bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modulation of Daily Precipitation over Southwest Asia by the Madden–Julian Oscillation
Authors: Barlow Mathew , Wheeler Matthew , Lyon Bradfield , Cullen Heidi 
Published: January 2006

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

AbstractHigh-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.
Identifying the Characteristics of Strong Southerly Wind Events at Casey Station in East Antarctica Using a Numerical Weather Prediction System
Authors: Adams Neil 
Published: January 2006

Abstract
Casey Station in East Antarctica is not often subject to strong southerly flow off the Antarctic continent but when such events occur, operations at the station are often adversely impacted. Not only are the dynamics of such events poorly understood, but the forecasting of such occurrences is difficult. The following study uses model output from a 12-month experiment using the Antarctic Limited-Area Prediction System (ALAPS) to advance the understanding of the dynamics of such events and postulates that what are often described as katabatic wind events are more likely to be synoptic in scale, with mid- and upper-level tropospheric dynamics forcing the surface layer flow. Strong surface layer flows that have a katabatic signature commonly develop on the steep Antarctic escarpment but rarely extend out over the coast in the Casey area, most probably as a result of cold air damming. However, the development of a strong south-southwesterly jet over Casey provides a mechanism whereby the katabatic can move out off the coast.
Numerical Simulations of the Island-Induced Circulations over the Island of Hawaii during HaRP
Authors: Yang Yang , Chen Yi-Leng , Fujioka Francis M. 
Published: January 2006

Abstract
The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)/land surface model (LSM) is used to simulate the diurnal island-scale circulations over the island of Hawaii during the Hawaiian Rainband Project (HaRP, 11 July–24 August 1990). The model is initialized with the NCEP–NCAR reanalysis data.
The diurnal variations of the land–sea thermal contrast at the land surface and the planetary boundary layer at Hilo, Hawaii, are well simulated. The main discrepancy occurs on the leeside areas of mountains or ridges below the trade wind inversion (2 km), where the simulated afternoon land–sea thermal contrast at the surface is 1°–3°C lower than observed mainly due to the misrepresentation of lava rocks by the bare ground category in the U.S. Geological Survey (USGS) data and stronger than observed simulated sea breezes bringing in relatively cool maritime air.
The flow deceleration and splitting of the incoming trade wind flow and the evolution of the diurnal circulation cells on the windward side, the thermally driven diurnal winds, and the wake circulations on the lee side are well simulated. The simulated diurnal variations in rainfall are also in good agreement with observations. However, the simulated winds in areas well exposed to the trade wind flow are weaker (1–3 m s−1) than observed mainly due to the underestimation of trade wind flow in the NCEP–NCAR reanalysis. The simulated rainfall over windward lowlands at night is underestimated and the maximum rainfall axis shifts farther toward the coast as compared with observations, due to an underestimation of orographic lifting aloft and a relatively large horizontal extent of the simulated katabatic flow because of the weaker- than-observed trade wind flow in the initial conditions.
In the afternoon hours on the windward side, the strongest winds (anabatic/sea breeze and trade wind flow) are simulated in low levels over land in response to the surface heating, with a westerly wind deviation beneath the mean trade wind inversion (2 km) and sinking motion over the adjacent oceans. The simulated low-level flow deceleration of the incoming trade wind flow is most significant in the early morning as a combination of island blocking and nocturnal cooling over land. At that time, the simulated upward motion representing the rising branch of the thermally direct circulation extends more than 40 km offshore.
Sensitivity tests show that with better surface conditions in the model coupled with the Oregon State University (OSU) LSM, the simulated thermal forcing over land is improved. The improvements in simulated ground temperature, land–sea thermal contrast at the land surface, and mixing ratio lead to better simulation of the strength of land/sea breezes over the island.