Showing papers on "Latent heat published in 2015"

Importance of latent heat release in ascending air streams for atmospheric blocking

Abstract: Atmospheric blocking can contribute to extreme weather events. A Lagrangian approach applied to reanalysis data shows that a large fraction of air masses are heated before entering a blocking system, pointing to a role for latent heating.

Energetics of the Core

Abstract: This chapter reviews theoretical approaches to calculating the energy and entropy budgets of the Earth's core. The energy budget allows the growth of the inner core with time to be calculated; the entropy budget determines whether or not a geodynamo can function. The present-day geodynamo is driven primarily by compositional convection, with latent heat, secular cooling, and (possibly) exsolution playing a subsidiary role. Prior to the onset of inner core solidification, the dynamo was likely driven purely by secular cooling and required a cooling rate roughly three times the present-day value to maintain the same rate of entropy production. The largest uncertainties in the present-day energy and entropy budgets arise from uncertainties in the core–mantle boundary (CMB) heat flow, the compositional density contrast between the outer and inner cores, the thermal conductivity of the core material, and the amount of ohmic dissipation generated by the dynamo. The present-day CMB heat flow is estimated at 12 ± 5 TW; heat flows < 5 TW are insufficient to drive the current dynamo. Prior to the formation of the inner core, sustaining a dynamo required a CMB heat flow of 15 TW or more, implying rapid core cooling and a molten lower mantle. Such high heat flows appear to be compatible with models of mantle convection. These high heat flows also imply an inner core age of < 0.7 Gyr. Adding potassium to the core results in lower initial core temperatures, but does not significantly alter the inner core age. Thermally or compositionally stratified regions may have existed (or still exist) within the core, but there is currently no consensus on their nature or effects on the dynamo.

A review of thermal energy storage technologies and control approaches for solar cooling

Abstract: This paper presents a review of thermal storage media and system design options suitable for solar cooling applications. The review covers solar cooling applications with heat input in the range of 60–250 °C. Special attention is given to high temperature (>100 °C) high efficiency cooling applications that have been largely ignored in existing reviews. Sensible and latent heat storage materials have been tabulated according to their suitability for double effect and triple effect chillers. A summary of system designs for water storage (sensible heat), and phase change material storage (latent heat) has been provided. The article summarizes literature related to solar thermal air-conditioning systems from a material level as well as plant level considerations. This includes evaluating various control strategies for managing the thermal store, that aid in optimal functioning of a solar air conditioning plant. Modeling approaches are reviewed for sizing the solar thermal store, highlighting the large difference seen in specific storage size when applied in different applications.

Enhancing Hydrologic Modelling in the Coupled Weather Research and Forecasting–Urban Modelling System

Abstract: Urbanization modifies surface energy and water budgets, and has significant impacts on local and regional hydroclimate. In recent decades, a number of urban canopy models have been developed and implemented into the Weather Research and Forecasting (WRF) model to capture urban land-surface processes. Most of these models are inadequate due to the lack of realistic representation of urban hydrological processes. Here, we implement physically-based parametrizations of urban hydrological processes into the single layer urban canopy model in the WRF model. The new single-layer urban canopy model features the integration of, (1) anthropogenic latent heat, (2) urban irrigation, (3) evaporation from paved surfaces, and (4) the urban oasis effect. The new WRF–urban modelling system is evaluated against field measurements for four different cities; results show that the model performance is substantially improved as compared to the current schemes, especially for latent heat flux. In particular, to evaluate the performance of green roofs as an urban heat island mitigation strategy, we integrate in the urban canopy model a multilayer green roof system, enabled by the physical urban hydrological schemes. Simulations show that green roofs are capable of reducing surface temperature and sensible heat flux as well as enhancing building energy efficiency.

Positive tropical marine low-cloud cover feedback inferred from cloud-controlling factors

Abstract: Differences in simulations of tropical marine low-cloud cover (LCC) feedback are sources of significant spread in temperature responses of climate models to anthropogenic forcing. Here we show that in models the feedback is mainly driven by three large-scale changes—a strengthening tropical inversion, increasing surface latent heat flux, and an increasing vertical moisture gradient. Variations in the LCC response to these changes alone account for most of the spread in model-projected 21st century LCC changes. A methodology is devised to constrain the LCC response observationally using sea surface temperature (SST) as a surrogate for the latent heat flux and moisture gradient. In models where the current climate's LCC sensitivities to inversion strength and SST variations are consistent with observed, LCC decreases systematically, which would increase absorption of solar radiation. These results support a positive LCC feedback. Correcting biases in the sensitivities will be an important step toward more credible simulation of cloud feedbacks.

The effect of groundwater interaction in North American regional climate simulations with WRF/Noah-MP

Abstract: The fluxes of water and energy between the land surface and atmosphere involve many complex non-linear processes. In this study, the Noah and Noah-MP land surface models with multiple groundwater sub-models are used to assess how the treatment of canopy processes and interactions with deep groundwater affect 6 month regional climate simulations in two contrasting years, 2002 and 2010. Unlike the free drainage models, the models with groundwater capability have upward flux from the aquifer at different periods in the simulation. The inter-model Noah-MP soil moisture and latent heat flux results are consistent with recharge differences: the stronger upward flux capability with interactive groundwater results in the highest soil moisture and latent heat flux of the Noah-MP models. The increased latent heat effect on increased precipitation is small, which may result from negligible differences in convective precipitation. The Noah-MP model, independent of groundwater option, improves upon a cold and dry bias in the spring Noah simulations both during the day and night. The results for summer are region dependent and also differ between year and time of day. For a majority of the simulation period, there is little groundwater effect on the Noah-MP near-surface diagnostic fields. However, when the Noah-MP model produces large warm/dry biases in the 2010 summer, the aquifer interactions in Noah-MP improve the air temperature bias by 1–2 °C and dew point temperature bias by 1 °C.

Heat utilisation technologies: A critical review of heat pipes

Abstract: In electrical or thermal appliances, heat (thermal energy) must either be added into or removed from a system to maintain operational stability. Heat pipes can enhance the heat transfer capabilities without needing a significant temperature gradient between heat sources and heat sinks. The effectiveness of heat pipes is due to the latent heat of phase change of the working fluid within (i) condensation and (ii) evaporation stages. The latent heat of phase change greatly exceeds the sensible heat capacity. Heat pipes may rely on gravity, wicks, centrifugal force or in some cases even a magnetic field to help return condensate flow from the condenser to the evaporator. Wicks in heat pipes are classified into three groups: sintered, groove and mesh types. This review attempts to cover various types of heat pipes such as thermal diodes, variable conductance, pulsating, etc. The application of nanotechnology in heat pipes can be separated into two groups: nanoparticles and nanobubbles, with the latter receiving considerably less attention than the former. The hybridisation of heat pipe technology is also possible and has been discussed along with its future research potential.

Energy and exergy performance assessments for latent heat thermal energy storage systems

Abstract: Latent heat thermal energy storage (TES) can be an efficient option to cater to fluctuating energy demands and at the same time to obtain a higher performance from the energy and exergy aspects. Latent heat TES storage performance is usually influenced by various operating conditions and design parameters during the energy/exergy stored and retrieved. The scope of investigation is to comprehensively review various useful techniques adopted in detail for energy and exergy performance enhancements, and provide the perspectives for researchers and engineers to design more efficient latent TES systems. Various influencing factors can be enlarged to include the heat transfer fluid (HTF) mass flow rate and inlet temperature, phase change material (PCM) melting temperature and number, additives for PCMs, storage unit dimension, heat exchanger surface enhancement, and sensible heating and sub-cooling, etc. The main perspectives and directions including heat transfer mechanism and optimized multiple PCM melting point are provided to enable further research.

1 – Introduction to thermal energy storage (TES) systems

Abstract: Thermal energy storage (TES) systems can store heat or cold to be used later, under varying conditions such as temperature, place or power. TES systems are divided in three types: sensible heat, latent heat, and thermochemical. Clues for each TES system are presented in this chapter and requirements for each technology and application are given. An overview of system types and description of particular and novel applications are presented. The potential for energy saving and climate change mitigation using TES with a 10-year scenario is presented for specific cases. Finally, the CO2 mitigation potential of TES in different applications is presented.

Vegetation controls on surface heat flux partitioning, and land‐atmosphere coupling

Abstract: We provide observational evidence that land-atmosphere coupling is underestimated by a conventional metric defined by the correlation between soil moisture and surface evaporative fraction (latent heat flux normalized by the sum of sensible and latent heat flux). Land-atmosphere coupling is 3 times stronger when using leaf area index as a correlate of evaporative fraction instead of soil moisture, in the Southern Great Plains. The role of vegetation was confirmed using adjacent flux measurement sites having identical atmospheric forcing but different vegetation phenology. Transpiration makes the relationship between evaporative fraction and soil moisture nonlinear and gives the appearance of weak coupling when using linear soil moisture metrics. Regions of substantial coupling extend to semiarid and humid continental climates across the United States, in terms of correlations between vegetation metrics and evaporative fraction. The hydrological cycle is more tightly constrained by the land surface than previously inferred from soil moisture.

Long-term climate forcing by atmospheric oxygen concentrations

Abstract: The percentage of oxygen in Earth’s atmosphere varied between 10% and 35% throughout the Phanerozoic. These changes have been linked to the evolution, radiation, and size of animals but have not been considered to affect climate. We conducted simulations showing that modulation of the partial pressure of oxygen (pO2), as a result of its contribution to atmospheric mass and density, influences the optical depth of the atmosphere. Under low pO2 and a reduced-density atmosphere, shortwave scattering by air molecules and clouds is less frequent, leading to a substantial increase in surface shortwave forcing. Through feedbacks involving latent heat fluxes to the atmosphere and marine stratus clouds, surface shortwave forcing drives increases in atmospheric water vapor and global precipitation, enhances greenhouse forcing, and raises global surface temperature. Our results implicate pO2 as an important factor in climate forcing throughout geologic time.

On the Rapid Intensification of Hurricane Wilma (2005). Part III: Effects of Latent Heat of Fusion

Abstract: The impacts of the latent heat of fusion on the rapid intensification (RI) of Hurricane Wilma (2005) are examined by comparing a 72-h control simulation (CTL) of the storm to a sensitivity simulation in which the latent heat of depositionis reduced by removing fusion heating (NFUS). Resultsshow that, while both storms undergo RI, the intensificationrate is substantially reducedin NFUS. At peakintensity, NFUSis weakerthan CTL by 30hPa in minimum central pressure and by 12ms 21 in maximum surface winds. The reduced rate of surface pressure falls in NFUS appears to result hydrostatically from less upper-level warming in the eye. It is shown that CTL generates more inner-core convective bursts (CBs) during RI, with higher altitudes of peak vertical motion in the eyewall, compared to NFUS. The latent heat of fusion contributes positively to sufficient eyewall conditional instability to support CB updrafts. Slantwise soundings taken in CB updraft cores reveal moist adiabatic lapse rates until 200hPa, where the updraft intensity peaks. These results suggest that CBsmayimpacthurricaneintensificationbyinducingcompensatingsubsidenceofthelower-stratosphericair, and the authors conclude that the development of more CBs inside the upper-level radius of maximum wind and at the higher altitude of latent heating all appear to be favorable for the RI of Wilma.

Unconventional experimental technologies available for phase change materials (PCM) characterization. Part 1. Thermophysical properties

Abstract: The use of thermal energy storage by phase change materials (PCM) is increasing in interest for building applications. For the deployment of the technology, appropriate characterization of PCM and hybrid PCM is essential, but it is not always possible to carry it out with conventional equipment, mainly due to the sample size. This paper shows equipment developed in different research centers and universities to analyze thermophysical properties, such as specific heat, latent heat and melting temperature, and thermal conductivity and diffusivity of PCM and hybrid PCM materials.

Quantifying the heat flux regulation of metropolitan land use/land cover components by coupling remote sensing modeling with in situ measurement

Abstract: Quantifying the effects of urban land use/land cover with regard to surface radiation and heat flux regulation is important to ecological planning and heat stress mitigation. To retrieve the spatial pattern of heat fluxes in the Beijing metropolitan area, China, a remote sensing-based energy balance model was calibrated with synchronously measured energy fluxes including net radiation, latent heat flux (LE), and sensible heat flux (H). Our model calibration approach avoided the uncertainties due to subjective judgments in previous empirical parameterization methods. The land surface temperature (LST), H, and Bowen ratio () of Beijing were found to increase along the outskirt-suburban-urban gradient, with strong spatial variation. LST and H were negatively correlated with vegetation fraction cover (VFC). For example, the modern high-rise residential areas with relatively higher VFC had lower H and than the traditional low-rise residential areas. Our findings that indicate thermal dissipation through vegetation transpiration might play an important role in urban heat regulation. Notably, the thermal dissipating strength of vegetation (calculated as LE/VFC) declined exponentially with increased VFC. For the purpose of heat stress regulation, we recommend upgrading the traditional low-rise residential areas to modern high-rise residential areas and focusing urban greenery projects in areas whose VFC<0.1, where the heat regulating service by urban vegetation could be twice as effective as in other places. Key Points

Effects of water clarity on lake stratification and lake‐atmosphere heat exchange

Abstract: Recent progress of including lake subroutines in numerical weather prediction (NWP) models has led to more accurate forecasts. In lake models, one essential parameter is water clarity, parameterized via the light extinction coefficient, Kd, for which a global constant value is usually used. We used direct eddy covariance fluxes and basic meteorological measurements coupled with lake water temperature and clarity measurements from a boreal lake to estimate the performance of two lake models, LAKE and FLake. These models represent two 1D modeling frameworks broadly used in NWP. The results show that the lake models are very sensitive to changes in Kd when it is lower than 0.5 m−1. The progress of thermal stratification depended strongly on Kd. In dark water simulations the mixed layer was shallower, longwave and turbulent heat losses higher and therefore the average water column temperatures lower than in clear water simulations. Thus, changes in water clarity can also affect the onset of ice cover. The more complex LAKE modeled the seasonal thermocline deepening whereas it remained virtually constant during summer in the FLake model. Both models overestimated the surface water temperatures by about 1°C and latent heat flux by >30%, but the variation in heat storage and sensible heat flux were adequately simulated. Our results suggest that, at least for humic lakes, a lake-specific, but not time-depending, constant value for Kd can be used and that a global mapping of Kd would be most beneficial in regions with relatively clear lakes, e.g. in lakes at high altitudes.

Automated calculation of surface energy fluxes with high-frequency lake buoy data

Abstract: Lake Heat Flux Analyzer is a program used for calculating the surface energy fluxes in lakes according to established literature methodologies. The program was developed in MATLAB for the rapid analysis of high-frequency data from instrumented lake buoys in support of the emerging field of aquatic sensor network science. To calculate the surface energy fluxes, the program requires a number of input variables, such as air and water temperature, relative humidity, wind speed, and short-wave radiation. Available outputs for Lake Heat Flux Analyzer include the surface fluxes of momentum, sensible heat and latent heat and their corresponding transfer coefficients, incoming and outgoing long-wave radiation. Lake Heat Flux Analyzer is open source and can be used to process data from multiple lakes rapidly. It provides a means of calculating the surface fluxes using a consistent method, thereby facilitating global comparisons of high-frequency data from lake buoys.