Showing papers on "Latent heat published in 1999"

A parameterization of snowpack and frozen ground intended for NCEP weather and climate models

Abstract: Extensions to the land surface scheme (LSS) in the National Centers for Environmental Prediction, regional, coupled, land-atmosphere weather prediction model, known as the mesoscale Eta model, are proposed and tested off-line in uncoupled mode to account for seasonal freezing and thawing of soils and snow-accumulation-ablation processes. An original model assumption that there is no significant heat transfer during redistribution of liquid water was relaxed by including a source/sink term in the heat transfer equation to account for latent heat during phase transitions of soil moisture. The parameterization uses the layer-integrated form of heat and water diffusion equations adopted by the original Eta-LSS. Therefore it simulates the total ice content of each selected soil layer. Infiltration reduction under frozen ground conditions was estimated by probabilistic averaging of spatially variable ice content of the soil profile. Off-line uncoupled tests of the new and original Eta-LSS were performed using experimental data from Rosemount, Minnesota. Simulated soil temperature and unfrozen water content matched observed data reasonably well. Neglecting frozen ground processes leads to significant underestimation/overestimation of soil temperature during soil freezing/thawing periods and underestimates total soil moisture content after extensive periods of soil freezing.

Heat Storage in Urban Areas: Local-Scale Observations and Evaluation of a Simple Model

Abstract: The flux density of sensible heat to or from storage in the physical mass of the city is determined for seven cities (Chicago, Illinois; Los Angeles, California; Mexico City, Distrito Federal; Miami, Florida; Sacramento, California; Tucson, Arizona; and Vancouver, British Columbia) in North America across a 308 latitudinal range. These cities have a variety of synoptic-scale climates and surface cover and structural morphologies. In all cases the ‘‘measured’’ storage heat flux is determined as the energy balance residual from direct observations of net all-wave radiation, and sensible and latent heat fluxes conducted using the same radiometer and eddy correlation techniques. Databases describing the surface characteristics around each site are developed from analysis of aerial photography and field surveys. Results indicate that storage heat flux is a significant component of the surface energy balance at all sites and is greatest at downtown and light industrial sites. Hysteresis behavior, of varying degrees, is seen at all locations. A simple objective hysteresis model (OHM), which calculates storage heat flux as a function of net all-wave radiation and the surface properties of the site, is found to perform well in the mean for most cases, with the notable exception of Tucson; but considerable scatter is observed at some sites. Some of this is attributed to the moisture, wind, and synoptic controls at each of the sites, and to hour-to-hour variability in the convective fluxes that the OHM does not simulate. Averaging over 2 to 3 h may be a more appropriate way to use the model. Caution should be used when employing the OHM in windy environments.

Direct numerical simulation of a confined three-dimensional gas mixing layer with one evaporating hydrocarbon-droplet-laden stream

Abstract: Direct numerical simulations are performed of a confined three-dimensional, temporally developing, initially isothermal gas mixing layer with one stream laden with as many as 7.3×105 evaporating hydrocarbon droplets, at moderate gas temperature and subsonic Mach number. Complete two-way phase couplings of mass, momentum and energy are incorporated which are based on a thermodynamically self-consistent specification of the vapour enthalpy, internal energy and latent heat of vaporization. Effects of the initial liquid mass loading ratio (ML), initial Stokes number (St0), initial droplet temperature and flow three-dimensionality on the mixing layer growth and development are discussed. The dominant parameter governing flow modulation is found to be the liquid mass loading ratio. Variations in the initial Stokes number over the range 0.5[les ]St0[les ]2.0 do not cause significant modulations of either first- or second-order gas phase statistics. The mixing layer growth rate and kinetic energy are increasingly attenuated for increasing liquid loadings in the range 0[les ]ML[les ]0.35. The laden stream becomes saturated before evaporation is completed for all but the smallest liquid loadings owing to: (i) latent heat effects which reduce the gas temperature, and (ii) build up of the evaporated vapour mass fraction. However, droplets continue to be entrained into the layer where they evaporate owing to contact with the relatively higher-temperature vapour-free gas stream. The droplets within the layer are observed to be centrifuged out of high-vorticity regions and to migrate towards high-strain regions of the flow. This results in the formation of concentration streaks in spanwise braid regions which are wrapped around the periphery of secondary streamwise vortices. Persistent regions of positive and negative slip velocity and slip temperature are identified. The velocity component variances in both the streamwise and spanwise directions are found to be larger for the droplets than for the gas phase on the unladen stream side of the layer; however, the cross-stream velocity and temperature variances are larger for the gas. Finally, both the mean streamwise gas velocity and droplet number density profiles are observed to coincide for all ML when the cross-stream coordinate is normalized by the instantaneous vorticity thickness; however, first-order thermodynamic profiles do not coincide.

Formation and spreading of Arabian Sea high-salinity water mass

Abstract: The formation and seasonal spreading of the Arabian Sea High-Salinity Water (ASHSW) mass were studied based on the monthly mean climatology of temperature and salinity in the Arabian Sea, north of the equator and west of 80°E, on a 2° × 2° grid. The ASHSW forms in the northern Arabian Sea during winter and spreads southward along a 24 sigma-t surface against the prevailing weak zonal currents. The eastern extent of the core is limited by the strong northward coastal current flowing along the west coast of India. During the southwest monsoon the northern part of the core shoals under the influence of the Findlater Jet, while the southern part deepens. Throughout the year the southward extent of the ASHSW is inhibited by the equatorial currents. The atmospheric forcing that leads to the formation of ASHSW was delineated using the monthly mean climatology of heat and freshwater fluxes. Monsoon winds dominate all the flux fields during summer (June-September), while latent heat release during the relative calm of the winter (November-February) monsoon, driven by cool, dry continental air from the north, results in an increased density of the surface layer. Thus excess evaporation over precipitation and turbulent heat loss exceeding the radiative heat gain cool the surface waters of the northern Arabian Sea during winter and drive convective formation of ASHSW.

Annual cycle of energy balance of Zongo Glacier, Cordillera Real, Bolivia

Abstract: An 18-month meteorological data set recorded at 5150 m above sea level (asl) on Zongo Glacier, in the tropical Andes of Bolivia, is used to derive the annual cycle of the local energy balance and to compare it to the local mass balance. The roughness parameters needed to calculate the turbulent fluxes over the surface are deduced from direct sublimation measurements performed regularly on the field site and serve as calibration parameters. For the hydrological year September 1996 to August 1997, net all-wave radiation (16.5 W m−2) is the main source of energy at the glacier surface and shows strong fluctuations in relation to the highly variable albedo. An important peculiarity of tropical glaciers is the negative latent heat flux (−17.7 W m−2) indicating strong sublimation, particularly during the dry season. The latent heat flux is reduced during the wet season because of a lower vertical gradient of humidity. The sensible heat flux (6.0 W m−2), continuously positive throughout the year, and the conductive heat flux in the snow/ice (2.8 W m−2) also bring energy to the surface. There is a good agreement between the monthly ablation calculated by the energy balance and the ablation evaluated from stake measurements. The seasonality of the proglacial stream runoff is controlled by the specific humidity, responsible for the sharing of the energy between sublimation and melting.

Free‐air CO2 enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat

Abstract: In order to determine the likely effects of the increasing atmospheric CO2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550 µmol/mol CO2 (or 200 µmol/mol above current ambient levels of about 360 µmol/mol) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, Rn; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime Ts about 0.6° and 1.1°C at high and low N, respectively. Daily total Rn was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively.

Turbulent fluxes during blowing snow: field tests of model sublimation predictions

Abstract: Sublimation fluxes during blowing snow have been estimated to return 10-50% of seasonal snowfall to the atmosphere in North American prairie and arctic environments. These fluxes are calculated as part of blowing snow two-phase particle transport models with provision for phase change based upon a particle-scale energy balance. Blowing snow models have normally been evaluated based upon their ability to reproduce diagnostic mass flux gradient measurements and regional-scale snow redistribution patterns and snow mass. Direct evidence is presented here that large latent heat fluxes (40-60 W m -2 ) that result in sublimation rates of 0.05-0.075 mm snow water equivalent hour -1 , are associated with mid-winter, high-latitude blowing snow events. For events with wind speeds above the threshold level for snow transport, these fluxes are in the range of those predicted by the Prairie Blowing Snow Model. The fluxes are well in excess of those found during spring snowmelt and which can be predicted by standard bulk aerodynamic transfer equations, suggesting that blowing snow physics will have to be incorporated in land surface schemes and hydrological models in order to properly represent snow surface mass and energy exchange during blowing snow events.

Key results and implications from phase 1(c) of the Project for Intercomparison of Land-surface Parametrization Schemes

Abstract: Using atmospheric forcing data generated from a general circulation climate model, sixteen land surface schemes participating in the Project for the Intercomparison of Land-surface Parametrization Schemes (PILPS) were run o!-line to equilibrium using forcing data from a GCM representative of a tropical forest and a mid-latitude grassland grid point. The values for each land surface parameter (roughness length, minimum stomatal resistance, soil depth etc.) were provided. Results were quality controlled and analyzed, focusing on the scatter simulated amongst the models. There were large di!erences in how the models' partitioned available energy between sensible and latent heat. Annually averaged, simulations for the tropical forest ranged by 79 W m~2 for the sensible heat #ux and 80 W m~2 for the latent heat#ux. For the grassland, simulations ranged by 34 W m~2 for the sensible heat #ux and 27 W m~2 for the latent heat #ux. Similarly large di!erences were found for simulated runo! and soil moisture and at the monthly time scale. The models' simulation of annually averaged e!ective radi- ative temperature varied with a range, between all the models, of 1.4 K for tropical forest and 2.2 K for the grassland. The simulation of latent and sensible heat #uxes by a standard &bucket' models was anomalous although this could be corrected by an additional res- istance term. These results imply that the current land surface models do not agree on the land surface climate

Sublimation from a seasonal snowpack at a continental, mid‐latitude alpine site

Abstract: Sublimation from the seasonal snowpack was calculated using the aerodynamic profile method at Niwot Ridge in the Colorado Front Range. Past studies of sublimation from snow have been inconclusive in determining both the rate and timing of the transfer of water between the snowpack and the atmosphere, primarily because they relied on one-dimensional measurements of turbulent fluxes or short term data sets. We calculated latent heat fluxes at ten minute intervals based on measurements of temperature, relative humidity and wind speed at heights of 0.5 m, 1.0 m and 2.0 m above the snowpack for nine months during the 1994- 1995 snow season. The meteorological instruments were raised or lowered daily to maintain a constant height above the snow surface. At each ten minute time step, the latent heat fluxes were converted directly into millimeters of sublimation or condensation. Total net sublimation for the snow season was 195 mm of water equivalent, or 15% of maximum snow accumulation at the stud site. The majorit y of this sublimation occurred during the snow accumulation season. Monthly losses to sublimation during the fall and winter ranged from 27 to 54 mm of water equivalent. The snowmelt season from May through mid-July showed net condensation to the snowpack ranging from 5 to 16 mm of water equivalent. Sublimation was sometimes episodic in nature, but often showed a diurnal periodicity with higher rates of sublimation during the day.

Energy balance and runoff seasonality of a Bolivian glacier

Abstract: The runoff of Zongo Glacier (Bolivia, 16°S) shows an appreciable seasonal variability, with low discharges in the dry season (May to August) and high values in the humid season (October to March). Incoming radiation, temperature and precipitation are poorly correlated with discharge and cannot explain the hydrological seasonality and the glacier's response to climate variability in the Tropical Andes. Since 1996, energy balance measurements have been carried out in the vicinity of the mean equilibrium line (5150 m a.s.l.) on Zongo Glacier (2.1 km 2 ). Comparisons are made with proglacial stream discharges recorded at the main hydrometric station. Each component of the energy balance (net radiation, turbulent heat fluxes, heat transfer into the ice and heat supplied by precipitation) is derived separately from the measurements, and the variability throughout the year is evaluated. Radiation and turbulent fluxes dominate the surface energy balance. Sensible heat flux is small and does not show a significant seasonal change. Latent heat flux is highly variable with low values during the accumulation season and high values during the dry period. This high sublimation loss during the dry season causes well-developed penitents at the glacier surface. In conclusion, incoming energy throughout the year is constant, with no large variations, and humidity controls the balance of this energy between sublimation and melting. During the accumulation season, sublimation is reduced because of a low gradient of vapour pressure and the energy supplied by radiation is directly consumed by melting, explaining why discharge is high. During the dry period, a large part of the energy supplied by radiation is used to sublimate snow or ice and therefore, energy available for melting is low, which leads to low melt rates. Due to the important role of humidity, tropical glaciers are likely to be the climatic indicators the most sensitive to climatic changes like the greenhouse effect.

Estimation of surface heat flux and an index of soil moisture using adjoint‐state surface energy balance

Abstract: Estimates of surface heat flux and an index of the surface control over evaporation are made based on radiometric observations of ground temperature. A variational data assimilation approach is used to include surface energy balance in the estimation procedure as a physical constraint (the adjoint technique). This technique formulates the estimation problem as a minimization of ground temperature forecast misfits against observations. The surface energy balance equation is incorporated as a physical constraint. Applications to the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) are presented. The procedure estimates of the surface control over latent heat flux match those values based on independent latent heat flux measurements. The estimates of surface heat flux, when compared with measurements, have a root-mean-square error of 20 W m−2. The need to discriminate between soil wetness and aerodynamic contributions to the surface control over evaporation is recognized.

Improved Wind Measurements on Research Aircraft

Abstract: Improved techniques for measuring horizontal and vertical wind components and state variables on research aircraft are presented. They include a filtering method for correcting ground speed and position Inertial Navigation System data with Global Positioning System data, use of moist-air thermodynamic properties in the true airspeed calculation, postflight calculation of the aircraft vertical velocity, and calibration of airflow attack and sideslip angles from the two air-data systems on each aircraft—a radome gust probe and a pair of fuselage-mounted Rosemount 858Y probes. Winds from the two air-data systems are compared for the National Oceanic and Atmospheric Administration WP-3D aircraft. As an evaluation of these techniques, data from the two aircraft during side-by-side low-level constant-heading runs are compared for mean and turbulent measurements of wind, ambient temperature, and absolute humidity. Small empirical offsets were determined and applied to the two latter scalars as well as to static and dynamic pressures. Median differences between mean horizontal wind components from nine comparisons were within 0.1 6 0.4 ms 21. Median differences in latent heat and sensible heat fluxes and momentum flux components were 3.5 6 15 Wm 22 ,0 6 2.5 Wm 22, and 0 6 0.015 Pa, respectively.

Paleoaltimetry incorporating atmospheric physics and botanical estimates of paleoclimate

Abstract: We present a method for deducing paleoaltitudes that incorporates basic physical principles of atmospheric science and inferences of paleoclimates from plant leaf physiognomy. We exploit the average west-to-east flow of the atmosphere at mid-latitudes and a thermodynamically conserved variable in the atmosphere—moist static energy (the combined internal, latent heat, and gravitational potential energy of moist air)—to develop a method that relies on a parameter that varies with height in the atmosphere in a predictable fashion. Because the surface distribution of moist static energy is constrained by atmospheric dynamics and thermodynamics, the combined internal and latent heat energies, also known as the moist enthalpy, should only vary with altitude provided we know the distribution of moist static energy. Thus, we avoid having to make assumptions about the mean annual temperature lapse rate, which varies spatially and temporally owing to unpredictable variations in atmospheric water vapor. To estimate a paleoaltitude, therefore, we require (1) a priori knowledge of the spatial distribution of moist static energy for the paleoclimate and (2) the ability to estimate paleoenthalpy for two isochronous locations: one at sea level, the other at some unknown elevation. To achieve this, we investigated the spatial distribution of moist static energy for the present-day climate of North America to estimate the deviations in moist static energy from zonal invariance. In parallel, we quantified the relation between moist enthalpy and plant leaf physiognomy of modern forests. Assuming that such deviations from zonal invariance and such relationships between physiognomy and enthalpy apply to ancient climates and fossil leaves, these investigations yield an uncertainty estimate of ±910 m in the paleoaltitude difference between two isochronous fossil assemblage locations.

Three-Dimensional Hydrothermal Model of Onondaga Lake, New York

Abstract: A three-dimensional time-dependent hydrodynamic model of Onondaga Lake, an inland lake in central New York, emphasizing the simulation of dynamics and thermal structure has been developed. The model is based on the ECOM family of models; this version, called ECOMsiz, employs a semi-implicit time splitting algorithm and a z-level vertical coordinate system. Proper assignment of boundary conditions, especially surface heat fluxes, has been found crucial in simulating the lake's hydrothermal dynamics. Formulas for atmospheric radiation and sensible and latent heat fluxes are introduced, which have been found most appropriate for evaluating the heat budget for this midlatitudinal urban lake. The ECOMsiz model has been calibrated and validated against data for two years, 1985 and 1989, representing a wide spectrum of atmospheric and hydrographic conditions in the lake. These two years, marked by significantly different freshwater inputs from tributary inflows, ionic waste loadings, wind forcing, and atmospheri...

A Consistent Model for the Large-Scale Steady Surface Atmospheric Circulation in the Tropics*

Abstract: The authors present a new model of the tropical surface circulation, forced by changes in sensible heat and evaporative flux anomalies that are associated with prescribed sea surface temperature anomalies. The model is similar to the Lindzen and Nigam (LN) boundary layer model, also driven by the above flux anomalies; but here, since the boundary layer is assumed well mixed and capped by an inversion, the model reduces to a twolayer, reduced-gravity system. Furthermore, the rate of exchange of mass across the boundary layer‐free atmosphere interface is dependent on the moisture budget in the boundary layer. When moist convection is diagnosed to occur, detrainment operates on the timescale associated with the life cycle of deep convection, approximately eight hours. Otherwise, the detrainment is assumed to be associated with the mixing out of the stable tropical boundary layer, which has a timescale of about one day. The model provides a diagnostic estimate of the anomalies in precipitation. However, it is assumed that the latent heat is released above the boundary layer, and it drives a circulation that does not impact the boundary layer. The authors discuss the derivations of the Gill‐Zebiak (GZ) and Lindzen‐Nigam models and highlight some apparent inconsistencies between their derivation and the values of several of the parameters that are required for these models to achieve realistic solutions for the circulations. Then, the new reduced-gravity boundary model equations are rewritten in the form of the GZ and LN models. Using realistic values for the parameters in the new model geometry, it is shown that the constants combine in the rewritten equations to produce the physically doubtful constants in the GZ and LN models, hence, the reason for the apparent success of these models.

Investigation of Turbulent Processes in the Lower Troposphere with Water Vapor DIAL and Radar–RASS

Abstract: High-resolution water vapor and wind measurements in the lower troposphere within the scope of the Baltic Sea Experiment (BALTEX) are presented. The measurements were performed during a field campaign with a new water vapor differential absorption lidar (DIAL) system and a radar–radio acoustic sounding system (radar–RASS). The analysis of a continuous 24-h measurement period shows that the resolution and the accuracy of these instruments are capable of measuring profiles of water vapor and vertical wind fluctuation moments up to the second order to investigate turbulent processes in the convective boundary layer. Besides the high-resolution data obtained with each instrument it is their combination that yields a further important variable in the boundary layer: the turbulent latent heat flux measured directly using the eddy correlation method. The results demonstrate that active remote sensing instruments like DIAL and radar–RASS can be routinely applied to provide previously unavailable datasets...

Mechanisms regulating sea-surface temperatures and deep convection in the tropics

Abstract: Scientific basis for the emergence of deep convection in the tropics at or above 28°C sea-surface temperature (SST), and its proximity to the highest observed SST of about 30°C, is explained from first principles of moist convection and TOGA-COARE data. Our calculations show that SST of 28-29°C is needed for charging the cloud-base airmass with the required moist static energy for clouds to reach the upper troposphere (i.e., 200 hPa). Besides reducing solar irradiation by cloud-cover, moist convection also produces cool and dry downdrafts, which promote oceanic cooling by increased sensible and latent heat fluxes at the surface. Consequently, the tropical ocean seesaws between the states of net energy absorber before, and net energy supplier after, the deep moist convection, which causes the SST to vacillate between 28° and 30°C. While dynamics of the large-scale circulation embodying the easterly waves and Madden-Julian Oscillations (MJOs) modulate moist convection, we show that the quasi-stationary vertical profile of moist static energy of the tropics is the ultimate cause of the upper limit on tropical SSTs.

Analysis of temperature and microstructure in the quenching of steel cylinders

Abstract: A finite-element procedure has been developed for temperature and microstructural analyses of quenching problems involving nonisothermal phase transformations. The finite-element analysis incorporates temperature-dependent material properties, time-temperature-transformation (TTT) diagrams to describe the microstructural evolution in steels, and the latent heat released during a phase change. The procedure is applied to calculate temperature, microstructure, and hardness distributions in 1080 steel cylinders quenched in water and in two polymeric quenchants. The calculated values for the three quantities are found to be in good agreement with corresponding measurements made in quenched 1080 steel cylinders. The effect of latent heat released during a phase change on the temperature and microstructural evolution is studied computationally. It is found that, when the latent heat is not included in the calculations, the resulting volume fractions of pearlite and matensite present in the quenched steel are nearly one order of magnitude different from the corresponding values calculated by including the latent heat in the formulation. Finally, the quenching of a large-diameter 1080 steel cylinder in water is analyzed to show the nonlinear effect of cylinder diameter on the temperatures and microstructures.

A Note on the Flux-Variance Similarity Relationships for Heat and Water Vapour in the Unstable Atmospheric Surface Layer

Abstract: Atmospheric surface layer (ASL) experiments over the past 10 years demonstrate that the flux-variance similarity functions for water vapour are consistently larger in magnitude than their temperature counterpart. In addition, latent heat flux calculations using the flux-variance method do not compare as favorably to eddy- correlation measurements when compared to their sensible heat counterpart. These two findings, in concert with measured heat to water vapour transport efficiencies in excess of unity, are commonly used as evidence of dissimilarity between heat and water vapour transport in the unstable atmospheric surface layer. In this note, it is demonstrated that even if near equality in flux-profile similarity functions for heat and water vapour is satisfied, the flux-variance similarity functions for water vapour are larger in magnitude than temperature for a planar, homogeneous, unstably-stratified turbulent boundary-layer flow.

Multi-objective conditioning of a simple SVAT model.

Abstract: It has previously been argued that current Soil Vegetation Atmosphere Transfer (SVAT) models are over-parameterised given the calibration data typically available. Using the Generalised Likelihood Uncertainty Estimation (GLUE) methodology, multiple feasible model parameter sets are here conditioned on latent heat fluxes and then additionally on the sensible and ground heat fluxes at a single site in Amazonia. The model conditioning schemes were then evaluated with a further data set collected at the same site according to their ability to reproduce the latent, sensible and ground heat fluxes. The results indicate that conditioning the model on only the latent heat flux component of the energy balance does not constrain satisfactorily the predictions of the other components of the energy balance. When conditioning on all heat flux objectives, significant additional constraint of the feasible parameter space is achieved with a consequent reduction in the predictive uncertainty. There are still, however, many parameter sets that adequately reproduce the calibration/validation data, leading to significant predictive uncertainty. Surface temperature measurements, whilst also subject to uncertainty, may be employed usefully in a multi-objective calibration of SWAT models.

Investigations of Humidity Skewness and Variance Profiles in the Convective Boundary Layer and Comparison of the Latter with Large Eddy Simulation Results

Abstract: Differential absorption lidar (DIAL) measurements of humidity variance and skewness profiles are presented. A detailed error analysis demonstrates that measurements throughout the convective boundary layer can be performed with an accuracy that is mainly limited by sampling errors due to turbulent statistics. The water vapor variance profiles are compared with results from a large eddy simulation (LES) model to estimate surface and entrainment latent heat fluxes. A promising agreement with surface latent heat fluxes derived by the bulk formula is achieved, but a strong deviation of the variance profiles from the LES results is found in the entrainment zone. Reasons for these discrepancies are discussed. A new approach for the investigation of top–down variance functions using the measured variance profiles is introduced. It is investigated whether the skewness profiles give further information on the turbulent transport of water vapor. This is accomplished by the analysis of the skewness budget e...