摘要:

AbstractA modified Eady channel model with oppositely sloping top and bottom Ekman layers is used to examine analytically and numerically the effect of the slope, Δ, with dissipation, δ, on baroclinic flows in linear and nonlinear systems. the spectral wave solution is truncated up to six components.In the linear system, when δ = 0, waves are dispersive and move westward owing to Δ. the maximum value of Δ for which unstable waves may exist, ΔCmax, is equal to the slope of the isentropes of the basic state. Δ exhibits only a weak stabilizing influence on short waves, while exhibiting a strong stabilizing influence on long waves when it exceeds a critical value. When δ ≠ = 0, a small δ has a destabilizing effect on short waves by shifting the short‐wave cut‐off in the shorter wave direction and extending Δcmaxto a larger value. However, the combined effect of Δ and δ strongly stabilizes long waves. the most unstable wavenumber may be shifted to a higher one by increasing Δ, in both inviscid and viscous cases. the relation between the most unstable wavenumber and Δ is similar to some annulus experiments with oppositely sloping boundaries. It is also similar to the relation between the most unstable wavenumber and β in β‐plane models.In a nonlinear system without wave‐wave interaction, Δ stabilizes the flow even for small δ and reduces the domain of vacillation and aperiodic flow regimes, while enlarging the domain of single‐wave steady state. It also qualitatively agrees with the results from β‐plane dissipative systems. Within the single‐wave steady state regime, the preferred nonlinear wavenumber may also be shifted to a higher one by increasing Δ. When wave‐wave interaction is allowed, a variety of flow regimes is observed in four truncated systems. There are single‐wave steady states, multi‐wave steady states where wave dispersion or structural vacillation may occur, multi‐wave vacillation, and aperiodic flows. In contrast to the system without wave‐wave interaction, the domain of vacillation is enlarged. A ‘frequency‐locking’ mechanism is found in multi‐wave steady states, where all three conditions

ISSN:0035-9009    

DOI:10.1002/qj.49711649102

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractRegimes of the northern extratropical circulation in winter are identified in this paper as clusters of atmospheric states in a low‐dimensional phase space generated by the leading EOFs of eddy geopotential fields. In order to define the clusters, our algorithm seeks points corresponding to local maxima for the density of atmospheric states; subsequently, a cluster is defined around each density maximum as that portion of the phase space in which the observed density can be locally approximated by a unimodal function. Two analyses were performed, using a 5‐dimensional and a 3‐dimensional space respectively, and they provided consistent results. Six clusters were found. the largest cluster includes 40% of the fields in our sample; its centroid is close to the climatological winter state, but it possesses a positive projection on the Pacific‐North American (PNA) pattern. the other five clusters represent anomalous flow regimes and include 52% of the fields. One of them shows a low amplitude of the planetary waves; the remaining four represent states with large wave amplitude but different phases. the variability between clusters accounts for the bimodality in the amplitude of planetary waves detected in previous observational studies. Our analysis reveals that this bimodality is much enhanced in the region of the phase space where the PNA index is negative, and the separation among the clusters is stronger. Finally, frequencies of transitions between clusters are presented, which show an asymmetric behaviour in the transitions between regimes with low and high amplitude of planetar

ISSN:0035-9009    

DOI:10.1002/qj.49711649103

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractAn attempt is made to simulate two general‐circulation‐model‐produced February climatologies in the tropics (from the GISS and the LMD GCMs), using a linear steady‐state multi‐layer primitive equations model which is forced thermally. A hypothesis‐testing strategy is used to quantify the simulations of the target climatologies taking into account the natural variability of the GCM fields. When the zonal‐mean flow and the diabatic heating field are prescribed from the GCM, the linear model is found to be generally consistent with both GCMs at the 5% level of statistical significance. the linear model simulation of the GCM is most successful for the vertical velocity. the horizontal wind field is moderately well simulated at low and high levels, but poorly in the mid troposphere. More than 40% of the spatial variance at 850 and 950mb between 30°S and 15°N is explained in the LMD GCM climatology whereas the corresponding figure for the GISS climatology is 34% at 850mb, falling off toward the surface. the linear model's skill at simulating the LMD climatology near the ground is little diminished when the forcing is restricted to the trade‐cumulus layer only. However, midtropospheric heating is found to be important for the zonal component of the flow at low levels, especially close to the equator. Sensitivity, especially at upper levels, was found to the inclusion of the zonal component of the mean wind in the linearization, but little to the mean meridional circulation. Sensitivity to the linear model dissipation

ISSN:0035-9009    

DOI:10.1002/qj.49711649104

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractLagged‐average forecast experiments, produced with the Meteorological Office 11‐level general circulation model, are used to assess the benefits arising from the use of ensemble forecasts in practical monthly prediction.As measured by anomaly correlation, forecast skill is consistently positive at days 6‐15, but very small on average beyond 20 days. For the monthly mean forecast, removal of an independent estimate of the model systematic error improves the average score significantly. Compared to the mean score amongst individual members, a modest increase in skill is achieved by ensemble averaging. Theoretically, however, there remains scope for a much greater impact in the future, through the use of models capable of more accurate forecasts.Local forecast skill is correlated with a measure of the agreement between ensemble members, although the overall relationship is weak beyond the medium range. Nevertheless, for areas of high agreement, the probability of obtaining useful skill is considerably higher than average at days 6‐15 and 11‐20.The experiments are also assessed as probability forecasts, using the Ranked Probability Score to measure skill. the ensemble probability forecast (EPF) is created by averaging the probability forecasts of the individual members. Relative to the average score for the latter, the EPF gives improved skill. For the medium‐range period of the forecast (days 1‐10), the use of an optimized EPF, calculated by weighting each ensemble member according to its age, leads to a small increase in skill compared to the individual forecast from the most recent analysis. This effect is not observed in the corresponding anomaly correlation results. the above results for local skill prediction are confirmed, and somewhat enhanced, in the probabilistic formulation, using the ensemble variance of forecast categories as the predictor. A simple method of issuing confidence‐based adjustments to the forecast probabiliti

ISSN:0035-9009    

DOI:10.1002/qj.49711649105

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractCurrently no theory of the stably stratified atmospheric boundary layer (SBL) is generally accepted as definitive even for idealized cases. Nieuwstadt's theory, though a promising candidate, faces objections relating to upper boundary and steady‐state conditions, internal wave effects and the consistency of the model outside the strong‐stability limit. the aim of this paper is to examine the objections, improve the model, draw further deductions and compare with numerical models.We shall deduce from Nieuwstadt's model thatBo=RfcG2|f|/√3, whereBois the surface buoyancy‐flux,Gthe geostrophic wind speed,fthe Coriolis parameter andRfcthe critical value of the flux Richardson numberRf. Higher‐order expansion shows this value is an upper bound corresponding to the stable limitL/h→ 0. This is consistent with a similar bound onBoderived from independent energy arguments. By contrast, within present idealizations there is no bound on the surface cooling rate.Comparisons with a new series of large eddy simulations (LES) and other numerical models support the interpretation of Nieuwstadt's SBL as an idealized limiting case. the theory explains from first principles the observed sensitivity to small slopes. Interaction between sharp inversions and slopes may cause turbulence in thin layers. Coupling with the surface boundary conditions is a likely cause of intermittent turbulence, and explains features of Brost and Wyngaard's second‐order closure study. the formal singularity at the top of the SBL gives time‐scales for approach to inertial and heat equilibrium. Both, for separate reasons, are O(|f|−1).A natural extension to moderately stable and near‐neutral conditions agrees well with numerical model results, and provides a complete prediction of SBL structure from given surface heat flux and synoptic pressure gradient. the near‐neutral regime is narrow at high Rossby number. Comparison with LES supports both the local scaling approach and the gross predictions of the theory. the model gives insight into the limitations of Rossby‐similarity formulae. In particular, restrictions of domain sizehd(or similar background stability effects) may not become negligible even whenh/hd→ 0. Hence matching to neutral conditions is important in predicting even quite stable boundary layers. Wave effects, though non‐local in some respects, do not fundamentally change the Nieuwstadt picture for the mean structure. Retention in the model of the original value forRfcis recommended even if wave radiation perturbs localRf.In summary, the adapted Nieuwstadt theory seems to provide a definitive framework for the idealized SBL, from which other ‘perturbation’ effects may be assessed. the value ofBo/G2|f| gives a criterion for (a) quasi‐stead

ISSN:0035-9009    

DOI:10.1002/qj.49711649106

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractTurbulence data collected in an area of complex terrain using a number of instrument packages attached to the tether cable of a captive balloon are presented. Eddy correlation measurements of the turbulent momentum flux are used to determine the magnitude of the roughness length from the drag coefficient and also to show that the roughness length is a useful concept in parametrizing the surface stress in such areas. the relationship between the terrain characteristics and the roughness length is examined using a simple formula, results from a numerical model and previous experimental estimates of the roughness length for complex terrain. It is found that a formula based on numerical model results for separated flows gives reasonable predictions and that the measured roughness length is representative of an area of order 100 km2.Non‐dimensionalized turbulence data are compared with similar data obtained over flat homogeneous surfaces. This comparison suggests that the structure of the turbulent boundary layer over complex terrain is remarkably similar to that over flat homogeneous surfaces. Because of the limitations of fixed point measurements in complex terrain, results from a numerical study of turbulent flow over two‐dimensional sinusoidal orography are shown to indicate the magnitudes of the perturbations introduced into the boundary‐layer flow by large undulations in the underlying surface. the model results demonstrate the validity of the analysis carried out with the

ISSN:0035-9009    

DOI:10.1002/qj.49711649107

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractA model of near‐neutral and convective steady‐state internal boundary layer evolution is presented. the model deals with the internal boundary layer that forms over land in coastal and lake areas during onshore winds. Near the ground, the growth of the internal boundary layer is controlled by friction velocity in accordance with surface layer theory. Farther downwind, the growth is determined by the atmospheric stability and friction velocity within the internal boundary layer, and the temperature gradient in the air above. the wind profile inside the internal boundary layer is assumed to follow Obukhov similarity theory. an expression for the strength of the inversion that caps the layer is derived and used in the model. A comparison is carried out with independent experimental observations of internal boundary layer growth in the sea‐land transition.Kinematic heat flux through the top of the internal boundary layer is described by the formulation −\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}h= 0.2\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}s+ 2.5u *3T/gh+ the Zilitinkevich correction. the terms 0.2\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}sand 2.5u *3T/ghaccount for convective and mechanical turbulence, respectively, and the Zilitinkevich correction is a turbulent kinetic‐energy storage term that ensures finite growth rate of the internal boundary layer near the ground. the relative importance of mechanical and convective turbulence as well as the Zilitinkevich correction is discussed. the Zilitinkevich correction dominates the growth process of the internal boundary layer when it is lower than roughly 50m. As the layer grows, the importance of the Zilitinkevich correction diminishes. Then mechanical turbulence dominates the growth process until the internal boundary layer has reached a height of approximately −1.4L. Further growth is controlled mainly by convective turbulence. Conditions of high wind speed and large values of the potential temperature gradient over water result in a deep zone where the growth of the internal boundary layer is controlled by mechanical turbulence. With near‐zero potential temperature gradient over water, the zone becomes shall

ISSN:0035-9009    

DOI:10.1002/qj.49711649108

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe outgoing long‐wave (OLR) and net solar (NSR) radiation at the top of the atmosphere, derived from the NOAA series satellites, are compared with those measured by the wide‐field‐of‐view Earth Radiation Budget (ERB) radiometers aboard Nimbus 7. For selected months between 1979 and 1983 important regional differences are found for both the OLR and NSR between the ERB observations and the NOAA, which have been spatially smoothed to approximate the resolution of the former. Knowledge of the systematic differences is utilized to derive linear regression equations, which adjust the smoothed NOAA data to match as well as possible the ERB measurements. These adjustment equations reduce the mean squared difference between the NOAA and ERB data for independent samples, not used in the statistical model development, by approximately 50% for both OLR and NSR. the adjustment equations are applied to the 2.5° NOAA data for June 1974 through May 1988. Long‐term zonal averages of the annual and monthly radiation budget at the top of the atmosphere are displayed and discussed. the adjusted data budgets are much more realistic than those derived from unadjusted data and suggest physical features not clearly evident in the previously published analyses based upon lower resol

ISSN:0035-9009    

DOI:10.1002/qj.49711649109

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractA statistical analysis of representative sets of rainfall data in the west Sahel region is performed and a jump transition in the late sixties is detected. It is suggested that the main features associated with the record can be accounted for by a fluctuation‐induced transition between two stable precipitation regimes: a ‘humid’ and a ‘dry’ state. A minimal model involving the dominant nonlinearities is constructed and its parameters determined from the data. an attempt is made to arrive at a statistical prediction of the duration of the alternate regimes through the relative stability of the two stable states as deduced from mean values and higher moments of residence times. the behaviour predicted by the model is in good qualitative agreement with t

ISSN:0035-9009    

DOI:10.1002/qj.49711649110

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractDirect field comparisons of precipitation gauges (PG) are difficult and costly. For this reason the development of correction models such as Folland's, based on wind tunnel tests, is very valuable. This note draws attention to some new wind tunnel data on the airflow over several types of gauge. the new data are in much better agreement with those of Robinson and Rodda (1969) than with those of Green and Helliwell (1972) supporting Folland's preference for the former data. In addition, the new data provide support for Folland's assumption that the airflows over the 5‐inch and Hellmann gauges are similar, though this appears to be more or less accidenta

ISSN:0035-9009    

DOI:10.1002/qj.49711649111

出版商:John Wiley&Sons, Ltd    

年代:1990

数据来源: WILEY