Showing papers on "Latent heat published in 1998"

Intercomparison of Bulk Aerodynamic Algorithms for the Computation of Sea Surface Fluxes Using TOGA COARE and TAO Data

Abstract: A bulk aerodynamic algorithm is developed for all stability conditions for the computation of ocean surface fluxes. It provides roughness lengths of wind, humidity, and temperature for a wind speed range from 0 to 18 ms 21: zo 5 0.013 /g 1 0.11n/u * and ln(zo/zot) 5 ln(zo/zoq) 5 2.67 2 2.57 as derived using the Tropical 2 1/4 u Re ** Oceans Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) data constrained by other observations under high wind conditions. Using the TOGA COARE ship data and the multiyear hourly TOGA Tropical Atmosphere‐Ocean moored buoy data, intercomparison of six different algorithms, which are widely used in research, operational forecasting, and data reanalysis, shows that algorithms differ significantly in heat and momentum fluxes under both very weak and very strong wind conditions, but agree with each other under moderate wind conditions. Algorithms agree better for wind stress than for heat fluxes. Based on past observations, probable deficiencies in roughness lengths (or neutral exchange coefficients) of some of the algorithms are identified along with possible solutions, and significant issues (particularly the trend of the neutral exchange coefficient for heat with wind speed under strong wind conditions) are raised for future experiments. The vapor pressure reduction of 2% over saline seawater has a significant impact on the computation of surface latent heat flux under strong wind conditions and should be considered in any bulk aerodynamic algorithm.

A Soil Moisture–Rainfall Feedback Mechanism: 1. Theory and observations

Abstract: This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large-scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper (Zheng and Eltahir, this issue) using a simple numerical model.

The Effect of Soil Thermal Conductivity Parameterization on Surface Energy Fluxes and Temperatures

Abstract: The sensitivity of sensible and latent heat fluxes and surface temperatures to the parameterization of the soil thermal conductivity is demonstrated using a soil vegetation atmosphere transfer scheme (SVATS) applied to intensive field campaigns (IFCs) 3 and 4 of the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE). In particular, the commonly used function for soil thermal conductivity presented by M. C. McCumber and R. A. Pielke results in overestimation during wet periods and underestimation during dry periods, as confirmed with thermal conductivity data collected at the FIFE site. The ground heat flux errors affect all components of the energy balance, but are partitioned primarily into the sensible heat flux and surface temperatures in the daytime. At nighttime, errors in the net radiation also become significant in relative terms, although all fluxes are small. In addition, this method erroneously enhances the spatial variability of fluxes assoc...

The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) phase 2(c) Red–Arkansas River basin experiment:: 3. Spatial and temporal analysis of water fluxes

Abstract: The energy components of sixteen Soil-Vegetation Atmospheric Transfer (SVAT) schemes were analyzed and intercompared using 10 years of surface meteorological and radiative forcing data from the Red-Arkansas River basin in the Southern Great Plains of the United States. Comparisons of simulated surface energy fluxes among models showed that the net radiation and surface temperature generally had the best agreement among the schemes. On an average (annual and monthly) basis, the estimated latent heat fluxes agreed (to within approximate estimation errors) with the latent heat fluxes derived from a radiosonde-based atmospheric budget method for slightly more than half of the schemes. The sensible heat fluxes had larger differences among the schemes than did the latent heat fluxes, and the model-simulated ground heat fluxes had large variations among the schemes. The spatial patterns of the model-computed net radiation and surface temperature were generally similar among the schemes, and appear reasonable and consistent with observations of related variables, such as surface air temperature. The spatial mean patterns of latent and sensible heat fluxes were less similar than for net radiation, and the spatial patterns of the ground heat flux vary greatly among the 16 schemes. Generally, there is less similarity among the models in the temporal (interannual) variability of surface fluxes and temperature than there is in the mean fields, even for schemes with similar mean fields.

FIFE Surface Climate and Site-Average Dataset 1987–89

Abstract: This paper analyzes the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment site-average datasets for near-surface meteorology, soil moisture, and temperature; the surface fluxes of radiation, sensible, and latent heat; and the ground heat flux, for the period May 1987‐November 1989. The diurnal and seasonal variation of surface albedo for this grassland site are discussed. The coupling of precipitation, soil moisture, evaporation, pressure height to the lifting condensation level, and equivalent potential temperature (uE) on seasonal and diurnal timescales is also discussed. The 1988 data confirm the authors’ result, shown earlier from the 1987 data that over moist soils increased evapotranspiration lowers afternoon lifting condensation level and increases afternoon uE, suggesting a mechanism for a local positive feedback between soil moisture and precipitation on horizontal scales greater than 200 km. The seasonal cycle of ground heat flux and soil temperature is examined and the authors show that the coupling in the warm months between uE and soil temperature on seasonal scales is similar over land to the coupling found over warm oceans despite very different controls on the surface fluxes. The boundary layer equilibrium over the ocean is contrasted with the diurnal cycle over land, which is soil moisture dependent.

Mixed Layer Modeling of Intraseasonal Variability in the Tropical Western Pacific and Indian Oceans

Abstract: Sea surface temperature (SST) variations associated with the atmospheric intraseasonal oscillation in the tropical Indian and western Pacific Oceans, are examined using a one-dimensional mixed layer model. Surface fluxes associated with 10 well-defined intraseasonal events from the period 1986‐93 are used to force the model. Surface winds from the European Centre for Medium-Range Weather Forecasts daily analyses and SST from the mixed layer model are used to compute latent and sensible heat fluxes and wind stress with the TOGA COARE bulk flux algorithm. Surface freshwater flux is estimated from the Microwave Sounding Unit precipitation data. Net shortwave radiation is estimated, via regression analysis, from outgoing longwave radiation. An idealized diurnal cycle of shortwave radiation is also imposed. The intraseasonal SST variation from the model, when forced by the surface fluxes estimated from gridded analyses, agrees well with the SST observed at a mooring during the COARE. The model was then integrated for the 10 well-defined intraseasonal events at grid points from 758 to 1758 Ea t 58S, which spans the warm pool of the equatorial Indian and western Pacific Oceans. The one-dimensional model is able to simulate the amplitude of the observed intraseasonal SST variation throughout this domain. Variations of shortwave radiation and latent heat flux are equally important for driving the SST variations in the western Pacific, while latent heat flux variations are less important in the Indian Ocean. The phasing of the intraseasonal variation of precipitation relative to wind stress results in little impact of the freshwater flux variation on the intraseasonally varying mixed layer. The diurnal cycle of shortwave radiation is found to significantly increase the intraseasonal amplitude of SST over that produced by daily mean insolation.

On water vapor transport in field soils

Abstract: Measurements of soil volumetric moisture content and temperature were made at 2, 4, 7, 10, and 15 cm below the surface of a bare field soil, over a 1-week period at 20-min intervals. The conductive heat and liquid moisture fluxes were calculated for the soil layer 7-10 cm below the surface, and the water vapor flux was then determined from both the energy transfer and mass transfer equations. Water vapor flux in this layer transported a significant amount of the total energy flux (up to 50%) and an appreciable amount of the total moisture flux (up to 25%). There was reasonable agreement between the water vapor flux calculated by the mass transfer equation and the vapor flux calculated by the energy equation. For at least 80 years it has been recognized that the move- ment of moisture and heat in the soil are coupled (Boucoyous, 1915). The total heat flux in the soil occurs not only from simple conduction but also from water movement in both the vapor and liquid states. Likewise, temperature gradients can drive mass transfer. Conceptually, the coupling of the heat and mass transfer equations can be seen as largely resulting from the water vapor flux. The movement of moisture from one location in the soil to another by evaporation and the subse- quent recondensation can contribute significantly to the net moisture movement in the soil. Additionally, because of the large value of the latent energy of vaporization of water, the water vapor transports significant energy when it evaporates and condenses. Various authors have examined the significance and magni- tude of the water vapor flux as it affects either the mass or energy balances in experimental studies (see Table 1 for sum- mary results). One of the first field-scale tests of the coupled effects of soil heat and moisture transport was done by Rose (1968a, b). Rose was interested in water vapor transport driven by temperature gradients and looked only at the mass balance. In his equation for the conservation of mass the only unknown term was the thermally driven vapor flux. Using the measured values of the other fluxes and the net change in moisture content, Rose solved for the thermal vapor flux, and found that the amount of water transported through a soil layer as vapor was on the same order of magnitude as the increase or de- crease of the volumetric moisture content in that layer. An- other study that looked at role of vapor flux in the soil moisture balance was done by Jackson and coworkers (Jackson, 1973; Jackson et al., 1974); they performed a similar experiment of combined measurements of soil temperature and volumetric moisture content in a field soil. Their calculated water vapor fluxes were on the same order as Rose's (1968b). Monji et al. (1990) also found large values of water vapor transport due to temperature gradients. A study which looked at the effect of vapor flux on the energy balance was that of Westcot and Wierenga (1974). In their combined modified-field experiment and computer model they calculated that heat transported by vapor flux was on the same order as heat flux by conduction and accounted for 40 - 60% of total heat flux in the top 2 cm of the soil and up to 20 -25% of total heat flux at a depth of 25 cm. When the heat transport by vapor flow was not included, the soil tem- perature was underestimated at the middle of the day. In the present study we examine the transport of water in a bare field soil (Yolo silt loam) using subsurface measurements of soil temperature and volumetric moisture content. Thermal conductivity and liquid water diffusivity are both calculated from well-established previous results for the soil used in the experiment. We show that there exists for these field experi- mental conditions a significant amount (40 - 60%) of heat flux due to vapor transport. The contribution of the water vapor flux to the total moisture flux is less (10 -30%) but still signif- icant. Unlike previous studies, which have looked at either mass or energy transport, we compare the vapor flux computed from the residual of the energy equation to the vapor flux computed from the mass equation. Reasonable agreement be- tween the two calculated time series of water vapor flux is found.

Turbulent Flux Measurements Above and Below the Overstory of a Boreal Aspen Forest

Abstract: Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.

Subgrid‐scale variability in the surface energy balance of arctic tundra

Abstract: Surface fluxes of energy, water vapor, and CO2 over homogeneous areas of the major tundra vegetation types in arctic Alaska were measured using a mobile eddy covariance tower for 5-day periods in the middle of the 1994 growing season. In order to account for differences in weather and time of season, data were analyzed in comparison to a nearby, fixed tower that operated throughout the summer. Among the different vegetation types, evaporation ranged from 1.3 to 2.7 mm d−1. Net carbon uptake ranged from 0.5 to 2.4 g C m−2 d−1. Ground heat flux consumed 10–33% of midday net radiation. Typically, 38% of the net radiation was partitioned into latent heat flux, while the fraction of net radiation removed from the surface in sensible heat flux varied from 16 to 50% among vegetation types. The largest differences among vegetation types in surface energy partitioning were related to variations in soil moisture, with midday Bowen ratios ranging from 0.37 over wet sedge tundra to 2.25 over dry heath. Direct effects of vegetation on the driving gradients for energy and water vapor exchange were important in shrub tundra: shading of the moss layer by the canopy reduced ground heat flux and increased sensible heat flux, while latent heat flux was similar to other mesic vegetation types because the moss layer accounted for a larger portion of total evaporation than did evapotranspiration by shrubs. Scaling up from the vegetation types to the Alaskan arctic using an area-weighted average of the observed energy partitioning gave results similar to regional energy budgets measured over larger, more heterogeneous areas of tundra. An extrapolation based on the hypothesis that climate variability could cause a large fraction of the current tussock tundra vegetation to be converted to shrub tundra resulted in modest changes in the regional energy balance. However, nonlinear variations of surface evaporation with leaf area and uncertainties regarding changes in moss cover suggest that additional field experiments as well as modeling efforts will be required to predict the potential for changes in arctic tundra vegetation to feed back on regional climate.

Formation mechanism of Japan Sea Proper Water in the flux center off Vladivostok

Abstract: It is known that wintertime air-sea interaction in the Japan Sea, enhanced by outbreaks of dry and cold air masses from the Eurasian continent, generates a characteristic water mass called Japan Sea Proper Water (JSPW) through deep convection. Using NASA scatterometer (NSCAT) wind vectors with high spatial resolution of 25 km, the role of the wind field over the Japan Sea in JSPW formation was investigated during the period of winter monsoon. It is revealed by NSCAT observations that the wintertime surface winds over the Japan Sea are strongly influenced by the upstream topography of the coastal region of the Eurasian continent. A strong wind area appears off Vladivostok, both in the snapshot and the monthly mean wind fields. The dynamics behind this strong wind flow may be attributed to the highly stratified surface wind being blocked by the coastal mountains and exiting through the narrow valley near Vladivostok into the Japan Sea. The NSCAT winds are coupled with European Centre for Medium-Range Weather Forecasts air temperature and humidity and Japan Meteorological Agency sea surface temperature (SST) data for turbulent flux estimation. Monthly mean wind speed, momentum flux, sensible heat flux, latent heat flux, and evaporation have peak values exceeding 9 m s -1 , 0.275 N m -2 , 170 W m -2 , 130 W m -2 , and 120 mm, respectively, in the strong wind area, which has a diameter of about 150 km. Multichannel sea surface temperature (MCSST) images of the Japan Sea for late January 1997 show that the cold SST (∼0°C) area extends from the coastal region to the outer sea off Vladivostok. The MCSSTs decreased by 1°C for January in this region. The cold SST region coincides with the strong wind area, and both locations agree well with the JSPW formation region, which, according to Sudo [1986], is north of 41°N between 132° and 134°E. The spatial agreement strongly suggests that the wind, enhanced by the topographic effect around Vladivostok, causes the large turbulent heat flux and evaporation in this area, which generates the coldest SST and dense water mass, i.e., JSPW. This may be a process of coastal topography-air-sea interaction that leads to deep-sea water formation in the Japan Sea. Because of the concentrated feature of turbulent fluxes and the inferred relation to the center of deep convection phenomenon, the strong wind area is called the flux center in this study.

Land‐surface water vapor and sensible heat flux: Spatial variability, homogeneity, and measurement scales

Abstract: Available methods to determine the sensible and latent heat fluxes from the Earth's land surfaces are still relatively imprecise; this is due primarily to the inherent irregularity of the turbulent transport mechanisms and also to the pronounced variability of most natural land surfaces. While great progress has been made in the study of turbulence, until recently, surface variability has received relatively little attention in this context. Some thoughts are provided on approaching this issue by focusing on the stochastic structure of the land surface within the framework of homogeneous, i.e., stationary, random functions. In addition, suggestions are made regarding the likely applicability of classical concepts by relating the relevant scales of surface variability structure to those characterizing turbulent transport in the atmospheric boundary layer.

Temporal Variability of the Energy Balance of Thick Arctic Pack Ice

Abstract: The temporal variability of the six terms of the energy balance equation for a slab of ice 3 m thick is calculated based on 45 yr of surface meteorological observations from the drifting ice stations of the former Soviet Union. The equation includes net radiation, sensible heat flux, latent heat flux, bottom heat flux, heat storage, and energy available for melting. The energy balance is determined with a time-dependent 10-layer thermodynamic model of the ice slab that determines the surface temperature and the ice temperature profile using 3-h forcing values. The observations used for the forcing values are the 2-m air temperature, relative humidity and wind speed, the cloud fraction, the snow depth and density, and the albedo of the nonponded ice. The downwelling radiative fluxes are estimated with parameterizations based on the cloud cover, the air temperature and humidity, and the solar angle. The linear relationship between the air temperature and both the cloud fraction and the wind speed i...

Shallow Cumulus Convection

Abstract: Shallow cumulus convection plays a crucial role in determining the vertical thermodynamic structure of the atmosphere and influences the large-scale circulation significantly in both tropics and mid-latitudes. This is most clearly demonstrated in the Hadley circulation over the tropical oceans, such as schematically illustrated in Fig. 1. In the subtropical belts, the surface evaporation from the ocean increases significantly due to the enhanced vertical transport of heat and moisture by shallow cumulus clouds, which are usually referred to as trade-wind cumuli. This enhanced moisture, collected in the trade-wind boundary layer, is transported by the trade winds towards the intertropical convergence zone (ITCZ), where it is finally released as latent heat in deep convective precipitating cumulus towers. Since this latent heat release is an important engine of the Hadley circulation, the enhanced surface evaporation upstream in the trade-wind region can be regarded as a fuel supply for this circulation. It is in this context that the presence of shallow cumuli, though small in individual size and non-precipitating, intensifies the large-scale circulation. Locally, the vertical transport of heat and moisture by the shallow cumulus clouds is also important to counteract the drying and warming effects of the large-scale subsidence induced by the Hadley circulation. As a result, a steady thermodynamic state of the vertical structure of the trade-wind region can be maintained.

Thermal and hydrologic dynamics of the active layer at a continuous permafrost site (Taymyr Peninsula, Siberia)

Abstract: The ground thermal and hydrologic regimes of a site located in the continuous permafrost landscape of Taymyr Peninsula, northern Siberia, were studied in 1994 and 1995. The aim was to quantify the seasonal fluxes of water and heat in the active layer from spring thaw to fall freeze-back. Liquid water content was measured in frozen and unfrozen soils using time domain reflectometry. Liquid water was present in frozen soil at temperatures down to −12°C, and its volumetric fraction increased with temperature before melting occurred. The ground thermal regime during spring thaw and fall freeze-back was dominated by latent heat fluxes that stabilized soil temperatures at 0°C for extended periods. The transfer of large amounts of latent heat released by freeze-back in the fall suggested convective heat transfer mechanisms. Seasonal fluxes of heat and water were well described using a simple zero-dimensional model of water and energy balance. The dominant heat sinks during the spring and summer were sensible and latent heat fluxes into the atmosphere. During fall freeze-back the dominant heat source was phase change. The soil heterogeneity strongly impacted hydrologic and thermal processes in the active layer. Two direct consequences were the development of preferential flowpaths and the preferential freezing of the profile during freeze-back.

Characteristics of resistance welding of lap shear coupons. Part I: Heat transfer

Abstract: A transient three-dimensional finite element model for resistance welding of thermoplastic matrix composite lap shear specimens was established. The basic model assumes orthotropic heat conduction in the composite parts and accounts for heat losses by radiation as well as natural convection, involving latent heat effects. The rough contact surface between the heating ply and composite adherends was simulated by means of a thermal gap-conductance concept. Heat generation through the bundles of fibres in the heating ply was modelled. Different welding configurations of lap shear specimens were evaluated, namely APC-2 laminate/PEEK film, APC-2 laminate/PEI film and CF-PEI laminate/PEI film. `Time to melt' and `time to cause thermal degradation' were predicted and compared with experimental data available in the literature, and close agreement was obtained. Localised thermal degradation at the bonding interface was identified for high input power levels, and heat flow to the bulk laminate for long processing times was exhibited.

Satellite-Derived Latent Heating Distribution and Environmental Influences in Hurricane Opal (1995)

Abstract: The total (i.e., convective and stratiform) latent heat release (LHR) cycle in the eyewall region of Hurricane Opal (October 1995) has been estimated using observations from the F-10, F-11, and F-13 Defense Meteorological Satellite Program Special Sensor Microwave/Imagers (SSM/Is). This LHR cycle occurred during the hurricane’s rapid intensification and decay stages (3–5 October 1995). The satellite observations revealed that there were at least two major episodes in which a period of elevated total LHR (i.e., convective burst) occurred in the eyewall region. During these convective bursts, Opal’s minimum pressure decreased by 50 mb and the LHR generated by convective processes increased, as greater amounts of latent heating occurred at middle and upper levels. It is hypothesized that the abundant release of latent heat in Opal’s middle- and upper-tropospheric region during these convective burst episodes allowed Opal’s eyewall to become more buoyant, enhanced the generation of kinetic energy and...

Evapotranspiration rates from wetlands with different disturbance histories: Indiana Dunes National Lakeshore

Abstract: This study examines the effects of different disturbance histories, specifically patterns of ditching and drainage on wetland evapotranspiration rates for two sites within the Indiana Dunes National Lakeshore, Indiana, USA. The effect of drainage at the disturbed site is to lower the water table and to reduce the depth and duration of inundation. Evapotranspiration was measured as its energy equivalent, the latent heat flux, within the framework of the surface energy balance using the eddy correlation approach. Results show that when standing water is present at each site, there is very similar flux partitioning; the latent heat flux accounts for approximately 50% of the net radiation (approximately 3.5 mm d−1), storage heat flux 30%, and sensible heat flux 20%. When the disturbed site has no standing water, evapotranspiration rates are maintained at the same level (3.5–3.75 mm d−1), but the storage heat flux drops significantly, and the sensible heat flux is enhanced. Excellent results are obtained with the Penman Monteith evapotranspiration model using a very small surface resistance (5 s m−1) when there is no standing water. Although the absolute magnitudes of the results reported are directly relevant only to similar sites in the Midwest USA in summer, the processes and controls described are representative of wetlands located near large water bodies (like Lake Michigan) subjected to diurnal lake (sea) breezes, with vascular vegetation, and a water table at or very close to the surface.