摘要:

AbstractAn intermediate‐complexity atmospheric model coupled with a simple land‐surface model and a mixed‐layer ocean model is used to investigate the processes involved in an idealized monsoon occurring on a single rectangular continent. Idealized divergences of ocean heat transports are specified as an annual average ‘Q‐flux’. In this simple coupled configuration, the mechanisms that affect land‐ocean contrast and, in turn, the seasonal movement of the continental convergence zones are examined. These include soil‐moisture feedbacks: cooling of tropical oceans by ocean transpoit; ventilation, defined as the import into continental regions of low moist static‐energy air from ocean regions where heat storage opposes summer warming; and the ‘interactive Rodwell‐Hoskins mechanism’, in which Rossby‐wave‐induced subsidence to the west of monsoon heating interacts with the convection zone. The fixed ocean transports have a substantial impact on the continental convection. If Q‐flux is set to zero, subtropical subsidence and ventilation tend to substantially limit the poleward movement of summer monsoon rainfall. When land hydrology feedbacks are active, the drying of subtropical continents disfavours continental convection even in the tropics. When ocean transports are included, tropical oceans are slightly disfavoured as regions for producing convection which, by contrast, favours continental convection. The monsoon circulation then produces moisture transport from the ocean regions that allows substantial progression of convection into the subtropics over the eastern portion of the continent. The western portion of the continent tends to have a dry region of characteristic shape. This east‐west asymmetry is partly due to the interactive Rodwell‐Hoskins mechanism. The ventilation is of at least equal importance in producing east‐west asymmetry and is the single most important process in limiting the poleward extent

ISSN:0035-9009    

DOI:10.1002/qj.49712757602

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractAnindex of sea surface temperature (SST) variability, δT, is introduced that measures the difference in SST across the separated Gulf Stream in late winter. By analysing a long observational record of SST and sea‐level pressure (SLP), it is shown that δT exhibits damped oscillations of decadal period, and covaries with the strength of a dipolar SLP anomaly reminiscent of the North Atlantic Oscillation (NAO). Analysis in the frequency domain shows a broad‐band ‘peak’ at 10–20 years in δT, with a continuous decrease of power on longer time‐scales. Similar spectral signatures are found in the northern part of the SLP dipole (the Greenland‐Icelandic Low region) but not in its southern part (the subtropical High region), whose power increases on long time‐scales.The observations are interpreted in the framework of a delayed‐oscillator model in which the ocean circulation introduces the delay, and modulates δT on decadal time‐scales. The decrease of power seen on long time‐scales (>25 years) in the δT index is captured by a model including wind‐driven ocean circulation, and arises primarily as a passive response of the latter to the NAO forcing. Variability of the ocean's meridional overturning circulation could also play a role in modulating δT on decadal time‐scales. If a small feedback of δT on the NAO pattern is introduced, the simple model can also reproduce the spectral structures seen in the SLP anomaly

ISSN:0035-9009    

DOI:10.1002/qj.49712757603

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe Etesians (northern sector winds), which blow over the Aegean Sea during summer, affect human activities in the area. The numerous islands of the Aegean and especially Crete (a mountainous island in the southern Aegean oriented perpendicular to the surface flow) seem to play an important role in the modification of the wind field during the Etesians. The Crete mountain ranges, surrounded as they are by water, are an excellent example of a major isolated topographic feature which significantly modifies the regional airflow and pressure; however, this modification can hardly be defined due to the lack of observing stations over the sea. For this reason, the available land surface and ship synoptic observations are used, together with ERS scatterometer wind data in order to identify the regions over the Aegean where the wind reaches its maximum intensity, and to assess the influence of Crete on the wind field. Moreover, numerical modelling is used to provide some further insight on the orographically disturbed wind flow. Sensitivity tests performed with the hydrostatic model BOLAM show that the interaction of the Etesian wind flow with the mountains of Crete produces deceleration of the Etesians up to almost 120 km upstream, the leftward deflection of the air as it approaches the mountains, and the associated intensification of the flow east of the island.

ISSN:0035-9009    

DOI:10.1002/qj.49712757604

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis paper demonstrates that it is possible to use measurements of the Doppler spectrum width from a high‐resolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones. Using the high‐resolution Chilbolton radar, this method has been applied, for the first time, to infer the rate at which kinetic energy is dissipated by turbulence within mid‐ and lower‐tropospheric frontal zones. Analysis of detailed measurements for one cold‐frontal region showed multiple shallow sheets of high dissipation rate, with spatially averaged values around 1 x 10−3m2s−3. Comparable values were also observed in a warm‐frontal zone. The magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts, in that the turbulence acted to destroy the frontal shear layer on a time‐scale of several hours. These issues have particular relevance to the representation of fronts within future very high‐resolution numerical weathe

ISSN:0035-9009    

DOI:10.1002/qj.49712757605

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractRadiosonde and National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data are utilized to consider aspects of large‐scale variability in tropopause height, temperature and pressure. This variability is related to coherent dynamical fluctuations in the troposphere and lower stratosphere through the use of linear correlation and regression. On interannual time‐scales, significant global‐scale tropopause fluctuations are tied to variability in sea surface temperature (SST) associated with the El Niňo/Southern Oscillation phenomenon. When SST is anomalously high in the central tropical Pacific, tropopause height (pressure) is high (low) throughout the Tropics, with largest perturbation amplitudes in the subtropical Pacific. At the same time, the tropopause is cold over the tropical and subtropical Pacific sector but warm elsewhere in the Tropics. Over the extratropics, wave‐like perturbations in the tropopause are seen, with anomalous cyclonic flow corresponding to a lower tropopause height and higher tropopause temperature and pressure, and vice versa. The sign of the temperature anomalies in the lower stratosphere tends to match that at the tropopause over much of the globe, with opposite‐signed anomalies in the upper troposphere. The vertical structure of these perturbations is consistent with the expected potential‐vorticity anomalies induced by quasi‐stationary Rossby waves and vertically propagating gravity waves forced by displacements of tropical convection. Similar relationships are associated with the eastward propprotion of tropical convection due to the Madden‐Julian Oscillation on intraseas

ISSN:0035-9009    

DOI:10.1002/qj.49712757606

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractEquatorial winds in the stratosphere are known to influence the frequency of stratospheric midwinter sudden warmings. Sudden warmings, in turn, influence the Earth's climate both through their direct influence on polar temperatures and through the temperature dependence of ozone depletion in the lower stratosphere. The conventional (Holton‐Tan) explanation for the equatorial influence on sudden warmings is in terms of the equatorial winds in the lower stratosphere (∼20‐30 km) acting as a waveguide for midlatitude planetarywave propagation. This study employs stratospheric‐temperature analyses and equatorial rocketsonde wind data extending to 58 km to diagnose the relationship between the northern‐hemisphere polar temperatures and equatorial zonal winds at all height levels in the stratosphere. In addition to the recognized Holton‐Tan relationship linking the polar temperatures to the quasi‐biennial oscillation in equatorial winds in the lower stratosphere, a strong correlation of polar temperatures with equatorial winds in the upper stratosphere is found. We suggest that this may be associated with the strength and vertical extent of the westerly phase of the semi‐annual oscillation in the upper stratosphere, although the observations alone cannot provide a conclusive, causal relationship. The main diagnostic tools employed are correlation studies and composite analysis. The results underline the need for continued high quality, equatorial wind measurements at all strat

ISSN:0035-9009    

DOI:10.1002/qj.49712757607

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractPrevious two‐dimensional model simulations using the Hines parametrization of unresolved gravity waves have produced long‐period oscillations in the tropics. Such oscillations include the quasi‐biennial oscillation (QBO) and the semi‐annual oscillation (SAO). In this paper simulations with a three‐dimensional mechanistic model are reported which show an even more realistic representation of the QBO. These simulations show the expected dependence of the period of the QBO on the momentum flux entering the stratosphere, but also show a hitherto unreported dependency of the period on the upper boundary condition. In these simulations the dependency is linked to changes in mesospheric vertical velocities associated wit

ISSN:0035-9009    

DOI:10.1002/qj.49712757608

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIf recent measurements of the fluxes of sea‐salt particles over the oceans as a function of wind speed are extrapolated to include the smallest particles detected over the oceans and in laboratory experiments on bursting bubbles, the fluxFof particles having radii betweenr0= 0.1 μm andrμm at 80% relative humidity may be represented byF= 2U3.5(e−0.2r0) – e−0.2r) m−2) s−1, whereUis the near‐surface wind speed in m s−1. The flux of particles ofr≤ 5 μm in winds of 16 m s−1is 20 000 m−2s−1. The corresponding local rate of production by air bubbles bursting in the foam patches of breaking waves that occupy, on average, only 5% of the ocean surface at this wind speed is 40 cm−2s−1. The average concentration of particles over the sea area producing whitecaps is:\documentclass{article}\pagestyle{empty}\begin{document}$$ N = \int\limits_{r_0 }^r {\frac{1}{{v_r }}\frac{{{\rm d}F}}{{{\rm d}r}}{\rm d}r = 0.4U\int\limits_{r_0 = 0.1\mu m}^{5\mu } {\frac{{\exp ( - 0.2r){\rm d}r}}{{v_r }}} } $$\end{document}wherevris the deposition velocity for particles of radiusr. In winds of 16 m s−1the total concentration of sea‐salt particles is calculated to be 40 cm−3. These, and the fluxes and concentrations calculated for other wind speeds, are consistent with measurements made at sea and in the laboratory by many scientists during the last 50 years.Recently O'Dowdet al.have reported on sea‐salt particles of dry radius as small as 0.03 μm, with radius 0.06 μm at 80% relative humidity and mass 5 × 10−16g. If the above equation is extended to include such small particles, it predicts a total concentration of 60 cm−3in winds of 17 m s−1, fairly close to the reported 70 cm−3.In general these computed and observed concentrations of sea‐salt particles are adequate to account for observed droplet concentrations in clouds over remote oceans uncontaminated by continental aerosols. However, on rather rare occasions droplet concentrations in maritime cumulus exceed 100 cm−3. Observations are cited to suggest that, in such cases, sea‐salt nuclei may be augmented by biogenic particles originating at the sea surface and by sulphate/sulphuric acid nuclei produced by the absorption of derivatives of dimethyl sulphide in cloud droplets.Locations such as Hawaii and Tasmania are occasionally contaminated by continental or local pollution which may account for the rare r

ISSN:0035-9009    

DOI:10.1002/qj.49712757609

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractObservations of near‐surface variables are used with the Met Office Surface Exchange Scheme to calculate the exchange of heat and moisture between the surface and the atmosphere for a grass‐field site at the Met Office, Cardington, UK. Different methods for modelling the stability near the surface are investigated with the surface temperatures and turbulent heat fluxes compared with data collected from the site. The results show that, whilst using Monin‐Obukhov similarity theory can give good results in stable conditions, the form of the stability functions can significantly affect the accuracy in certain stability regimes. The use of log‐linear stability functions can cause the turbulent fluxes of heat and moisture to cut off at stabilities which are too close to neutral compared with the observations. The results also show that unrealistically high surface temperatures are obtained in clear‐sky nocturnal conditions with the standard surface energy balance. An alternative energy balance which includes a canopy of vegetation above the underlying soil gives better agreement with the observations in these c

ISSN:0035-9009    

DOI:10.1002/qj.49712757610

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractBaroclinic instability of a two‐level primitive equation model is represented by a dynamical system consisting of the four interacting fields of barotropic and baroclinic vorticity, divergence and temperature. The instability mechanism is illustrated in terms of a positive feedback loop with a succession of tendencies. It occurs at wavelengths longer than the short wave cut‐off, for which vortex stretching prevails over vertical temperature advection. Quasi‐geostrophy reduces the positive feedback loop to the interaction of the barotropic and baroclinic vorticity fields which are diagnostically linked to divergence and temperature by the omega equation and by geostrophy, respect

ISSN:0035-9009    

DOI:10.1002/qj.49712757611

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY