Mean Temperature Fields Research Articles

Impact of modifying the longwave water vapor continuum absorption model on community Earth system model simulations

The far‐infrared (wavelengths longer than 17μm) has been shown to be extremely important for radiative processes in the earth's atmosphere. The strength of the water vapor continuum absorption in this spectral region has largely been predicted using observations at other wavelengths that have been extrapolated using semiempirical approaches such as the Clough‐Kneizys‐Davies (CKD) family of models. Recent field experiments using new far‐infrared instrumentation have supported a factor of 2 decrease in the modeled strength of the foreign continuum at 50μm and a factor of 1.5 increase in the self‐continuum at 24μm in the Clough‐Kneizys‐Davies continuum model (CKD v2.4); these changes are incorporated in the Mlawer‐Tobin‐CKD continuum model (MT_CKD v2.4). The water vapor continuum in the Community Earth System Model (CESM v1.0) was modified to use the newer model, and the impacts of this change were investigated by comparing output from the original and modified CESM for 20 year integrations with prescribed sea surface temperatures. The change results in an increase in the net upward longwave flux of order 0.5 W m−2between 300 and 400 mb, and a decrease in this flux of about the same magnitude for altitudes below 600 mb. The radiative impact results in a small but statistically significant change in the mean temperature and humidity fields, and also a slight decrease (order 0.5%) of high‐cloud amount. The change in the cloud amount modified the longwave cloud radiative forcing, which partially offset the radiative heating caused by the change in the water vapor continuum absorption model.

Heat transport in partially saturated heterogeneous aquifers

SUMMARY This paper presents a stochastic analysis of the behaviour of heat transfer by soil moisture in a partially saturated heterogeneous aquifer. A spectral approach based on Fourier–Stieltjes representations for the perturbed quantities is used to develop closed-form expressions that describe the field-scale heat advection and the variability of temperature profile. It is shown that the field-scale heat advection is related to the variability of the specific discharge, which, in turn, is related to the variation of hydraulic conductivity. Our analysis is, therefore, focused on the impact of the correlation scale of the log saturated hydraulic conductivity field on these analytical results. It is found that the correlation scale of log saturated hydraulic conductivity has an influence on enhancing the heat advection and the variability of the mean temperature field.

The three-dimensional distribution of clouds around Southern Hemisphere extratropical cyclones

[1] The organization and, for the first time, the three-dimensional structure of clouds associated with the Southern Hemisphere cyclones are studied using active observations from the CloudSat and CALIPSO satellites. First, a composite cyclone is constructed from more than 800 individual cases in the years 2007 and 2008 using the cyclone centre as the composite reference point. It is shown that the three-dimensional cloud distribution around the composite cyclone agrees well with conceptual models of extratropical cyclones. Composite mean fields of sea level pressure, vertical motion, potential temperature and relative humidity are superposed on the three-dimensional cloud structure to better define the relationship between the clouds and dynamical properties of extratropical cyclones. The methodology used here reveals the relationship between dynamical and cloud processes in three dimensions around cyclones and provides the foundation for in-depth evaluations of the ability of climate models to simulate the cloud and dynamical structures of Southern Hemisphere extratropical cyclones.

Large Eddy Simulation of combustion instabilities in a lean partially premixed swirled flame

This paper investigates one issue related to Large Eddy Simulation (LES) of self-excited combustion instabilities in gas-fueled swirled burners: the effects of incomplete mixing between fuel and air at the combustion chamber inlet. Perfect premixing of the gases entering the combustion chamber is rarely achieved in practical applications and this study investigates its impact by comparing LES assuming perfect premixing and LES where the fuel jets are resolved so that fuel/air mixing is explicitely computed. This work demonstrates that the perfect premixing assumption is reasonable for stable flows but is not acceptable to predict self-excited unstable cases. This is shown by comparing LES and experimental fields in terms of mean and RMS fields of temperature, species, velocities as well as mixture fraction pdfs and unsteady activity for two regimes: a stable one at equivalence ratio 0.83 and an unstable one at 0.7.

Analysis of a porous-inclined slider bearing lubricated with magnetic fluid considering thermal effects with slip velocity

A theoretical model of a porous-inclined slider bearing lubricated with magnetic fluid has been considered together with slip velocity boundary condition. Our aim is to study the influence of various dimensionless parameters arising out of the analysis of the model. By assuming the viscosity µ = µ0 exp [-β(tm - t0)] of magnetic fluid, the expressions for mean temperature and load capacity have been obtained. It has been observed that both mean temperature field and load capacity are the functions of slip parameter, magnetic parameter, thermal parameter and permeability parameter. The dependence of the mean temperature field as well as of load capacity on these parameters has been seen graphically.

Spatial-Scale Dependence of Climate Model Performance in the CMIP3 Ensemble

Abstract About 20 global climate models have been run for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) to predict climate change due to anthropogenic activities. Evaluating these models is an important step to establish confidence in climate projections. Model evaluation, however, is often performed on a gridpoint basis despite the fact that models are known to often be unreliable at such small spatial scales. In this study, the annual mean values of surface air temperature and precipitation are analyzed. Using a spatial smoothing technique with a variable-scale parameter it is shown that the intermodel spread, as well as model errors from observations, is reduced as the characteristic smoothing scale increases. At the same time, the ability to reproduce small-scale features is reduced and the simulated patterns become fuzzy. Depending on the variable of interest, the location, and the way that data are aggregated, different optimal smoothing scales from the gridpoint size to about 2000 km are found to give good agreement with present-day observation yet retain most regional features of the climate signal. Higher model resolution surprisingly does not imply much better agreement with temperature observations, in particular with stronger smoothing, and resolving smaller scales therefore does not necessarily seem to improve the simulation of large-scale climate features. Similarities in mean temperature and precipitation fields for a pair of models in the ensemble persist locally for about a century into the future, providing some justification for subtracting control errors in the models. Large-scale to global errors, however, are not well preserved over time, consistent with a poor constraint of the present-day climate on the simulated global temperature and precipitation response.

Quality Assessment of a 1985–2007 Mediterranean Sea Reanalysis

Abstract A simulation and two reanalyses from 1985 to 2007 have been produced for the Mediterranean Sea using different assimilation schemes: a reduced-order optimal interpolation [System for Ocean Forecast and Analysis (SOFA)] and a three-dimensional variational scheme (OceanVar). The observational dataset consists of vertical temperature and salinity in situ profiles and along-track satellite sea level anomalies; daily mean fields of satellite sea surface temperature are used for correcting the air–sea fluxes. This paper assesses the quality of the reanalyses with respect to observations and the simulation. Both the SOFA and OceanVar schemes give very similar root-mean-square errors and biases for temperature and salinity fields compared with the assimilated observations. The largest errors are at the thermocline level and in regions of large eddy field variability. However, OceanVar gives 20% better results for sea level anomaly root-mean-square error.

A new burner for studying auto-ignition in turbulent dilute sprays

This paper introduces a new burner to study the auto-ignition characteristics of dilute turbulent spray flames. The spray is formed upstream of the exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The fluid dynamics is kept intentionally simple and the boundary conditions are well-defined such that the burner lends itself easily to computations. The stability characteristics are presented here for a range of liquid fuels as a function of the controlling parameters. For each fuel, three types of flames are identified based on their visual appearance. These flames are all lifted but differ in the shape of the leading edge and heat release zones. These characteristics are similar for all fuels and three flames, one from each type, are selected for further study. Measurements of mean temperature, velocity and droplet fields as well as high-speed LIF images of OH, Mie scattering from droplets and chemiluminescence of CH ∗ are made in the selected flames of ethanol. It is found that the leading edge of the flame, which is a hardly visible light-blue zone close to the jet exit plane, is marked by the presence of OH (but not CH ∗), and slight increases in peak temperature above the levels of the co-flow. Further downstream, there exists an intense blue region that is characterized by higher OH levels, significant emission of CH ∗, and temperatures that are much higher than those of the co-flow. This is the region where the bulk of the heat release is taking place. Pockets of CH ∗ are seen to appear in this region and grow as they are advected further downstream.

Reconstruction of Global Monthly Upper-Level Temperature and Geopotential Height Fields Back to 1880

Abstract This work presents statistically reconstructed global monthly mean fields of temperature and geopotential height (GPH) up to 100 hPa for the period 1880–1957. For the statistical model several thousand predictors were used, comprising a large amount of historical upper-air data as well as data from the earth’s surface. In the calibration period (1958–2001), the statistical models were fit using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) as the predictand. After the weighting of the predictors, principal component (PC) analyses were performed on both the predictand and predictor dataset. Multiple linear regression models relate each principal component time series from the predictand with an optimal subset of principal component time series from the predictor. To assess the quality of the reconstructions, statistical split-sample validation (SSV) experiments were performed within the calibration period. Furthermore, the reconstructions were compared with independent historical upper-air and total ozone data. Based on the SSV experiment, this study obtained good reconstructions for temperature and GPH in the Northern Hemisphere; however, the skill in the tropics and the Southern Hemisphere was much lower. The reconstruction skill shows a clear annual cycle with the highest values in January. The lower levels were better reconstructed except in the tropics where the highest levels showed the best skill. With the inclusion of a considerable amount of upper-air data after 1939 the skill increased substantially. The fields were analyzed for selected months in the 1920s and 1930s to demonstrate the usefulness of the reconstructions. It is shown that the reconstructions are able to capture regional-to-global dynamical features.

Using satellite altimetry to correct mean temperature and salinity fields derived from Argo floats in the ocean regions around Australia

We present results from a suite of methods using in situ temperature and salinity data, and satellite altimetric observations to obtain an enhanced set of mean fields of temperature, salinity (down to 2000-m depth) and steric height (0/2000 m) for a time-specific period (1992–2007). Firstly, the improved global sampling resulting from the introduction of the Argo program, enables a representative determination of the large-scale mean oceanic structure. However, shortcomings in the coverage remain. High variability western boundary current eddy fields, continental slope and shelf boundaries may all be below their optimal sampling requirements. We describe a simple method to supplement and improve standard spatial interpolation schemes and apply them to the available data within the waters surrounding Australia (100°E–180°W; 50°S–10°N). This region includes a major current system, the East Australian Current (EAC), complex topography, unique boundary currents such as the Leeuwin Current, and large ENSO related interannual variability in the southwest Pacific. We use satellite altimetry sea level anomalies (SLA) to directly correct sampling errors in in situ derived mean surface steric height and subsurface temperature and salinity fields. The surface correction is projected through the water column (using an empirical model) to modify the mean subsurface temperature and salinity fields. The errors inherent in all these calculations are examined. The spatial distribution of the barotropic–baroclinic balance is obtained for the region and a ‘baroclinic factor’ to convert the altimetry SLA into an equivalent in situ height is determined. The mean fields in the EAC region are compared with independent estimates on repeated XBT sections, a mooring array and full-depth CTD transects.

Turbulent Spray Flames of Acetone and Ethanol Approaching Extinction

This paper presents measurements of mean temperature, axial velocity, turbulence, and droplets fields in pilot-stabilized jet flames of dilute sprays where acetone or ethanol is used as liquid fuel. Laser-induced fluorescence (LIF) is employed to image hydroxyl radical OH which marks the existence of hot regions or reaction zones. Depending on the fuel used, droplets within the flow are delineated by imaging LIF from acetone or Mie scattering. As the jet velocity is increased, the flames gradually approach blow-off in a region further downstream from the stabilizing pilot. It is found that the mean velocity, turbulence, and droplet fields are somewhat similar for both ethanol and acetone flames, and these do not change much with increasing jet velocity, particularly when normalized with the relevant parameters. However, the temperature and reactive fields are varying and undergo departure indicative of nonpremixed to premixed flame behavior depending on the vapor pressure of the fuel and proximity to blow-off. Broad regions of OH as well as breaks in the OH profile marking possible local extinction are observed in the ethanol flames.

Estimating ocean climatologies for short periods: A simple technique for removing the effect of eddies from temperature and salinity profiles

[1] Existing temperature and salinity ocean climatologies are usually mean fields based on observations collected over many decades. Because of low frequency variability of major features such as the Gulf Stream it is often more appropriate to define the mean for shorter periods, but there are subsequently fewer observations and more problems with aliasing of mesoscale variability. We present a method for removing eddy-related noise from observations using satellite altimeter sea surface height measurements. We demonstrate the technique using Argo observations from the northwest Atlantic and produce mean temperature and salinity fields for an eight year period. The reduction in variance achieved is quantified. Comparison with an existing climatology shows that there is good agreement between the two analyses but that the representation of the Gulf Stream is more realistic in the new climatology.

Rossby Waves and the Interannual and Interdecadal Variability of Temperature and Salinity off California

Abstract Previous work has shown that large-scale interannual Rossby waves, largely remotely generated by equatorial winds, propagate westward from the coast off southern California. These waves have a large-scale anomalous alongshore velocity field that is proportional to the time derivative of the interannual sea level anomaly. Using these results, a theory is developed for interannual perturbations to a mean density field that varies both vertically and alongshore, like that for the California Current region off southern California. Because both the anomalous vertical and alongshore currents are proportional to the time derivative of the interannual sea level, the theory suggests that the anomalous currents associated with the Rossby waves, acting on the mean temperature field, should induce temperature fluctuations proportional to the anomalous dynamic height. The alongshore and vertical advections contribute to the temperature fluctuations in the same sense, a higher-than-normal sea level, for example, resulting in downward and poleward displacement of warmer water and a local higher-than-normal temperature. Near the surface, alongshore advection dominates vertical advection but both contribute comparably near the thermocline and below. The correlation of observed temperature and dynamic height anomalies from the California Cooperative Oceanic Fisheries Investigation (CalCOFI) data is positive, which is consistent with the theory. The correlation is highest (r ≈ 0.8) near 100-m depth in the thermocline. Although the correlation falls toward the surface, it is still between 0.5 and 0.6, suggesting that the advection mechanism is a major contributor to the temperature anomalies there. The anomalous Rossby wave currents, acting on the mean background salinity gradient, also induce salinity anomalies. At halocline depths of 100–200 m, consistent with the theory, the correlation of observed CalCOFI salinity and dynamic height anomalies is negative and large in magnitude (r ≈ −0.8). However, the surface salinity anomaly is not due to Rossby wave dynamics; instead, much of it is driven by the alongshore wind stress, which it lags by 4 months.

Numerical studies of flow over a sill: sensitivity of the non-hydrostatic effects to the grid size

A non-hydrostatic terrain-following model in cross sectional form is applied to study the processes in the lee of a sill in an idealized stratified fjord during super-critical tidal inflow. A sequence of numerical studies with horizontal grid sizes in the range from 100 to 1.5625 m are performed. All experiments are repeated using both hydrostatic and non-hydrostatic versions of the model, allowing a systematic study of possible non-hydrostatic pressure effects and also of the sensitivity of these effects to the horizontal grid size. The length scales and periods of the internal waves in the lee of the sill are gradually reduced and the amplitudes of these waves are increased as the grid size is reduced from 100 down to 12.5 m. With a further reduction in grid size, more short time and space scale motions become superimposed on the internal waves. Associated with the internal wave activity, there is a deeper separation point that is fairly robust to all parameters investigated. Another separation point nearer to the top of the sill appears in the numerical results from the high-resolution studies with the non-hydrostatic model. Associated with this shallower separation point, an overturning vortex appears in the same set of numerical solutions. This vortex grows in strength with reduced grid size in the non-hydrostatic experiments. The effects of the non-hydrostatic pressure on the velocity and temperature fields grow with reduced grid size. In the experiments with horizontal grid sizes equal to 100 or 50 m, the non-hydrostatic pressure effects are small. For smaller grid sizes, the time mean velocity and temperature fields are also clearly affected by the non-hydrostatic pressure adjustments.

On the homogeneity and interpretation of precipitable water time series derived from global GPS observations

Observations of the Global Positioning System (GPS) were reanalyzed over the period from 1994 to 2004 in a joint project of the technical universities in Dresden and Munich. The estimated tropospheric parameters were converted into precipitable water (PW) using surface pressure observations from the World Meteorological Organization and atmospheric mean temperature fields from the European Centre for Medium‐Range Weather Forecasts. For the first time a systematic study of the homogeneity of global GPS‐derived precipitable water time series was carried out regarding the influence of changes in the GPS antennas and radomes as well as changes in the number of recorded observations. The focus of this study is on interannual changes in precipitable water. Over Europe, large parts of North America, and Iceland and in the region south of 30°S, these changes are very small. The range of the PW variations on interannual time scales is less than 2 mm in these areas. However, in the southeastern part of North America and north Australia, these anomalies in precipitable water show a range of up to 6 mm. In the tropics, PW anomalies with a range of up to 10 mm were found. GPS PW was compared with a modeled PW assuming water vapor saturation. This shows that GPS PW of stations located in the middle and high northern and southern latitudes is consistent with the temperature‐related saturation values of water vapor. In the tropics and subtropics the annual temperature variations are low. In these regions the variations in the PW can be dominated by other factors, including water vapor transport. At seasonal time scales the water vapor transport can be associated with atmospheric circulation such as monsoonal flow.

A Spatio-Temporal Model for Mean, Anomaly, and Trend Fields of North Atlantic Sea Surface Temperature

We consider the problem of fitting a statistical model to 30 years of sea surface temperature records collected over a large portion of the Northern Atlantic. The observations were collected sparsely in space and time with different levels of accuracy. The purpose of the model is to produce an atlas of oceanic properties, including climatological mean fields, estimates of historical trends, and a spatio-temporal reconstruction of the anomalies, i.e., the transient deviations from the climatological mean. These products are of interest to climate change and climate variability research, numerical modeling, and remote sensing analyses. Our model improves upon the current tools used by oceanographers in that it constructs instantaneous temperature fields before averaging them into the climatology, thus giving equal weight to all years in the time frame, regardless of the temporal distribution of data. It also accounts for nonisotropic and nonstationary space and time dependencies, owing to its use of discrete process convolutions. Particular attention is given to the handling of massive datasets such as the one under study. This is achieved by considering compact support kernels that allow an efficient parallelization of the Markov chain Monte Carlo method used in the estimation of the model parameters. Resulting monthly climatologies are compared with those of the World Ocean Atlas 2001, version 2. Different water masses appear better separated in our climatology, and a close link emerges between the kernels' shape and the dominating patterns of ocean currents. The subpolar and the temperate North Atlantic display opposite trends, with the former mainly cooling over the years and the latter mainly warming, especially in the Gulf Stream region. Long-term changes in annual cycles are also detected. As in any hierarchical Bayesian model, parameter estimates come with credibility intervals, which are useful to compare results with other approaches and detect areas where sampling campaigns are needed the most.

Diagnostic Analysis of Tidal Winds and the Eliassen–Palm Flux Divergence in the Mesosphere and Lower Thermosphere from TIMED/SABER Temperatures

Abstract For migrating tides or fast-moving planetary waves, polarization relations derived from the linear wave equations are required to accurately derive the wind components from the temperature field. A common problem in diagnosing winds from the measured temperature is the error amplification associated with apparent singularities in the wave polarization relations. The authors have developed a spectral module that accurately derives tidal winds from the measured tidal temperature field and effectively eliminates the error amplification near the apparent singularities. The algorithm is used to perform a diagnostic analysis of tidal winds and the Eliassen–Palm (EP) flux divergence in the mesosphere and lower thermosphere (MLT) based on the zonal mean and tidal temperature fields derived from 6 yr of temperature measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite. The derived zonal mean wind and diurnal tidal amplitude reveal new insights into the mesospheric biennial oscillation (MBO) that exists in the MLT at both equatorial and midlatitude regions. The equatorial MBO in the zonal mean wind is present in the entire mesosphere from 50 to 90 km. The equatorial MBO in the temperature amplitude of the diurnal tide occurs near the mesopause region between 80 and 90 km and is largely coincident with the downward phase propagation of the equatorial MBO in the zonal mean wind, indicating a possible mechanism of wave–mean flow interaction between the two. On the other hand, the newly discovered midlatitude MBOs in zonal mean wind and the meridional wind in diurnal tide occur at different altitudes, suggesting possibly a remote forcing–response relationship. The acceleration or deceleration of the zonal mean wind due to EP flux divergence that is contributed by the migrating tides peaks at midlatitudes with a typical value of 10–20 m s−1 day−1 around 95 km.

ITF and Poleward Heat Transport of Indian Ocean

A sigma coordinate model is used to study the heat transport variability and the influence of Indonesian Throughflow (ITF) heat transport on the Indian Ocean. The vertical mixing coefficients were provided by the turbulence scheme. The horizontal mixing coefficients for momentum (viscosity) and tracers (diffusivity) were calculated by a Smagorinsky-type formulation. The model is initialized with the Levitus94 climatological data set for annual mean temperature and salinity fields and is forced by seasonal and yearly varying da Silva SMD 1994 and Hellerman and Rosenstein (1983) wind data set. The spin-up of the model has been carried out separately for open and closed. The ITF transport is found around 10 Sv (Svedrup) (1 Sv = 106 m3/s), and the heat transport across the equator is found at its maximum amplitude 1.3 PW (Peta Watt) (1 PW = 1015 Watts) in March and −1.6 PW in July. It is found that the overall effect of the ITF heat transport on the Indian Ocean is dominant on the south of 10°S, and this has a small effect between equator and 10°S.

Effects of mesh resolution on large eddy simulation of reacting flows in complex geometry combustors

The power of current parallel computers is becoming sufficient to apply large eddy simulation (LES) tools to reacting flows not only in academic configurations but also in real gas turbine chambers. The most limiting factor in performing LES of real systems is the mesh size, which directly controls the overall cost of the simulation, so that the effects of mesh resolution on LES results become a key issue. In the present work, an unstructured compressible LES solver is used to compute the reacting flow in a domain corresponding to a sector of a realistic helicopter chamber. Three grids ranging from 1.2 to 44 million elements are used for LES and results are compared in terms of mean and fluctuating fields as well as of pressure spectra. Results show that the mean temperature, reaction rate, and velocity fields are almost insensitive to the grid size. The RMS field of the resolved velocity is also reasonably independent of the mesh, while the RMS fields of temperature exhibit more sensitivity to the grid, as expected from the fact that most of the combustion process proceeds at small scales. The acoustic field exhibits a limited sensitivity to the mesh, suggesting that LES is adapted to the computation of combustion instabilities in complex systems.

Three-Dimensional Acoustic Travel-Time Tomography of the Atmosphere

Acoustic travel-time tomography is a remote technique that can be used for reconstruction of temperature and wind velocity fields in the atmosphere. In the literature, results of two-dimensional tomography of the atmosphere have been reported so far. This paper presents an algorithm for reconstruction of temperature and wind velocity fields in three-dimensional tomography. In the algorithm, stratification of the atmosphere is taken into account explicitly. This allows high accurate reconstruction of stratified mean temperature and wind velocity fields. The fields of fluctuations are reconstructed with the use of three-dimensional time-dependent stochastic inversion. Estimation of the errors of reconstruction is also presented. A numerical experiment with large-eddy simulation is used to confirm the efficiency of the developed theory.