摘要:

AbstractThe global climatology of monthly heat storage and heat transport are presented using data from the Master Oceanographic Observations Data Set for the period 1949–1980. Heat storage and the rate of heat storage change are calculated for the Pacific, Atlantic and Indian Oceans. Heat transports on a monthly basis are calculated using surface energy flux data from a previous study together with the computed heat storage change. There is a large annual cycle in heat storage change at almost all latitudes with the largest amplitude in latitudes where the western boundary currents dominate. There is also a considerable annual cycle in the heat transport estimates. The heat transport in the Atlantic is northward most of the year except in January when there is significant southward transport. The largest northward transport is found at 5°N during March with a magnitude of about 2.0PW (1 PW =1015W). In the Pacific Ocean, northward transport dominates north of 20°N with a maximum of about 1.5PW at 25°N in July‐September. South of 20°N, there is significant southward transport in July through September with a maximum of 1.5PW at 10°S in August. Overall, the largest northward transport in the Pacific is about 2.1 PW in March at 10°N. Heat transport in the Indian Ocean is mostly southward with a maximum of about 2.5 PW in May at 20°S except for a small northward transport in January‐February north of 5°S. Although the uncertainties involved with the transport estimates are large, the transport values presented here compare rather well with direct estimates and with

ISSN:0035-9009    

DOI:10.1002/qj.49711548502

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe effects of the sea surface temperature (s.s.t.) front at the edge of the Gulf Stream on the marine atmospheric boundary layer (MABL) are investigated using a numerical model to study the modification effects of an oceanic front on the MABL structure. The situation simulated is flow from over cold shelf water to over the warm water of the Gulf Stream. The initial temperature and humidity profiles of the air are specified to be near neutral over the cold water and are therefore typical of undisturbed conditions. The differential in s.s.t. across the oceanic front creates a horizontal variation in the surface perturbation pressure and the stability. The surface perturbation pressure and turbulent fluxes modulate the flow and produce horizontal variations in horizontal wind components with associated vertical motions. A thermally direct cell is produced as a result of the s.s.t. difference across the front. The isotherms slope upward towards the warm water. Entrainment of inversion layer air and upward vertical motion over the warm water cause the MABL to be deeper there. A layer of cloud forms over warm water and is associated with mixed layer deepening rather than lowering of the condensation level. Turbulent fluxes in the MABL show considerable spatial variation. Surface stress is much larger over the front and over the warm water than over the cold water. This is mostly caused by wind speed changes associated with the front. Changes in the drag coefficient due to changes in surface roughness and stability are much less important.Mean budgets for temperature and total water indicate that there is a balance between horizontal advection and turbulent flux divergence. TheUmomentum budget shows that once the geostrophic balance terms are subtracted, the balance is mainly between the pressure gradient force associated with the induced temperature field and turbulent friction, with horizontal advection and the Coriolis force acting on the geostrophic departure playing minor roles. TheVmomentum budget shows a balance between horizontal advection, Coriolis force and friction.Although there are few data for comparison, the results are in qualitative agreement with observations in the area. This study shows that the s.s.t. front at the Gulf Stream edge produces marked local changes in the nearby atmospheric surface layer.

ISSN:0035-9009    

DOI:10.1002/qj.49711548503

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractIt is shown that both the steady and the vacillatory states of a two‐layer, quasi‐geostrophic, forced dissipative topographic model with a high spectral resolution can be truly intransitive. There exists only an odd number of equilibria in this geophysical system, as predicted by Benjamin's general theorem for the Navier‐Stokes equation. The overall findings constitute a generalization of previous results for a barotropic model.The analyses specifically establish for the case of a single‐wave topography that, in response to a barotropic forcing, there exist: (1) a low threshold value of the topographic height required for the existence of hysteresis: (2) a higher threshold value for the occurrence of weakly vacillating states with a high frequency; (3) a detuning effect of the nonlinearity due to an increase of the equivalent resonance frequency; (4) a baroclinization effect of an asymmetric friction; and (5) a robustness of the multiple equilibria for an additional weak baroclinic forcing.Under near‐resonance conditions of the topographic waves for the case of a simple multi‐wave topography, as defined according to the linear dynamics, none of the equilibria is stable. The strong wave‐wave interaction in such a situation sustains a dynamically different pronounced vacillation with a dominant long period in the order of tens of days. The existence of multiple vacillations with distinctly different structure and period is established. Outside that range of parametric conditions, there is only one stable equilibrium state.It is demonstrated that the time‐mean wavy flow of a decidedly nonlinear time‐dependent response can be reproduced, in a high degree of resemblance, by tuning the dissipation parameters in a counterpart linearized model that makes use of the ‘known’ mean zonal flow. It would be misleading, however, to conclude, on the basis of such an agreement, that the linear dynamics really accounts for the actual time‐mean wavy flow. This example serves to reveal the potential dangers in interpreting an observed mean flow as consisting of linear planetary waves on a pre‐determined zona

ISSN:0035-9009    

DOI:10.1002/qj.49711548504

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA nonlinear systems analysis is applied to the tracks of 249 tropical cyclones (with a six‐hour sampling time) in the Australian tropics for the period 1959–1980. First estimates are obtained of the degree of their chaotic, or irregular, behaviour. The degree of chaos is estimated by normalizing all trajectories to a common initial position and measuring the average rate at which initially close pairs of pieces of trajectories diverge.It was found from the correlation integrals calculated for the tropical cyclone tracks that the dimensionality of the underlying dynamical processes appears to be between six and eight, and that the time scale for e‐folding error growth is about one day. The results of this study therefore suggest that the movement of Australian region tropical cyclones is predictable deterministically up to about 24 hours in advance. Beyond that limit, consideration must be given to statistically based techniques.These findings were supported further by comparing the rate of growth of the observed Australian region tropical cyclone position variance with that derived from a random walk model superimposed on a mean drift. The correspondence was very close, with both the empirical and theoretical position variances growing linearly in time after approximately the first 18 to 24 hours, confirming that stochastic models have a role to play in forecasts beyond 24

ISSN:0035-9009    

DOI:10.1002/qj.49711548505

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractSince the disaster in The Netherlands caused by a mesoscale gale on 12 May 1983, an hourly nowcasting and very short‐range forecasting system has been developed using all available hourly surface observations of pressure and wind over the North Sea and the surrounding countries.Hourly analyses and prognoses are produced using hourly interpolated background fields and lateral boundaries from the fine mesh United Kingdom prognoses, which are received from Bracknell twice a day based on 00 and 12 GMT up to 36 hours in advance at 3‐hour intervals.The computational area consists of a grid of 35 × 35 points with a grid distance of 50 km covering the North Sea and surroundings. The update and forecast model is a two‐parameter model based on filtered equations with the geopotential height fields at 500 and 1000 mb as input and output. It is called VIMOLA (vertically integrated model on limited area). A new method is used for the analysis at 1000 mb. The observations of pressure and wind are directly assimilated onto the computational grid, and in this way wind measurements are converted into pressure gradients. All 10‐minute‐mean winds are taken into account. The analysis system is called APW (assimilation of pressure and wind).Every hour analyses and prognoses are produced up to nine hours in advance. The prognoses are merged with the fine mesh U.K. prognoses, i.e. the final prognoses are a linear combination of VIMOLA prognoses and U.K. prognoses. For one hour in advance no merging takes place and after nine hours the final prognoses are identical to the U.K. prognoses.For The Netherlands and surrounding regions winds computed from the forecasts have been verified against the observations. Winds from the U.K. prognoses have been verified in the same way. The U.K. prognoses show a very good quality and a small error growth during the first 24 hours. However, the hourly incorporation of the observations gives an improvement, especially for the first hours of the forecast period.Because no upper air observations are needed, a very short data cut‐off time can be applied (20–45 minutes after the time of observation). In practice, the system is suitable for operational gale warnings and tidal surge forecasts. Due to the hourly character, the system is able to detect temporal variations in the surface wind field such as the diurnal cycle over land and air mass variability. Practical examples are given in this paper, including some results obtained for the storm of 15–16 October 1987, which hit the south‐easte

ISSN:0035-9009    

DOI:10.1002/qj.49711548506

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractLaboratory experiments have been performed to study rime growth and its relation to secondary ice crystal production (the Hallett‐Mossop process). The rime was grown in a wind tunnel on a fixed vertical fibre 0.5 mm in diameter. The range of conditions extended from −1.0°C to −13°C in temperature, from 20cms−1to 200cms−1in air speed, and liquid water content up to 1.5 gm−3. The droplet size distributions were 5 to 25 μm and 8 to 50 μm, peaking at diameters of 9 and 22 μm respectively. Under conditions favourable for secondary ice production, the accreting droplets spread out on the ice surface; individual frozen droplets formed only at the lower temperatures. A separate experiment showed that supercooled water droplets froze as cones on a flat surface above −10.°C. At temperatures between −1. and −4.°C the spreading ratio of supercooled droplets on the basal surface of ice was less than on other faces, with lower molecular packing density. It is hypothesized that spreading takes place in a quasi‐liquid layer which is thinner on the basal surface. Splinter formation by pressure build‐up inside individual frozen droplets is therefore unlikely to be responsible for shatter; a new model is suggested in which thermal gradients give stresses leading to

ISSN:0035-9009    

DOI:10.1002/qj.49711548507

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe vertical structure equation is a singular Sturm‐Liouville problem whose eigenfunctions describe the vertical dependence of the normal modes of the primitive equations linearized about a given thermal profile. The eigenvalues give the equivalent depths of the modes. We study, for arbitrary thermal profiles, the spectrum of the vertical structure equation and the appropriateness of various upper boundary conditions.Our results depend critically upon whether or not the thermal profile is such that the basic state atmosphere is bounded. This is not surprising since, as we point out, the vertical structure equation is not meaningful at large heights because of the traditional shallowness approximations which are used to derive the primitive equations. The nature of the spectrum of a singular Sturm‐Liouville problem depends only on the behaviour of the coefficients of the differential equation near the singular boundary. Spectral results therefore have no physical significance for unbounded atmospheres.For all bounded atmospheres we show that the spectrum is totally discrete, regardless of details of the thermal profile. For the barotropic equivalent depth, which corresponds to the lowest eigenvalue, we obtain upper and lower bounds which depend only on the surface temperature and the atmosphere height. All eigenfunctions are bounded, but always have first derivatives which become unbounded near the top. We prove that the commonly invoked upper boundary condition that vertical velocity must vanish as pressure tends to zero, as well as a number of alternative conditions, are well posed.For unbounded atmospheres, on the other hand, we show that typically there is a continuous spectrum, that the boundary condition of vanishing vertical velocity is not well posed, and that the eigenfunctions. if any. are unboun

ISSN:0035-9009    

DOI:10.1002/qj.49711548508

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA formal BBGKY kinetic theory for inhomogeneous 2‐D fluid turbulence is developed from the Liouville equation for Fourier mode amplitudes. The analysis indicates that all members of the hierarchy are present except in the special case of unforced barotropic flow where only the two‐coefficient distributions are explicitly absent. A graphical representation of the BBGKY hierarchy is presented, providing a vivid picture of the interaction processes involved. Equations representing the turbulence dynamics of correlations are derived for binary‐ and triad‐wave groups and exhibit a highly nonlinear structure. It is shown that, in the inhomogeneous case, binary‐wave interaction via the mean flow plays an important role, in contrast with accepted homogeneous theories. A phenomenological approximation belonging to the eddy‐damped class of turbulence models is developed and yields a set of kinetic equations for which conservation laws and an H‐theorem can be established. The kinetic equations can be recast into a familiar Fokker‐Planck form written in intrinsic coordinates and demonstrate the approach to absolute equilibrium for the case of forced barotropic inhomogeneous flow. A certain information measure is shown to be a maximum att= 0 for the exact dynamics and suggests that forcing gives rise to an increased rate at which info

ISSN:0035-9009    

DOI:10.1002/qj.49711548509

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA filtered system of equations is derived, using the ideas of normal mode initialization. Thisslow equationsystem models the low frequency rotational atmospheric motions; there are no solutions corresponding to gravity waves. The prognostic element of the system is an equation expressing the conservation of potential vorticity. The slow equations differ from the general balance system at second order in the Rossby number, and are free from the spurious solutions found in that system. Integration of a barotropic model with the slow equations shows them to be highly accurate when compared with the primitive equations. Since the slow equation model has no high frequency solutions and is free from data shock, it may be useful for continuous assimilation of observational data.

ISSN:0035-9009    

DOI:10.1002/qj.49711548510

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49711548512

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY