Showing papers on "Latent heat published in 1991"

Eral source-based method for solidification phase change

Abstract: After a brief review of current source-based methods for modeling solidification phase change systems, a new source-based method for the treatment of latent heat evolution is presented. The essential feature of the proposed method is linearization of the discretized source term. This results in a robust and accurate computational method that can deal efficiently with a wide range of latent heat evolution mechanisms (i.e., liquid fraction temperature relationships). The proposed method is illustrated on application to a test problem in which various liquid fraction temperature relationships are employed.

The effect of snow cover on the climate

Abstract: Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.

Daily evaporation over a region from lower boundary layer profiles measured with radiosondes

Abstract: Regional daily evaporation was estimated by means of continuous measurements of the available energy flux at the surface and one or more instantaneous determinations of the evaporative fraction (EF). The data were obtained during the First ISLSCP Field Experiment (FIFE) in northeastern Kansas. EF, which is the ratio of the latent heat flux and the available energy flux, was assumed to be constant during the daylight hours; thus it was determined from only a few instantaneous surface flux values calculated on the basis of radiosonde profiles in the surface layer, together with remotely sensed surface temperature. Comparison of 23 estimated and measured daytime evaporation values showed good correlation (r = 0.97), although the evaporation was underestimated by about 5% on average. In the estimation of daily evaporation, nighttime evaporation must also be considered; measurements by means of the eddy correlation method showed that on average it accounted for some 8% of the total daily evaporation.

Entropy budget of the atmosphere

Abstract: The balance equations for the entropy in the atmosphere are presented and discussed. Using observed energy fluxes and atmospheric temperatures, we present estimates of the various terms in the global mean entropy budget. The largest boundary fluxes of entropy are associated with the emission of longwave radiation. The fluxes of entropy associated with turbulent and molecular diffusion are found to be much smaller. On the planetary scale the mean outgoing flux of entropy at the top of the atmosphere is found to be about 22 times larger than the mean incoming flux of entropy through solar radiation. The rates of entropy production and destruction by the various irreversible processes that occur in the atmosphere are also computed. The entropy production terms involved in the release of latent heat and the absorption of solar radiation are, by far, the largest sources of entropy for the atmosphere, whereas the production of entropy associated with the absorption of longwave radiation is an order of magnitude smaller. The destruction is mainly accomplished through cooling by outgoing longwave radiation. The contributions of the sensible heat fluxes and friction are relatively small. Regional contributions to the total entropy generation in the atmosphere are studied by considering an equatorial region bounded by two latitudinal walls at 15oN and 15oS and a polar region poleward of 70oN. The rates of entropy generation by the various diabatic processes are highest in the equatorial region; part of the generated entropy is exported to higher latitudes. Most natural phenomena occurring in the climate system are characterized by great irreversibility. For example, the turbulent motion in the planetary boundary layer does not spontaneously develop into the large-scale organized flow of the general circulation; a cloud cannot be reconstituted from the same water it lost previously through precipitation; rivers do not flow backward from the sea to their headwaters; and ocean water does not decompose spontaneously into oxygen and hydrogen. We will analyze the entropy budget of the atmosphere since it is important for the study of the thermodynamical behavior of the climatic system and, in particular, for atmospheric phenomena. The second law for an isolated system,

A study of the inertial-dissipation method for computing air-sea fluxes

Abstract: The inertial-dissipation method has long been used to estimate air-sea fluxes from ships because it does not require correction for ship motion. A detailed comparison of the inertial-dissipation fluxes with the direct covariance method is given, using data from the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. In this experiment, inertial-dissipation packages were deployed at the end of a 17 m boom, in a region relatively free of flow distortion; and on a mast 7 m above the platform (26 m above the sea surface) in a region of considerable flow distortion. An error analysis of the inertial-dissipation method indicates that stress is most accurately measured in near-neutral conditions, whereas scalar fluxes are most accurately measured in near-neutral and unstable conditions. It is also shown that the inertial-dissipation stress estimates are much less affected by the flow distortion caused by the platform as well as by the boom itself. The inertial-dissipation (boom and mast) and boom covariance estimates of stress agree within ±20%. The latent heat flux estimates agree within approximately ±45%. The sensible heat flux estimates agree within ±26% after correction for velocity contamination of the sonic temperature spectra. The larger uncertainty in the latent heat fluxes is due to poor performance of our Lyman-α hygrometers in the sea spray environment. Improved parameterizations for the stability dependence of the dimensionless humidity and temperature structure functions are given. These functions are used to find a best fit for effective Kolmogorov constants of 0.55 for velocity (assuming a balance of production and dissipation of turbulent kinetic energy) and 0.79 for temperature and humidity. A Kolmogorov constant of 0.51 implies a production-dissipation imbalance of approximately 12% in unstable conditions.

A physical model for the volume and composition of melt produced by hydrous fluxing above subduction zones

Abstract: Thermal models of subduction zones, restrict the melt source region to a domain at sufficiently high temperature with water present (either as a free phase or in hydrous minerals). Water, released into the mantle by slab dehydration, traverses the wedge horizontally by a combination of (i) vertical movement as a fluid phase and (ii) fixed in amphiboles carried by the induced mantle flow; only in mantle hotter than amphibole stability can melts escape upwards. We develop a one-dimensional model for the source region fluxed with water. The induced mantle flow advects heat laterally to balance the latent heat of melting, in a column where the liquidus of the melt is depressed by its water content. Melt flux, fraction, temperature and water content are calculated assuming steady state. Melt compositions are predicted from the melt fraction distribution as a function of depth, constrained by the experimental data of Green. On investigating a range of plausible models, we find that the average degrees of melting predicted vary from ca. 2 to 8%. The predicted primary magmas are mafic high magnesium basalts with water contents ranging from 1.6 to 6 wt%, and temperatures from 1160 to 1290 degrees C. Models with shallower depths of segregation have higher degrees of melting and lower water contents. The volumes predicted by the physical model are a strong function of the water flux assumed to enter the source region. Previous estimates of arc growth would suggest either low water fluxes or that not all the melt reaches the arc crust.

Measurements of sensible and latent heat flux in the western equatorial Pacific Ocean

Abstract: Micrometeorological measurements, including direct eddy-correlation measurements of heat and moisture fluxes, have been made from shipboard under light-wind conditions in the western equatorial Pacific warm pool. Air-sea temperature differences were typically 1.5°–2°C, that is, 1°–1.5°C larger than long-term averages from merchant ship data. A sea surface “cool skin” of about 0.3°C was observed. Bulk transfer coefficients for both fluxes agree well with the predictions of Liu et al. (1979) in the convective wind speed regime below 4 m s−1. Between 4 and 6 m s−1 values of the neutral exchange coefficients were CEN - 0.89 × 10−3; CHN = 1.03 × 10−3; CDN = 1.16 × 10−3. At zero mean wind speed, latent heat flux is maintained at about 25 W m−2 by convective exchange. Inertial dissipation estimates of the latent heat flux are about 20% lower than the directly measured eddy-correlation values below 4 m s−1.

A diagnostic study of winter diabatic heating in the Mediterranean in relation to cyclones

Abstract: SUMMARY Diabatic heating and net condensation rates during winter are calculated over the Mediterranean and surrounding areas from the 1982-88 ECMWF initialized analyses using the residual method for the heat and moisture budgets. The steady-state contributions over the Mediterranean are found to be prominent as in the tropics, but contrary to the case of the tropics the transients’ contributions in the Mediterranean, attributed to cyclonic activity, are not at all negligible. A composite Cyprus low was constituted from 67 typical cyclones in the eastern Mediterranean. The diabatic heating and net condensation distribution indicate vigorous sea fluxes especially in the lee of the Turkish mountains over the Aegean Sea where the air-sea contrast is maximized. In contrast with other regions, the maximum in the vertical profile of diabatic heating due to latent heat release is found at a lower altitude of about 850 hPa. It is suggested that this fact has important implications in modelling Mediterranean cyclones.

Soil and Canopy Energy Balances of a Row Crop at Partial Cover

Abstract: When crops are grown in a row configuration, heat and mass transfer within the soil-canopy system influence the energy and water balance of the crop. Field experiments were conducted near Lubbock, TX, to examine the energy balance of the soil and canopy separately, in cotton (Gossypium hirsutum L.) under a variety of aerial to obtain the field energy balance, including total latent heat flux (LE). Latent heat flux from the crop canopy (LE c ) was determined from sap flow measurements of transpiration. Latent heat flux from the soil (LE s ) was computed as the difference between LE and LE c (...)

Numerical Simulation of a Subtropical Squall Line over the Taiwan Strait

Abstract: The present 2D nonhydrostatic, time-dependent numerical cloud model, which is applied to the structure and development of a subtropical squall line observed during the Taiwan Area Mesoscale Experiment, includes (1) a parameterized ice-phase microphysical scheme, (2) longwave and shortwave radiative-transfer processes, and (3) heat and moisture fluxes from the ocean surface. A comparative study is made of the vertical transport of horizontal momentum and of the latent heat release by the simulated subtropical squall system, as well as the heat release of squall systems occurring in other geographic locations.

Estimation of Heat-Transfer Coefficients, the Upward Heat Flow, and Evaporation in a Solar Still

Abstract: The present work enables prediction of the performance of a solar still through simple calculations. Estimation of the temperature of the glass cover by an empirical relation developed in this work permits calculation of the heat-transfer coefficients, the upward heat flow, and evaporation. Since some of the heat-transfer coefficients vary substantially and nonlinearly with temperature, the empirical relation developed for glass cover temperature is based on an approximate solution of the heat balance equation. Hence, the overall upward heat flow factor is obtained with a maximum absolute error of three percent compared to the value obtained through a numerical solution of the heat balance equation along with the relations for vapor pressure and latent heat. The fraction of upward heat flow utilized for evaporation is determined with a maximum absolute error of 0.5 percent. The range of variables covered is 30{degrees}C to 80{degrees}C in water temperature, 5W/m{sup 2}K to 40W/M{sup 2}K in wind heat-transfer coefficient, and 5{degrees}C to 40{degrees}C in ambient temperature.

Coupled Temperature/Capillary Potential Variations in Unsaturated Soil

Abstract: A new form of a capillary potential/temperature formulation of heat and mass transfer in unsaturated soil is presented. Moisture transfer in both the vapor and liquid phases and heat transfer by means of conduction and latent heat of vaporization effects are accommodated in a one‐dimensional approach. A numerical solution of the complete formulation is then developed, based on the use of the finite‐element method, to describe spatial variations and a finite‐difference time‐stepping scheme to model transient behavior. The application of the model is then presented, making use of material parameters that have been independently established for all the soil properties specified in the formulation. A comprehensive, consistent approach is therefore claimed to be proposed. The results obtained are shown to be qualitatively correct throughout, conforming with physically realistic behavior. For a number of aspects of the work, it proved possible to examine precisely the validity of the results achieved, and in al...

The Effect of Heterogeneous Soil Moisture on a Summer Baroclinic Circulation in the Central United States

Abstract: Thermally induced circulations, similar to sea breezes, may be established in the presence of horizontal gradients in soil moisture, soil type, vegetation, or snow cover. The expense of extensive observational networks and the relatively small-scale circulations involved has made examining these circulations very difficult. Recent numerical studies have indicated that sharp gradients in soil or vegetation properties may induce mesoscale circulations in the absence of synoptic forcing. The current study employed a three-dimensional, hydrostatic mesoscale model to evaluate the effects of horizontally heterogeneous soil moisture and soil type on the passage of a summer cold front in the central United States. Grid-scale condensation, precipitation, latent heat release, and cumulus conviction are not accounted for in this model; moisture was affected only by advection, diffusion, and evaporation. Numerical simulations demonstrated that evaporation of soil moisture significantly affected the boundary ...

A modified profile method for determining the vertical fluxes of NO, NO2, Ozone and HNO3 in the atmospheric surface layer.

Abstract: A modified profile method for determining the vertical deposition (or/and exhalation) fluxes of NO, NO2, ozone, and HNO3 in the atmospheric surface layer is presented. This method is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these trace gases. The analysis (aerodynamic profile method) includes a detailed determination of the micrometeorological quantities (such as the friction velocity, the fluxes of sensible and latent heat, the roughness length and the zero plane displacement), and of the height-invariant fluxes of the composed chemically conservative trace gases with ‘group’ concentrations c1=[NO]+[NO2]+[HNO3], c2=[NO2]+[O3]+3/2·[HNO3], and c3=[NO]−[O3]−1/2·[HNO3]. The fluxes of the ‘individual’ species are finally determined by the numerical solution of a system of coupled nonlinear ordinary differential equations for the concentrations of ozone and HNO3 (‘decoding’ method). The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The model requires only the vertical profile data of wind velocity, temperature and humidity and concentrations of NO, NO2, ozone, and HNO3.

EVAPORATION FROM A BLANKET BOG IN A FOGGY COASTAL ENVIRONMENT (Research Note)

Abstract: Frequent fog severely restricts evaporation from blanket bogs in Newfoundland because it more than halves the radiant energy input, and it eliminates the vapor pressure deficit, resulting in evaporation at the equilibrium rate (average a = 0.99 during fog). During these periods, there is no surface resistance to evaporation because the bog has been wetted by fog drip, and although the latent heat flux dominates over sensible heat (average p = 0.8), both are small. In contrast, the surface dries during clear periods, increasing the surface resistance to evaporation so that sensible heat becomes more important (p = 1.05). When the mosses are dry, evaporation is below the equilibrium rate (a = 0.87), although the higher available energy ensures that actual evaporation is higher. During clear periods, daily evaporation averaged 2.5 mm, compared to 1.1 and 0.7 mm for fog and rain, respectively. The suppressed evaporation at this site is important in maintaining appropriate hydrological conditions for blanket bog development.

Annual mean surface heat fluxes in the Arabian Gulf and the net heat transport through the Strait of Hormuz

Abstract: An attempt is made to assess the climatological air‐sea fluxes in the Arabian (Persian) Gulf using meteorological observations. The annual mean values of upward heat transfer due to solar radiation, sensible heat, latent heat and infrared radiation fluxes are ‐212, ‐1, 168 and 66 W m−2, respectively. Based on the surface inflow and subsurface outflow through the Strait of Hormuz, the annual average heat transport into the Gulf is 25 W m−2. This is in fair agreement with the resulting upward flux of 21 W m−2 at the sea surface in the Gulf.

Evaporation from a blanket bog in a foggy coastal environment

Abstract: Frequent fog severely restricts evaporation from blanket bogs in Newfoundland because it more than halves the radiant energy input, and it eliminates the vapor pressure deficit, resulting in evaporation at the equilibrium rate (average α = 0.99 during fog). During these periods, there is no surface resistance to evaporation because the bog has been wetted by fog drip, and although the latent heat flux dominates over sensible heat (average β = 0.8), both are small. In contrast, the surface dries during clear periods, increasing the surface resistance to evaporation so that sensible heat becomes more important (β = 1.05). When the mosses are dry, evaporation is below the equilibrium rate (α = 0.87), although the higher available energy ensures that actual evaporation is higher. During clear periods, daily evaporation averaged 2.5 mm, compared to 1.1 and 0.7 mm for fog and rain, respectively. The suppressed evaporation at this site is important in maintaining appropriate hydrological conditions for blanket bog development.

Multispectral satellite data in the context of land surface heat balance

Abstract: Multispectral satellite data are demonstrated to be an important potential contributor to the understanding and completeness of heat balance analysis. Satellite observations are presented, including visible, near-IR, IR, and microwave bands, which estimate surface characteristics and surface fluctuations. The relationship of the interannual variations of the satellite data and the land surface changes is discussed. It is shown that spatially representative values of global fluxes and parameters from multispectral data can consistently enhance the results of heat balance analysis. Daily net radiation can be estimated with a 10 percent error, while the error for albedo would be higher. Computations for all heat fluxes except the latent heat flux are shown. Soil moisture and surface temperature estimates can be derived from microwave and IR observations, respectively. Spectral reflectances in the visible and near-IR bands are suggested as important heat-balance indices for future consideration.

Heat fluxes and roll circulations over the western Gulf Stream during an intense cold-air outbreak

Abstract: Turbulence and heat fluxes in the marine atmospheric boundary layer (MABL) for three aircraft stacks near the western Gulf Stream front, observed during the Genesis of Atlantic Lows Experiment (GALE) January 28, 1986 cold-air outbreak, has been studied using mixed-layer scaling. The GOES image and stability parameter indicates that these three stacks were in the roll vortex regime. The turbulence structure in the MABL is studied for this case, as well as the significance of roll vortices to heat fluxes. The roll circulations are shown to contribute significantly to the sensible (temperature) and latent heat (moisture) fluxes with importance increasing upward. The results suggest that the entrainment at the MABL top might affect the the budgets of temperature and humidity fluxes in the lower MABL, but not in the unstable surface layer.

A Coupled Soil Moisture and Surface Temperature Prediction Model

Abstract: A model for soil moisture and soil surface temperature prediction for bare soil is considered in this paper. In describing evaporation rate. soil structure and moisture were taken into account as much as possible. Soil moisture prediction was carried out using Sellers’ method. Hydraullic properties were determined both known values for the given type of soil and the empirical equations of Clapp and Hornberger. Soil surface temperature prediction were made using the “force-restore” method. Sensible and latent heat fluxes were determined using a resistance representation taking into consideration atmospheric stability. The model results were compared with some field data of soil temperature and latent and sensible heat fluxes. The agreement between the model-calculated and the observed heat fluxes was fairly good. Soil moisture and temperature prediction were satisfactory, although an excessive drying occurred at the beginning of a five-day simulation period. Some sensitivity analyses were also mad...

A Second-Law Study on Packed Bed Energy Storage Systems Utilizing Phase-Change Materials

Abstract: Thermal modeling of packed bed, thermal energy storage systems has traditionally been limited to first-law considerations. The exceptions include a few second-law studies of sensible heat storage systems and the latent heat storage systems. The cited second-law studies treat the storage and removal processes essentially as batch heating and cooling. The approximation effectively ignores the significant temperature gradient, especially in the axial direction, in the storage medium over a substantial portion of both the storage and removal processes. The results presented in this paper are for a more comprehensive model of the packed bed storage system utilizing encapsulated phase-change materials. The fundamental equations for the system are similar to those of Schumann, except that a transient conduction equation is included for intraparticle conduction in each pellet. The equations are solved numerically, and the media temperatures obtained are used for the determination of the exergy (or availability) disposition in complete storage-removal cycles. One major conclusion of the study from both the first-law and second-law perspectives is that the principal advantage in the use of phase-change storage material is the enhanced storage capacity, compared with the same size of packed bed utilizing a sensible heat storage material. Thermodynamically, however, it does notmore » appear that the system employing phase-change storage material will always, or necessarily, be superior to that using a sensible heat-storage material. The latter conclusion is reached only on the basis of the second-law evaluation.« less

Thermal behaviour of glacier and laboratory ice

Abstract: Water is present in glaciers in the form of veins at the three-grain junctions. This water remains unfrozen even many degrees below the normal freezing point, mainly because it contains much of the soluble impurity content of a glacier, but also because of the microscopic curvature of the ice–water interfaces. As the temperature is lowered and the veins shrink, the concentration of impurities in them increases, and the curvature effect also increases. The predicted relation between vein size and temperature has now been verified by laboratory experiments. Because of the latent heat of the vein water, the ice behaves macroscopically as a continuum with an anomalous specific heat capacity that depends strongly on temperature. From this point of view, a polythermal glacier is a single medium with continuously varying properties, rather than consisting of distinct cold and temperate phases with sharp boundaries between them. The paper sets up differential equations for heat diffusion in such a continuum. To explain the local uniformity of the vein system seen under the microscope, it is found necessary to include the effect of diffusion of solutes along the veins. Solutions are presented for a model in which two semi-infinite slabs, initially having different temperatures, impurity concentrations and vein sizes, are instantaneously brought into contact. In this way, transition thicknesses between cold and temperate ice are estimated, and also the velocities of various kinds of waves that are generated from the original discontinuity at the interface.

Variations in boundary layer properties associated with clouds and transient weather disturbances at the South Pole during winter

Abstract: Both the increasing concentrations of greenhouse gases and potential changes in cloud distributions are likely to affect the surface energy budget of the polar regions. Changes in the polar atmosphere are linked to dynamical processes that control the transport of mass, heat, and moisture from lower latitudes and in turn, feed back into the global circulation. Radiation and meteorological data collected at the South Pole during the 1986 austral winter are analyzed to gain a better understanding of the relationships between cloud radiative effects, transport processes and the vertical distribution of temperature and wind. An algorithm is developed to characterize the quasi-permanent surface-based temperature inversion and the 'warm' radiatively active layer above it. Mean winter temperature and wind profiles for clear and overcast conditions are combined with surface radiation measurements and synoptic circulation patterns to study the mechanisms that cause periodic weakening of the inversion. Results support previous studies that ascribe this weakening to (1) warm air advection, (2) downward vertical mixing of sensible and latent heat, and (3) longwave cloud radiative heating. The integrity of the inversion depends on the combined effects of all three mechanisms. Parameters representing the intensity of the inversion and the bulk wind shear throughmore » the lower troposphere are suggested as appropriate indices for the detection of climate change in the region of the Antarctic Plateau.« less

A model for heat and water balance estimation and its application to land use and climate variation

Abstract: This paper presents a method, which has been recently developed in the Department of Agrometeorology of the Poznan Agricultural University, for estimating heat balance components based on standard meteorological data, plant development stage, and land-use conditions. Estimates of latent heat flux components made it possible to obtain values of areal evapotranspiration and surface runoff. The method was applied in four catchment areas in Central Poland under present climatic conditions as well as under new, expected climatic conditions caused by an increase of ‘greenhouse gases’ in the atmosphere. Some changes in land use were also considered. The results show significant changes in the water balance when comparing present and future conditions of precipitation and evapotranspiration.