摘要:

AbstractThe detailed photochemistry of methane oxidation has been studied in a coupled chemical/dynamical model of the middle atmosphere. The photochemistry of formaldehyde plays an important role in determining the production of water vapour from methane oxidation. At high latitudes, the production and transport of molecular hydrogen is particularly important in determining the water vapour distribution. It is shown that the ratio of the methane vertical gradient to the water vapour vertical gradient at any particular latitude should not be expected to be precisely two, due both to photochemical and dynamical effects. Modelled H2O profiles are compared with measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment at various latitudes. Molecular hydrogen is shown to be responsible for the formation of a secondary maximum displayed by the model water vapour profiles in high latitude summer, a feature also found in the LIMS data.

ISSN:0035-9009    

DOI:10.1002/qj.49711448002

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractExperiments with a primitive equation model of the stratosphere and mesosphere are described which investigate the effect on the stratosphere of the growth of localized, stationary forcing in the troposphere. Two basic states are used which are representative of stratospheric flow in early and late winter in the southern hemisphere. Simple mechanistic models are then introduced to examine the wave phenomena which occur during the first 10 days of the experiments when the dynamics are known to be linear.The localized forcing grows to a peak amplitude of 600 gpm and half‐width of 15° of latitude, values typical of the mid‐latitude upper troposphere. Stratospheric travelling waves are excited with phase velocities which are sensitive to the zonal mean winds and to the speed of the switch‐on of the forcing, and wave activity spreads quickly from the localized forcing with a typical vertical group velocity of 10 km d−1. Strong horizontal curvature in the basic states can profoundly affect the vertical and horizontal structures of the wavetrains: mid‐latitude regions of negative northward potential vorticity gradient in the stratosphere (as seen in the southern hemisphere in late winter) lead to the confinement and amplification of waves in high latitudes. Three‐dimensional diagnostics of wave activity propagation reveal wave reflections missed by zonally‐averaged EP flux diagnostics.Significant wave reflection and trapping occur on the flow from late winter, eventually producing a steady linear wavetrain with an intense local centre of circulation in the upper stratosphere, the maximum anomaly being ∼1000gpm at 1mb. It is argued that such wavetrains are unrealistic because nonlinear ageostrophic winds drive the dynamics away from any lin

ISSN:0035-9009    

DOI:10.1002/qj.49711448003

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAt the time of the onset of the monsoon in June 1979 the sea surface temperatures in the eastern Arabian Sea were higher than normal. A prediction experiment designed to reveal the effects of this anomaly on the onset is described. This involves two 8‐day forecasts made using the same model and the same initial conditions. They differ only in using different sea surface temperatures in the eastern Arabian Sea. A better prediction of the onset is obtained by using the anomalously high temperatures rather than the normal climatological values. In particular, the development of a tropical storm over the Arabian Sea, the strengthening of the Somali jet, and the northward movement of the rainfall over India are all better predicted. The mechanism of the onset in the model is discussed and two important feedback loops are identified, one involving moisture‐flux convergence and the other involving the surface fluxes of sensible and latent heat. The most important factor in both loops is the release of latent heat over the Arabian Sea. A linear model is used to show that the onset may be thought of as a response to this heating. The roles of barotropic and baroclinic processes in the onset are also discus

ISSN:0035-9009    

DOI:10.1002/qj.49711448004

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA two‐dimensional primitive equation model conserving potential vorticity is used to simulate the effect of a horizontal barotropic deformation field on a pre‐existing isopycnic potential vorticity gradient. The novel feature of the prognostic model is that it accurately describes the development of the density and velocity distributions up to local Rossby number 0.5 and beyond. The model coordinates are based in the vertical on quasi‐isopycnic vertical coordinates and in the horizontal on geostrophic coordinates. The model is integrated forward in time for three days creating a density front and a jet of 0.4 m s−1in the surface layers. Cross‐jet velocities are one order of magnitude smaller. Conservation of potential vorticity leads to a modulation of spacing between isopycnals as both the cyclonic and anticyclonic vorticity associated with the horizontal shear of the accelerating jet increase. Vortex stretching and compression sustain a strong vertical circulation. The paper shows how the jet kinetic energy is limited by the amount of available potential energy in a catchment area defined by the extent of the region of confluence. The jet becomes shallower as frontogenesis approaches this limit.The model is also applicable to the development of permanent streams in the large‐scale circulation of the upper ocean, by slow (order one month) frontogenesis due to gyre‐scale deformation acting on an isopycnic potential vorticity gradient in the seasonal thermocline.The vigorous upwelling on the anticyclonic side of the jet raises nutrients through the euphotic zone, increasing primary productivity. This explains observed meoscale modulatio

ISSN:0035-9009    

DOI:10.1002/qj.49711448005

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe model is an explicit solution of the equations which define the conservation of heat and water vapour in the system consisting of uniform vegetation and soil. The vegetation has two layers, the first extending from a reference height in the atmosphere to the virtual sink for momentum and the second from the virtual sink to the soil surface. Soil between the surface and the damping depth is divided into an upper, completely dry layer and a lower wet layer. Throughout the system, differences of potential per unit flux are specified by resistances, notably the conventional surface resistance governing the loss of vapour from foliage and a new soil resistance, assumed proportional to the accumulated loss of water by evaporation from the soil surface, which is proportional to the square root of elapsed time. Since the total energy available to the system is limited by absorption of radiation, the increase of transpiration from foliage as it expands decreases evaporation from the soil. Conversely, as soil dries, transpiration rate per unit of foliage area increases. This interaction of vapour fluxes is governed by the behaviour of the saturation vapour pressure deficit within the vegetation, identified as a major variable.General implications of the model are consistent with observations and estimates of evaporation rate from a stand of wheat grown in Arizona agree well with measurements from lysimeters. The model is applicable to a wide range of problems in agricultural meteorology and hydrology. With simplification, it could be used to obtain surface exchanges of sensible and latent heat in atmospheric general circulation models.

ISSN:0035-9009    

DOI:10.1002/qj.49711448006

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractIt is argued that the best area average of the roughness length zo in heterogeneous terrain is that which, if it applied in homogeneous terrain, would produce the correct spatial average value of the surface stress. A heuristic argument is presented to show that this effective value of z0, zeff0, can be obtained by averaging drag coefficients based on a ‘blending’ height. The blending height of aboutL/200, whereLis the horizontal scale of the roughness variations, is the characteristic height at which the flow changes from equilibrium with the local surface to independence of horizontal position. These calculations are compared with numerical simulations of planetary boundary layer flow over variations in roughness length and show good agreement. The values of surface stress which result are always greater than those which would be deduced by assuming a local flow equilibrium through the whole depth of the boundary layer. For variations of z0 on short length scales, zeff0 approaches the largest values of z0 within the averaging a

ISSN:0035-9009    

DOI:10.1002/qj.49711448007

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA simple model of linearized, inviscid baroclinic instability in an adiabatic, hydrostatic, compressible atmosphere of arbitrary (though finite) depth, based on the well‐known Eady model, is used to investigate the variation of growth rates and favoured horizontal length scales as functions of δ, the ratio of the model depthDto the density scale height Hs. Both geometric height coordinates (withw= 0 horizontal boundary conditions) and log‐pressure (with ω = 0 boundary conditions) are considered. For δ>3 and a given zonal velocity scale, growth rates are significantly reduced relative to comparable instabilities in an incompressible fluid (δ = 0), and may be suppressed altogether in a laterally‐bounded channel for large enough δ at a given value of static stability. Where instabilitydoesoccur, the length scales favoured are longer than for an incompressible fluid, and are generally comparable to a deformation radius based onD(rather thanHs). The relationship between these results and those obtained in comparable recent studies (which have found scales comparable to a deformation radius based onHs to be important) is examined. Some implications for the role of baroclinic instability in the atmospheres of the major planets are also

ISSN:0035-9009    

DOI:10.1002/qj.49711448008

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractTropical deforestation, by changing land surfaces, may have important consequences for the climate system. Predicting even the local, immediate effects of replacing tropical broadleaf forest with impoverished grassland has been difficult, because the land‐surface parametrization schemes used previously in climate models have been inadequate. The forest canopy is particularly important for the surface‐energy budget in tropical regions, and models neglecting the occurrence of such a canopy may give an unrealistic partitioning between various surface‐energy fluxes. Inclusion of a land‐surface scheme with a vegetation canopy into a version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) with a diurnal as well as a seasonal cycle permits an exploratory study of the possible effects of tropical deforestation. In a 13‐month integration that assumes that all of the Amazon tropical forest in South America is replaced by impoverished grassland, surface hydrological and temperature effects dominate the response. Reduced mixing and less interception and evaporation from the canopy cause runoff to increase and surface temperatures to rise by 3–5 K. The period of driest soil is increased in the model from one month to several, but the possibility that this change is random cannot be excluded. Increased temperatures and drier soil could have a detrimental impact on survival of the remaining forest and on attempts at cultivation in deforested areas. The land‐surface model, driven in a stand‐alone mode by prescribed atmospheric conditions and with an imposed seasonal cycle of rainfall, mimics the seasonal cycle of soil moisture and runoff found in the CCM. Hence, it is used to estimate the relative contribution of the various changes imposed to simulate deforestation in the CCM with respect to the model's response at the surface. The change in surface roughness interacting with the canopy hydrology is evidently a major factor in determining the surface response to deforestation. However, the response to change in roughness is less pronounced for

ISSN:0035-9009    

DOI:10.1002/qj.49711448009

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAn estimate of the mean effect of ensemble‐averaging on forecast skill, under idealized ‘perfect model’ conditions, is obtained from a set of eight independent 50‐day winter ensemble forecast experiments made with a hemispheric version of the Meteorological Office (UKMO) 5‐level general circulation model. Each ensemble forecast consisted of seven individual integrations. Initial conditions for these were obtained by adding spatially correlated perturbations to a given wintertime analysis, and a further integration created in the same manner was used to represent nature, giving the perfect model approach.The ensemble‐mean forecast shows a clear improvement in amplitude and phase skill compared with individual forecasts, the period of significant predictability for daily fields being increased by 50%. The improvement in skill is consistent with simple theoretical estimates based on the perfect model assumption. These calculations are used to deduce how ensemble‐mean forecast skill should vary with the size of ensemble. The superiority of the ensemble‐mean is maintained when forecasts are spatially smoothed or time‐averaged.The spread of an ensemble distribution can in principle give ana prioriindication of forecast skill. A moderate level of correlation between ensemble spread and the forecast skill of the ensemble‐mean is found on the hemispheric scale.The extent to which the potential benefits of ensemble forecasting may be achieved in reality depends on the model's practical forecast skill. Since the practical skill of the 5‐level model is rather low, an ensemble‐mean forecast is on average no better than an individual forecast up to the normal limit of deterministic predictability. However, in four experiments where the individual forecasts show skill beyond this point, the ensemble‐mean forecast does give increased skill.Spatial variations in both the practical and perfect model skills of an ensemble‐mean anomaly field are found to be related to corresponding variations in the statistical significance of the anomaly field. For example, the average perfect model skill, in regions where the ensemble‐mean anomaly is significantly different from zero, exceeds the full field skill in all experiments for forecast days 1–15, and in a

ISSN:0035-9009    

DOI:10.1002/qj.49711448010

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractRawinsonde observations combined with surface synoptic reports and satellite data, indicate that in Israel rain often falls from clouds with top temperatures ≥− 10° C. This suggests the presence of relatively large cloud droplets and/or high concentrations of ice particles at these temperatures, and a relatively efficient natural precipitation‐forming p

ISSN:0035-9009    

DOI:10.1002/qj.49711448011

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY