Urban Water Storage Capacity Inferred From Observed Evapotranspiration Recession.

Water storage plays an important role in mitigating heat and flooding in urban areas. Assessment of the water storage capacity of cities remains challenging due to the inherent heterogeneity of the urban surface. Traditionally, effective storage has been estimated from runoff. Here, we present a novel approach to estimate effective water storage capacity from recession rates of observed evaporation during precipitation-free periods. We test this approach for cities at neighborhood scale with eddy-covariance based latent heat flux observations from 14 contrasting sites with different local climate zones, vegetation cover and characteristics, and climates. Based on analysis of 583 drydowns, we find storage capacities to vary between 1.3 and 28.4 mm, corresponding to e-folding timescales of 1.8-20.1 days. This makes the urban storage capacity at least five times smaller than all the observed values for natural ecosystems, reflecting an evaporation regime characterized by extreme water limitation.

Warming effects on the urban hydrology in cold climate regions.

While approximately 338 million people in the Northern hemisphere live in regions that are regularly snow covered in winter, there is little hydro-climatologic knowledge in the cities impacted by snow. Using observations and modelling we have evaluated the energy and water exchanges of four cities that are exposed to wintertime snow. We show that the presence of snow critically changes the impact that city design has on the local-scale hydrology and climate. After snow melt, the cities return to being strongly controlled by the proportion of built and vegetated surfaces. However in winter, the presence of snow masks the influence of the built and vegetated fractions. We show how inter-year variability of wintertime temperature can modify this effect of snow. With increasing temperatures, these cities could be pushed towards very different partitioning between runoff and evapotranspiration. We derive the dependency of wintertime runoff on this warming effect in combination with the effect of urban densification.

Enhanced air pollution via aerosol-boundary layer feedback in China.

Severe air pollution episodes have been frequent in China during the recent years. While high emissions are the primary reason for increasing pollutant concentrations, the ultimate cause for the most severe pollution episodes has remained unclear. Here we show that a high concentration of particulate matter (PM) will enhance the stability of an urban boundary layer, which in turn decreases the boundary layer height and consequently cause further increases in PM concentrations. We estimate the strength of this positive feedback mechanism by combining a new theoretical framework with ambient observations. We show that the feedback remains moderate at fine PM concentrations lower than about 200 µg m(-3), but that it becomes increasingly effective at higher PM loadings resulting from the combined effect of high surface PM emissions and massive secondary PM production within the boundary layer. Our analysis explains why air pollution episodes are particularly serious and severe in megacities and during the days when synoptic weather conditions stay constant.

Magnitude of dietary effects on plasma cholesterol concentration: role of sex and apolipoprotein E phenotype.

The effects of fat-controlled, low-cholesterol and high-fat, high-cholesterol diets pursued for 4 weeks on plasma lipids and lipoproteins were studied in 44 healthy middle-aged subjects (22 women and 22 men). All the calories were supplied from the hospital kitchen. When the subjects were switched from the fat-controlled, low-cholesterol diet to the high-fat, high-cholesterol diet the average increase in total cholesterol was 1.2 mmol/l (28%), ranging from 0.2 to 2.7 mmol/l (4-56%). At the same time the average increase in LDL cholesterol was 1.0 mmol/l (39%), ranging from 0.1 to 2.4 mmol/l (3-90%). Interestingly, the men responded to the dietary changes more sensitively than the women. The increase in total cholesterol from the low-fat to the high-fat diet was 31% for the men and 25% for the women (P less than 0.05), the corresponding increases in LDL cholesterol being 42% and 37%, respectively (P less than 0.05). A marked increase in HDL cholesterol was observed when the subjects were switched from the low-fat to the high-fat diet, the increase being 30% for the men and 20% for the women. The absolute and percentage lipid changes on the two diets were equal in the subjects with the common apolipoprotein E phenotype 3/3 and in those homozygous and heterozygous for the epsilon 4 allele (E4/4 and E4/3).(ABSTRACT TRUNCATED AT 250 WORDS)