Latent heat

About: Latent heat is a research topic. Over the lifetime, 13503 publications have been published within this topic receiving 302811 citations.

Vegetation controls on surface heat flux partitioning, and land‐atmosphere coupling

Abstract: We provide observational evidence that land-atmosphere coupling is underestimated by a conventional metric defined by the correlation between soil moisture and surface evaporative fraction (latent heat flux normalized by the sum of sensible and latent heat flux). Land-atmosphere coupling is 3 times stronger when using leaf area index as a correlate of evaporative fraction instead of soil moisture, in the Southern Great Plains. The role of vegetation was confirmed using adjacent flux measurement sites having identical atmospheric forcing but different vegetation phenology. Transpiration makes the relationship between evaporative fraction and soil moisture nonlinear and gives the appearance of weak coupling when using linear soil moisture metrics. Regions of substantial coupling extend to semiarid and humid continental climates across the United States, in terms of correlations between vegetation metrics and evaporative fraction. The hydrological cycle is more tightly constrained by the land surface than previously inferred from soil moisture.

Ecosystem gas exchange in a California grassland : seasonal patterns and implications for scaling

Abstract: We used the eddy covariance technique to measure exchanges of water vapor, energy, and CO 2 between California serpentine grassland and the atmosphere. Even though the system was built around an inexpensive, one-dimensional sonic anemometer and a closed-path CO 2 analyzer, energy balance closure was accurate to ± 20% at a net radiation of 500 W/m 2 , and the spectra and cospectra indicated only modest information loss from incompletely resolved high-frequency turbulence. In the early and middle parts of the growing season, net radiation, latent heat, and sensible heat all had similar diurnal dynamics, with latent heat accounting for 60% of the net radiation. Late in the growing season, energy dissipation by latent heat dropped dramatically, even though the vapor pressure gradient remained high. The Ω factor, an index of the role of canopy conductance in regulating transpiration (Jarvis and McNaughton 1986), decreased from 0.8 early in the growing season (indicating predominant control of transpiration by net radiation) to 0.1 late in the growing season (indicating a shift to control of transpiration by canopy conductance and vapor pressure deficit). Canopy conductance was a linear function of the product of net photosynthesis and relative humidity, divided by the CO 2 concentration, as predicted by Ball (1988). The slope of the relationship, however, was greater early in the growing season than at other times. Whole-ecosystem carbon exchange rates were modest, with midday net photosynthesis reaching maximum values of 6-8 μmol.m -2 .s -1 in early April. Diurnal variation in photosynthesis roughly paralleled variation in photosynthetically active photon flux density (PFD), but with the daily maximum increasing with canopy development early in the growing season and decreasing with drought at the end of the growing season. Photosynthesis did not clearly saturate at high levels of PFD. Ecosystem dark respiration increased strongly (Q 10 =4.6) with increasing soil surface temperature. The efficiency with which absorbed radiation was used in ecosystem photosynthesis, integrated over entire days, was 0.0115 ± 0.0015 mol CO 2 /mol PFD (20% of the values measured for healthy, single leaves under low light conditions) until late in the growing season, when the efficiency fell sharply. Using simple assumptions to extrapolate measurements from 11 d to the entire growing season, we estimate ecosystem annual gross primary production to be 11.1 mol CO 2 /m 2 , yielding an annual net primary production of 133 g biomass/m 2 . This is near the center of the range of published values from aboveground harvest studies.

Modelling of the available latent heat of a CO2 hydrate slurry in an experimental loop applied to secondary refrigeration

Abstract: The present study investigates the suitability of CO2 hydrate for a use as phase change material in two-phase secondary refrigeration. Unlike the generation of the classical two-phase refrigerants, power limited by mechanical parts, hydrate slurry production has the advantage of being performed using a nonmechanical process. Nevertheless, in order to be efficient, the hydrate slurry needs to fulfil two major conditions: a high latent heat of melting of the solid phase and appropriate flowing conditions of the slurry carrying a sufficient amount of solid. Consequently, in the present work, multi-cycle differential scanning calorimetry (DSC) measurements were performed and confirmed a value of CO2 hydrate dissociation enthalpy of approximately View the MathML source500 kJ kgw−1, one and a half higher than that of ice View the MathML source(333 kJ kgw−1). Moreover, an experimental loop made it possible to study the CO2 hydrates in suspension in a carrying liquid and to model the available enthalpy of the system related to the solid fraction of the slurry.

Summertime Response of the Tropical Atmosphere to the Indian Ocean Dipole Sea Surface Temperature Anomalies

Abstract: The anomalous response of the tropical atmosphere to the recently discovered Indian Ocean Dipole has been studied in the present article, using an Atmospheric General Circulation Model (AGCM), and the NCEP/NCAR Reanalysis. Our AGCM study shows that the response of the atmosphere to the IOD is of dipole—like pattern in the circulation, and is baroclinic. An anomalous circulation in the zonalvertical plane is induced, with the subsidence over colder pole and the upward motion over the warmer pole, modulating the Walker circulation over the equatorial Indian Ocean, as observed. An anomalous circulation in the meridional-vertical plane from the eastern pole of the dipole towards India, and the Bay of Bengal is simulated, which affects the Indian summer monsoon rainfall. The model atmosphere is heated by the anomalous surplus of the latent heat, the net long-wave radiation, and sensible heat fluxes over the warmer pole of the IOD. This results in an excess of net vertically integrated moisture convergence, and excess latent heat release in the atmospheric column above the warm pole. This energy, together with the anomalous convergence of enthalpy, is converted into anomalous mechanical energy and leads to the divergence of mechanical energy flux. This anomalous divergence of the mechanical energy in the upper tropospheric part of the column causes propagation of the disturbances to the surrounding regions. The mean seasonal circulation during the boreal summer is crucial for maintenance of the anomalous energy distribution and propagation during the IOD event. Transient high frequency variability does not appear to contribute much to the energy conversions.