摘要:

AbstractGreenland is about the same size as the plateau of Tibet. It is, by contrast, a cold source and a complete obstruction to the advance of air at sea level. Air flowing from the ice plateau originates at around plateau level. The general slope towards the north and west causes a katabatic land breeze on the west coast. The so‐called Iceland low in its movements from Cape Farewell more than half way to Spitzbergen causes a prevailing wind from the north or north‐east along the lee coast which drives ice as far as Cape Farewell all year round. The earth's rotation causes the ocean current to turn the cape and occasionally carry ice 100‐200 km up the west coast. But for most of the year the wind is from the north while the salty water moves from the south.When cirrus moves inland it is not accompanied by air at sea level and when air descends from the plateau the flow at cirrus level is usually upslope so that a trail of orographic cirrus leaves the coast.The Iceland low frequently carries warm air across the Faeroes and Jan Mayen to the east coast at Scoresbysund and even to the west of Iceland and at the same time draws arctic air from the Davis Strait across the Atlantic to Britain to form an inverted front on which a row of small cyclones forms.The general picture formed from looking at about nine years of satellite pictures is that the weather goes on around Greenland and is steered by it, but only occasionally penetrates over the plateau and then neither far nor for long. While Spitzbergen, Iceland, and other territories around are some of the cloudiest places on earth, Greenland is a very sunny place, and yet it remains a very significant source of

ISSN:0035-9009    

DOI:10.1002/qj.49711447902

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe feasibility of the implicit (parametrized) compared to the explicit (grid‐resolved) approach for meso‐β‐scale models is examined using two different mesoscale precipitating weather systems (MPWSs): a squall line and a mesoscale convective complex (MCC) with an embedded meso‐vortex, that were responsible for the 1977 Johnstown, Pennsylvania, flood events. The prognostic explicit scheme contains predictive equations for cloud water and rainwater incorporating liquid water evaporation and hydrostatic water loading, and it is tested with grid sizes of 25 and 12‐5 km, respectively. Due to a near‐saturated and moist adiabatic initial environment associated with the MPWSs, there is no notable delay in the initiation of the grid‐box saturation. It is found that the explicit convective scheme fails to reproduce the convective precipitation related to the squall line and MCC, while it overpredicts the stratiform rainfall associated with the meso‐vortex. On the other hand, the implicit convective scheme reproduces the size, orientation and evolution of the squall line, but totally misses the meso‐vortex. A semi‐implicit control simulation (implicit plus diagnostic explicit) reproduces very well both the squall line and MCC. The results indicate that even with a grid resolution on the order of 10 km, an implicit convective scheme is very important for the numerical prediction of convectively driven precipitating systems.The roles of different model physics in controlling the frequent model ‘blow‐ups’ are also examined. It is found that if the semi‐implicit or the explicit convective scheme is used alone, the model sustains different degrees of overdevelopment of the mesocyclogenesis due to the CISK‐like feedback process among latent heat release, larger‐scale moisture convergence and the surface pressure fall. Model simulation with the diagnostic explicit scheme is the most affected by this instability, followed by the prognostic explicit scheme. The parametrized convection tends to reduce or eliminate this problem by elevating the heating and moistening maximum and removing moisture that otherwise would be used for explicit condensation. The liquid water evaporation is very effective in an unsaturated environment whereas the hydrostatic water loading has a significant retarding effect under saturated conditions.Intercomparisons of six experimental simulations in addition to the control run indicate that the combination of the implicit and prognostic explicit schemes (full physics) is superior in reproducing the basic sequence of convective and stratiform rainfall and the extent of the mesocyclogenesis associated with the Johnstown MPWSs. Because of their different roles in handling various types and scales of precipitation, the full physics approach appears to allow a broader scale interaction between the parametrized convection and mesoscale environment and to have the greatest potential success in predicting MPWSs that occur under

ISSN:0035-9009    

DOI:10.1002/qj.49711447903

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA detailed synoptic and mesoscale analysis is conducted of an intense convective outbreak which struck much of Israel, Jordan, Lebanon and Syria in early March 1980. Severe winter weather caused considerable personal hardship and economic disruption over a wide area.The planetary‐scale circulation featured a confluent flow pattern across North Africa and southern Europe downstream of a block just west of the Greenwich meridian. A strong subtropical jet stream was maintained across North Africa in the confluent flow regime. Synoptic‐scale disturbances from the Atlantic were diverted northward across Scandinavia from where they plunged south‐eastward toward the Mediterranean to the west of the Black Sea. A series of three disturbances, the last being the most powerful, swept through the eastern Mediterranean as the vortex settled south‐eastward toward the Middle East.Our analysis of the available evidence has established that three physical mechanisms contributed primarily to the extreme nature of the event. First, the steady south‐eastward movement of mobile short wave troughs resulted in the progressive cooling of the lower and middle troposphere over the eastern Mediterranean Sea and the Middle East. Second, sea surface heat and moisture fluxes contributed to destabilization and the generation of extreme instability where the cold air from higher latitudes had a long overwater fetch. Third, exceptionally heavy precipitation fell where a strong low‐level moist, unstable westerly air stream impinged on the coastal mountains. The strong westerly flow was generated to the south of an intense subsynoptic‐scale vortex that formed in the lower and mid troposphere as the updraught region of a powerful short‐wave‐trough/ jet‐streak system aloft ingested air that had been

ISSN:0035-9009    

DOI:10.1002/qj.49711447904

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractSeveral studies have linked a wintertime cycle of deep, moist near‐equatorial convection to surges of cold air from Asia. According to an early hypothesis two successive streams of surge air reach north Borneo and offshore waters of the South China Sea. The first, which follows a long over‐water trajectory, is warm by the time it arrives off Borneo. The second, which follows a short over‐water trajectory, still is cool when it arrives off Borneo. More recent hypotheses suggest only the second stage of the surge involves an airstream from Asia and drying rather than cooling is the predominant downstream thermodynamic effect on this airstream.By means of trajectory calculations and an airmass transformation (AMT) model we test these ideas for a moderate cold surge from December 1978. In particular, we contrast the roles, in the transformation process, of surface fluxes of latent and sensible heat and of boundary layer entrainment. At no time through the course of the surge did we find boundary layer trajectories of the second (short path) type. Nevertheless, late in the course of the case study surge dry air did appear at the surface off the south‐east coast of Vietnam. Neither sea surface temperature nor initial airmass structure could account for the local nature of the drying. The explanation most consistent both with observations and AMT model results is entrainment of air that had been dried by local subsidence at and above the top of the marine boundar

ISSN:0035-9009    

DOI:10.1002/qj.49711447905

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe main aim of this study is to establish the ability of a simple parametrization to represent adequately the microphysical processes acting in frontal rainbands. In order to do this we compare it with a more detailed parametrization in conditions typical of warm frontal and narrow cold frontal rainbands. Simulations using a 2‐dimensional diagnostic model indicate that a mixed phase parametrization is required in dynamical models of high resolutio

ISSN:0035-9009    

DOI:10.1002/qj.49711447906

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAircraft measurements of broad‐band solar irradiances together with radiance measurements in the 1‐2 to 2‐1 μm region are presented for a stratocumulus sheet. Measurements of the solar albedo and cloud physics parameters indicate that the cloud was very uniform and optically thick. Broad‐band observations of albedo agreed well with calculations but observed absorption was about double the model values. Narrow‐band observations of vertical reflectance at three wavelengths suggested a marked increase in effective cloud droplet radius along the cloud, which was not observed. Some evidence that the discrepancy might be due to cloud top unevenness is

ISSN:0035-9009    

DOI:10.1002/qj.49711447907

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAircraft measurements of broad‐band infrared and solar irradiances together with radiance measurements at discrete angles and wavelengths are used, combined with microphysical observations, to investigate the optical properties of cirrus cloud.Results are found to be consistent with approximations appropriate to large particles with strong absorption in the thermal infrared. The observed angular scattering pattern is compared with other limited observational results and found to be consistent with them. Implications for predicting cloud albedo are discussed. An effective radius for the ice particles is also defined to enable solar absorption within the cloud to be predicted. Observations in the near‐infrared are shown to be consistent with this definition although the agreement is dependent on a number of assumpti

ISSN:0035-9009    

DOI:10.1002/qj.49711447908

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractWith a view to improving climate prediction, we test various models for estimating solar radiation at sea from standard meteorological surface observations against long series of radiation measurements at several Ocean Weather Stations and at an offshore island meteorological station. The widely‐used Budyko formula is found to err by up to 32% in estimating long‐term mean insolation, and has even larger errors in reproducing seasonal variations. Various models which estimate solar radiation from solar elevation and from hourly cloud amount and type, using empirical or simple physical formulae, are evaluated. None of these models estimate mean insolation better than an existing formula which relies only on noon solar elevation and daily mean cloud amount, although the hourly models do reproduce seasonal, monthly and daily variations better. None of the formulae using standard surface observations is able to achieve the 10 Wm−2accuracy in long‐term mean insolation which is required for climate pre

ISSN:0035-9009    

DOI:10.1002/qj.49711447909

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThis paper is a study of the transport properties of large‐scale quasi‐geostrophic flow, when it is forced by an unstable shear of much greater horizontal scale than the scale of the resultant eddies. One of the purposes is thereby to test certain phenomenological parametrization schemes which predict the transport of potential vorticity or heat, or which predict the time‐mean state of the zonally averaged atmosphere. Models used are highly nonlinear spectral quasi‐geostrophic models, with sufficient resolution to resolve the energy‐containing scales and partially resolve an enstrophy inertial range. A scale separation is enforced between the mean and eddy flow, this being a necessary condition for transfer theories to work. An expression is derived for the rate of change of the mean flow in such cases. As a quantitative predictor of the time‐mean state of the zonal flow, baroclinic adjustment is found to work only when nonlinear wave‐wave interaction between eddies is small. In such cases, the amplitude of the eddy flow is determined by the forcing on the zonal flow, which itself equilibrates close to the critical value for linear instability. In fully nonlinear models if the zonal flow is strongly forced the mean flow can be highly supercritical. The flux of potential vorticity is found to depend strongly and monotonically on the mean shear, although no obvious simple relationship is found relating it to potential vorticity gradient. In part because potential vorticity is not a passive scalar, using phenomenological transfer coefficients is by no means straightforward. Such transfer relationships are seen to be in some ways u

ISSN:0035-9009    

DOI:10.1002/qj.49711447910

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractSome practical problems in objective analysis for numerical weather prediction are best solved by using nonlinear analysis equations. They include the existence of nonlinear prior constraints on the analysis, and the use of observations which are nonlinearly related to the analysis parameters, or which have non‐Gaussian error distributions. Bayesian methods are used to derive equations for the optimal (maximum likelihood) nonlinear analysis. It is shown how to incorporate a strong constraint that the four‐dimensional evolution of the analysis should be consistent with a NWP model, by reduction of the control variable to the space‐dimensioned initial field for the model. The iterative solution of the nonlinear analysis equation then involves the integration of the NWP model, and its adjoint.The behaviour of the nonlinear equations is demonstrated with a simple one‐dimensional shallow‐water model. It is shown that time‐tendency information, and indirect observations such as wind speed, or the movement of a tracer, can be used in the analysis. The resulting forecasts are better than those made from an analysis from a traditional analysis‐forecast cycle. The nonlinear method is shown to be capable of ‘moving’ a discontinuity similar to a front, to fit observations defining its position, thus giving an analysis with more detail than would be expected from the spatial resolution of the observations. The incorporation of additional nonlinear constraints, such as that used in initialization, is demonstrated. The method can be used to effectively reject observations with gross errors, by specifying a non‐Gaussian error distribution. However, this generates multiple minima which complicate the search for the best analysis, so the complex decision‐taking algorithms associated with other methods of quality control are not avoided.The convergence properties of iterative methods of solution, and approximations to the ideal equations, are studied, in order to provide some indication as to whether the nonlinear effects might be allowed for in a prac

ISSN:0035-9009    

DOI:10.1002/qj.49711447911

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY