bluuebunny/crossref_metadata_2025_split huggingface dataset:
Climate modelling: Achievements and prospects
Authors: Carson D. J. 
Published: February 2007

AbstractThe significant progress made with coupled (atmosphere‐ocean) general‐circulation models (CGCMs) over the past decade or so is discussed in the context of the reports of the Intergovernmental Panel on Climate Change (IPCC). In their first report in 1990, most climate‐change GCMs were atmospheric GCMs coupled to ‘slab’ ocean models and being used only to determine the ‘equilibrium’ climate response to specified increases in carbon dioxide. the wide range of estimates of their ‘climate sensitivity’ is discussed, especially in the context of the uncertainty introduced by model sensitivity to representations of cloud. the modelling focus then moved to transient‐response, climate‐change experiments. These and the related modelling problems are noted; as are the simultaneous growing awareness and improved quantification of a range of natural and human‐induced radiative forcings. the consequent inclusion of this information in CGCMs to study the detection and attribution of climate change, leading to the IPCC statement in their 1995 report that ‘the balance of evidence suggests that there is a discernible human influence on global climate’, is also discussed.A significant modelling breakthrough since the 1995 report is the capability of a few CGCMs to produce stable and realistic climate (control) simulations without using artificial ‘flux adjustments’. This is illustrated with results from such a model, the Hadley Centre HadCM3. In conclusion, a brief perspective is given of high‐priority modelling developments and applications to be addressed over the next decade.
An examination of the accuracy of the linearization of a mesoscale model with moist physics
Authors: Errico Ronald M. , Raeder Kevin D. 
Published: February 2007

AbstractThe accuracy of tangent linear and adjoint versions of a primitive‐equation model with moist physics is examined with respect to growing perturbations having significant initial magnitudes. the Jacobians for the convective parametrizations are approximated using a perturbation method. These Jacobians are then quality controlled to ensure that the approximations are suitable. Results show that: (1) linearization of the diabatic moist physics can have a significant impact; (2) even where such impacts are large, the linearized versions of the model can yield good approximations to the nonlinear behaviour for significant perturbations, especially if there is sufficient dynamical influence; (3) poor approximations can be obtained when convection dominates the results; and (4) a straightforward linearization of some parametrization schemes may be inadequate. the results are encouraging for quantitative applications of some moist adjoint models to extratropical cyclones in the winter, but suggest some tangent linear approximations may be unsuitable in the tropics or over continents in the summer, except if qualitative agreements with nonlinear results are suficient. Detailed comparisons of linear and nonlinear results should be made, particularly using optimal perturbations, prior to any applications of tangent linear or adjoint models.
An intercomparison of radiatively driven entrainment and turbulence in a smoke cloud, as simulated by different numerical models
Authors: Bretherton C. S. , Macvean M. K. , Bechtold P. , Chlond A. , Cotton W. R. , Cuxart J. , Cuijpers H. , Mhairoutdinov M. , Kosovic B. , Lewellen D. , Moeng C.‐H. , Siebesma P. , Stevens B. , Stevens D. E. , Sykes I. , Wyant M. C. 
Published: February 2007

AbstractAs part of a programme of intercomparison of eddy‐resolving and one‐dimensional (1‐D) boundary‐layer models, a convective boundary‐layer filled with radiatively active ‘smoke’ was simulated. the programme is sponsored by the Global Energy and Water Experiment Cloud Systems Study. Cloud‐top‐cooling rates weire chosen to be comparable with those observed in marine stratocumulus, while avoiding evaporative feedbacks on entrainment and turbulence that are also important in liquid‐water clouds. the radiative‐cooling rate hacl a specified dependence on the smoke profile, so that differences between simulations could only be a result of different numerical representations of fluid motion and subgrid‐scale turbulence. At a workshop in De Bilt, the Netherlands in August 1995, results from numerous groups around the world were compared with each other and with a previously investigated laboratory analogue to the smoke cloud.The intercomparison results show that models must be run with higher vertical resolution in the inversion than is customary at present, in order to accurately simulate the entrainment rate into cloud‐topped bounday‐layers under strong inversions. In three‐dimensional (3‐D) models using a vertical grid spacing of 5‐12.5 m, sufficient to resolve the horizontal variability of inversion height, entrainment rates were 10‐50% larger than: he range consistent with the laboratory experiments. With a larger vertical grid spacing of 25 m, 1‐D, 2‐D and 3‐D models all overestimated the entrainment rate by more than 50%. 3‐D models with monotone advection‐schemes overestimated entrainment slightly more than those with non‐monotone schemes, at least when 25 m vertical grid‐spacing was used. However, results from non‐monotone schemes had several undesirable features associated hith the generation of undershoots and overshoots, most notably spurious turbulent mixing above the smoke layer. the 1‐D models tended to underestimate turbulent kinetic energy (TKE) but performed reasonably well given tkeir simplicity. 2‐D models produced too much entrainment and considerably overestimated TKE, compared with 3‐D models with the same numerical formulation.Based on a simple scaling‐argument, we propose that the minimum vertical grid‐spacing required to obtain an accurate entrainment‐rate is of the order of the horizontal fluctuations in inversion height, which is proportional to the layer‐averaged TKE and inversely proportional to the inversion strength.
A broad‐scale circulation index for the interannual variability of the Indian summer monsoon
Authors: Goswami B. N. , Krishnamurthy V. , Annmalai H. 
Published: February 2007

AbstractA broad‐scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain‐gauge data with satellite estimates of precipitation for the period 1979‐96, it is shown that the variability of precipitation on seasonal to interannual time‐scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70°E–110°E, 10°N–30°N. the EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. the June‐to‐September climatological mean EIMR is found to be larger than that of the IMR even though the former is averaged over a larger area. the dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north‐western Pacific region (110°E–160°E, 10°N–30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E–110°E, 10°S–5°N).It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. an index of the monsoon Hadley (MH) circulation is defined as the meridional wind‐shear anomaly (between 850 hPa and 200 hPa) averaged over the same domain as the EIMR, Using circulation data from two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium‐Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi‐biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Niño and Southern Oscillation during the latest eight years.
Slow and ultraslow equatorial kelvin waves: The UARS‐CLAES view
Authors: Canziani Pablo O. 
Published: February 2007

AbstractTemperature retrievals from the Cryogenic Limb Array Etalon Spectrophotometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS) are used to determine the behaviour of the slower Kelvin‐wave modes in the lower and middle stratosphere. the CLAES dataset permits their study during the easterly phase of the quasi‐biennial oscillation and the onset and descent of the westerly phase.Given the sampling and operational characteristics of the CLAES, which result in breaks in the time series, it was necessary to implement unequally spaced data series spectral methods, as an intermediate step in the application of the Hayashi method for bi‐dimensional spectral analysis. Comparisons with results for fast and ultrafast Kelvin waves obtained from retrievals by other sensors on board the UARS are very good, confirming the validity of this approach, which does not require interpolations of any kind to fill in the missing data before the use of the spectral methods.The wide‐band reconstruction of the time series allowed the observation of slow Kelvin waves (approximately 15‐day periods, zonal wave numbers 1 and 2) and ‘ultraslow’ Kelvin waves (approximately 25 to 30‐day periods, zonal wave number 1) in the lower stratosphere, for the period January 1992‐May 1993. Slow wave‐number‐2 events occur at the same time as ‘ultraslow’ wave events, in a way similar to that previously observed between fast and ultrafast Kelvin waves in the middle and upper stratosphere. It was possible to determine the behaviour of ‘ultraslow’ Kelvin waves in the CLAES observation period. These waves had only been previously reported in radiosonde data.Results for the slow Kelvin waves are in good agreement with previous observations, mostly obtained from radiosonde data. the effect of the shears, both vertical and meridional, upon the vertical propagation of the observed Kelvin waves is discussed.
Severe flooding over central and southern greece associated with pre‐cold frontal orographic lifting
Authors: Kotroni V. , Lagouvardos K. , Kallos G. , Ziakopoulos D. 
Published: February 2007

AbstractAn extreme rainfall event, associated with a cold front, which occurred on 11‐12 January 1997 over Greece is investigated through observational analysis and numerical simulations. the analysed storm system produced large amounts of rainfall, with devastating impacts over central and southern Greece, where at some sites 24‐hour accumulated precipitation exceeded 300 mm. the model analysis was performed with the three‐dimensional non‐hydrostatic Colorado State University Regional Atmospheric Modeling System. the model results caught the larger‐scale features present in the observational analyses while developing more realistic finer‐scale features. Sensitivity tests are also performed in order to investigate the role of the topography of the Greek peninsula on the enhancement of convection. Indeed, the orientation of the flow was optimal for advection of moist air with a maximized fetch over the sea, and at an angle of impact optimum for orographic lifting ahead of the main topographic axes of continental Greece.
Observations of oblate hail using dual polarization radar and implications for hail‐detection schemes
Authors: Smyth T. J. , Blackman T. M. , Illingworth A. J. 
Published: February 2007

AbstractCoincidental radar measurements and in situ ground‐truth observations of large, oblate hail has allowed new insights into the fall mode of oblate hailstones. the effect of the fall mode upon hail‐detection algorithms using multi‐parameter radar is investigated and discussed, as is the vexed problem of extracting realistic rainfall rates in regions where hail and rain coexist. Observations presented here show that oblate hail can be associated with nonzero values of differential reflectivity, Zdr, which, together with the coincidental values of the linear depolarization ratio, suggest some degree of alignment. Although large hail can dominate backscatter through the diameter‐to‐the‐sixth‐power dependence, even if it is aligned it makes a negligible contribution to specific differential phase, Kdp. Rainfall rates extracted using Kdp in the presence of hail are more robust than those using the contaminated reflectivity in the horizontal polarization, Zh, and Zdr measurements, but proposed hail‐detection algorithms which compare the observed values of Kdp and Zh do seem error prone. the hailstones measured in this study were of sufficient dimensions to allow Mie scattering to occur at S‐band (wavelength λ = 9.75 cm). This served to complicate the interpretation of the differential‐phase measurements. A new hail‐detection algorithm has been formulated which compares the differential phase predicted from Zh and Zdr with that actually observed. This avoids the problem of estimating Kdp from the derivative of a noisy differential‐phase profile.
The simulation of the Asian summer monsoon, and its sensitivity to horizontal resolution, in the UK meteorological office unified model
Authors: Martin G. M. 
Published: February 2007

AbstractThe quality of the simulation of the Asian summer monsoon in the climate version of the UK Meteorological Office's Unified Model, and the impact upon this of increased horizontal resolution, is investigated using two atmosphere‐only model runs forced with observed sea surface temperatures (SSTs) and sea ice extents. the runs each cover the period from 1979 to 1988, but have different horizontal resolutions, with one at climate resolution (2.5° latitude by 3.75° longitude, about 300 km at midlatitudes) and the other at global forecast model resolution (0.833° latitude by 1.25° longitude, approximately 100 km at midlatitudes).The characteristic monsoon circulation and the spatial distribution of precipitation are in good agreement with observations. However, the model has a tendency to overestimate the strength of the monsoon, and also exhibits an early monsoon onset. the large‐scale interannual variations in circulation appear to be simulated reasonably well (as far as can be determined using this short dataset), although the magnitude of the interannual variability of precipitation is overestimated. However, the regional circulation and precipitation changes between El Niño and La Niña years show some significant differences between the model and the observations. the dominant mode of intraseasonal variability seen in both model simulations is, in agreement with observations, associated with the active/break cycle of the monsoon (although this only explains about 10% of the total variance in both simulations). There is some evidence that the SST changes associated with El Niño may produce a coherent forcing of the secondary (east‐west) mode of intraseasonal variability during the onset phase of the monsoon in the model. However, comparison with observations suggests that this may not be representative of what occurs in the real atmosphere. There is no evidence that the SST variations are causing the system to prefer either the active or the break monsoon phase, as was suggested by Palmer.With increased model horizontal resolution, extra detail is provided in the precipitation distribution, but the mean monsoon simulation is scarcely altered and the systematic errors remain. the interannual variations in circulation and precipitation appear not to be greatly altered, and the overall pattern of intraseasonal variability is also unaffected. This study suggests that the systematic errors in the monsoon simulation are not a result of poor horizontal resolution, but may be due to problems with the model physics.
Soil‐moisture nudging experiments with a single‐column version of the ECMWF model
Authors: Hu Yuchun , Gao Xiaogang , Shuttleworth W. James , Gupta Hoshin , Mahfouf Jean‐Francois , Viterbo Pedro 
Published: February 2007

AbstractThe soil‐moisture nudging technique suggests using model forecast errors in near‐surface air temperature and relative humidity to re‐initialize (update) soil moisture in atmospheric models. This study investigates the application of soil‐moisture nudging using a single‐column version of the European Centre for Medium‐Range Weather Forecasts (ECMWF) model. the model was applied at 16 sites selected to sample a range of climates and land covers across the globe, with atmospheric forcing taken from the ECMWF operational analysis for 15 June 1994 and 15 December 1994. When observation errors are set to zero, the Optimal Interpolation technique for deriving estimates of nudging coefficients shows computational instability because strong correlation between near‐surface temperature and near‐surface relative humidity forecast errors makes the coefficient matrix of the linear equations ill‐posed. Therefore, Principal Component Analysis (PCA) is used as a pre‐processor to identify the independent and dependent components of temperature and relative humidity errors. With PCA, the soilmoisture nudging coefficients appropriate to the dominant principal component become stable and effective for correcting soil moisture within days. Such coefficients, when derived for the northern hemisphere summer over the 16 sites, are sufficiently consistent to propose using their all‐site, daily‐average values in a globally applicable soil‐moisture analysis scheme. Tests of this method at the First ISLSCP (International Satellite Land Surface Climatology Program) Field Experiment (FIFE) site confirm that soil‐moisture nudging can provide good estimates of near‐surface weather variables and surface fluxes in a numerical weather prediction (NWP) model which gives poor simulation of precipitation (thereby poor soil moisture). However, the PCA method is unable to give an accurate determination of the location of the soil moisture within soil layers that are accessible to the atmosphere. Further, these tests show that, if the NWP model has good simulation of precipitation but poor simulation of surface radiation, soil‐moisture nudging could wrongly vary the soil moisture so as to provide good estimates of the surface sensible‐heat flux, but poor estimates of surface latent‐heat flux. an approach to distinguish error sources from surface radiation and soil moisture is necessary for further improvement.
A similarity hypothesis for shallow‐cumulus transports
Authors: Grant A. L. M. , Brown A. R. 
Published: February 2007

AbstractA similarity theory for cumulus convection is proposed, and applied to the problem of cumulus transports, using data from several large‐eddy simulations to test the theory. the parameters in the similarity hypothesis include the cloud‐base mass flux, the buoyancy of a parcel undergoing reversible ascent, through the cloud layer and the depth of the cloud layer. Using arguments based on the turbulence kinetic‐energy budget a velocity‐scale is derived, in addition to the conventional Convective Available Potential Energy scale. This new scale is analogous to the convective velocity‐scale used in boundary‐layer theories, but incorporates the effects of latent‐heat release. the ratio between the cloud‐base mass flux and the velocity‐scale is found to be a key parameter in describing cumulus convection.It is shown that the similarity hypothesis can be applied to mass‐flux schemes to determine the fractional entrainment rate. For this the entrainment rate is assumed to be related to the rate of generation of turbulent kinetic energy in the cloud ensemble. the resulting scaling is tested against fractional entrainment rates diagnosed from the large‐eddy simulations.
Momentum budgets over idealized orography with a non‐hydrostatic anelastic model. I: Two‐dimensional flows
Authors: Héreil Philippe , Stein Joël 
Published: February 2007

AbstractNumerical simulations of two‐dimensional (2D) mountain waves are carried out with a non‐hydrostatic anelastic model to investigate the interaction between 2D nonlinear orographic effects and the synoptic flow. the problem is idealized to isolate the essential features of real orographic flows. For each simulation, the complete momentum equations of the model are summed in different domains of variable size representative of a grid mesh of a general‐circulation model. the resulting budgets of momentum, instantaneous or averaged in time, are used to test the hypothesis of current drag parametrizations in three nonlinear regimes of 2D orographic flows. For mountain waves breaking in the troposphere, the mean flow is decelerated in the vertical between the ground and the theoretical breaking level. In the horizontal, the deceleration of the large‐scale flow is uniformly distributed by non‐resolved acoustic waves. When non‐Boussinesq effects lead mountain waves to break in the lower stratosphere, the large‐scale flow is decelerated on a vertical wavelength centred on the breaking level. In the case of non‐breaking trapped lee waves, the deceleration of the mean flow is strongly dependent of the non‐hydrostatic character of the primary propagating wave. Off‐line tests of two drag parametrizations show that some adaptations are necessary to improve the prediction of the impact of subgrid‐scale orographic effects on the large‐scale flow for the set of investigated idealized orographic events.