis. However, the authors also demonstrate that a different mode of nonoccluding cyclic mesocyclogenesis may occur in certain environments.

Straight hodographs (unidirectional shear) produce only nonoccluding cyclic mesocyclogenesis. Introducing some curvature by adding a quarter circle of turning at low levels results in steady, nonoccluding, and occluding modes. When a higher degree of curvature is introduced—for example, turning through half and three-quarter circles—the tendency for nonoccluding behavior is diminished. None of the full-circle hodographs exhibited cycling during 4 h of simulation. Overall, within a given storm, the preferred mode of cycling is related principally to hodograph shape and magnitude of the ambient vertical shear.
Influence of Environmental Vertical Wind Shear on the Intensity of Hurricane-Strength Tropical Cyclones in the Australian Region
Authors: Paterson Linda A. , Hanstrum Barry N. , Davidson Noel E. , Weber Harry C.
Published: January 2006

NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4.

Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h.

A review of synoptic patterns during intensification-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.
Evaluation of the NCEP–NCAR and ECMWF 15- and 40-Yr Reanalyses Using Rawinsonde Data from Two Independent Arctic Field Experiments*
Authors: Bromwich David H. , Wang Sheng-Hung
Published: January 2006

Many aspects of reanalysis data are of high quality over regions with sufficiently dense data, but the accuracy is uncertain over areas with sparse observations. NCEP–NCAR reanalysis (NNR) and ECMWF 15/40-Yr Re-Analysis (ERA-15 and ERA-40) variables are compared to two independent rawinsonde datasets from the periphery of the Arctic Ocean during the late 1980s and early 1990s: the Coordinated Eastern Arctic Research Experiment (CEAREX) and the Lead Experiment (LeadEx). The study is prompted by J. A. Francis who found that the NNR and ERA-15 upper-level winds are very different from those observed during these two field experiments.

All three reanalyses display large biases in comparisons of the wind components and wind speeds with CEAREX observations, particularly above the 500-hPa level, but exhibit smaller discrepancies with respect to the LeadEx data, generally consistent with the previous findings of J. A. Francis. However, all three reanalyses well capture the wind variability during both experiment periods. For the geopotential height, temperature, and moisture fields, the reanalyses demonstrate close agreement with the CEAREX rawinsonde observations. From comparisons with surrounding fixed rawinsonde stations and examination of the average vertical wind speed shear, it is concluded that the CEAREX upper-level wind speeds (especially above the 500-hPa level) are erroneous and average about half of the actual values. Thus, this evaluation suggests that the three reanalyses perform reliably for tropospheric-state variables from the edge of the Arctic Ocean during the modern satellite era.
Dynamic Assimilation of MODIS-Retrieved Humidity Profiles within a Regional Model for High-Latitude Forecast Applications
Authors: Fan Xingang , Tilley Jeffrey S.
Published: January 2006

A “hot start” technique is applied to the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) to dynamically assimilate cloud properties and humidity profiles retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the NASA Earth Observing System polar-orbiting satellites. The assimilation approach has been studied through extensive numerical experimentation for high-latitude rain events to demonstrate the feasibility and the benefit of the approach on the model cloud and precipitation simulation/forecast.

The ingestion of MODIS-retrieved cloud and clear-air humidity information impacts MM5 cloud fields on both a microphysical and macrophysical level. From short-term (6–12 h) forecast experiments conducted for a preliminary test case and 16 extensive summer and winter experiments, the following primary conclusions have been reached. 1) It is feasible to introduce MODIS-retrieved cloud-top properties and humidity profiles into the MM5 model in a hot start mode without disrupting model stability and evolutionary continuity. 2) The introduction of high-resolution MODIS information produced more accurate humidity fields and resulted in increased mesoscale structure in the cloud and precipitation fields. 3) The opportunistic ingestion of MODIS data at its observation time into the model leads to improved 6–12-h model precipitation forecasts with respect to not only the frequency of occurrences, but also the magnitude of precipitation amounts. 4) Verification with three-dimensional analyses indicates some improvement in model forecasts of temperature, wind, pressure perturbation, and sea level pressure as well. 5) Verification with surface station observations indicates that model forecasts of 2-m temperature, 2-m relative humidity, 10-m winds, and sea level pressure are also improved, most notably for the summer cases. The largest improvement in forecast skill is for 2-m relative humidity (12%).
Modeling Linear Kinematic Features in Sea Ice
Authors: Hutchings Jennifer K. , Heil Petra , Hibler William D.
Published: January 2006

Sea ice deformation is localized in narrow zones of high strain rate that extend hundreds of kilometers, for example, across the Arctic Basin. This paper demonstrates that these failure zones may be modeled with a viscous–plastic sea ice model, using an isotropic rheology. If the ice is assumed to be heterogeneous at the grid scale, and allowed to weaken in time, intersecting failure zones propagate across the region. The direction of failure propagation depends upon the stress applied to the ice (wind stress and boundary conditions) and the rheological model describing plastic failure of the ice. The spacing between failure zones is controlled by the magnitude of the wind stress and the distribution describing spatial variability of ice strength. Sea ice motion and deformation oscillate at close to a 12-h period throughout the Arctic and Antarctic pack ice. This oscillation is found at all spatial scales from hundreds of kilometers to the lead scale. It is shown that with an inertial embedded model, sea ice deformation rotates between pairs of fault patterns with a semidiurnal period. It is well known that linear zones of deformation exist at many spatial scales throughout the Arctic Basin. The model presented in this paper may be scaled to simulate these features.
Plant- and Soil-Parameter-Caused Uncertainty of Predicted Surface Fluxes
Authors: Mölders Nicole
Published: January 2006

Simulated surface fluxes depend on one or more empirical plant or soil parameters that have a standard deviation (std dev). Thus, simulated fluxes will have a stochastic error (or std dev) resulting from the parameters’ std dev. Gaussian error propagation (GEP) principles are used to calculate the std dev for fluxes predicted by the hydro–thermodynamic soil–vegetation scheme to identify prediction limitations due to stochastic errors, parameterization weaknesses, and critical parameters, and to prioritize which parameters to measure with higher accuracy.

Relative errors of net radiation, sensible, latent, and ground heat flux, on average, are 7%, 10%, 6%, and 26%, respectively. The analysis identified the parameterization of thermal conductivity as the dominant influence on the std dev of ground heat flux. For net radiation, critical parameters are vegetation fraction and ground emissivity; for sensible and latent heat fluxes, vegetation fraction. Minimum stomatal resistance and leaf area index dominate the std dev of stomatal resistance for most vegetation and soil types. The empirical parameters with the highest relative error are not necessarily the greatest contributors to the std dev of the predicted flux. Based on the analysis high priority should be given to measurements of vegetation fraction, ground emissivity, minimum stomatal resistance, leaf area index in general, and the permanent wilting point and field capacity for clay and clay loam. Moreover, further specification of clay-type soils and tundra-type vegetation may improve the accuracy of the lower boundary condition in Arctic numerical weather prediction. Since GEP showed itself able to identify critical parameters and (parts of) parameterizations, GEP analysis could form a basis for parameterization intercomparisons and for parameter determination aimed at improving models.
Northern High-Latitude Precipitation as Depicted by Atmospheric Reanalyses and Satellite Retrievals
Authors: Serreze Mark C. , Barrett Andrew P. , Lo Fiona
Published: January 2006

Monthly precipitation based on forecasts from the new 40-yr ECMWF Re-Analysis (ERA-40) is evaluated for the north polar region (the region north of 45°N), the terrestrial Arctic drainage, and its four major watersheds: the Ob, Yenisey, Lena, and Mackenzie basins. Corresponding evaluations are performed for precipitation from the NCEP–NCAR reanalysis, the earlier 15-yr ERA (ERA-15), and satellite-derived estimates from the Global Precipitation Climatology Project (GPCP). Evaluations rely on an improved gridded dataset of precipitation derived from monthly gauge data during the period 1979–93. The available number of gauges has declined since 1993, making it difficult to perform evaluations for later years. ERA-40 depicts monthly precipitation much better than NCEP–NCAR. This is with respect to both lower mean biases and higher squared correlations between modeled and observed grid-cell time series. Squared correlations between monthly time series of ERA-40 and observed precipitation, averaged over the four major Arctic watersheds, typically range from 0.60 to 0.90. Performance over the central Arctic Ocean is poor in winter and spring, but improves in summer and autumn when precipitation amounts are higher. While the overall performance of ERA-40 is better than NCEP–NCAR, it offers no obvious improvement over ERA-15. In some respects, ERA-15 performs slightly better in summer. This lack of improvement may relate to difficulties in assimilating satellite radiances. All of the reanalyses provide better depictions of monthly precipitation than do the GPCP satellite retrievals. This applies to both land areas and the Arctic Ocean. There is no clear improvement in the GPCP estimates after 1987 when the Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) data began to be used. The GPCP estimates are best in summer.
High-Resolution Regional Climate Simulations over Iceland Using Polar MM5*
Authors: Bromwich David H. , Bai Lesheng , Bjarnason Gudmundur G.
Published: January 2006

High-resolution regional climate simulations of Iceland for 1991–2000 have been performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesocale Model (MM5) modified for use in polar regions (Polar MM5) with three nested domains and short-duration integrations. The simulated results are compared with monthly mean surface observations from Iceland for 1991–2000 to demonstrate the high level of model performance; correlation coefficients exceed 0.9 for most variables considered.

The simulation results are used to analyze the near-surface climate over Iceland. The simulated near-surface winds in winter are primarily katabatic. The land–sea-breeze circulation is clearly evident in summer. The land is colder than the ocean during winter, with a strong (weak) temperature gradient along the southern (northern) coast. This temperature pattern over the sloping terrain forces the katabatic wind. The diurnal cycle of near-surface air temperature is marked in summer over the land areas, which drives the land–sea breeze. The near-surface climate variations for extremes of the North Atlantic Oscillation (NAO) index during winter and summer result from the large-scale atmospheric advection conditions.

The time-averaged mesoscale precipitation distribution over Iceland is reasonably well simulated by Polar MM5. Winter precipitation rates are double those during the summer, reflecting the much greater winter cyclonic activity. The simulated interannual precipitation variations during winter for 1991–2000 agree with those observed from snow accumulation measurements on the Vatnajökull ice cap. The winter precipitation decrease for 1991–2000 dominates the annual signal for all of Iceland except the northeastern and eastern parts where the precipitation increases. The large precipitation trends (decadal decrease of up to 50%) are caused by the eastward shift and weakening of the Icelandic low during the 1990s, as a result of changes in the NAO modulation of regional climate.