Showing papers on "Latent heat published in 1988"

A four-layer model for the heat budget of homogeneous land surfaces

Abstract: The present model envisions the heat balance of a homogeneous land surface in terms of available energy, a set of driving potentials, and parameters for the physical state of the soil and vegetation. Two unique features of the model are: (1) the expression of the interaction of evaporation from the soil and from foliage by changes in the value of the saturation vapor pressure deficit of air in the canopy (the conclusions of this interaction being consistent with field observations); and (2) the treatment of sensible and latent heat exchange between the atmosphere and a soil consisting of two discrete layers.

Applied Modeling of the Nighttime Surface Energy Balance over Land

Abstract: In this paper a semiempirical scheme is proposed which relates the nocturnal surface fluxes of sensible heat, latent heat, and momentum to routine weather data. The main components of the surface radiation and energy balance over land are described on a half-hourly basis. Observations over a grass-covered surface at Cabauw are used to investigate topics proposed in the literature, and to develop new parameterizations. The input data of the scheme are total cloud cover, wind speed, air temperature, and specific humidity deficit at single heights in the atmospheric surface layer. A semiempirical expression is proposed for the estimation of the soil heat flux. Also the relation between the surface radiation temperature and the temperature at the level of the roughness length is described semiempirically. It is found that their difference is considerable, especially for low wind speeds. The output of the scheme is presented in terms of the main forcing terms. On average, the agreement of the model qu...

Improved Ground Hydrology Calculations for Global Climate Models (GCMs): Soil Water Movement and Evapotranspiration

Abstract: A physically based ground hydrology model is developed to improve the land-surface sensible and latent heat calculations in global climate models (GCMs). The processes of transpiration, evaporation from intercepted precipitation and dew, evaporation from bare soil, infiltration, soil water flow, and runoff are explicitly included in the model. The amount of detail in the hydrologic calculations is restricted to a level appropriate for use in a GCM, but each of the aforementioned processes is modeled on the basis of the underlying physical principles. Data from the Goddard Institute for Space Studies (GISS) GCM are used as inputs for off-line tests of the ground hydrology model in four 8° × 10° regions (Brazil, Sahel, Sahara, and India). Soil and vegetation input parameters are calculated as area-weighted means over the 8° × 10° gridhox. This compositing procedure is tested by comparing resulting hydrological quantities to ground hydrology model calculations performed on the 1° × 1° cells which co...

Preparation and melting of amorphous silicon by molecular-dynamics simulations

Abstract: Molecular-dynamics simulations of amorphous silicon are described, using the two- and three-body interaction potentials constructed by Stillinger and Weber. The amorphous-material preparation procedure is described and the structural and dynamical properties of the amorphous phase are simulated and analyzed. The characteristics of the simulated amorphous phase compare favorably with experimental data and with structural models. Upon rapid heating, the amorphous sample melts via a first-order transition at a temperature of 230 K below the melting temperature of the crystalline material with a latent heat approximately equal to 0.2\char21{}0.4 of the latent heat of the crystalline-liquid transition. The nature of the transition and the magnitude of the latent heat maintain, but shift to a lower temperature, for melting of fully relaxed amorphous configurations at elevated temperatures.

Moisture and latent heat flux variabilities in the tropical Pacific derived from satellite data

Abstract: This paper describes a method of determining latent heat flux and the ocean-atmosphere moisture from sea surface temperature, precipitable water, and surface wind speed data derived from 1980-1983 observations of SMMR aboard Nimbus 7 above tropical Pacific. The observation period included a very intense El Nino-Southern Oscillation (ENSO) episode. It was found that, during the early phase of the 1982-1983 ENSO, a surface convergence center moved east leading the anomalous equatorial westerlies. At this center, the low wind and high humidity caused negative (low) latent heat flux anomalies, despite anomalously high sea surface temperatures. Latent heat flux was found to play an important role in the seasonal cooling of the upper ocean, except in areas covered by major surface convergence zones and in areas of ocean upwelling.

Dri-Pc humidity and temperature controller

Abstract: A temperature and humidity control system for natatoriums. The system provides a constant volume of air circulation which is used to control temperature and humidity in the natatorium while inhibiting the accumulation of moisture on the surfaces defining the natatorium. Usually, ambient air is introduced into the system and moisture latent air discharged from the system under controlled conditions. When the humidity of the ambient air is too great the system operates in a self-contained cycle. Return air from the natatorium flowing through the system is treated to maximize its relative humidity. Subsequently, it flows across a cooling coil. This cooled air flows through a heat exchange module where it is placed in exchange relationship with the air being drawn from the natatorium. It cools the air drawn from the natatorium to maximize its relative humidity and this air in turn flows across the refrigeration coil removing substantial quantities of moisture. Combining the refrigeration coil with the heat transfer module and the other components in the system, the sensible/latent heat split is shifted towards higher latent output increasing the overall efficiency of the system as a dehumidification device.

On the numerical modelling of heat transfer during solidification processes

Abstract: An extensive amount of work has been published on the treatment of heat transfer associated with phase change. A few recent advances are discussed in this paper with some emphasis on the phase change of metals. The apparent capacity, effective capacity, enthalpy, post iterative, source based and semi-analytical methods are discussed and relative advantages and disadvantages of each are analysed. Recent developments in nodelling the flow during pouring and natural convection with applications of two widely used convective liffusive codes, the Los Alamos MAC and the Imperial College TEACH, are presented. An alternative tream function–vorticity approach is also discussed. Applications of these methods to turbulent convection during mould filling and continuous casting are presented. Areas of interest for further research work are identified as modelling of turbulence in liquid metals, flow through mushy regions and improvement of the performance of weak methods in multidimensional problems when the ratio of latent heat to sensible heat is large.

Structure and growth of the mixing layer over the Amazonian rain forest

Abstract: The structure and growth of the atmospheric mixed layer over the Amazonian rain forest were examined using measurements obtained during the NASA Amazon Boundary Layer Experiment. Measurements of temperature, moisture, and horizontal wind were carried out in and above the mixed layer by means of a tethered balloon, rawinsonde, and aircraft; fluxes of sensible and latent heat were measured at the top of the canopy. It was found that the mixing layer grows rapidly, at 5-8 cm/sec, soon after sunrise to a mean maximum height of 1200 m by 1300 LT; during undisturbed conditions, mixed layer heights of 1000 are common between 1000 and 1600 LT. No horizontal inhomogeneities in the mixed layer structure or depth were found over large distances. A simple mixed layer model was applied to show how fluxes of species might be estimated using only quantities measured at the surface and prescribing an initial condition and boundary condition for the mixed layer.

Method and apparatus of indirect-evaporation cooling

Abstract: The within invention improves on the indirect evaporative cooling method and apparatus by making use of a working fluid that is pre-cooled with and without desiccants before it is passed through a Wet Channel where evaporative fluid is on the walls to take heat and store it in the working fluid as increased latent heat. The heat transfer across the membrane between the Dry Channel and the Wet Channel may have dry, solid desiccant or liquid desiccant and may have perforations, pores or capillary pathways. The evaporative fluid may be water, fuel, or any substance that has the capacity to take heat as latent heat. The Wet Channel or excess cooled fluid is in heat transfer contact with a Product Channel where Product Fluid is cooled without adding any humidity. An alternative embodiment for heat transfer between adjacent channels is with heat pipes.

High efficiency sensible and latent heat exchange media with selected transfer for a total energy recovery wheel

Abstract: A sensible and latent heat exchange media (10) has a gas permeable matrix (12). The gas permeable matrix (12) provides passageways (14) through which an air stream can flow through the sensible and latent heat exchange media (10). The gas permeable matrix (12) is formed of a sensible heat exchange material (16) that is capable of absorbing sensible heat from a warm air stream and releasing the absorbed sensible heat into a cool air stream as the air streams flow through the sensible and latent heat exchange media (10). A layer of a coating composition (28) of a molecular sieve is applied to at least a portion of the surface of the sensible heat exchange material (16). The molecular sieve has a plurality of pores of a substantially uniform size such that the molecular sieve is capable of adsorbing moisture from a humid air stream flowing through the sensible and latent heat exchange media (10), and is capable of releasing the adsorbed moisture into a dry air stream flowing through the sensible and latent heat exchange media (10), but is not capable of adsorbing contaminants from either of the air streams. The sensible and latent heat exchange media (10) is extremely efficient in transferring both sensible and latent heat between air streams.

The Heat Balance of the Icy Slope of Adelie Land, Eastern Antarctica

Abstract: A complete but budget investigation was carried out in summer at a site in Adelie Land, some 100 km from the edge of the Antarctic ice sheet. For an average day, the all wave radiation budget based on the fluxes toward the surface being positive was positive for about 11 h, which is a short time considering that the sun was above the horizon between 22 and 24 h a day during the observational period. It is a result of the high albedo, which, on average, was found to be about 83%. Furthermore, with increasing cloudiness, a more positive radiation budget was found, which is in contrast to most studies at lower latitudes. The heat flux in and out of the snow cover was small, and showed a typical sinusoidal diurnal variation. The mean daily values of snow heat flux were negative, as the snow cover was warmed during the observational period. The latent heat flux was negative, on the average, as sublimation took place for most of the time. Deposition was observed only on a few nights. The sensible heat ...

Thermal modelling of stepwise anatexis in a thrust-thickened sialic crust

Abstract: One-dimensional modelling of the thermal history of a sialic crust thickened by multiple overstack thrusting of upper crustal material shows that anatexis is likely. Both the uplift rate and the length of the incubation period between end of tectonism and start of uplift are important controls on the amount and temperature of the melt. Heat of fusion does not significantly affect the long-term thermal structure of the crust if the melt is not extracted because only a small fraction of conductive heat is converted to latent heat, though short-term thermal effects of latent heat can be locally important.Model results show that commonly <15% of mantle heat flux is converted to latent heat; even during peak melting in the most productive models, less than half of incremental mantle flux is converted. The results have obvious implications on the acceptability of proposed heat sources for crustal anatexis. Fusion could retard crustal temperature rise by nearly 100°C, but the system would recover except for situations of very rapid uplift. Understanding of the thermal evolution of a burial-uplift system requires knowledge not only of the timing of anatexis but of the pooling and movement of the magma, as well as the duration and nature of the incubation period; we are poorly equipped to measure these events. The model predicts that the characteristic time for anatexis in a thickened sialic crust is several tens of millions of years, comparable to the time lapse between orogenies; in making geological interpretations of magmatism, this time lag must be considered.

Latent energy of deformation of bisphenol A polycarbonate

Abstract: The thermodynamic behavior of poly(bisphenol A carbonate) (PC) during uniaxial cold drawing and the properties of the drawn polymer were examined. Isothermal deformation calorimetric measurements were made during the drawing process. The deformation calorimeter measures heat, work, and internal energy changes for deformation. It was found that PC exhibited nonideal plasticity with approximately 50–80% of the work of deformation dissipated as heat. The remainder of the work of deformation was stored as a latent internal energy change. The value of the internal energy change was dependent on strain rate at 20°C but was not strongly dependent on temperature in the range 20–65°C. Thermomechanical measurements on cold-drawn PC samples demonstrated striking behavior at temperatures far below the glass transition temperature Tg. Stress-temperature experiments showed that the stress increased for uniaxially constrained samples, and this stress increase began at temperatures just above the deformation temperature. Additional experiments indicated that the changes which took place during cold drawing were physical in nature and were thermoreversible. These changes in physical properties are related to those which occur due to physical aging below Tg.

Direct and Indirect Effects of Latent Heat Release on a Synoptic-Scale Wave System

Abstract: The primary goal of this paper is to diagnose, the “direct” and “indirect” effects of latent heat release on a synoptic-scale wave system containing an extratropical cyclone that developed over the eastern United States. To achieve this goal, comparisons are made between MOIST (full model physics) and DRY (latent heating removed) predictions of the wave system during the period 27–29 February 1984 using the National Meteorological Center's Limited-Area Fine Mesh (LFM) model. Both the MOIST and DRY models predict significant cyclone systems, suggesting that the background adiabatic forcing is quite important. However, the DRY model predict a weaker cyclone. The direct and indirect latent heating influences are diagnosed using eddy energy quantities and the extended height tendency equation. Direct effects are restricted to the diabatic generation of available potential energy and height tendencies forced by diabatic heating. Results show that latent heating exerts an important direct influence on ...

The effect of a sea surface temperature anomaly on a prediction of the onset of the south-west monsoon over India

Abstract: At the time of the onset of the monsoon in June 1979 the sea surface temperatures in the eastern Arabian Sea were higher than normal. A prediction experiment designed to reveal the effects of this anomaly on the onset is described. This involves two 8-day forecasts made using the same model and the same initial conditions. They differ only in using different sea surface temperatures in the eastern Arabian Sea. A better prediction of the onset is obtained by using the anomalously high temperatures rather than the normal climatological values. In particular, the development of a tropical storm over the Arabian Sea, the strengthening of the Somali jet, and the northward movement of the rainfall over India are all better predicted. The mechanism of the onset in the model is discussed and two important feedback loops are identified, one involving moisture-flux convergence and the other involving the surface fluxes of sensible and latent heat. The most important factor in both loops is the release of latent heat over the Arabian Sea. A linear model is used to show that the onset may be thought of as a response to this heating. The roles of barotropic and baroclinic processes in the onset are also discussed.

Tests of three urban energy balance models

Abstract: Observations of surface characteristics, meteorological conditions and energy balance components from Vancouver, B.C. are used to test the validity of the output from three one-dimensional surface energy balance models. The results show that whereas all of the models provide good simulations of net radiation, none can consistently predict the turbulent fluxes of sensible and latent heat using easily available input data. Inability to handle the role of water availability and its impact on evapotranspiration is identified as the principal problem.

Energy flux partitioning over the Amazon forest

Abstract: The present study involved determination of the experimental energy receipt partitioning over the tropical Amazon forest. Diurnal variation of net radiation (Q*), sensible heat flux (QH) and latent heat flux (QE) is presented. The daytimeQE is in phase withQ* and it is always an important term in the energy balance. The daily averagedQE is 59 to 100% of the dailyQ* whereasQH is 5 to 28% at the Amazon forest site (2° 57′ S; 59° 57′ W) for the sample periods. The results present evidence thatQE over the Amazon forest is greater thanQ* in the afternoon hours. The role of sensible heat advection in maintaining largeQE over the forest surface is discussed. Hourly Bowen ratio (β) values for two campaigns of the Amazon forest micrometeorological experiment are presented. During daylight hours, the differences inβ are not significant, and exhibit a systematic pattern. The only time that the variation in Bowen ratio increased significantly was at sunrise and sunset when the thermal structure of the air was changing from a strong inversion to lapse and vice versa. The diurnalβ values changed from −3.50 to 0.85. The mean hourlyβ calculated from values from 07.00 to 16.00 h, varied from 0.05 to 0.85.

Intercomparison of three urban climate models

Abstract: An intercomparison of the surface energy budgets from three urban climate models was made to assess the comparability of results, and to evaluate the surface energy fluxes from each model. The three models selected spanned the continuum of approaches currently employed in the treatment of the effects of urban geometry. The first model was an urban canopy-layer model which explicitly examined urban canyon geometry. The second model treated the city as a warm, rough, moist plate but included greatly simplified parameterizations of urban geometry. Neither model included a dynamic link to the urban boundary-layer. The third model was a one-dimensional urban boundary-layer model which utilized a simple warm, rough, moist plate approach but included a dynamic coupling of the urban surface layer to the urban boundary-layer. Results showed considerable disagreement between the three models in regards to the individual energy fluxes. Average rankings of the energy fluxes in terms of comparability from high-to-low similarity were: (1) solar radiation, (2) sensible heat flux, (3) conduction, (4) latent heat flux, (5) longwave re-radiation, and (6) longwave radiation input. In general, the urban canopy-layer model provided more realistic results, although each model demonstrated strong and weak points. Results indicate that current urban boundary-layer models may produce surface energy budgets with lower sensible heat fluxes and substantially higher latent heat fluxes than is supported by field evidence from the literature.

Solar Desalination With Latent Heat Recovery

Abstract: Unlike conventional solar stills, the present system utilizes the latent heat of condensation and the sensible heat of the discarded seawater. The performance was optimized analytically and the system is presently under construction in Egypt. The system consists of a humidifier, a solar still or channel, a condenser, and a pond. In the humidifier, ambient air is humidified and heated by a warm brine from the pond. If the brine outlet temperature is higher than the ambient temperature, it goes back to the pond, if not, it is discarded. The solar still is a long glass-covered channel, about 200 meters long.

Extension of Penman's formulae to multi-layer models

Abstract: In this paper, the well-established multi-layer model originally devised by Waggoner and Reifsnyder (1968) is used. This steady-state model based on an electrical analogue simulates the energy exchange between the vegetation and the atmosphere. A purely mathematical development of the basic equations of this model yields explicit expressions of the total fluxes of sensible and latent heat at the top of the canopy as a function of the net radiation absorbed in each layer, the soil heat flux, the water vapour pressure deficit at a reference height and the whole set of elementary conductances (stomatal, boundary-layer and aerodynamic). These new equations can be considered as a generalization of the familiar Penman's formulae to a multi-layer model.

Modelling surface energy fluxes and temperatures in dry and wet bare soils

Abstract: A physically‐based numerical model was developed to estimate the temporal course of the surface energy flux densities and the soil temperatures in dry and wet bare soils. Aerodynamic heat, vapour and momentum transfer theory was used to calculate the sensible and latent heat flux densities at the surface under diabatic and adiabatic conditions. A finite‐difference solution of the differential equation describing one‐dimensional heat transfer was used to calculate the surface soil heat flux density and soil profile temperatures. The surface temperature was determined iteratively by the simultaneous solution of equations describing radiative, heat and momentum transfer at the surface. The model was tested with measurements from energy balance studies conducted on a dry, sandy soil and a wet, silt loam soil, and was found to predict accurately the surface energy fluxes and soil temperatures over three‐day periods under conditions of potential and negligible evaporation. The sensitivity of the model ...

The role of advection in the energy balance of late‐lying snowfields: Niwot Ridge, Front Range, Colorado

Abstract: Measured profiles of temperature, humidity, and wind above alpine tundra are used as upwind boundary conditions in a numerical model for simulating turbulent transfer over late-laying snow. Resulting simulations indicate that (1) sensible heat flux comprises the major source of turbulent melt energy, although its relative importance vis-a-vis latent heat flux depends upon ambient weather conditions, and (2) advected heat contributes greatly to the energy balance of late-lying alpine snow. Near the leading edge of a snowfield, advection may contribute more than 30 MJ m−2 d−1 of melt energy on a very windy day and more than 12 MJ m−2 d−1 on a relatively windless day. The totals decrease to circa 5 MJ m−2 d−1 and 2 MJ m−2 d−1 at a distance of 1000 m from the leading edge. These findings suggest that the sensible heat flux may exceed net radiation as the major source of energy available for melting late-lying snow in windy alpine environments.

Thermal analysis of heat storage canisters for a solar dynamic, space power system

Abstract: A thermal analysis was performed of a thermal energy storage canister of a type suggested for use in a solar receiver for an orbiting Brayton cycle power system. Energy storage for the eclipse portion of the cycle is provided by the latent heat of a eutectic mixture of LiF and CaF2 contained in the canister. The chief motivation for the study is the prediction of vapor void effects on temperature profiles and the identification of possible differences between ground test data and projected behavior in microgravity. The first phase of this study is based on a two-dimensional, cylindrical coordinates model using an interim procedure for describing void behavor in 1-g and microgravity. The thermal analysis includes the effects of solidification front behavior, conduction in liquid/solid salt and canister materials, void growth and shrinkage, radiant heat transfer across the void, and convection in the melt due to Marangoni-induced flow and, in 1-g, flow due to density gradients. A number of significant differences between 1-g and o-g behavior were found. This resulted from differences in void location relative to the maximum heat flux and a significantly smaller effective conductance in 0-g due to the absence of gravity-induced convection.