Observations indicate earlier spring snowmelt in the northern hemisphere. We hypothesize that increased temperatures and decreased precipitation due to a positive trend in the winter Arctic ...Oscillation (AO) have advanced the date of snowmelt. To test this, we modeled snowmelt using the Simple Biosphere model, Version 2 (SiB2) and the National Centers for Environmental Prediction (NCEP) reanalysis from 1958–2002. The simulated snowmelt dates are consistent with dates derived from National Oceanic and Atmospheric Administration (NOAA) weekly snow charts. The winter AO exerts the strongest influence on the timing of snowmelt in northern Europe, with a weaker influence in eastern Siberia and almost no influence in North America. The winter AO trend can statistically explain 20–70% of simulated snowmelt trends in northern Europe.
Low-level endotoxemia has been identified as a powerful risk factor for atherosclerosis. However, little is known about the mechanisms that regulate endotoxin responsiveness in vascular cells. We ...conducted experiments to compare the relative responses of human coronary artery endothelial cells (HCAEC) and smooth muscle cells (HCASMC) to very low levels of endotoxin, and to elucidate the mechanisms that regulate endotoxin responsiveness in vascular cells. Endotoxin (</=1 ng/ml) caused production of chemotactic cytokines in HCAEC. Endotoxin-induced cytokine production was maximal at LPS-binding protein:soluble CD14 ratios <1, typically observed in individuals with subclinical infection; higher LPS-binding protein:soluble CD14 ratios were inhibitory. Endotoxin potently activated HCASMC, with cytokine release >10-fold higher in magnitude at >10-fold lower threshold concentrations (10-30 pg/ml) compared with HCAEC. This remarkable sensitivity of HCASMC to very low endotoxin concentrations, comparable to that found in circulating monocytes, was not due to differential expression of TLR4, which was detected in HCAEC, HCASMC, and intact coronary arteries. Surprisingly, membrane-bound CD14 was detected in seven different lines of HCASMC, conferring responsiveness to endotoxin and to lipoteichoic acid, a product of Gram-positive bacteria, in these cells. These results suggest that the low levels of endotoxin associated with increased risk for atherosclerosis are sufficient to produce inflammatory responses in coronary artery cells. Because CD14 recognizes a diverse array of inflammatory mediators and functions as a pattern recognition molecule in inflammatory cells, expression of membrane-bound CD14 in HCASMC implies a potentially broader role for these cells in transducing innate immune responses in the vasculature.
Various observations show trends toward warmer and earlier springs in the Northern Hemisphere. We hypothesize that the positive trend in the winter Arctic Oscillation (AO) has led to higher winter ...temperatures, advanced spring, and increased seasonal amplitudes in atmospheric CO2. To test this hypothesis, we modeled leaf‐out and terrestrial carbon fluxes using the Simple Biosphere model, Version 2 (SiB2) and the National Centers for Environmental Prediction (NCEP) reanalysis for 1958–2002. We found that our modeled leaf‐out trends were consistent with observed phenology and that the winter AO trend can statistically explain 20–70% of the modeled leaf‐out trends in the eastern United States and northern Europe. We also found that warmer winter temperatures associated with the positive trend in the winter AO increased winter respiration. At the same time, these warmer winter temperatures advanced the date of leaf‐out, increasing the total spring uptake of atmospheric CO2 by plants. These seasonally asymmetric trends toward increased respiration in winter and increased photosynthesis in spring can help explain the trend toward increased seasonal amplitudes in observed atmospheric CO2 concentration.
Ecosystem fluxes of energy, water, and CO2 result in spatial and temporal variations in atmospheric properties. In principle, these variations can be used to quantify the fluxes through inverse ...modelling of atmospheric transport, and can improve the understanding of processes and falsifiability of models. We investigated the influence of ecosystem fluxes on atmospheric CO2 in the vicinity of the WLEF‐TV tower in Wisconsin using an ecophysiological model (Simple Biosphere, SiB2) coupled to an atmospheric model (Regional Atmospheric Modelling System). Model parameters were specified from satellite imagery and soil texture data. In a companion paper, simulated fluxes in the immediate tower vicinity have been compared to eddy covariance fluxes measured at the tower, with meteorology specified from tower sensors. Results were encouraging with respect to the ability of the model to capture observed diurnal cycles of fluxes. Here, the effects of fluxes in the tower footprint were also investigated by coupling SiB2 to a high‐resolution atmospheric simulation, so that the model physiology could affect the meteorological environment. These experiments were successful in reproducing observed fluxes and concentration gradients during the day and at night, but revealed problems during transitions at sunrise and sunset that appear to be related to the canopy radiation parameterization in SiB2.
SiB2, the second-generation land-surface parameterization developed by Sellers et al., has been incorporated into the Colorado State University general circulation model and tested in multidecade ...simulations. The control run uses a "bucket" hydrology but employs the same surface albedo and surface roughness distributions as the SiB2 run. Results show that SiB2 leads to a general warming of the continents, as evidenced in the ground temperature, surface air temperature, and boundary-layer-mean potential temperature. The surface sensible heat flux increases and the latent heat flux decreases. This warming occurs virtually everywhere but is most spectacular over Siberia in winter. Precipitation generally decreases over land but increases in the monsoon regions, especially the Amazon basin in January and equatorial Africa and Southeast Asia in July. Evaporation decreases considerably, especially in dry regions such as the Sahara. The excess of precipitation over evaporation increases in the monsoon regions. The precipitable water (vertically integrated water vapor content) generally decreases over land but increases in the monsoon regions. The mixing ratio of the boundary-layer air decreases over nearly all continental areas, however, including the monsoon regions. The average (composite) maximum boundary-layer depth over the diurnal cycle increases in the monsoon regions, as does the average PBL turbulence kinetic energy. The average boundary-layer wind speed also increases over most continental regions. Groundwater content generally increases in rainy regions and decreases in dry regions, so that SiB2 has a tendency to increase its spatial variability. SiB2 leads to a general reduction of cloudiness over land. The net surface longwave cooling of the surface increases quite dramatically over land, in accordance with the increased surface temperatures and decreased cloudiness. The solar radiation absorbed at the ground also increases. SiB2 has modest effects on the simulated general circulation of the atmosphere. Its most important impacts on the model are to improve the simulations of surface temperature and snow cover and to enable the simulation of the net rate of terrestrial carbon assimilation.
Accurate simulation of variations in planetary boundary layer (PBL) depth is important for weather prediction and climate studies as well as for carbon cycle analysis. The PBL is difficult to ...represent in global models because of the need to represent strong gradients associated with capping inversions at arbitrary heights everywhere. Bulk parameterizations of boundary layer processes are therefore an attractive solution. We evaluated a bulk PBL parameterization locally by prescribing horizontal advective tendencies from high-frequency regional meteorological analyses, and running the PBL formulation in a single-column version of a climate model. We compared simulated variations in PBL depth to observations of radar reflectivity and vertical profiles of CO
2 made at a tall tower in northern Wisconsin during 1999. The model captures many features of the observed diurnal and synoptic variability, but tends to underestimate mid-day maxima in PBL depth. Observed late afternoon collapse of the PBL due to decoupling from an underlying stable surface layer is not simulated. The model underestimates mid-day mixing during calm conditions, suggesting underestimation of buoyancy forcing. Conversely, it overestimates PBL depth under windy conditions, suggesting the parameterization is overly sensitive to shear forcing. Global model simulations cannot be compared to specific dates, but monthly mean diurnal cycles show reasonably good agreement to observations at this site. The simulated PBL in the GCM is generally too shallow at mid-day during the summer months, but is well simulated in spring (when it is deeper than summer) and autumn (when it is shallower than in summer). Seasonal rectifier forcing is slightly underestimated by the model at this site.
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