Showing papers on "Latent heat published in 2010"

A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS)

Abstract: This paper reviews the development of latent heat thermal energy storage systems studied detailing various phase change materials (PCMs) investigated over the last three decades, the heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy and the formulation of the phase change problem. It also examines the geometry and configurations of PCM containers and a series of numerical and experimental tests undertaken to assess the effects of parameters such as the inlet temperature and the mass flow rate of the heat transfer fluid (HTF). It is concluded that most of the phase change problems have been carried out at temperature ranges between 0 °C and 60 °C suitable for domestic heating applications. In terms of problem formulation, the common approach has been the use of enthalpy formulation. Heat transfer in the phase change problem was previously formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for. There is no standard method (such as British Standards or EU standards) developed to test for PCMs, making it difficult for comparison to be made to assess the suitability of PCMs to particular applications. A unified platform such as British Standards, EU standards needs to be developed to ensure same or similar procedure and analysis (performance curves) to allow comparison and knowledge gained from one test to be applied to another.

Thermophysical properties of seawater: a review of existing correlations and data

Abstract: Correlations and data for the thermophysical properties of seawater are reviewed. Properties examined include density, specific heat capacity, thermal conductivity, dynamic viscosity, surface tension, vapor pressure, boiling point elevation, latent heat of vaporization, specifi c enthalpy, specific entropy and osmotic coefficient. These properties include those needed for design of thermal and membrane desalination processes. Results are presented in terms of regression equations as functions of temperature and salinity. The available correlations for each property are summarized with their range of validity and accuracy. Best-fi tted new correlations are obtained from available data for density, dynamic viscosity, surface tension, boiling point elevation, specifi c enthalpy, specific entropy and osmotic coefficient after appropriate conversion of temperature and salinity scales to the most recent standards. In addition, a model for latent heat of vaporization is suggested. Comparisons are carried out amo...

The International Urban Energy Balance Models Comparison Project: First Results from Phase 1

Abstract: A large number of urban surface energy balance models now exist with different assumptions about the important features of the surface and exchange processes that need to be incorporated To date, no comparison of these models has been conducted; in contrast, models for natural surfaces have been compared extensively as part of the Project for Intercomparison of Land-surface Parameterization Schemes Here, the methods and first results from an extensive international comparison of 33 models are presented The aim of the comparison overall is to understand the complexity required to model energy and water exchanges in urban areas The degree of complexity included in the models is outlined and impacts on model performance are discussed During the comparison there have been significant developments in the models with resulting improvements in performance (root-mean-square error falling by up to two-thirds) Evaluation is based on a dataset containing net all-wave radiation, sensible heat, and latent heat flux observations for an industrial area in Vancouver, British Columbia, Canada The aim of the comparison is twofold: to identify those modeling approaches that minimize the errors in the simulated fluxes of the urban energy balance and to determine the degree of model complexity required for accurate simulations There is evidence that some classes of models perform better for individual fluxes but no model performs best or worst for all fluxes In general, the simpler models perform as well as the more complex models based on all statistical measures Generally the schemes have best overall capability to model net all-wave radiation and least capability to model latent heat flux

Do surfaces with mixed hydrophilic and hydrophobic areas enhance pool boiling

Abstract: We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7° wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are, up to respectively, 65% and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer. © 2010 American Institute of Physics. doi:10.1063/1.3485057 Boiling is an efficient process to transfer large amounts of heat at a prescribed temperature because of the large latent heat of vaporization. The term flow boiling describes the boiling of liquids forced to move along hot surfaces, while in pool boiling, the topic handled in this paper, the liquid is stagnant and in contact with a hot solid surface. 1 Besides the common experience of boiling water in an electric kettle, pool boiling has applications in metallurgy, high performance heat exchangers, and immersion cooling of electronics. Pool boiling performance is measured with two parameters, the heat transfer coefficient HTC and the critical heat flux CHF. The CHF is measured by increasing the surface temperature until a transition from high HTC to very low HTC occurs. This signifies the formation of a vapor film insulating the liquid from the heated surface, a phenomenon called dry out. Several characteristics determine the performance of a boiling surface. Nucleation sites in appropriate number and dimensions need to be provided such as cavities, rough areas, or hydrophobic islands. 2 As of today, the performance of boiling surfaces has been increased by using wicking structures to prevent dry out, 3 by increasing the surface area with fins or fluidized bed, 3‐6 and by enhancing the wettability of the surface. 5‐10 The latter strategy is justified by experiments of Wang and Dhir, 11 showing that the CHF was

Water vapor and the dynamics of climate changes

Abstract: [1] Water vapor is not only Earth's dominant greenhouse gas. Through the release of latent heat when it condenses, it also plays an active role in dynamic processes that shape the global circulation of the atmosphere and thus climate. Here we present an overview of how latent heat release affects atmosphere dynamics in a broad range of climates, ranging from extremely cold to extremely warm. Contrary to widely held beliefs, atmospheric circulation statistics can change nonmonotonically with global-mean surface temperature, in part because of dynamic effects of water vapor. For example, the strengths of the tropical Hadley circulation and of zonally asymmetric tropical circulations, as well as the kinetic energy of extratropical baroclinic eddies, can be lower than they presently are both in much warmer climates and in much colder climates. We discuss how latent heat release is implicated in such circulation changes, particularly through its effect on the atmospheric static stability, and we illustrate the circulation changes through simulations with an idealized general circulation model. This allows us to explore a continuum of climates, to constrain macroscopic laws governing this climatic continuum, and to place past and possible future climate changes in a broader context.

Thermophysical Comparison of Five Commercial Paraffin Waxes as Latent Heat Storage Materials

Abstract: Thermophysical properties of phase change materials (PCM) are of utmost importance in latent heat thermal energy storage (LHTES) applications. Therefore, an experimental study is conducted in order to determine thermophysical properties of five technical grade paraffin waxes produced by major Croatian oil company, INA d.d. Rijeka. The temperatures and enthalpies of melting and solidification (latent heat capacity) and specific heat capacities of solid and liquid paraffin waxes were measured by differential scanning calorimetry (DSC). The thermal diffusivity of paraffin waxes was determined utilizing transient method. The importance of eliminating phase transformation interferences to thermophysical properties determination is addressed. The densities and the coefficient of thermal expansion were measured using Archimedes methods. A self-adopted simple and inexpensive laboratory procedure for the determination of liquid density as a temperature function is presented. Finally, the thermal conductivities have been calculated from measured densities, heat capacities and diffusivities. Based on results obtained, the investigated paraffin waxes were evaluated in regard to their applicability as PCM for LHTES.

Do Undiluted Convective Plumes Exist in the Upper Tropical Troposphere

Abstract: Using a passive tracer, entrainment is studied in cloud-resolving simulations of deep convection in radiative‐convective equilibrium. It is found that the convective flux of undiluted parcels decays with height exponentially, indicating a constant probability per vertical distance of mixing with environmental air. This probability per distance is sufficiently large that undiluted updrafts are negligible above a height of 4‐5 km and virtually absent above 10 km. These results are shown to be independent of the horizontal grid size within the range of 3.2 km to 100 m. Plumes that do reach the tropopause are found to be highly diluted. An equivalent potential temperature is defined that is exactly conserved for all reversible adiabatic transformations, including those with ice. Using this conserved variable, it is shown that the latent heat of fusion (from both freezing and deposition) causes only a small increase in the level of neutral buoyancy near the tropopause. In fact, when taken to sufficiently low pressures, a parcel with an ice phase ends up colder than it would without an ice phase. Nevertheless, the contribution from fusion to a parcel’s kinetic energy is quite large. Using an ensemble of tracers, information is encoded in parcels at the cloud base and decoded where the parcel is observed in the free troposphere. Using this technique, clouds at the tropopause are diagnosed for their cloud-base temperature, specific humidity, and vertical velocity. Using these as the initial values for a Lagrangian parcel model, it is shown that fusion provides the kinetic energy required for diluted parcels to reach the tropopause.

Persistent unstable atmospheric boundary layer enhances sensible and latent heat loss in a tropical great lake: Lake Tanganyika

Abstract: [1] Energy fluxes across the surface of lakes regulate heat storage and affect the water balance. Sensible and latent heat fluxes are affected by atmospheric stability, especially for large lakes. We examined the effect of atmospheric stability on the heat fluxes on seasonal time scales at Lake Tanganyika, East Africa, by estimating hourly sensible and latent heat fluxes and net radiation using thermistor chains and meteorological stations. The atmosphere was almost always unstable, in contrast to the atmosphere above North American Great Lakes which is unstable in winter and stable in summer. Persistent atmospheric instability resulted in a 13% and 18% increase in the annual mean heat loss by latent and sensible heat fluxes, respectively, relative to conditions of neutral stability. The persistent unstable atmosphere is caused by a higher water surface temperature compared with air temperature, which we argue is the case in general in (sub)tropical lakes. Low humidity further enhanced the frequency of unstable conditions and enhanced the exchange of heat and vapor from the lake to the atmosphere. The estimated heat fluxes were sensitive to the temporal scale of data inputs and to the local values of parameters such as air density. To our knowledge this is the first paper that demonstrates and quantifies the effect of atmospheric stability on latent and sensible heat fluxes from a lake on an annual basis, using data collected from the lake surface.

The global atmospheric water cycle

Abstract: Water vapour plays a key role in the Earth's energy balance. Almost 50% of the absorbed solar radiation at the surface is used to cool the surface, through evaporation, and warm the atmosphere, through release of latent heat. Latent heat is the single largest factor in warming the atmosphere and in transporting heat from low to high latitudes. Water vapour is also the dominant greenhouse gas and contributes to a warming of the climate system by some 24°C (Kondratev 1972). However, water vapour is a passive component in the troposphere as it is uniquely determined by temperature and should therefore be seen as a part of the climate feedback system. In this short overview, we will first describe the water on planet Earth and the role of the hydrological cycle: the way water vapour is transported between oceans and continents and the return of water via rivers to the oceans. Generally water vapour is well observed and analysed; however, there are considerable obstacles to observing precipitation, in particular over the oceans. The response of the hydrological cycle to global warming is far reaching. Because different physical processes control the change in water vapour and evaporation/precipitation, this leads to a more extreme distribution of precipitation making, in general, wet areas wetter and dry areas dryer. Another consequence is a transition towards more intense precipitation. It is to be expected that the changes in the hydrological cycle as a consequence of climate warming may be more severe that the temperature changes.

Waste heat transportation system, using phase change material (PCM) from steelworks to chemical plant

Abstract: This paper describes the feasibility of a latent heat transportation (LHT) system that uses phase change material (PCM) to recover waste heat at temperatures over 300 °C in steelworks and supplies it to a distillation tower of benzene, toluene, and xylene (BTX). The operating data in the proposed system – as well as in a conventional heat supply system and a sensible heat transportation (SHT) system – were calculated based on heat and material balances in order to evaluate energy requirements, exergy loss, and CO 2 emissions. The results showed that an LHT system using NaOH with a solid–solid transformation temperature of 293 °C and a melting point of 320 °C as PCM has 2.76 times the amount of heat-storage density of an SHT system; additionally, it has only 8.6% of the energy requirements, 37.9% of the exergy loss, and 17.5% of the CO 2 emissions of a conventional system that lack heat-recovery capabilities. The results suggest the possibility of transporting heat via high-temperature PCM from steelworks to chemical plants. Such infrastructure would offer many benefits such as energy savings, reductions in exergy loss and CO 2 emissions, and the benefits associated with coproduction.

Heat storage in forest biomass improves energy balance closure

Abstract: . Temperature measurements in trunks and branches in a mature ca. 100 years-old mixed pine and spruce forest in central Sweden were used to estimate the heat storage in the tree biomass. The estimated heat flux in the sample trees and data on biomass distributions were used to scale up to stand level biomass heat fluxes. The rate of change of sensible and latent heat storage in the air layer below the level of the flux measurements was estimated from air temperature and humidity profile measurements and soil heat flux was estimated from heat flux plates and soil temperature measurements. The fluxes of sensible and latent heat from the forest were measured with an eddy covariance system in a tower. The analysis was made for a two-month period in summer of 1995. The tree biomass heat flux was the largest of the estimated storage components and varied between 40 and −35 W m−2 on summer days with nice weather. Averaged over two months the diurnal maximum of total heat storage was 45 W m−2 and the minimum was −35 W m−2. The soil heat flux and the sensible heat storage in air were out of phase with the biomass flux and they reached maximum values that were about 75% of the maximum of the tree biomass heat storage. The energy balance closure improved significantly when the total heat storage was added to the turbulent fluxes. The slope of a regression line with sum of fluxes and storage as independent and net radiation as dependent variable, increased from 0.86 to 0.95 for half-hourly data and the scatter was also reduced. The most significant finding was, however, that during nights with strongly stable conditions when the sensible heat flux dropped to nearly zero, the total storage matched the net radiation very well. Another interesting result was that the mean energy imbalance started to increase when the Richardson number became more negative than ca. −0.1. In fact, the largest energy deficit occurred at maximum instability. Our conclusion is that eddy covariance measurements can function well during stable conditions but that the functioning under strong instabilities might be a so far unforeseen problem.

Cooling vests with phase change material packs: the effects of temperature gradient, mass and covering area.

Abstract: Phase change material (PCM) absorbs or releases latent heat when it changes phases, making thermal-regulated clothing possible. The objective of this study was to quantify the relationships between PCM cooling rate and temperature gradient, mass, and covering area on a thermal manikin in a climatic chamber. Three melting temperatures (24, 28, 32 °C) of the PCMs, different mass, covering areas, and two manikin temperatures (34 and 38 °C) were used. The results showed that the cooling rate of the PCM vests tested is positively correlated with the temperature gradient between the thermal manikin and the melting temperature of the PCMs. The required temperature gradient is suggested to be greater than 6 °C when PCM vests are used in hot climates. With the same temperature gradient, the cooling rate is mainly determined by the covering area. The duration of the cooling effect is dependent on PCM mass and the latent heat

Application of the Priestley–Taylor Approach in a Two-Source Surface Energy Balance Model

Abstract: The Priestley‐Taylor (PT) approximation for computing evapotranspiration was initially developed for conditions of a horizontally uniform saturated surface sufficiently extended to obviate any significant advection of energy. Nevertheless, the PT approach has been effectively implemented within the framework of a thermal-based two-source model (TSM) of the surface energy balance, yielding reasonable latent heat flux estimates over a range in vegetative cover and climate conditions. In the TSM, however, the PT approach is applied only to the canopy component of the latent heat flux, which may behave more conservatively than the bulk (soil 1 canopy) system. The objective of this research is to investigate the response of the canopy and bulk PT parameters to varying leaf area index (LAI) and vapor pressure deficit (VPD) in both natural and agricultural vegetated systems, to better understand the utility and limitations of this approximation within the context of the TSM. Micrometeorological flux measurements collected at multiple sites under a wide range of atmospheric conditions were used to implement an optimization scheme, assessing the value of the PT parameter for best performance of the TSM. Overall, the findings suggest that within the context of the TSM, the optimal canopy PT coefficient for agricultural crops appears to have a fairly conservative value of ;1.2 except when under very high vapor pressure deficit (VPD) conditions, when its value increases. For natural vegetation (primarily grasslands), the optimal canopy PT coefficient assumed lower values on average (;0.9) and dropped even further at high values of VPD. This analysis provides some insight as to why the PT approach, initially developed for regional estimates of potential evapotranspiration, can be used successfully in the TSM scheme to yield reliable heat flux estimates over a variety of land cover types.

Solar micro-energy harvesting based on thermoelectric and latent heat effects. Part II: Experimental analysis

Abstract: This article presents a new method of harvesting ambient renewable micro-energy by using both thermoelectric and latent heat effects. We designed a prototype work unit made of phase change material (PCM) and a thermoelectric generator (TEG) and used a numerical method to inspect system performance. Special attention was paid to the implementation of ambient loading (including solar radiation, temperature variation and wind speed) in the numerical simulation process. The calculated results show that the proposed work unit (TEG & PCM) can harvest micro-energy day and night, thanks to the heat storage function of the PCM which is placed behind the TEG.

Shallow and Deep Latent Heating Modes over Tropical Oceans Observed with TRMM PR Spectral Latent Heating Data

Abstract: The global hydrological cycle is central to the Earth's climate system, with rainfall and the physics of its formation acting as the key links in the cycle. Two-thirds of global rainfall occurs in the Tropics. Associated with this rainfall is a vast amount of heat, which is known as latent heat. It arises mainly due to the phase change of water vapor condensing into liquid droplets; three-fourths of the total heat energy available to the Earth's atmosphere comes from tropical rainfall. In addition, fresh water provided by tropical rainfall and its variability exerts a large impact upon the structure and motions of the upper ocean layer. Three-dimensional distributions of latent heating estimated from Tropical Rainfall Measuring Mission Precipitation Radar (TRMM PR)utilizing the Spectral Latent Heating (SLH) algorithm are analyzed. Mass-weighted and vertically integrated latent heating averaged over the tropical oceans is estimated as approx.72.6 J/s (approx.2.51 mm/day), and that over tropical land is approx.73.7 J/s (approx.2.55 mm/day), for 30degN-30degS. It is shown that non-drizzle precipitation over tropical and subtropical oceans consists of two dominant modes of rainfall systems, deep systems and congestus. A rough estimate of shallow mode contribution against the total heating is about 46.7 % for the average tropical oceans, which is substantially larger than 23.7 % over tropical land. While cumulus congestus heating linearly correlates with the SST, deep mode is dynamically bounded by large-scale subsidence. It is notable that substantial amount of rain, as large as 2.38 mm day-1 in average, is brought from congestus clouds under the large-scale subsiding circulation. It is also notable that even in the region with SST warmer than 28 oC, large-scale subsidence effectively suppresses the deep convection, remaining the heating by congestus clouds. Our results support that the entrainment of mid-to-lower-tropospheric dry air, which accompanies the large-scale subsidence is the major factor suppressing the deep convection. Therefore, representation of the realistic entrainment is very important for proper reproduction of precipitation distribution and resultant large-scale circulation.

Longitudinal variation of tides in the MLT region: 2. Relative effects of solar radiative and latent heating

Abstract: [1] Part 2 of our study examines the relative importance of radiative heating and latent heating in accounting for vertically propagating tides that impose longitude variability on mesosphere-lower thermosphere (MLT) winds, temperatures, and densities. Our results are based upon numerical simulations using the Global-Scale Wave Model (GSWM) and new tidal heating rates derived from International Satellite Cloud Climatology Project (ISCCP) radiative fluxes (see part 1), Tropical Rainfall Measuring Mission (TRMM) latent heating profiles, and TRMM rainfall rates. Contrary to previous results and general perceptions, we demonstrate that radiative heating is more important than latent heating in accounting for MLT longitude variability due to tides although latent heating causes some large nonmigrating tidal oscillations such as DE3. Through comparison with TIMED SABER temperature measurements, the model results are shown to approximate many observed features of this longitude variability.

Thermodynamic analysis of snowball Earth hysteresis experiment: Efficiency, entropy production and irreversibility

Abstract: We present an extensive thermodynamic analysis of a hysteresis experiment performed on a simplified yet Earth-like climate model. We slowly vary the solar constant by 20% around the present value and detect that for a large range of values of the solar constant the realization of snowball or of regular climate conditions depends on the history of the system. Using recent results on the global climate thermodynamics, we show that the two regimes feature radically different properties. The efficiency of the climate machine monotonically increases with decreasing solar constant in present climate conditions, whereas the opposite takes place in snowball conditions. Instead, entropy production is monotonically increasing with the solar constant in both branches of climate conditions, and its value is about four times larger in the warm branch than in the corresponding cold state. Finally, the degree of irreversibility of the system, measured as the fraction of excess entropy production due to irreversible heat transport processes, is much higher in the warm climate conditions, with an explosive growth in the upper range of the considered values of solar constants. Whereas in the cold climate regime a dominating role is played by changes in the meridional albedo contrast, in the warm climate regime changes in the intensity of latent heat fluxes are crucial for determining the observed properties. This substantiates the importance of addressing correctly the variations of the hydrological cycle in a changing climate. An interpretation of the climate transitions at the tipping points based upon macro-scale thermodynamic properties is also proposed. Our results support the adoption of a new generation of diagnostic tools based on the second law of thermodynamics for auditing climate models and outline a set of parametrizations to be used in conceptual and intermediate-complexity models or for the reconstruction of the past climate conditions. Copyright © 2010 Royal Meteorological Society

Quantifying contributions to polar warming amplification in an idealized coupled general circulation model

Abstract: An idealized coupled general circulation model is used to demonstrate that the surface warming due to the doubling of CO2 can still be stronger in high latitudes than in low latitudes even without the negative evaporation feedback in low latitudes and positive ice-albedo feedback in high latitudes, as well as without the poleward latent heat transport. The new climate feedback analysis method formulated in Lu and Cai (Clim Dyn 32:873–885, 2009) is used to isolate contributions from both radiative and non-radiative feedback processes to the total temperature change obtained with the coupled GCM. These partial temperature changes are additive and their sum is convergent to the total temperature change. The radiative energy flux perturbations due to the doubling of CO2 and water vapor feedback lead to a stronger warming in low latitudes than in high latitudes at the surface and throughout the entire troposphere. In the vertical, the temperature changes due to the doubling of CO2 and water vapor feedback are maximum near the surface and decrease with height at all latitudes. The simultaneous warming reduction in low latitudes and amplification in high latitudes by the enhanced poleward dry static energy transport reverses the poleward decreasing warming pattern at the surface and in the lower troposphere, but it is not able to do so in the upper troposphere. The enhanced vertical moist convection in the tropics acts to amplify the warming in the upper troposphere at an expense of reducing the warming in the lower troposphere and surface warming in the tropics. As a result, the final warming pattern shows the co-existence of a reduction of the meridional temperature gradient at the surface and in the lower troposphere with an increase of the meridional temperature gradient in the upper troposphere. In the tropics, the total warming in the upper troposphere is stronger than the surface warming.

The Annual Surface Energy Budget of a High-Arctic Permafrost Site on Svalbard, Norway

Abstract: . Independent measurements of radiation, sensible and latent heat fluxes and the ground heat flux are used to describe the annual cycle of the surface energy budget at a high-arctic permafrost site on Svalbard. During summer, the net short-wave radiation is the dominant energy source, while well developed turbulent processes and the heat flux in the ground lead to a cooling of the surface. About 15% of the net radiation is consumed by the seasonal thawing of the active layer in July and August. The Bowen ratio is found to vary between 0.25 and 2, depending on water content of the uppermost soil layer. During the polar night in winter, the net long-wave radiation is the dominant energy loss channel for the surface, which is mainly compensated by the sensible heat flux and, to a lesser extent, by the ground heat flux, which originates from the refreezing of the active layer. The average annual sensible heat flux of −6.9 Wm−2 is composed of strong positive fluxes in July and August, while negative fluxes dominate during the rest of the year. With 6.8 Wm−2, the latent heat flux more or less compensates the sensible heat flux in the annual average. Strong evaporation occurs during the snow melt period and particularly during the snow-free period in summer and fall. When the ground is covered by snow, latent heat fluxes through sublimation of snow are recorded, but are insignificant for the average surface energy budget. The near-surface atmospheric stratification is found to be predominantly unstable to neutral, when the ground is snow-free, and stable to neutral for snow-covered ground. Due to long-lasting near-surface inversions in winter, an average temperature difference of approximately 3 K exists between the air temperature at 10 m height and the surface temperature of the snow. As such comprehensive data sets are sparse for the Arctic, they are of great value to improve process understanding and support modeling efforts on the present-day and future arctic climate and permafrost conditions.

Sources and sinks of climate entropy

Abstract: A satisfactory climate model must have similar sources and sinks of entropy to those of the atmosphere itself. This paper examines some of the theoretical and numerical issues involved in entropy sources and sinks. A balance equation is developed that involves a balance between entropy destruction by emission and absorption of radiation and entropy production by irreversible molecular processes. Estimating the thermal molecular source from turbulent enthalpy fluxes is shown to be consistent with the second law of thermodynamics only for free convection. Four entropy sources connected with the dissipation of kinetic energy are discussed. Moist convection is treated in terms of the mixing of warm/dry and cool/moist air masses. Questions are raised about the validity of treating latent heat as an external heat source acting upon a dry atmosphere. Theoretical estimates of entropy sources are compared with earlier estimates by Peixoto and others, and to the sources and sinks of entropy in a Goddard Institute of Space Studies climate control run. There are many discrepancies, particularly for moist convection. Our knowledge of atmospheric sources and sinks of entropy is evidently poor and, even if they were known, it would be difficult to construct a climate model with sources and sinks as precise as Johnson considers to be necessary.

Observation analysis of land‐atmosphere interactions over the Loess Plateau of northwest China

Abstract: Arid and semi-arid areas of the Loess Plateau over northwestern China are one of the dust aerosol source regions featured by its unique underlying surface. These areas, suffering the severe aridity trend in past decades, are also known as the transitional zone of climate and ecosystem change. To better understand the basic characteristics of the land surface energy budget, seasonal and diurnal variations of moisture and heat flux over this region, field observations collected at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL, 35 degrees 57' N, 104 degrees 08' E, Elev. 1965.8 m) from January 2007 to December 2008 were analyzed systematically, especially focusing on land surface energy partitioning and energy balance. The results indicate that all four radiative components had distinct seasonal and diurnal cycles, except for the diurnal variation of downward longwave radiation. They maintained high values during the growing season and low values during the non-growing season. The highest daily mean value of DSR (downward shortwave radiation, 369.2 Wm(-2)), DLR (downward longwave radiation, 386.8 Wm(-2)) and ULR (upward longwave radiation, 484.2 Wm(-2)) measured in summer while the highest daily mean value of USR (upward shortwave radiation, 150.1 Wm(-2)) occurred in winter as the snow cover. The highest surface albedo was also found in winter as a result of the snow cover. Surface albedo was lower in the growing season (wet season) due to the larger vegetation fraction and wetter soil. The components of the land surface energy budget varied seasonally except for the surface soil heat flux, and all showed strong diurnal cycles. Net radiation increased from winter to summer and decreased from summer to winter associated with the variation of DSR. Sensible (latent) heat flux was the main consumer of the available energy in winter and spring (summer and autumn). The energy imbalance problem was also identified. When the soil heat storage in the surface soil and vegetation canopy was neglected, the energy imbalance ratio was about 22%. While given the surface heat storage calculated by the thermal diffusion equation and correction method (TDEC), the imbalance ratio was only 14%. Furthermore, taking the soil heat storage into account, this ratio was only 8% in spring, and 15% in summer and autumn. Compared with the bare surface layer in spring, it is likely that a part of energy was stored in the vegetation canopy in summer and autumn. In addition, the sensible and latent heat fluxes over different land surface types of the Loess Plateau are analyzed. Sensible and latent heat fluxes are utterly different substantially over those different underlying surfaces due to the factors such as vegetation, precipitation, and soil moisture.

On the corrections of ERA-40 surface flux products consistent with the Mediterranean heat and water budgets and the connection between basin surface total heat flux and NAO

Abstract: [1] This is a study of heat fluxes and heat budget of the Mediterranean Sea using the European Centre for Medium-Range Weather Forecasts (ECMWF) 45 year reanalysis data set ERA-40. The simple use of the ERA-40 surface flux components fails to close the budget and, in particular, the shortwave radiation flux is found to be underestimated with respect to observed data by about 10%. The heat flux terms are recomputed and corrected in order to close the heat and freshwater budgets of the Mediterranean basin over the period 1958 to 2001, thus producing a corrected ERA-40 surface flux data set. Various satellite and in situ observational data are used to construct spatially varying corrections to the ERA-40 products needed to compute the air-sea fluxes. The corrected interannual and climatological net surface heat and freshwater fluxes are −7 W/m2 and −0.64 m/yr, respectively, which are regarded as satisfactorily closing the Mediterranean heat and water budgets. It is also argued that there is an important contribution from large heat losses associated with a few severe winters over the Mediterranean Sea. This is shown to be related to wind regime anomalies, which strongly affect the latent heat of evaporation that is mainly responsible for the interannual modulation of the total heat flux. Furthermore, the surface total heat flux anomaly time series is compared with the North Atlantic Oscillation (NAO) index, and the result is a positive correlation with ocean warming for positive NAO index periods and ocean cooling associated with negative index periods.