摘要:

AbstractThis paper is an analysis of equilibrium evaporation and its role in the energy balance of a terrestrial surface, as described by combination theory. Three themes are covered: first, a brief historical review identifies multiple definitions of the concept of equilibrium evaporation. Second, these are formalized by developing the basic principles of combination theory with minimum approximation. Several measures are utilized to do this: linearization is avoided, radiative and storage coupling are incorporated systematically, and actual and linearized saturation deficits are distinguished. The formalism is used to analyse several algebraically defined states and limits for the surface energy balance. Third, the thermodynamic foundation of equilibrium evaporation is analysed by studying surface‐atmosphere feedbacks in arbitrary closed and open evaporating systems. It is shown that under steady energy supply any closed evaporating system evolves towards a quasi‐steady state in which the Bowen ratio takes the equilibrium value 1/εv, where εvis the ratio of the latent‐ and sensible‐heat contents of saturated air with temperature, evaluated at the volume‐averaged temperature in the closed system. This applies whether the system is well‐mixed or imperfectly mixed, and whatever the internal distribution of surface fluxes and surface and aerodynamic resistances. In contrast, open systems cannot reach such an equilibrium. This evolutionary definition of equilibrium evaporation differs from an alternative algebraic definition, the fully decoupled limit. The differences between the two definitions are identified, and the evolutionary definition is shown to be more fundamental. Thus, the correct temperature for evaluating ε in determining equilibrium evaporation is the volume‐averaged temperature in a closed region, which in the case of a convective boundary layer is well approximated by the mixed

ISSN:0035-9009    

DOI:10.1002/qj.49712757402

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIn this study, theoretical models of Schumann, Sykeset al., Beljaars, and Zilitinkevichet al.are examined, compared with data, and evaluated with regard to the calculation of the minimum friction velocity and the heat transfer coefficient. All data employed in earlier papers, namely those from meteorological campaigns SCOPE, TOGA COARE and BOREX‐95, and the Schmidt and Schumann and Sykes and Henn large‐eddy simulations (LESs), are considered. To achieve objective comparison between different formulae, empirical coefficients are recalculated by fitting theoretical curves separately for field data and for data from LESs. Despite essential differences in the shapes of the vertical profiles and the surface‐layer height formulations applied in different models, practically all of them perform rather similarly and in fairly good correspondence with the chosen data set. However, a remarkable systematic difference is observed between data from measurements, on the one hand, and LES data, on the other. It is argued that this difference results from a contribution from uncounted mean‐wind shear to the friction velocity in the field experiments. By this expedient, applicability of LESs to the resistance and heat‐mass transfer problem is confirmed, and empirical coefficients in the resistance and heat transfer formulations ar

ISSN:0035-9009    

DOI:10.1002/qj.49712757403

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis paper describes a study in which, for the first time, advanced systems‐engineering parameter‐estimation techniques were applied to data from several field studies to estimate the preferred set of parameters for some of the most common biomes represented in an advanced Soil‐Vegetation‐Atmosphere Transfer (SVAT) scheme (BATS2, a recent version of the Biosphere‐Atmosphere Transfer Scheme); the effect on modelled climate was also investigated. Observational data from field sites in Brazil, Canada, Arizona and Kansas/Oklahoma in the USA, and the Netherlands were chosen as representative of tropical rain forest, coniferous forest, semi‐arid vegetation, agricultural crops, and grassland biomes, respectively. Together, these five biomes make up 50% of the land area represented in BATS2. Multi‐criteria calibration algorithms do not produce a unique set of model parameters and, when different combinations of the available objective functions at each site are considered, the number of solutions increases substantially. The need for a single parameter‐set for each site (biome) is an important practical issue that was necessarily addressed in this study. A procedure was defined in which optimized parameter‐sets were successively discarded by successively applying a cut‐off threshold to single observable objective functions following a preference hierarchy. In this study, only the vegetation‐related parameters are calibrated for each of the five biomes and implemented into BATS2; however, in a separate experiment, the effect of including soil parameters in the optimization was investigated. When the calibrated parameters are adopted and used in BATS2, there are significant changes between the climates calculated in an eight‐year run with Version 3 of the Community Climate Model and in an equivalent eight‐year run in which the original default parameters were used. The overall conclusion of this exploratory study is that advanced parameter‐estimation techniques and appropriate field data can be used successfully to improve representation of surface exchanges and the modelled climate given by a GCM, by defining appropriate values for vegetation‐related paramete

ISSN:0035-9009    

DOI:10.1002/qj.49712757404

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIdealized simulations are performed in order to examine which scales of orography (relative to the grid scale) can credibly be represented in numerical models. A wide range of flow regimes is considered. These include neutral flow over small hills embedded in the boundary layer and highly stable flow over and around large mountains. In none of these cases do simulations with only two grid points per hill show any appreciable skill. Four and six points per hill give qualitatively reasonable results, but still tend to underestimate the drag relative to that obtained in the best resolved simulations. Implications for the smoothing of the resolved orography fields (and for the inputs to the sub‐grid parametrizations) in large‐scale models are discus

ISSN:0035-9009    

DOI:10.1002/qj.49712757405

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractCloud‐resolving model (CRM) simulations of deep convection in a variety of environmental conditions are used to analyse assumptions in bulk‐updraught convection parametrization schemes, which are commonly employed in climate and weather‐prediction models. It is shown that the mass‐flux approach can faithfully represent the effect of convection on the large‐scale fields, provided that the mass flux and other bulk properties of the updraughts (and downdraughts) are accurately predicted. Entrainment and detrainment (which govern the shape of the mass‐flux profile and the dilution of the bulk updraught with environment air) are calculated from CRM simulations. Limitations in the basic premiss of the classical entraining/detraining plume model are revealed and an alternative method of predicting bulk‐updraught temperature

ISSN:0035-9009    

DOI:10.1002/qj.49712757406

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA novel approach to the parametrization of convective heating‐moistening is proposed. It is observationally known that convective heating is balanced overall by adiabatic cooling due to the resolved‐scale upward motion (large‐scale forcing) during the convective situations, with a corresponding balance also found in the moisture budget. A parametrization may be tuned to satisfy this first‐order balance. However, a small difference between these two terms defines the actual temporal evolution of the temperature and the moisture fields. Hence, the errors due to the cumulus parametrization are often the same order of magnitude as the temporal change itself, in spite of this first‐order tuning. In the present paper, it is proposed that this small ‘residual’ difference be directly represented by a cumulus parametrization. This idea is pursued by adopting a simple Newtonian relaxation formulation in the same spirit as the Betts‐Miller cumulus parametrization. Both the relaxation time‐scale and the reference profile are defined by the optimal fitting of convective‐forcing data diagnosed over the Intensive Flux Array domain during the TOGA‐COARE period for both the total and the residual convective heating parametrizations. Performance of these two types of parametrizations under this optimization is compared using single‐c

ISSN:0035-9009    

DOI:10.1002/qj.49712757407

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe anelastic sigma‐coordinate version of the pseudo‐anelastic Miller‐Pearce model is developed. The pseudo‐anelastic model is characterized by the presence of external‐mode acoustic perturbations which propagate in the horizontal, compromising the model performance by the need to use smaller time steps than would otherwise be required. The approach developed in this study allows for the filtering of acoustic waves and adjustment of surface pressure perturbations. The approach is tested in a new numerical implementation of an adiabatic sigma‐coordinate non‐hydrostatic model and results are found to be comparable to the unadjusted version, and to theory when applicable. The new code is found to allow for significantly larger time steps whilst still retaining numerical stability, showing the possibility of further model

ISSN:0035-9009    

DOI:10.1002/qj.49712757408

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the structure and growth rates of baroclinic normal modes. A linearized GCM is used to find the most unstable normal‐mode disturbances for a zonally symmetric basic state consisting of a zonal jet and meridional circulation. Normal‐mode growth rates are reduced by up to 20% for realistic strengths of meridional overturning, and normal‐mode structures are significantly affected. For example, the normal‐mode poleward tilt with height varies, affecting the conversion of basic‐state available energy. A mechanism for the change in poleward tilt is proposed. Ageostrophic meridional circulations may play a significant role in determining the magnitude and length of mid‐latitude s

ISSN:0035-9009    

DOI:10.1002/qj.49712757409

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the nonlinear development of normal modes. The nonlinear growth and decay phases of the baroclinic life cycle and, in particular, the vertical propagation and subsequent meridional advection of Rossby waves are very sensitive to the exact form of the zonally symmetric basic state. This leads to changes in the associated energetics and eddy fluxes. In order to represent the passage of a disturbance though a jet, a time‐varying zonally symmetric meridional circulation was implemented. Many of the factors identified in the time invariant work, such as the normal‐mode latitudinal and meridional advection, were again found to be important. Ageostrophic meridional circulations may play an important role in determining the magnitude and length of mid‐latitude storm t

ISSN:0035-9009    

DOI:10.1002/qj.49712757410

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractLow‐frequency variability and teleconnection patterns induced by two storm tracks in idealized experiments depend on their zonal distance. A separation of about 150°, which resembles the northern hemisphere storm‐track distribution, reveals teleconnection patterns which are similar to the Pacific North American (PNA) pattern and the North Atlantic Oscillation (NAO). The NAO‐like structure is associated with retrograde travelling Rossby waves; barotropic stream‐function tendencies show that the interaction of the stationary eddies with the low‐frequency flow and the high‐frequency eddies contribute to the amplification of the pattern. Its decay is due to their interaction with the zonal‐mean flow, and the low‐frequency contribution to the divergence term. The PNA‐like pattern has a longer memory and is linked to a quasi‐stationary wave. The stationary‐wave activity flux shows the dominating influence of baroclinic processes and the other storm track further upstream. The correlation between the teleconnection indices is significant for storm‐track distances between 180° and 160°, and vani

ISSN:0035-9009    

DOI:10.1002/qj.49712757411

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY