Search

The dynamical core of an atmospheric general circulation model is engineered to satisfy a delicate balance between numerical stability, computational cost, and an accurate representation of the equations of motion. It generally contains either explicitly added or inherent numerical diffusion mechanisms to control the buildup of energy or enstrophy at the smallest scales. The diffusion fosters computational stability and is sometimes also viewed as a substitute for unresolved subgrid-scale processes. A particular form of explicitly added diffusion is horizontal divergence damping. In this paper a von Neumann stability analysis of horizontal divergence damping on a latitude-longitude grid is performed. Stability restrictions are derived for the damping coefficients of both second- and fourth-order divergence damping. The accuracy of the theoretical analysis is verified through the use of idealized dynamical core test cases that include the simulation of gravity waves and a baroclinic wave. The tests are applied to the finite-volume dynamical core of NCAR's Community Atmosphere Model version 5 (CAM5). Investigation of the amplification factor for the divergence damping mechanisms explains how small-scale meridional waves found in a baroclinic wave test case are not eliminated by the damping. [ABSTRACT FROM AUTHOR]

The humidity data retrieved from the TIROS Operational Vertical Sounder (TOVS) measurements is assimilated into the Goddard Earth Observing System (GEOS) data assimilation system. The study focuses on the impact of the TOVS humidity on assimilated humidity, precipitation, clouds, and radiation. The GEOS assimilation system utilizes the TOVS humidity effectively at levels below 300 mb, while the net impact on the 300-mb humidity is much less. It has been demonstrated that the impact results from direct and indirect effects. The direct effect is the analysis increment introduced by the humidity data, which draws the assimilated humidity toward the data. The indirect effect is realized through the interactions of humidity with physical processes, mainly with moist convection. The indirect effect is often opposite to the direct effect in the current assimilation system. The direct effect is dominant in the lower and middle troposphere while in the upper troposphere the indirect effect is more important. The impact of the TOVS humidity on the GEOS precipitation, clouds, and radiation is also significant due to strong interactions with convection and other physical processes. There is clear evidence indicating that tuning of physical parameterizations explicitly in the data assimilation mode is necessary for optimal use of the TOVS data in the assimilation system. [ABSTRACT FROM AUTHOR]

Compares winds derived from a stratospheric and tropospheric data assimilation system (STRATAN) with winds derived from National Meteorological Center/Climate Analysis Center (NMC/CAC) heights. STRATAN winds at middle latitudes in lower stratosphere; Usefulness of STRATAN winds in tropics.

An obituary for Jerry D. Mahlman, retired director of the U.S. National Oceanic and Atmospheric Administration's (NOAA) Ceophysical Fluid Dynamics Laboratory (CFDL) and lecturer in the Atmospheric and Oceanic Sciences Program at Princeton University, is presented.

The original publication of the article contains a typo in the first author's name. This should be "Xiaolong" instead of "Xialong". [ABSTRACT FROM AUTHOR]

The processes that lead to the spatial and temporal evolution of the Bermuda high (BH) during July and August (JA) are investigated on the basis of linear regression analysis. The analysis is based on a Bermuda high index (BHI): the difference in standardized, deseasonalized, and detrended sea level pressure (SLP) between northeast of Bermuda (40°N, 60°W) and New Orleans (30°N, 90°W). Negative values of BHI indicate a westward expansion of the Bermuda high relative to its climatological-mean location and reduced precipitation in the southeastern United States (SEUS), whereas positive values correspond to BH contraction and enhanced precipitation in the SEUS. Linear regression of the 200-hPa geopotential height based on the BHI reveals the existence of a Rossby wave train that extends zonally from the eastern North Pacific to the eastern North Atlantic. The troughs and ridges associated with this wave train are spatially collocated with the climatological-mean jet stream, indicating that the jet serves as their waveguide. Anomalous troughing in the SEUS associated with this wave train is linked to the contraction of the Bermuda high during JA. The enhanced precipitation is associated with anomalous ascent to the east and south of this trough where anomalous warm advection is observed. Based on these results, it is hypothesized that this Rossby wave train may partially explain the occurrence of suppressed precipitation tied to midsummer drought in the SEUS during July and August. It is found that the BHI has trended from negative to positive in recent decades, suggesting that it may be influenced by low-frequency variability. [ABSTRACT FROM AUTHOR]

The article reports on several of the factors affecting climate with emphasis on the definitions given to climate. According to the author, the various definitions given to climate affects the general concept about climate as well as the diagnostics and remediation biases in climate models. Definitions of weather conditions are characterized by their statistical description of both the mean state as well as deviations form the state. As a matter of fact, one of the common practice in defining climates is to evaluate the sensitivity of some mean circulation feature with respect to a change in model formulation.

Analyses of simple models of moist tropical motion systems reveal that the column-mean moist static potential vorticity (MSPV) can explain their propagation and growth. The MSPV is akin to the equivalent PV except it uses moist static energy (MSE) instead of the equivalent potential temperature. Examination of an MSPV budget that is scaled for moist off-equatorial synoptic-scale systems reveals that α, the ratio between the vertical gradients of latent and dry static energies, describes the relative contribution of dry and moist advective processes to the evolution of MSPV. Horizontal advection of the moist component of MSPV, a process akin to horizontal MSE advection, governs the evolution of synoptic-scale systems in regions of high humidity. On the other hand, horizontal advection of dry PV predominates in a dry atmosphere. Derivation of a "moist static" wave activity density budget reveals that α also describes the relative importance of moist and dry processes to wave activity amplification and decay. Linear regression analysis of the MSPV budget in eastern Pacific easterly waves shows that the MSPV anomalies originate over the eastern Caribbean and propagate westward due to dry PV advection. They are amplified by the fluxes of the moist component of MSPV over the Caribbean Sea and over the eastern Pacific from 105° to 130°W, underscoring the importance of moist processes in these waves. On the other hand, dry PV convergence amplifies the waves from 90° to 100°W, likely as a result of the barotropic energy conversions that occur in this region. [ABSTRACT FROM AUTHOR]

Consumers of climate model information face difficulty in assessing which models and projections are best for their particular needs. This difficulty stems from the abundance of climate information, as well as the relative inaccessibility or unavailability of information concerning a given model's quality, trade-offs, and suitability for a particular geographic region or decision-making application. Consumer reports have traditionally provided potential consumers with background knowledge and a review of available products and services to help to make decisions. As a knowledge broker for climate information in the Great Lakes region, the Great Lakes Integrated Sciences and Assessments (GLISA) team has developed a suite of climate model consumer-report-style documents to help climate information consumers make decisions when selecting models and projections for their work. To develop the reports, GLISA reviewed examples of consumer reports from other sectors, relied on the feedback and advice of our ongoing Practitioner Working Group composed of real-world consumers, and incorporated otherwise-unavailable information from model developers. Due to close, continuing partnership with our Practitioner Working Group, the content and the formatting of our climate model consumer reports respond directly to the needs of consumers. Our reports therefore evolve based on needs of the practitioners as well as with the knowledge generated by our research in usability of climate knowledge. We pose that climate model consumer reports, especially when developed in the context of trusted user-knowledge broker relationships, contribute to making climate information more relevant to and usable by practitioners. [ABSTRACT FROM AUTHOR]

Numerical models of fluid flows calculate the resolved flow at a given grid resolution. The smallest wave resolved by the numerical scheme is deemed the effective resolution. Advection schemes are an important part of the numerical models used for computational fluid dynamics. For example, in atmospheric dynamical cores they control the transport of tracers. For linear schemes solving the advection equation, the effective resolution can be calculated analytically using dispersion analysis. Here, a numerical test is developed that can calculate the effective resolution of any scheme (linear or non-linear) for the advection equation. The tests are focused on the use of non-linear limiters for advection schemes. It is found that the effective resolution of such non-linear schemes is very dependent on the number of time steps. Initially, schemes with limiters introduce large errors. Therefore, their effective resolution is poor over a small number of time steps. As the number of time steps increases the error of non-linear schemes grows at a smaller rate than that of the linear schemes which improves their effective resolution considerably. The tests highlight that a scheme that produces large errors over one time step might not produce a large accumulated error over a number of time steps. The results show that, in terms of effective-resolution, there is little benefit in using higher than third-order numerical accuracy with traditional limiters. The use of weighted essentially non-oscillatory (WENO) schemes, or relaxed and quasi-monotonic limiters, which allow smooth extrema, can eliminate this reduction in effective resolution and enable higher than third-order accuracy. [ABSTRACT FROM AUTHOR]

The effective resolution of a numerical scheme describes the smallest spatial scale (largest wavenumber) that is completely resolved by that scheme. Using dispersion relation analysis allows the effective resolution of a numerical scheme for the advection equation to be calculated. The advection equation is a fundamental building block of dynamical cores of atmospheric and ocean models, and this analysis provides an indication of the effective resolution of the numerical methods used by dynamical cores. Using a variety of finite-difference schemes, the effect on effective resolution of using explicit diffusion and hyper-diffusion terms is examined. The choice of order-of-accuracy, and the time-stepping of the numerical scheme is also investigated with regard to effective resolution. Finally, we apply this analysis to methods that are commonly used in dynamical cores of atmospheric general circulation models, namely semi-Lagrangian and finite-volume methods. [ABSTRACT FROM AUTHOR]

Abstract: Understanding regional-scale water resource systems requires understanding coupled hydrologic and climate interactions. The traditional approach in the hydrologic sciences and engineering fields has been to either treat the atmosphere as a forcing condition on the hydrologic model, or to adopt a specific hydrologic model design in order to be interoperable with a climate model. We propose here a different approach that follows a service-oriented architecture and uses standard interfaces and tools: the Earth System Modeling Framework (ESMF) from the weather and climate community and the Open Modeling Interface (OpenMI) from the hydrologic community. A novel technical challenge of this work is that the climate model runs on a high performance computer and the hydrologic model runs on a personal computer. In order to complete a two-way coupling, issues with security and job scheduling had to be overcome. The resulting application demonstrates interoperability across disciplinary boundaries and has the potential to address emerging questions about climate impacts on local water resource systems. The approach also has the potential to be adapted for other climate impacts applications that involve different communities, multiple frameworks, and models running on different computing platforms. We present along with the results of our coupled modeling system a scaling analysis that indicates how the system will behave as geographic extents and model resolutions are changed to address regional-scale water resources management problems. [Copyright &y& Elsevier]

Abstract: Background: Because of the warming climate urban temperature patterns have been receiving increased attention. Temperature within urban areas can vary depending on land cover, meteorological and other factors. High resolution satellite data can be used to understand this intra-urban variability, although they have been primarily studied to characterize urban heat islands at a larger spatial scale. Objective: This study examined whether satellite-derived impervious surface and meteorological conditions from multiple sites can improve characterization of spatial variability of temperature within an urban area. Methods: Temperature was measured at 17 outdoor sites throughout the Detroit metropolitan area during the summer of 2008. Kriging and linear regression were applied to daily temperatures and secondary information, including impervious surface and distance-to-water. Performance of models in predicting measured temperatures was evaluated by cross-validation. Variograms derived from several scenarios were compared to determine whether high-resolution impervious surface information could capture fine-scale spatial structure of temperature in the study area. Results: Temperatures measured at the sites were significantly different from each other, and all kriging techniques generally performed better than the two linear regression models. Impervious surface values and distance-to-water generally improved predictions slightly. Restricting models to days with lake breezes and with less cloud cover also somewhat improved the predictions. In addition, incorporating high-resolution impervious surface information into cokriging or universal kriging enhanced the ability to characterize fine-scale spatial structure of temperature. Conclusions: Meteorological and satellite-derived data can better characterize spatial variability in temperature across a metropolitan region. The data sources and methods we used can be applied in epidemiological studies and public health interventions to protect vulnerable populations from extreme heat events. [Copyright &y& Elsevier]

Foodborne diseases have large economic and societal impacts worldwide. To evaluate how the risks of foodborne diseases might change in response to climate change, credible and usable climate information tailored to the specific application question is needed. Global Climate Model (GCM) data generally need to, both, be downscaled to the scales of the application to be usable, and represent, well, the key characteristics that inflict health impacts. This study presents an evaluation of temperature-based heat indices for the Washington D.C. area derived from statistically downscaled GCM simulations for 1971-2000--a necessary step in establishing the credibility of these data. The indices approximate high weekly mean temperatures linked previously to occurrences of Salmonella infections. Due to bias-correction, included in the Asynchronous Regional Regression Model (ARRM) and the Bias Correction Constructed Analogs (BCCA) downscaling methods, the observed 30-year means of the heat indices were reproduced reasonably well. In April and May, however, some of the statistically downscaled data misrepresent the increase in the number of hot days towards the summer months. This study demonstrates the dependence of the outcomes to the selection of downscaled climate data and the potential for misinterpretation of future estimates of Salmonella infections.