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Physical Parametrizations (physical + parametrization)
Selected AbstractsFASTEX IOP 18: A very deep tropopause fold.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 577 2001I: Synoptic description, modelling Abstract The life cycle of a very deep tropopause fold (820 hPa) is documented with aircraft and ship observations during the Intensive Observing Period 18 of the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The initial setting involves a coherent tropopause disturbance and an associated Arctic tropopause fold. The confluence episode that results from the phasing up of the tropopause disturbance and a southern ridge, ends in the formation of an intense jet streak, the dynamics of which are associated with the development of a polar tropopause fold. A diagnostic analysis suggests that the final dramatic stratospheric intrusion is the consequence of the vertical superposition of the Arctic and polar tropopause folds. The Mesoscale Non-Hydrostatic (Meso-NH) model is used to discuss this hypothesis. Mixing of the passive stratospheric tracer within the marine boundary layer is investigated with sensitivity tests which unplug, in turn, the model physical parametrizations. Finally, upper-level forcings associated with the development of the tropopause fold are investigated in detail in a companion paper. [source] The vertical resolution sensitivity of simulated equilibrium temperature and water-vapour profilesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 565 2000Adrian. Abstract Variability of atmospheric water vapour is the most important climate feedback in present climate models. Thus, it is of crucial importance to understand the sensitivity of water vapour to model attributes, such as physical parametrizations and resolution. Here we attempt to determine the minimum vertical resolution necessary for accurate prediction of water vapour. To address this issue, we have run two single-column models to tropical radiative,convective equilibrium states and have examined the sensitivity of the equilibrium profiles to vertical resolution. Both column models produce reasonable equilibrium states of temperature and moisture. Convergence of the profiles was achieved in both models using a uniform vertical resolution of around 25 hPa. Coarser resolution leads to significant errors in both the water vapour and temperature profiles, with a resolution of 100 hPa proving completely inadequate. However, fixing the boundary-layer resolution and altering only the free-tropospheric resolution significantly reduces sensitivity to vertical resolution in one of the column models, in both water and temperature, highlighting the importance of resolving boundary-layer processes. Additional experiments show that the height of the simulated tropopause is sensitive to upper-tropospheric vertical resolution. At resolutions higher than 33 hPa, one of the models developed a high degree of vertical structure in the vapour profile, resulting directly from the complex array of microphysical processes included in the stratiform cloud parametrization, some of which were only resolved at high resolutions. This structure was completely absent at lower resolutions, casting some doubt on the approach of using relatively complicated cloud schemes at low vertical resolutions. [source] The ECMWF operational implementation of four-dimensional variational assimilation.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 564 2000II: Experimental results with improved physics Abstract A comprehensive set of physical parametrizations has been linearized for use in the European Centre for Medium-Range Weather Forecasts (ECMWF's) incremental four-dimensional variational (4D-Var) system described in Part I. The following processes are represented: vertical diffusion, subgrid-scale orographic effects, large-scale precipitation, deep moist convection and long-wave radiation. The tangent-linear approximation is examined for finite-size perturbations. Significant improvements are illustrated for surface wind and specific humidity with respect to a simplified vertical diffusion scheme. Singular vectors computed over 6 hours (compatible with the 4D-Var assimilation window) have lower amplification rates when the improved physical package is included, due to a more realistic description of dissipative processes, even though latent-heat release contributes to amplify the potential energy of perturbations in rainy areas. A direct consequence is a larger value of the observation term of the cost-function at the end of the minimization process when improved physics is included in 4D-Var. However, the larger departure of the analysis state from observations in the lower-resolution inner-loop is in better agreement with the behaviour of the full nonlinear model at high resolution. More precisely, the improved physics produces smaller discontinuities in the value of the cost-function when going from low to high resolution. In order to reduce the computational cost of the linear physics, a new configuration of the incremental 4D-Var system using two outer-loops is defined. In a first outer-loop, a minimization is performed at low resolution with simplified physics (50 iterations), while in the second loop a second minimization is performed with improved physics (20 iterations) after an update of the model trajectory at high resolution. In this configuration the extra cost of the physics is only 25%, and results from a 2-week assimilation period show positive impacts in terms of quality of the forecasts in the Tropics (reduced spin-down of precipitation, lower root-mean-square errors in wind scores). This 4D-Var configuration with improved physics and two outer-loops was implemented operationally at ECMWF in November 1997. [source] A standard test for AGCMs including their physical parametrizations: I: the proposalATMOSPHERIC SCIENCE LETTERS, Issue 2 2000R. B. Neale Abstract To assist model intercomparison and development of a set of eight numerical experiments is proposed as a test-bed for the interaction of dynamics and physical parameterizations in atmospheric GCMs. The framework for the experiments is that of an aqua-planet and the prescribed sea-surface temperatures (SSTs) are highly idealized. Copyright © 2001 Royal Meteorological Society. [source] A standard test for AGCMs including their physical parametrizations.ATMOSPHERIC SCIENCE LETTERS, Issue 2 2000II: results for the Met Office Model Abstract Example results are shown for the proposed aqua-planet experiments using a version of The Met Office Unified Model (UM). The zonal mean circulation exhibits strong sensitivity to the latitudinal distribution of sea-surface temperatures (SST). Longitudinal variation of SST yields information on the linearity and distribution of the convective response. Copyright © 2001 Royal Meteorological Society. [source] Simulation of the Asian summer monsoon in five European general circulation modelsATMOSPHERIC SCIENCE LETTERS, Issue 1 2000G. M. Martin Abstract A comparison is made of the mean monsoon climatology in five different general circulation models (GCMs) which have been used by the participants of a project, funded by the European Union, entitled Studies of the Influence, Hydrology and Variability of the Asian summer monsoon (SHIVA). The models differ considerably, in horizontal and vertical resolution, numerical schemes and physical parametrizations, so that it is impossible to isolate the cause of differences in their monsoon simulations. Instead, the purpose of this comparison is to document and compare the representation of the mean monsoon in models which are being used to investigate the characteristics of the monsoon, its variability and its response to different boundary forcings. All of the models produce a reasonable representation of the monsoon circulation, although there are regional variations in the magnitude and pattern of the flow at both 850 hPa and 200 hPa. Considerable differences between the models are seen in the amount and distribution of precipitation. The models all reproduce the basic monsoon seasonal variation, although the timing of the onset and retreat, and the maxima in the winds and precipitation during the established phase, differ between them. There are corresponding differences in the evolution of the atmospheric structure between the pre-monsoon season and its established phase. It is hoped that this study will set in context the investigations of the monsoon system and its impacts carried out using these models, both during SHIVA and in the future. Copyright © 2000 Royal Meteorological Society. [source] | ||||||||||