Upward Heat Loss Research Articles

Rapid restratification of the ocean surface boundary layer during the suppressed phase of the MJO in austral spring

Rapid restratification of the ocean surface boundary layer in the Indonesian-Australian Basin was captured in austral spring 2018, under the conditions of low wind speed and clear sky during the suppressed phase of Madden–Julian Oscillations (MJOs). Despite sunny days, strong diurnal variations of sea surface temperature (SST) were not observed until the wind speed became extremely low, because the decreasing wind speed modulated the latent heat flux. Combined with the horizontal advection of ocean current, the reduced upward heat loss inhibited the nighttime convective mixing and facilitated the restratification of the subsurface ocean layers. The surface mixed layer was thus shoaled up to 40 m in two days. The restratified upper ocean then sustained high SSTs by trapping heat near the sea surface until the onset of the MJO convection. This restratification process might be initialized under the atmospheric downwelling conditions during the suppressed phase of MJOs. The resulted high SSTs may affect the development and trajectories of MJOs, by enhancing air-sea heat and moisture fluxes as the winds pick up. Simulating this detailed interaction between the near-surface ocean and atmospheric features of MJOs remains a challenge, but with sufficient vertical resolution and realistic initial conditions, several features of the observations can be well captured.

Sea‐Ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

AbstractMuch of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or freshwater fluxes. However, these mechanisms so far failed to fully explain the surface trends and the concurrent subsurface warming (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind‐driven northward sea‐ice transport, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward sea‐ice transport, a strengthened salinity‐dominated stratification cools the open‐ocean surface waters between the Subantarctic Front and the sea‐ice edge. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea‐ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice‐shelf melting. Moreover, the subsurface warming could account for about 8 ± 2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea‐ice changes thereby may have contributed to the slowdown of global surface warming over this period. Our conclusions are robust across all considered sensitivity cases, although the trend magnitude is sensitive to forcing uncertainties and the model's mean state. It remains unclear whether these sea‐ice induced changes are associated with natural variability or reflect a response to anthropogenic forcing.

Heat loss from the Atlantic water layer in the northern Kara Sea: causes and consequences

Abstract. A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 18 cm s−1) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although we cannot substantiate our conclusions by direct observation-based estimates of mixing rates in the area, we hypothesize that the enhanced vertical mixing along the St. Anna Trough eastern flank favors the upward heat loss from the intermediate warm Atlantic water layer. Modeling results support this hypothesis. The upward heat flux inferred from hydrographic data and model simulations is of O(30–100) W m−2. The region of lowered sea ice thickness and concentration seen both in sea ice remote sensing observations and model simulations marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin. In fact, the sea ice shows a delayed freeze-up onset during fall and a reduction in the sea ice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough.

To Study Different Losses and Loss Co-Efficient on Solar Flat Plate Collector at 900 Yaw Angle

The flat plate collector is a device used for harnessing the solar energy. The efficiency of this device is dependent on the various heat losses. These losses are bottom loss, side loss, edge loss, corner loss, sealing loss & top loss. These losses are measured as well as recorded. There are forty eight thermocouples located at different points. The input energy is through the electric heaters to the collector. These heaters are kept below the absorber. The capacity of each heater is 0.5 kW. These heaters are two in number. The wind velocity is measured by the vane type anemometer. The input heat flux is kept constant & wind velocity is varied. The evaluation of various heat losses is important for the performance evaluation of the collector. Generally the upward heat losses play an important role in the total heat losses. The present paper is an experimental simulation of a horizontal single glazed flat plate collector. The losses are measured under steady state conditions at different wind velocity & keeping 900yaw angle and at zero tilt angle.

Atmosphere–Ocean Coupled Process along Coastal Areas of the Yellow and East China Seas in Winter

Abstract A regional atmosphere–ocean coupled model is developed, based on the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model in conjunction with the Princeton Ocean Model, to investigate atmosphere–ocean coupled processes that might occur over the Yellow and East China Sea shelves in winter. To examine how the coupled processes actually work in the ocean, sea surface temperatures (SSTs) computed in both coupled and uncoupled models are compared with SSTs synthesized from multiple satellite observations. The results indicate that the coupled model significantly improves the negative SST bias in shallow waters around the Chinese coast produced by the uncoupled model. Cool and dry northerly winds from the Asian landmass reduce SST in these shallow waters through intensive upward heat loss. Thereafter, the horizontal gradient of sea level pressure (SLP) around the Chinese coast moderates because the land–ocean heat contrast weakens owing to the reduced SST in the coastal waters. As a result, the wind speed weakens, in line with the moderated horizontal SLP gradient. Moreover, northerly winds can reduce the transport of cool and dry air from the Asian landmass. Hence, upward heat flux around the coastal waters is reduced because of the weakening of the northerly winds and the decreased cool and dry air. This negative feedback thereby suppresses excessive SST cooling along the Chinese coast during winter.

Solid particle effects on heat transfer in a multi-layered molten pool with gas injection

In the very unlikely event of a severe reactor accident involving core melt and pressure vessel failure, it is important to identify the circumstances that would allow the molten core material to cool down and resolidify, bringing core debris to a stable coolable state. To achieve this, it has been proposed to flood the cavity with water from above forming a layered structure where upward heat loss from the molten pool to the water will cause the core material to quench and solidify. In this situation the molten pool would become a three-phase mixture: e.g., a solid and liquid slurry formed by the molten pool as it cools to a temperature below the temperature of liquidus, agitated by the gases formed in the concrete ablation process. The present work quantifies the partition of the heat losses upward and downward in this multi-layered configuration, considering the influence of the viscosity and the solid fraction in the pool, from test data obtained from intermediate scale experiments at the University of Wisconsin-Madison. These experimental results show heat transfer behavior for multi-layered pools for a range of viscosities and solid fractions. These results are compared to previous experimental studies and well known correlations and models.

Investigations on an air-heating system having thermal storage

In the present communication, the authors propose and analyze an air-heating system which differs from conventional systems in the sense that it stores a substantial amount of heat and hence can be used in the night hours also. The system essentially consists of a rectangular metallic tray which is blackened and glazed; the tray is filled with water and air flows in contact with its underside. Explicit expressions are obtained for the parameters describing the system performance in two cases, viz. (i) when the system remains uncovered at all times, and (ii) when an insulation cover is used at the top to reduce the upward heat losses during off/low sunshine hours. Numerical calculations are also carried out corresponding to a typical set of data.

The use of moderate vacuum environments as a means of increasing the collection efficiencies and operating temperatures of flat-plate solar collectors

It is shown that evacuating a flat-plate solar collector to a pressure 1–25 torr results in elimination of the natural convection heat loss from the absorber for absorber-to-cover spacings up to 15 cm. This mode of heat transfer then reduces to pure conduction through the air space between the absorber and the cover. The effect of this reduction on the total upward heat loss from the collector is considered for a variety of collector operating conditions and is shown to be especially pronounced for collectors employing wavelength-selective surfaces (high absorptance for solar radiation, but low emittance for the energy re-radiated by the absorber). Computer simulations of collector performance for the Dallas, Texas area indicate that the combination of a moderate vacuum and a selective surface (α = 0·90, ϵ = 0·15) can increase daily energy collection as much as 278 per cent over that obtained with a non-vacuum collector using a flat-black (α = ϵ = 0·95) surface and can make it possible to operate at a temperature of 150°C with a daily energy collection efficiency of more than 40 per cent. The theoretical predictions are supported by the results of twelve experiments with a no-load solar tester. At an absorber-to-cover spacing of 7·5 cm, the steady-state temperature of a moderately selective absorber (α = 0·75, ϵ = 0·3) was increased from 115°C at atmospheric pressure to 179°C at a pressure of 25 torr.

Performance correlation of horizontal plastic solar water heaters

Statistical and experimental methods are used to determine the optical efficiency and the heat lost by radiation and convection in a horizontal plastic solar hot water heater. The heater consisted of a water filled polyethylene bag with a clear top and black bottom. It was insulated on the sides and bottom by styrofoam and covered on the top with one to three layers of clear Tedlar. The effective sun angle, effective sky temperature, ambient temperature and the number of covers are included as variables. The upward heat losses were higher than those predicted by the heat transfer coefficients of Hottel and Woertz.