The distribution of positive and negative turbulent heat diffusivity under urban pollution conditions

<p>Turbulent diffusion efficiently transports momentum, heat, and matter and affects their transfers between the surface and the atmosphere. As an important parameter in describing turbulent diffusion, turbulent heat diffusivity K<sub>H</sub> has scarcely been studied in the context of frequent urban pollution in recent years. In this study, K<sub>H</sub> under urban pollution conditions was directly calculated based on the K-theory. We found an obvious diurnal variation in K<sub>H</sub> and its varying vertical distributions for each case and with time. Interestingly, the height of negative K<sub>H</sub> rises gradually after sunrise, peaks at noon, and falls near sunset. Negative K<sub>H</sub> is unusually significant at sunrise and sunset and approximately 140 m during most of the night. The magnitude and fluctuation in K<sub>H</sub> are smaller in the pollutant accumulation stage (CS) at all levels than in the pollutant transport stage (TS) and pollutant removal stage (RS). Turbulent diffusion may greatly affect PM<sub>2.5</sub> concentration at the CS because of the negative correlation between PM<sub>2.5</sub> concentration and the absolute value of K<sub>H</sub> at the CS accompanied by weak wind speed. The applicability of the K-theory is not very good during either day or at night. Note that these problems are inherent in K-theory when characterizing complex systems, such as turbulent diffusion, and require new frameworks or parameterization schemes. These findings may provide valuable insights for improving or establishing a new parameterization scheme for K<sub>H</sub> and promote the study of turbulent diffusion, air quality forecasting, and weather and climate modeling.</p>

The influence of the ocean circulation state on ocean carbon storage and CO₂ drawdown potential in an Earth system model

During the four most recent glacial cycles, atmospheric CO2 during glacial maxima has been lowered by about 90–100 ppm with respect to interglacials. There is widespread consensus that most of this carbon was partitioned in the ocean. It is, however, still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to explore the sensitivity of ocean carbon storage to ocean circulation state. We carry out a set of simulations in which we run the model to pre-industrial equilibrium, but in which we achieve different states of ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air–sea CO2 disequilibrium. We analyse changes in total ocean carbon storage and separate it into contributions by the solubility pump, the biological pump and the CO2 disequilibrium component. We also relate these contributions to differences in the strength of the ocean overturning circulation. Depending on which ocean forcing parameter is tuned, the origin of the change in carbon storage is different. When wind stress or ocean diapycnal diffusivity is changed, the response of the biological pump gives the most important effect on ocean carbon storage, whereas when atmospheric heat diffusivity or ocean isopycnal diffusivity is changed, the solubility pump and the disequilibrium component are also important and sometimes dominant. Despite this complexity, we obtain a negative linear relationship between total ocean carbon and the combined strength of the northern and southern overturning cells. This relationship is robust to different reservoirs dominating the response to different forcing mechanisms. Finally, we conduct a drawdown experiment in which we investigate the capacity for increased carbon storage by artificially maximising the efficiency of the biological pump in our ensemble members. We conclude that different initial states for an ocean model result in different capacities for ocean carbon storage due to differences in the ocean circulation state and the origin of the carbon in the initial ocean carbon reservoir. This could explain why it is difficult to achieve comparable responses of the ocean carbon pumps in model inter-comparison studies in which the initial states vary between models. We show that this effect of the initial state is quantifiable. The drawdown experiment highlights the importance of the strength of the biological pump in the control state for model studies of increased biological efficiency.

SOME MOISTURE DEPENDENT THERMAL PROPERTIES AND BULK DENSITY OF Prosopis Africana SEEDS (OKPEYE)

The thermal heat conductivity, specific heat capacity, thermal heat diffusivity and bulk density of Prosopis africana seeds were determined as a function of moisture content. Specific heat capacity was measured by the method of mixture while the thermal heat conductivity was measured by the guarded hot plate method. Thermal heat diffusivity was calculated from the experimental results obtained from specific heat capacity, thermal heat conductivity and bulk density. The bulk density for Prosopis africana (PA) seeds decreased from 890kg m-3 to 590kg m-3 as moisture content increased from 4 to 20% wet basis (w.b). Specific heat capacity increased from 2760J kg-1 ºC-1 to 2960J kg-1 ºC-1with increasing moisture content. The thermal heat conductivity ranged between 0.70 and 0.90W m-1oC-1 when moisture content rose from 4% to 20% (w.b). Thermal heat diffusivity increased from 2.7 10-7 to 4.2 10-7m2 s-1 as moisture content increased from 4 to 20% (w.b). The values obtained for these thermal properties and bulk density could be useful for design of systems for heat treatment of Prosopis africana seeds.  http://dx.doi.org/10.4314/njt.v36i3.38

The influence of the ocean circulation state on ocean carbon storage and CO₂ drawdown potential in an Earth system model

During the four most recent glacial cycles, atmospheric CO2 during glacial maxima has been lowered by about 90–100 ppm with respect to interglacials. There is widespread consensus that most of this carbon was partitioned in the ocean. It is however still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to constrain the range in ocean carbon storage for an ensemble of ocean circulation equilibrium states. We do a set of simulations where we run the model to pre-industrial equilibrium, but where we achieve different ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air-sea CO2 disequilibrium. We analyse changes in total ocean carbon storage and separate it into contributions by the solubility pump, the biological pump and the CO2 disequilibrium component. We also relate these contributions to differences in strength of ocean overturning circulation. In cases with weaker circulation, we see that the ocean's capacity for carbon storage is larger. Depending on which ocean forcing parameter that is tuned, the origin of the change in carbon storage is different. When wind stress or ocean vertical diffusivity is changed, the response of the biological pump gives the most important effect on ocean carbon storage, whereas when atmospheric heat diffusivity or ocean horizontal diffusivity is changed, the solubility pump and the disequilibrium component are also important and sometimes dominant. Finally, we do a drawdown experiment, where we investigate the capacity for increased carbon storage by maximising the efficiency of the biological pump in our ensemble members. We conclude that different initial states for an ocean model result in different capacities for ocean carbon storage, due to differences in the ocean circulation state. This could explain why it is difficult to achieve comparable responses of the ocean carbon pumps in model intercomparison studies, where the initial states vary between models. The drawdown experiment highlights the importance of the strength of the biological pump in the control state for model studies of increased biological efficiency.