摘要:

AbstractThis study investigates the variability and predictability of snow depth anomalies over the Eurasian continent at the end of winter, as represented in 12 ensembles of General Circulation Model simulations performed at the European Centre for Medium‐Range Weather Forecasts. Each ensemble includes nine integrations performed with the same prescribed sea surface temperature, but started from time‐lagged initial conditions. An empirical orthogonal function (EOF) analysis shows that the leading EOF of Eurasian snow depth in March has a zonally‐oriented dipole structure, with a band of positive anomalies covering northern Europe and Siberia, and negative anomalies over central Europe, the Himalayas and north China. A significant relationship is found between the positive/negative phase of this snow‐depth anomaly and warm/cold El Niño Southern Oscillation events. The positive phase of the snow‐depth EOF1 is associated with a wintertime circulation characterized by a strengthening of the westerly winds over Europe and Siberia; in the upper troposphere, this westerly anomaly is accompanied by negative zonal wind anomalies over Eurasia around 30‐40°N and positive zonal wind anomalies between the equator and 25°N over Africa and south‐east Asia. A good degree of predictability is found in the snow‐related circulation anomalies; considering 500‐hPa height, 850‐hPa zonal wind and 200‐hPa zonal wind, the interannual variations of the ensemble‐mean fields show a correlation of 48%, 56% and 65% (respectively) with the corresponding observed anomalies over the eastern half (0° to 180° E) of the northern hemisphere. The tropical component of the zonal wind anomaly associated with snow‐depth EOF1 is strongly predictable; it shows a marked persistence from winter to the early summer, and affects the large‐scale circulation over south Asia in the early and centr

ISSN:0035-9009    

DOI:10.1002/qj.49712656212

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractWe investigate the dynamics of an isentropic layer of air, governed by the hydrostatic primitive equations. In such a layer the horizontal velocity is independent of height if this is assumed to be the case at some initial time. From the mass‐conservation equation in pressure coordinates it follows that ω, the material derivative of the pressure, varies linearly with pressure. From the mass‐conservation equation in height coordinates the vertical velocity ω is obtained. The resulting expression is shown to be in accordance with Richardson's equation for the vertical velocity in a general hydrostatic atmosphere, when particularized to an isentropic

ISSN:0035-9009    

DOI:10.1002/qj.49712656213

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA primitive‐equation global model of the middle atmosphere is used to investigate the response of the extratropical stratosphere to different levels of wave forcing from steady perturbations of the geopotential height near the tropopause. The response of the stratosphere is compared to that in quasi‐geostrophic beta‐plane models used in previous studies.The primitive‐equation model exhibits three flow regimes under perpetual‐January conditions: strong westerly, steady flow for small wave‐amplitude forcing, strong westerly but unsteady flow for moderate wave‐amplitude forcing and oscillations between easterly and westerly flow for large wave‐amplitude forcing. The regimes for low and high forcing are analogous to solutions of the simpler Holton‐Mass (HM) quasi‐geostrophic model. The moderate‐forcing regime does not occur in the HM model and it is attributed to instability of the strongly sheared flow generated by planetary waves in the upper stratosphere. We also show how the observed patterns of interannual variability in the winter stratosphere can be explained in terms of these three flow regimes: in the northern hemisphere the flow often enters the high‐forcing regime, where variations in conditions in the early‐winter flow or quasi‐steady upper‐tropospheric planetary‐wave amplitudes make similar contributions to the interannual variability in the stratospheric circulation. For the southern hemisphere, we suggest that the flow alternates between the low‐ and moderate‐forcing regimes through year‐to‐year changes in the amplitude of quasi‐steady waves near the tropopause. This mechanism produces large enough changes to explain the interannual

ISSN:0035-9009    

DOI:10.1002/qj.49712656214

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThis paper systematically investigates differences between the thermal and the dynamical tropopause for upper‐tropospheric balanced flow anomalies. Idealized cyclonic and anticyclonic anomalies are considered, which are either axisymmetric or plane symmetric. Given a distribution of potential vorticity (PV), the inversion problem is solved numerically to obtain the corresponding balanced flow (i.e. wind and temperature). The control parameter is the aspect ratio of the PV anomaly, which governs the partitioning into a thermal and a dynamical anomaly. For PV anomalies of intermediate and tall aspect ratios, the location of the thermal tropopause differs significantly from the location of the dynamical tropopause. The thermal tropopause is rather indistinct for intermediate aspect ratios, while it is sharp and well defined for both tall and shallow anomalies. A barotropic deformation flow field superimposed on a plane symmetric anomaly induces an ageostrophic wind which modifies the static stability throughout the PV anomaly such that the thermal and dynamical tropopauses evolve differently. Recent observations concerning the correlation between the thermal and ozone tropopauses can be interpreted consistently in terms of the present result

ISSN:0035-9009    

DOI:10.1002/qj.49712656215

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractSince the stratosphere lies above the troposphere, naive expectation would associate mass transfer from the latter into the former with ascent. This note shows that, in the context of the mass circulation associated with large scales in mid latitudes, the transfer into the stratosphere can actually be associated with descent relative to isentropes. The diabatic effects which accompany the transfer of air from the troposphere to the stratosphere are such as to give the air a downward motion, but, the motion of the tropopause is faster so that the descending air is overtaken by the tropopause and transferred into the stratosphere. Combined with the conclusions of an earlier study, these results suggest that the dryness of a substantial portion of the air in the lowermost extra‐tropical stratosphere is caused by its passage through extremely low tropopause temperatures associated with transient mid‐latitude high‐pressure sy

ISSN:0035-9009    

DOI:10.1002/qj.49712656216

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractBayes's theorem is applied to the problem of analysing temperature and moisture in a volume of air given a single observation of precipitation amount, utilizing a model of non‐convective precipitation and prior estimates of the fields. Results using different statistics and shapes of probability distributions are examined. These include normal, truncated normal, and log normal distributions with special treatment of the value zero. The uncertainly of the model's formulation is considered in addition to uncertainty of observations. The posterior distribution is multi‐modal due to the model's formulation using a conditional expression. The dominant mode may be predicted as a non‐precipitating slate by the model, although the observation indicates precipitation is present. Means and modes of posterior distributions depend sensitively both on the assumed statistics and the shapes of the underlying distributions. The results suggest that the usual minimization of a cost‐function should not be used cavalierly to assimilate precipitation obser

ISSN:0035-9009    

DOI:10.1002/qj.49712656217

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe importance of an accurate specification of short‐range forecast‐error structures for the successful assimilation of satellite sounding data is discussed, with special emphasis on the role of vertical (inter‐level) error correlations. Two different statistical approaches for estimating these correlations are examined. The first is based on comparing radiosonde profiles with the short‐range forecast. The second compares forecasts of different lengths which verify at the same time. It will be shown that, in some aspects, the two approaches produce consistent results which suggests that significant geographic and seasonal variability exists in the vertical error correlations. The effect of not modelling this variability in the analysis of TOVS (TIROS‐N (Television Infra‐Red Observation Satellite) Operational Vertical Sounder) radiance data is simulated using a linear retrieval estimator, and it is shown that it may result in a serious misinterpretation of the information in the o

ISSN:0035-9009    

DOI:10.1002/qj.49712656218

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712656219

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712656201

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY