摘要:

AbstractPrevious studies and different methods of estimating short‐range forecast errors are summarized. Zonally and temporally averaged statistics based on differences of one‐ and two‐day forecasts valid at the same time are presented and an attempt is made to explain many of the features by reference to dynamical concepts.Vertical correlation length‐scale tends to increase with horizontal correlation scale but to be very short in the Tropics; horizontal scale is longest in the Tropics and in the stratosphere. The variations in vertical correlation are much more pronounced for largely balanced variables such as rotational wind and temperature than they are for divergent wind or humidity. The extratropics are dominated by an equivalent barotropic mode with the level of the maximum wind amplitude (and the zero crossing of the temperature correlation) being determined by the tropopause. Surface pressure is negatively correlated with low‐level temperature as expected (except over the Antarctic plateau where it is positively correlated); it is also negatively correlated with temperatures near/above the tropopause in the extratropics.The covariance model used in The Met. Office Global Three‐Dimensional Variational (3D‐Var) Data Assimilation system represents the variation of vertical covariances with latitude reasonably well, but the longer horizontal scales in the stratosphere are not currently reproduced. The implied covariances used operationally have been modified so that the correlation length‐scales, both horizontal and vertical, are somewhat shorter than those direct from the forecast differences. Recent changes to the representation are briefly described, with an indication of their impact on the forecasts. The impacts are significant relative to other changes tested, and the covariance model has played a major role in the successful implementation and subsequent improvement of ou

ISSN:0035-9009    

DOI:10.1002/qj.49712757112

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe Singular Evolutive Extended Kalman (SEEK) filter introduced by Phamet al.is applied to a primitive‐equation model in order to reconstruct the mesoscale circulation typical of the mid‐latitude ocean from altimetric data. The SEEK filter is a variant of the Kalman‐filter algorithm based on two concepts: the order reduction of the initial‐error covariance matrix, and the dynamical evolution of the reduced‐order basis. This makes the method potentially suitable for problems with a high number of degrees of freedom.Previous work has shown the ability of a steady version of the filter to improve the vertical structure of the ocean thermocline in the case of the quasi‐linear dynamics associated with the equatorial tropical Pacific Ocean, and the need to combine the dynamical evolution of the basis with an adaptive scheme in a mid‐latitude ocean model of the Gulf Stream region.This work examines the potential advantages of the dynamical evolution of the basis functions with simple assimilation experiments. It demonstrates the ability of the method to propagate in time the statistical properties of the system when the filter is initialized properly. However, the lack of robustness of the filter is investigated theoretically and experimentally, showing the need to consider variants of the method when the filter is not properl

ISSN:0035-9009    

DOI:10.1002/qj.49712757113

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe characteristics of roughness length for temperature,zT, over melting snow and ice are examined on the base of data gathered during one summer season on the Greenland ice sheet. Despite fixed surface temperature and homogeneous surface conditions,zTis highly variable, taking on values between 10−5and 1 m. Mean value and median are computed as 2 × 10−2m and 1 × 10−3m. It thus turns out thatzTis about 10 to 100 times larger than the aerodynamic roughness length,z0. This disparity cannot be explained in terms of the roughness Reynolds number. Rather, the data show that over melting snow and ice, and for slightly stable conditions,zTis uniquely determined by the temperature

ISSN:0035-9009    

DOI:10.1002/qj.49712757114

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712757115

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712757116

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712757117

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712757101

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY