Body force circulations in a compressible atmosphere: Key concepts

The body force circulation problem of Eliassen is extended to spherical geometry and a quasi-compressible atmosphere using the zonally symmetric tidal theory. The concept of body force circulation is generalized to include the effects of mechanical friction and Newtonian cooling. This viewpoint is conceptually advantageous when the circulation is driven by body forces against radiative relaxation. The resulting linear theory is qualitatively useful in middle atmosphere applications, including the equatorial momentum source for which an analytic solution has not been given previously. Further generalizations of the theory are possible by including dynamical and photochemical feedback effects.     

Temperature and heat flux spectra in the turbulent buoyancy subrange

The physical nature of motions with scales intermediate between approximately isotropic turbulence and quasi-linear internal gravity waves is not understood at the present time. Such motions play an important role in the energetics of small scales processes, both in the ocean and in the atmosphere, and in vertical transport of heat and constituents. This scale range is currently interpreted either as a saturated gravity waves field or as a buoyancy range of turbulence.We first discuss some distinctive predictions of the classical (Lumley, Phillips) buoyancy range theory, recently improved (Weinstock, Dalaudier and Sidi) to describe potential energy associated with temperature fluctuations. This theory predicts the existence of a spectral gap in the temperature spectra and of an upward mass flux (downward buoyancy and heat fluxes), strongly increasing towards large scales. These predictions are contrasted with an alternate theory, assuming energetically insignificant buoyancy flux, proposed by Holloway.Then we present experimental evidences of such characteristic features obtained in the lower stratosphere with an instrumented balloon. Spectra of temperature, vertical velocity, and cospectra of both, obtained in homogeneous, weakly turbulent regions, are compared with theoretical predictions. These results are strongly consistent with the improved classical buoyancy range theory and support the existence of a significant downward heat flux in the buoyancy range.The theoretical implications of the understanding of this scale range are discussed. Many experimental evidences consistently show the need for an anisotropic theory of the buoyancy range of turbulence.     

Monitoring,causality, and uncertainty in a stratospheric context

Our increasingly complex understanding of stratospheric chemistry and transport processes leaves us with various theoretical possibilities of appreciable and perhaps serious environmental impact due to human activities. These possibilities raise policy questions in which the economic and other costs of regulating human activities must be weighed against the possible consequences of no such regulation. The natural variability of the atmosphere, the physical and other limitations on our global sampling and monitoring abilities, and the difficulties in establishing causal connections leave us in a state of uncertainty as to the reality and magnitude of at least some of these theoretical environmental impacts. Policy-makers must make decisions in the face of these uncertainties.The proper role of scientists as such in narrowing and quantifying the uncertainties is discussed, with particular regard to the evidence that cultural and other biases often affect individual scientists' conclusions. Conscious efforts are needed to minimize bias, quantify uncertainties, and speed up the process of scientific consensus-building. A careful distinction should be drawn between scientifically determined probabilities, and cost-benefit analyses which necessarily involve value judgments.     

Hemispheric simulation of the Asian summer monsoon

A three-level, -plane, filtered model is used to simulate the Northern Hemisphere summer monsoon. A time-averaged initial state, devoid of sub-planetary scale waves, is integrated through 30 days on a 5° latitude-longitude grid. Day 25 through day 30 integrations are then repeated on a 2.5° grid. The planetary-scale waves are forced by time-independent, spatially varying diabatic heating. Energy is extracted via internal and surface frictional processes. Orography is excluded to simplify synoptic-scale energy sources.During integration the model energy first increases, but stabilizes near day 10. Subsequent flow patterns closely resemble the hemisphere summer monsoon. Climatological features remain quasi-stationary. At 200 mb high pressure dominates the land area, large-scale troughs are found over the Atlantic and Pacific Oceans, the easterly jet forms south of Asia, and subtropical jets develop in the westerlies. At 800 mb subtropical highs dominate the oceans and the monsoon trough develops over the Asian land mass. The planetary scales at all levels develop a realistic cellular structure from the passage of transient synoptic-scale features, e.g., a baroclinic cyclone track develops near 55°N and westward propagating waves form in the easterlies.Barotropic redistribution of kinetic energy is examined over a low-latitude zonal strip using a Fourier wave-space. In contrast to higher latitudes where the zonal flow and both longer and shorter waves are fed by barotropic energy redistribution from the baroclinically unstable wavelengths, the low-latitude waves have a planetary-scale kinetic energy source. Wave numbers 1 and 2 maintain both the zonal flow and all shorter scales via barotropic transfers. Transient and standing wave processes are examined individually and in combination.Wave energy accumulates at wave numbers 7 and 8 at 200 mb and at wave number 11 in the lower troposphere. The 800-mb waves are thermally indirect and in the mean they give energy to the zonal flow. These characteristics agree with atmospheric observation. The energy source for these waves is the three wave barotropic transfer. The implications of examining barotropic processes in a Fourier wave-space, vice the more common approach of separating the flow into a mean plus a deviation are discussed.     

Quantifying sedimentation patterns of small landslide-dammed lakes in the central Oregon Coast Range

Understanding sedimentation patterns in small coastal watersheds due to landscape perturbations is critical for connecting hillslope and fluvial processes, in addition to managing aquatic habitats for anadromous fish and other aquatic species in the Oregon Coast Range (OCR). Changes in sedimentation patterns spanning the last 250 years are preserved in two landslide-dammed lakes in small watersheds (< 10 km²) underlain by the Tyee Formation in the central OCR. Dendrochronology of drowned Douglas-fir stumps in both lakes provided precise timing of the damming and formation of the lakes, with Klickitat Lake forming in winter ad 1751/52 and Wasson Lake in winter ad 1819/20. Perturbations from wildfires, logging and road development, and previously underappreciated snow events affect sedimentation rates in the lakes to different degrees, and are identified in the sediment record using cesium-137 (¹³⁷Cs), high-resolution charcoal stratigraphy, local fire records, and aerial photography. Each lake has variable sedimentation accumulation rates (0.05–4.4 cm yr⁻¹) and mass accumulation rates (0.02–1.42 g cm⁻² yr⁻¹). Sedimentation rates remained low from the landslide-damming events until the mid-19th century, when they increased following stand-replacing wildfires. Aside from a sediment remobilization triggered by human modification of the landslide dam at Klickitat Lake around 1960, the largest peaks in mass accumulation rates in the mid-20th century at both lakes in the early 1950s precede major road construction and logging activity in the watersheds. Subsequent sedimentation rates are lower, but variable, and possible effects of logging and road development might be exacerbated by abnormal precipitation and heavy snow events. A comparison of previous studies of landslide-dammed lakes in larger watershed of the OCR are consistent with our findings of increased sedimentation in the mid-20th century, as well as higher sedimentation rates in the debris-flow dominated southern Tyee Formation than in the lower-relief northern Tyee Formation.     

Meso-scale circulation at the intermediate-depth east of Mindanao observed by Argo profiling floats

The meso-scale circulation at the intermediate depth east of Mindanao is studied using Argo profiling floats observations. The trajectories and the parking-depth velocities of Argo floats show that the intermediate-depth circulation east of Mindanao contains significant meso-scale features that are highly variable both in space and in time. Both cyclonic and anticyclonic eddies at the intermediate depth (1000–2000 m) are indicated by the trajectories east of Mindanao. The mean tangential velocities of these eddies are about 10 cm/s at 2000 m and over 20 cm/s at 1000 m, which indicates that the geostrophic calculation may contain large errors due to the vigorous eddy activity at the reference levels. The analyses also suggest that these eddies might play an important role in mass and vorticity balances of the intermediate-depth circulation east of Mindanao.     

Cooling of the mesosphere and lower thermosphere due to doubling of CO2

A new parameterization of infrared radiative transfer in the 15-m CO₂ band has been incorporated into the Spectral mesosphere/lower thermosphere model (SMLTM). The parameterization is applicable to calculations of heating rates above approximately 15 km for arbitrary vertical profiles of the CO₂ concentration corresponding to the surface mixing ratio in the range 150–720 ppm. The sensitivity of the mesosphere and lower thermosphere (MLT) to doubling of CO₂ has been studied. The thermal response in the MLT is mostly negative (cooling) and much stronger than in the lower atmosphere. An average cooling at the stratopause is about 14 K. It gradually decreases to approximately 8 K in the upper mesosphere and again increases to about 40–50 K in the thermosphere. The cooling and associated thermal shrinking result in a substantial density reduction in the MLT that reaches 40–45% in the thermosphere. Various radiative, chemical, and dynamical feedbacks potentially important for the thermal response in the MLT are discussed. It is noted that the results of simulations are strikingly similar to observations of long-term trends in the MLT. This suggests that during the last 3–4 decades the thermal structure in the real upper atmosphere has undergone substantial changes driven by forcing comparable with that due to doubling of CO₂.     

Variations of near-surface atmospheric CO2 and H2O concentrations during summer on Muztagata

Expeditions during the summers of 2002 and 2003 implemented continuous monitoring of near-surface (2 m height) atmospheric CO₂ and H₂O concentrations at the 4500 m elevation on Muztagata. The resultant data sets reveal a slight decrease of CO₂ concentrations (of about 5 μmol·mol^-1) and changes in the diurnal variations from the end of June to the middle August. The daily maximum CO₂ concentrations occur between 02:30-05:30 AM (local time) and the minimum levels occur between 12:00-15:30 PM. The atmospheric CO₂ concentrations in the summer of 2002 were around 5 μmol·mol^-1 lower than those during the same period of 2003, whereas the diurnal amplitude was higher. In contrast, we found that the daily mean atmospheric H₂O content in 2003 was much lower than that in 2002 and there exists a striking negative correlation between CO₂ and H₂O concentrations. We therefore suggest that the near-surface atmospheric CO₂ concentration is affected not only by photosynthesis and respiration, but also by the air H₂O content in the glaciated region around Muztagata.     

Impact of precipitation seasonality changes on isotopic signals in polar ice cores: a multi-model analysis

For Central Greenland, water isotope analysis indicates a temperature difference of about 10°C since the Last Glacial Maximum (LGM). However, borehole thermometry and gas diffusion thermometry indicate that LGM surface temperatures were about 20°C colder than today. Two general circulation model studies have shown that changes in the seasonal precipitation timing in Central Greenland might have caused a warm bias in the LGM water isotope proxy temperatures, and that this bias could explain the difference in the estimated paleotemperatures. Here we present an analysis of a number of atmospheric general circulation model simulations mostly done within the framework of the Paleoclimate Modeling Intercomparison Project. The models suggest that the seasonal cycle of precipitation and surface mass balance over Central Greenland at the LGM might have been very different from today. This supports the idea that the accuracy of the water isotope thermometry at the LGM in Greenland might be compromised as a result of a modified surface mass balance seasonality. However, the models disagree on the amplitude and sign of the bias. For Central East Antarctica, a strong seasonality effect on the LGM isotopic signal is not simulated by any of the analyzed models. For the mid-Holocene (6 kyr BP) the models suggest relatively weak isotope paleothermometry biases linked to changes in the surface mass balance seasonality over both ice sheets.