bluuebunny/crossref_metadata_2025_split huggingface dataset:
Response of Idealized Baroclinic Wave Life Cycles to Stratospheric Flow Conditions
Authors: Kunz Torben , Fraedrich Klaus , Lunkeit Frank 
Published: January 2009

Abstract
Dynamical stratosphere–troposphere coupling through a response of baroclinic waves to lower stratospheric flow conditions is investigated from an initial value approach. A series of adiabatic and frictionless nonlinear baroclinic wave life cycles in a midlatitude tropospheric jet with different initial zonal flow conditions in the stratosphere is simulated, using a dry primitive equation model with spherical geometry. When a stratospheric jet, located at various latitudes between 35° and 70°, is removed from the initial conditions, the wavenumber-6 life cycle behavior changes from the well-known LC1 to LC2 evolution, characterized by anticyclonic and cyclonic wave breaking, respectively. Linear theory, in terms of refractive index and the structure of the corresponding fastest-growing normal mode, is found to be unable to explain this stratosphere-induced LC1 to LC2 transition. This implies that altered nonlinear wave–mean flow interactions are important. The most significant stratosphere-induced change that extends into the nonlinear baroclinic growth stage is a region of downward wave propagation in the lower stratosphere associated with positive values of the squared refractive index near 20 km. Furthermore, it is demonstrated that the difference between the response of the tropospheric circulation to LC1 and LC2 life cycles closely resembles the meridional and vertical structure of the North Atlantic Oscillation (NAO), with positive (negative) NAO-like anomalies being driven by LC1 (LC2). Thus, a weakened stratospheric jet induces the generation of negative NAO-like anomalies in the troposphere, consistent with the observed stratosphere–NAO connection.
Systematic Metastable Atmospheric Regime Identification in an AGCM
Authors: Franzke Christian , Horenko Illia , Majda Andrew J. , Klein Rupert 
Published: January 2009

Abstract
In this study the authors apply a recently developed clustering method for the systematic identification of metastable atmospheric regimes in high-dimensional datasets generated by atmospheric models. The novelty of this approach is that it decomposes the phase space in, possibly, overlapping clusters and simultaneously estimates the most likely switching sequence among the clusters. The parameters of the clustering and switching are estimated by a finite element approach. The switching among the clusters can be described by a Markov transition matrix. Possible metastable regime behavior is assessed by inspecting the eigenspectrum of the associated transition probability matrix.
The recently introduced metastable data-analysis method is applied to high-dimensional datasets produced by a barotropic model and a comprehensive atmospheric general circulation model (GCM). Significant and dynamically relevant metastable regimes are successfully identified in both models. The metastable regimes in the barotropic model correspond to blocked and zonal states. Similar regime states were already previously identified in highly reduced phase spaces of just one and two dimensions in the same model.
Next, the clustering method is applied to a comprehensive atmospheric GCM in which seven significant flow regimes are identified. The spatial structures of the regimes correspond to, among others, both phases of the Northern Annular Mode and Pacific blocking. The regimes are maintained predominantly by transient eddy fluxes of low-pass-filtered anomalies. It is demonstrated how the dynamical description of the slow process switching between the regimes can be acquired from the analysis results, and an investigation of the resulting simplified dynamical model with respect to predictability is performed.
A predictability study shows that a simple Markov model is able to predict the regimes up to six days ahead, comparable to the ability of high-resolution state-of-the-art numerical weather prediction models to accurately predict the onset and decay of blockings. The implications of the results for derivation of reduced models for extended-range predictability are discussed.
Surface-Based Convective Potential in the Contiguous United States in a Business-as-Usual Future Climate
Authors: Van Klooster Sara L. , Roebber Paul J. 
Published: January 2009

Abstract
To date, neither observational studies nor direct climate model simulations have been able to document trends in the frequency or severity of deep moist convection associated with global climate change. The lack of such evidence is not unexpected as the observational record is insufficiently long and computational limitations prevent modeling at the scales necessary to simulate explicitly such phenomena. Nonetheless, severe deep moist convection represents an important aspect of regional climate, particularly in the central United States, where damage, injuries, and fatalities are a frequent result of such phenomena. Accordingly, any comprehensive assessment of the regional effects of climate change must account for these effects.
In this work, the authors present a “perfect prog” approach to estimating the potential for surface-based convective initiation and severity based upon the large-scale variables well resolved by climate model simulations. This approach allows for the development of a stable estimation scheme that can be applied to any climate model simulation, presently and into the future. The scheme is applied for the contiguous United States using the output from the Parallel Climate Model, with the Intergovernmental Panel on Climate Change third assessment A2 (business as usual) as input. For this run, relative to interannual variability, the potential frequency of deep moist convection does not change, but the potential for severe convection is found to increase east of the Rocky Mountains and most notably in the “tornado alley” region of the U.S. Midwest. This increase in severe potential is mostly tied to increases in thermodynamic instability as a result of ongoing warm season surface warming and moistening. Finally, approaches toward improving such estimation methods are briefly discussed.
The Roles of Intraseasonal Kelvin Waves and Tropical Instability Waves in SST Variability along the Equatorial Pacific in an Isopycnal Ocean Model
Authors: Jiang Chuan Li , Thompson Lu Anne , Kelly Kathryn A. , Cronin Meghan F. 
Published: January 2009

Abstract
The roles of intraseasonal Kelvin waves and tropical instability waves (TIWs) in the intraseasonal and low-frequency mixed-layer temperature budget were examined in an isopycnal ocean model forced by QuikSCAT winds from 2000 to 2004. Correlations between temperature tendency and other terms of the intraseasonal budget compare well with previous results using Tropical Atmosphere Ocean (TAO) observations: the net heat flux has the largest correlation in the western Pacific and zonal advection has the largest correlation in the central Pacific. In the central Pacific, the intraseasonal variations in zonal advection were due to both the zonal background velocity acting on the Kelvin wave temperature anomaly and the Kelvin wave’s anomalous velocity acting on the background temperature. In the eastern Pacific, three of the four temperature budget terms have comparable correlations. In particular, the vertical processes acting on the shallow thermocline cause large SST anomalies in phase with the intraseasonal thermocline anomalies.
On intraseasonal time scales, the influence of individual composite upwelling and downwelling Kelvin waves cancel each other. However, because the intraseasonal SST anomalies increase to the east, a zonal gradient of SST is generated that is in phase with intraseasonal zonal velocity. Consequently, heat advection by the Kelvin waves rectifies into lower frequencies in the eastern Pacific. Rectification resulting from TIWs was also seen. The prevalence of intraseasonal Kelvin waves and the zonal structure of intraseasonal SST from 2002 to early 2004 suggested that they might be important in setting the eastern Pacific SST on interannual time scales.
A Comparison of Climate Feedback Strength between CO2 Doubling and LGM Experiments
Authors: Yoshimori Masakazu , Yokohata Tokuta , Abe-Ouchi Ayako 
Published: January 2009

Abstract
Studies of the climate in the past potentially provide a constraint on the uncertainty of climate sensitivity, but previous studies warn against a simple scaling to the future. Climate sensitivity is determined by a number of feedback processes, and they may vary according to climate states and forcings. In this study, the similarities and differences in feedbacks for CO2 doubling, a Last Glacial Maximum (LGM), and LGM greenhouse gas (GHG) forcing experiments are investigated using an atmospheric general circulation model coupled to a slab ocean model. After computing the radiative forcing, the individual feedback strengths of water vapor, lapse-rate, albedo, and cloud feedbacks are evaluated explicitly. For this particular model, the difference in the climate sensitivity between the experiments is attributed to the shortwave cloud feedback, in which there is a tendency for it to become weaker or even negative in cooling experiments. No significant difference is found in the water vapor feedback between warming and cooling experiments by GHGs. The weaker positive water vapor feedback in the LGM experiment resulting from a relatively weaker tropical forcing is compensated for by the stronger positive lapse-rate feedback resulting from a relatively stronger extratropical forcing. A hypothesis is proposed that explains the asymmetric cloud response between the warming and cooling experiments associated with a displacement of the region of mixed-phase clouds. The difference in the total feedback strength between the experiments is, however, relatively small compared to the current intermodel spread, and does not necessarily preclude the use of LGM climate as a future constraint.
Evaluating Enhanced Hydrological Representations in Noah LSM over Transition Zones: Implications for Model Development
Authors: Rosero Enrique , Yang Zong-Liang , Gulden Lindsey E. , Niu Guo-Yue , Gochis David J. 
Published: January 2009

Abstract
The authors introduce and compare the performance of the unified Noah land surface model (LSM) and its augments with physically based, more conceptually realistic hydrologic parameterizations. Forty-five days of 30-min data collected over nine sites in transition zones are used to evaluate (i) their benchmark, the standard Noah LSM release 2.7 (STD); (ii) a version equipped with a short-term phenology module (DV); and (iii) one that couples a lumped, unconfined aquifer model to the model soil column (GW). Their model intercomparison, enhanced by multiobjective calibration and model sensitivity analysis, shows that, under the evaluation conditions, the current set of enhancements to Noah fails to yield significant improvement in the accuracy of simulated, high-frequency, warm-season turbulent fluxes, and near-surface states across these sites. Qualitatively, the versions of DV and GW implemented degrade model robustness, as defined by the sensitivity of model performance to uncertain parameters. Quantitatively, calibrated DV and GW show only slight improvement in the skill of the model over calibrated STD. Then, multiple model realizations are compared to explicitly account for parameter uncertainty. Model performance, robustness, and fitness are quantified for use across varied sites. The authors show that the least complex benchmark LSM (STD) remains as the most fit version of the model for broad application. Although GW typically performs best when simulating evaporative fraction (EF), 24-h change in soil wetness (ΔW30), and soil wetness, it is only about half as robust as STD, which also performs relatively well for all three criteria. GW’s superior performance results from bias correction, not from improved soil moisture dynamics. DV performs better than STD in simulating EF and ΔW30 at the wettest site, because DV tends to enhance transpiration and canopy evaporation at the expense of direct soil evaporation. This same model structure limits performance at the driest site, where STD performs best. This dichotomous performance suggests that the formulations that determine the partitioning of LE flux need to be modified for broader applicability. Thus, this work poses a caveat for simple “plug and play” of functional modules between LSMs and showcases the utility of rigorous testing during model development.
Evaluation and Comparison of Microphysical Algorithms in ARW-WRF Model Simulations of Atmospheric River Events Affecting the California Coast
Authors: Jankov Isidora , Bao Jian-Wen , Neiman Paul J. , Schultz Paul J. , Yuan Huiling , White Allen B. 
Published: January 2009

Abstract
Numerical prediction of precipitation associated with five cool-season atmospheric river events in northern California was analyzed and compared to observations. The model simulations were performed by using the Advanced Research Weather Research and Forecasting Model (ARW-WRF) with four different microphysical parameterizations. This was done as a part of the 2005–06 field phase of the Hydrometeorological Test Bed project, for which special profilers, soundings, and surface observations were implemented. Using these unique datasets, the meteorology of atmospheric river events was described in terms of dynamical processes and the microphysical structure of the cloud systems that produced most of the surface precipitation. Events were categorized as “bright band” (BB) or “nonbright band” (NBB), the differences being the presence of significant amounts of ice aloft (or lack thereof) and a signature of higher reflectivity collocated with the melting layer produced by frozen precipitating particles descending through the 0°C isotherm.
The model was reasonably successful at predicting the timing of surface fronts, the development and evolution of low-level jets associated with latent heating processes and terrain interaction, and wind flow signatures consistent with deep-layer thermal advection. However, the model showed the tendency to overestimate the duration and intensity of the impinging low-level winds. In general, all model configurations overestimated precipitation, especially in the case of BB events. Nonetheless, large differences in precipitation distribution and cloud structure among model runs using various microphysical parameterization schemes were noted.
The Impact of Abyssal Mixing Parameterizations in an Ocean General Circulation Model
Authors: Jayne Steven R. 
Published: January 2009

Abstract
A parameterization of vertical diffusivity in ocean general circulation models has been implemented in the ocean model component of the Community Climate System Model (CCSM). The parameterization represents the dynamics of the mixing in the abyssal ocean arising from the breaking of internal waves generated by the tides forcing stratified flow over rough topography. This parameterization is explored over a range of parameters and compared to the more traditional ad hoc specification of the vertical diffusivity.
Diapycnal mixing in the ocean is thought to be one of the primary controls on the meridional overturning circulation and the poleward heat transport by the ocean. When compared to the traditional approach with uniform mixing, the new mixing parameterization has a noticeable impact on the meridional overturning circulation; while the upper limb of the meridional overturning circulation appears to be only weakly impacted by the transition to the new parameterization, the deep meridional overturning circulation is significantly strengthened by the change. The poleward ocean heat transport does not appear to be strongly affected by the mixing in the abyssal ocean for reasonable parameter ranges. The transport of the Antarctic Circumpolar Current through the Drake Passage is related to the amount of mixing in the deep ocean. The new parameterization is found to be energetically consistent with the known constraints on the ocean energy budget.
Removal of Contaminated Pixels from the Desert Target for AVHRR Vicarious Calibration
Authors: Yu Fangfang , Wu Xiangqian 
Published: January 2009

Abstract
Desert-based vicarious calibration plays an important role in generating long-term reliable satellite radiances for the visible and near-infrared channels of the Advanced Very High Resolution Radiometer (AVHRR). Lacking an onboard calibration device, the AVHRR relies on reflected radiances from a target site, for example, a large desert, to calibrate its solar reflective channels. While the radiometric characteristics of the desert may be assumed to be stable, the reflected radiances from the target can occasionally be affected by the presence of clouds, sand storms, vegetation, and wet surfaces. These contaminated pixels must be properly identified and removed to ensure calibration performance. This paper describes an algorithm for removing the contaminated pixels from AVHRR measurements taken over the Libyan Desert based on the characteristics of consistent normalized difference vegetation index (NDVI) land-cover stratification. An NDVI histogram-determined threshold is first applied to screen pixels contaminated with vegetation in each individual AVHRR observation. The resulting analyses show that the vegetation growth inside the desert target has a negligibly small impact on the AVHRR operational calibration results. Two criteria based on the maximum NDVI compositing technique are then employed to remove pixels contaminated with clouds, severe sand storms, and wet sand surfaces. Compared to other cloud-screening methods, this algorithm is capable of not only identifying high-reflectance clouds, but also removing the low reflectance of wet surfaces and the nearly indifferent reflectance of severe dust storms. The use of clear pixels appears to improve AVHRR calibration accuracy in the first 3–4 yr after satellite launch.
CO2 Retrieval over Clouds from the OCO Mission: Model Simulations and Error Analysis
Authors: Vidot Jérôme , Bennartz Ralf , O’Dell Christopher W. , Preusker René , Lindstrot Rasmus , Heidinger Andrew K. 
Published: January 2009

Abstract
Spectral characteristics of the future Orbiting Carbon Observatory (OCO) sensor, which will be launched in January 2009, were used to infer the carbon dioxide column-averaged mixing ratio over liquid water clouds over ocean by means of radiative transfer simulations and an inversion process based on optimal estimation theory. Before retrieving the carbon dioxide column-averaged mixing ratio over clouds, cloud properties such as cloud optical depth, cloud effective radius, and cloud-top pressure must be known. Cloud properties were not included in the prior in the inversion but are retrieved within the algorithm. The high spectral resolution of the OCO bands in the oxygen absorption spectral region around 0.76 μm, the weak CO2 absorption band around 1.61 μm, and the strong CO2 absorption band around 2.06 μm were used. The retrieval of all parameters relied on an optimal estimation technique that allows an objective selection of the channels needed to reach OCO’s requirement accuracy. The errors due to the radiometric noise, uncertainties in temperature profile, surface pressure, spectral shift, and presence of cirrus above the liquid water clouds were quantified. Cirrus clouds and spectral shifts are the major sources of errors in the retrieval. An accurate spectral characterization of the OCO bands and an effective mask for pixels contaminated by cirrus would mostly eliminate these errors.
Anomalous Spiking in Spectra of XCTD Temperature Profiles
Authors: Gille Sarah T. , Lombrozo Aaron , Sprintall Janet , Stephenson Gordon , Scarlet Richard 
Published: January 2009

Abstract
The high vertical resolution of temperature and salinity measurements from expendable conductivity–temperature–depth (XCTD) instruments can be useful for inferring small-scale mixing rates in the ocean. However, XCTD temperature profiles show distinct spectral spikes at frequencies of 5 and 10 Hz, corresponding to 1 and 2 cycles per five measurement points. Peaks at these same frequencies are often present in the conductivity spectra as well. The spectral spikes occur in XCTD profiles from both the subtropical and subpolar regions. They appear to originate as digital electronic noise within the probes. A finite impulse response filter design procedure was used to develop filters that could remove the spectral spikes while retaining as much high vertical resolution as possible. For most purposes, the application of an 11-point, least squares, low-pass filter proves sufficient for removing the spectral energy at 5 and 10 Hz, and results in an effective minimum vertical resolution of about 0.7 m.
On the Reliability and Calibration of Ensemble Forecasts
Authors: Johnson Christine , Bowler Neill 
Published: January 2009

Abstract
An important aspect of ensemble forecasting is that the resulting probabilities are reliable (i.e., the forecast probabilities match the observed frequencies). In the medium-range forecasting context, the literature tends to focus on the requirement that, for a reliable ensemble, the ensemble spread should be representative of the uncertainty in the mean, whereas in the seasonal forecasting context, the literature tends to focus on the requirement that, for a reliable ensemble, the ensemble forecasts should have the same climatological variance as the truth. In this note, the authors emphasize that both of these requirements are necessary for reliability and they clarify that a popular calibration method actually achieves both of these requirements.