Simultaneous Atlantic–Pacific blocking and the Northern Annular ModeWoollings, Tim; Hoskins, Brian
doi: 10.1002/qj.310pmid: N/A
A synoptic situation termed ‘high‐latitude blocking’ (HLB) is shown to occur frequently in both the Atlantic and Pacific sectors, and to result in flow anomalies very similar to those associated with the negative phase of the Northern Annular Mode (NAM) in the respective sector. There is a weak but significant link between the occurrence of HLB in the two sectors, with Atlantic HLB tending to lead Pacific HLB by 1–3 days. This link arises from rare events in which both sectors are almost simultaneously affected by a large‐scale wave‐breaking event which distorts the polar trough over Northern Canada. In several cases the tropospheric wave‐breaking occurs in tandem with a large‐scale disturbance of the stratospheric polar vortex.
Ensemble simulations of the cold European winter of 2005‐2006Scaife, A. A.; Knight, J. R.
doi: 10.1002/qj.312pmid: N/A
There is only limited understanding of the processes driving year‐to‐year variability in European winter climate and the skill of seasonal forecasts for Europe in winter is generally low. The winter of 2005‐2006 is a useful case‐study because it was the coldest winter in large parts of western Europe for over a decade, and the coldest in central England since 1995‐1996. Here, we present results of experiments with a range of general circulation models to investigate the importance of both the Atlantic Ocean and stratospheric circulation in producing the unusually cold winter of 2005‐2006. We use models with different combinations of horizontal and stratospheric vertical resolution, allowing the sensitivity of the response to model formulation to be tested. The response to Atlantic sea‐surface temperature (SST) anomalies is improved in a more recent model with higher horizontal resolution. The results show that both Atlantic SSTs and the January 2006 sudden stratospheric warming are likely to have contributed to the cold 2005‐2006 European winter. © Crown Copyright 2008. Reproduced with the permission of HMSO. Published by John Wiley & Sons Ltd.
Tropical‐cyclone intensification and predictability in a minimal three‐dimensional modelShin, Seoleun; Smith, Roger K.
doi: 10.1002/qj.327pmid: N/A
We investigate the amplification and predictability of tropical cyclones in the context of a minimal, three‐dimensional numerical model. In the prototype problem for intensification, starting with a tropical storm strength vortex in a quiescent environment on an f‐plane, the emergent flow in the inner region of the vortex becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The details of the intensification process, including the asymmetric structures that develop, are highly sensitive to small perturbations in the low‐level moisture field at the initial time. This sensitivity is manifest in a significant spread in the intensity of vortices from an ensemble of calculations in which random moisture perturbations are added in the lowest model level. Similar experiments are carried out on a β‐plane and in the case where there is an anticyclonic shear flow at upper levels. The former set shows no significant difference from the f‐plane calculations in the evolution of intensity, but the latter set shows a significantly weaker vortex, contrary to a broadly held hypothesis that upper‐level outflow channels are favourable to intensification. Copyright © 2008 Royal Meteorological Society
Impact of wind bogus and cloud‐ and rain‐affected SSM/I data on tropical cyclone analyses and forecastsMontroty, Rémi; Rabier, Florence; Westrelin, Samuel; Faure, Ghislain; Viltard, Nicolas
doi: 10.1002/qj.308pmid: N/A
In the context of its responsibilities as a Regional Specialised Meteorological Centre (RSMC) for the Southwest Indian Ocean, Météo‐France operates a tropical cyclone warning centre which sends advisories to all the concerned countries in the area. In assistance to the forecasters and as part of the operational suite for the short‐range forecasts, a limited‐area model (ALADIN Réunion) is run with four daily analyses. Assimilated observations include conventional observations – radiosondes, buoys, surface stations, aircraft reports, upper‐wind reports – and satellite observations. The latter include QuikSCAT surface winds, atmospheric motion vectors from geostationary satellites, and radiances from polar‐orbiting satellites. Assimilation of satellite radiance data is done in clear‐sky conditions, and thus cannot be used in the vicinity of tropical cyclones. Two new sources of pseudo‐observations are investigated that can bring new information content to those regions: Total Column Water Vapour (TCWV) pseudo‐observations deduced from cloudy/rainy SSM/I data, as well as a 3D wind bogus. The TCWV algorithm is obtained from SSM/I brightness temperatures through a simple statistical regression from the 1D‐Var analyses of the European Centre for Medium‐Range Weather Forecasts, which are derived from complex inversion methods using moist physics and radiative transfer models. The 3D wind bogus is derived from structural information contained in the tropical cyclone advisories issued by the RSMC and contains a low‐level vortex composed of two concentric rings of eight winds each, at each of the surface, 850, 700 and 500 hPa levels. Forecast scores and fit of the model to the observations are improved and indicate a positive impact of those new datasets. Structural validation is investigated through the comparison of model and TMI ‘observed’ rain rates: it is found that assimilating and cycling cloudy/rainy TCWV helps achieve more realistic cyclonic features. Copyright © 2008 Royal Meteorological Society
An atmospheric model of intermediate complexity for data assimilation studiesEhrendorfer, Martin; Errico, Ronald M.
doi: 10.1002/qj.329pmid: N/A
Atmospheric models of intermediate complexity play an important role when studying atmospheric phenomena. Their complexity is between highly truncated low‐dimensional ‘toy’ models and modern general circulation or numerical weather prediction models. By design, computational cost associated with intermediate models is much reduced while at the same time some important aspects of atmospheric behaviour are still reasonably realistically described. Performing numerical experimentation with such models in the contexts of data assimilation, predictability, and atmospheric dynamics can produce informative results regarding those aspects for comparatively low cost. Nevertheless, as with any model‐based study, the degree to which results so obtained may be generalized to more realistic conditions remains somewhat uncertain and dependent on the specific questions being considered. An intermediate‐complexity model, named AMIC (Atmospheric Model of Intermediate Complexity) based on the nonlinear quasi‐geostrophic potential vorticity equation is presented. This global model uses a spectral dynamical core, and contains ‘physical processes’, such as climatological forcing, diffusion, and damping, designed to reasonably match AMIC's behaviour with observed atmospheric properties. While AMIC has variable horizontal and vertical resolution, the properties of AMIC are studied here for two specific resolutions (T45L6 and T106L9) and these are compared against atmospheric properties in terms of energy spectra, time‐mean and transient behaviour, and singular‐vector perturbation growth. The model's behaviour is reasonably realistic, except for its transient activity being somewhat weak, especially in the southern (summer) hemisphere. AMIC is also suited for some data assimilation and predictability studies since it contains complete tangent‐linear and adjoint models. Copyright © 2008 Royal Meteorological Society
The impact of satellite retrievals in a global sea‐surface‐temperature analysisBrasnett, Bruce
doi: 10.1002/qj.319pmid: N/A
An analysis of sea surface temperature (SST) is described. It incorporates in situ observations and retrievals from one microwave and three infrared sensors. Statistical interpolation is used to update the analysis daily on a global grid with a resolution of 1/3°. The background or first‐guess field is essentially the analysis from the previous day. Satellite retrievals and buoy observations undergo a thinning and all data is subjected to a careful quality control. A scheme to remove large‐scale biases from the satellite data is included, and its impact is assessed.
Ocean altimeter assimilation with observational‐ and model‐bias correctionLea, D. J.; Drecourt, J.‐P.; Haines, K.; Martin, M. J.
doi: 10.1002/qj.320pmid: N/A
We implement a combined on‐line model‐ and observation‐bias correction scheme in the UK Met Office Forecasting Ocean Assimilation Model (FOAM) Unified Model ocean data assimilation system. The observation bias is designed to estimate the error in the mean dynamic topography that is used for altimeter data assimilation. In future, this mean dynamic topography and its errors may be derived from the Gravity field and steady‐state Ocean Circulation Explorer (GOCE) mission geoid data. The mean dynamic topography field is added to the altimeter data supplied as sea‐level anomalies, giving the absolute dynamic topography. The model‐bias scheme separately estimates the remaining bias in the model's sea surface height field. The final unbiased estimate of the absolute dynamic topography is assimilated into the FOAM model by adjusting the subsurface density field using the Cooper and Haines scheme. Various diagnostics, including the observation minus background statistics, show that both model‐ and observation‐bias correction schemes improve the assimilation results. Combining the schemes provides better results than either used alone. Copyright © 2008 Royal Meteorological Society and Crown Copyright, 2008
Impact of initial condition uncertainties on the predictability of heavy rainfall in the Mediterranean: a case studyArgence, Sébastien; Lambert, Dominique; Richard, Evelyne; Chaboureau, Jean‐Pierre; Söhne, Nathalie
doi: 10.1002/qj.314pmid: N/A
This study explores the predictability of a heavy rainfall event that struck North Africa on 9 and 10 November 2001. This case is a paradigm of Mediterranean extreme events characterized by the presence of a deep upper‐level trough associated with an intense cyclone which developed over the Western Mediterranean basin. Using the French non‐hydrostatic mesoscale model MESO‐NH, numerical experiments starting from various initial atmospheric states were conducted to assess the impact of initial condition uncertainties on the precipitation and cloud cover forecast. To generate a set of perturbed atmospheric states, a simple date‐shifting initialization method was used. Two sets of simulations were run, using lateral boundaries and initial conditions derived from both the French operational global assimilation system Action de Recherche Petite Echelle Grande Echelle (ARPEGE) and the European Centre for Medium‐Range Weather Forecasts (ECMWF) system. Initial perturbations applied to the upper‐level trough propagated and intensified throughout the simulation, leading to some discrepancy in the forecast of the low‐level cyclone. While it was found that the upper‐level trough and the surface cyclone controlled the location of the overall precipitation pattern, the predictability of smaller‐scale features such as localized heavy rainfall was directly related to specific mesoscale structures. The low‐level jet associated with the surface cyclone and the location and the intensity of the surface‐low both impact upon the triggering and the sustainment of the convective cells. In consequence, small‐scale perturbations of these mesoscale features led to large errors in the precipitation forecast, especially in the Algiers area. Copyright © 2008 Royal Meteorological Society