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Molteni F.,European Center for Medium Range Weather Forecasts Reading | Farneti R.,Abdus Salam International Center For Theoretical Physics | Kucharski F.,Abdus Salam International Center For Theoretical Physics | Stockdale T.N.,European Center for Medium Range Weather Forecasts Reading
Geophysical Research Letters | Year: 2017

It is widely accepted that natural decadal variability played a major role in the slowdown in global warming observed in the 21st century, with sea surface cooling in the tropical Pacific recognized as a major contributor. However, the warming pause was most pronounced during boreal winter, with Northern Hemisphere flow anomalies also playing a role. Here we quantify the contribution of extratropical heat exchanges by comparing geopotential and temperature anomalies simulated by ensembles of seasonal forecasts with similar ocean temperature but different heat fluxes north of 40°N, as a result of planetary wave variability. We show that an important part of heat flux anomalies is associated with decadal variations in the phase of a specific planetary wave pattern. In model simulations covering the last three decades, this variability pattern accounts for a decrease of 0.35°C/decade in the post-1998 wintertime temperature trend over northern continents. ©2017. American Geophysical Union.

PubMed | Environment Canada, University of Washington, Pacific Northwest National Laboratory, University of California at Irvine and 9 more.
Type: Journal Article | Journal: Journal of geophysical research. Atmospheres : JGR | Year: 2016

An analysis of diabatic heating and moistening processes from 12 to 36h lead time forecasts from 12 Global Circulation Models are presented as part of the Vertical structure and physical processes of the Madden-Julian Oscillation (MJO) project. A lead time of 12-36h is chosen to constrain the large-scale dynamics and thermodynamics to be close to observations while avoiding being too close to the initial spin-up of the models as they adjust to being driven from the Years of Tropical Convection (YOTC) analysis. A comparison of the vertical velocity and rainfall with the observations and YOTC analysis suggests that the phases of convection associated with the MJO are constrained in most models at this lead time although the rainfall in the suppressed phase is typically overestimated. Although the large-scale dynamics is reasonably constrained, moistening and heating profiles have large intermodel spread. In particular, there are large spreads in convective heating and moistening at midlevels during the transition to active convection. Radiative heating and cloud parameters have the largest relative spread across models at upper levels during the active phase. A detailed analysis of time step behavior shows that some models show strong intermittency in rainfall and differences in the precipitation and dynamics relationship between models. The wealth of model outputs archived during this project is a very valuable resource for model developers beyond the study of the MJO. In addition, the findings of this study can inform the design of process model experiments, and inform the priorities for field experiments and future observing systems.

PubMed | Environment Canada, University of Reading and European Center for Medium Range Weather Forecasts Reading
Type: Journal Article | Journal: Journal of advances in modeling earth systems | Year: 2016

A recent intercomparison exercise proposed by the Working Group for Numerical Experimentation (WGNE) revealed that the parameterized, or unresolved, surface stress in weather forecast models is highly model-dependent, especially over orography. Models of comparable resolution differ over land by as much as 20% in zonal mean total subgrid surface stress (

Joos H.,ETH Zurich | Forbes R.M.,European Center for Medium range Weather Forecasts Reading
Quarterly Journal of the Royal Meteorological Society | Year: 2016

The influence of microphysical processes on the upper-level flow features associated with an extratropical cyclone is investigated with the ECMWF global atmospheric model. A control simulation with the model version operational at ECMWF during 2014/early 2015 and a simulation with new parametrizations of rain autoconversion/accretion, rain evaporation and snow riming operational since May 2015 are compared in detail. In order to investigate the impact of each microphysical process separately, the diabatic heating rate for each microphysical process and the associated change in potential vorticity (PV) is calculated and compared for the two simulations. The influence on the upper-level ridge building and the downstream flow evolution is investigated. The changes in the microphysical parametrization led to differences in the position of the warm conveyor belt (WCB) outflow. The WCB reaches the upper troposphere with low PV values and shifts the location of the tropopause in a slightly different way in the two simulations. Although these differences are relatively small at the beginning, they are advected downstream and amplify, leading to distinct differences in the upper-level PV pattern. These results highlight the importance of correct representation of microphysical processes for large-scale flow features. Additionally they emphasise the need for detailed microphysical measurements in extratropical cyclones in order to better understand and constrain the microphysical processes in NWP models. © 2016 Royal Meteorological Society.

Hawkins E.,University of Reading | Tietsche S.,University of Reading | Day J.J.,University of Reading | Melia N.,University of Reading | And 2 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2015

Using lessons from idealised predictability experiments, we discuss some issues and perspectives on the design of operational seasonal to inter-annual Arctic sea-ice prediction systems. We first review the opportunities to use a hierarchy of different types of experiment to learn about the predictability of Arctic climate. We also examine key issues for ensemble system design, such as measuring skill, the role of ensemble size and generation of ensemble members. When assessing the potential skill of a set of prediction experiments, using more than one metric is essential as different choices can significantly alter conclusions about the presence or lack of skill. We find that increasing both the number of hindcasts and ensemble size is important for reliably assessing the correlation and expected error in forecasts. For other metrics, such as dispersion, increasing ensemble size is most important. Probabilistic measures of skill can also provide useful information about the reliability of forecasts. In addition, various methods for generating the different ensemble members are tested. The range of techniques can produce surprisingly different ensemble spread characteristics. The lessons learnt should help inform the design of future operational prediction systems. © 2015 Royal Meteorological Society.

Pithan F.,University of Reading | Shepherd T.G.,University of Reading | Zappa G.,University of Reading | Sandu I.,European Center for Medium Range Weather Forecasts Reading
Geophysical Research Letters | Year: 2016

State-of-the art climate models generally struggle to represent important features of the large-scale circulation. Common model deficiencies include an equatorward bias in the location of the midlatitude westerlies and an overly zonal orientation of the North Atlantic storm track. Orography is known to strongly affect the atmospheric circulation and is notoriously difficult to represent in coarse-resolution climate models. Yet how the representation of orography affects circulation biases in current climate models is not understood. Here we show that the effects of switching off the parameterization of drag from low-level orographic blocking in one climate model resemble the biases of the Coupled Model Intercomparison Project Phase 5 ensemble: An overly zonal wintertime North Atlantic storm track and less European blocking events, and an equatorward shift in the Southern Hemispheric jet and increase in the Southern Annular Mode time scale. This suggests that typical circulation biases in coarse-resolution climate models may be alleviated by improved parameterizations of low-level drag. © 2016. American Geophysical Union. All Rights Reserved.

Grazzini F.,ARPA SIMC Bologna Italy | Vitart F.,European Center for Medium Range Weather Forecasts Reading
Quarterly Journal of the Royal Meteorological Society | Year: 2015

Historically, the objective identification of atmospheric wave-packets has been very elusive. However, interest in these important sources of atmospheric variability has recently increased, and some automated tracking methods have been proposed. The Rossby wave packet (RWP) tracking algorithms opened the way to different types of investigation, ranging from climatology and predictability to assessing the impact of climate change on wave packet characteristics. The present study investigates the relationship between predictability (intrinsic and practical, i.e. predictive skill in a numerical weather prediction model) and the properties of RWPs, such as temporal duration, spatial extension and their area of genesis. Results suggest a significant correlation between RWP length and medium-range skill over Europe and the Northern Hemisphere. Analysis of an ensemble system shows that the spread decreases when long-living RWPs are present in the forecast, supporting the hypothesis that part of the observed increase in skill could indeed be attributed to higher intrinsic predictability induced by RWPs. Higher than average medium-range forecast skill scores are often associated with the presence of long-lasting RWPs (duration of at least 8 days) in the initial conditions, with a source often located in the west Pacific. On the contrary, bad medium-range forecast skill scores tend to be associated with shorter RWPs coming from the central USA or western Atlantic. An analysis of the probabilistic skill scores confirms that predictive skill increases with the presence of long RWPs from the west Pacific, up to week 3. © 2015 Royal Meteorological Society.

Janiskova M.,European Center for Medium Range Weather Forecasts Reading
Quarterly Journal of the Royal Meteorological Society | Year: 2015

Space-borne active instruments, providing a vertically resolved characterization of clouds, promise a new dimension of information to be used in numerical weather prediction systems. Research activities are ongoing at the European Centre for Medium-Range Weather Forecasts to exploit these data for monitoring and assimilation purposes. Using currently available observations from CloudSat and CALIPSO, a technique combining one-dimensional variational (1D-Var) assimilation with four-dimensional variational (4D-Var) data assimilation has been used to study the impact of cloud-related observations on analyses and subsequent forecasts. Temperature and specific humidity vertical profiles retrieved from 1D-Var using observations of cloud radar reflectivity and lidar backscatter, either separately or in combination, were used as pseudo-observations in the 4D-Var system. Results indicate that 1D-Var analyses get closer to assimilated and also independent observations when appropriate quality control, bias correction and error estimate are applied. The performed 1D+4D-Var assimilation experiments also suggest a slight positive impact of the new observations on the subsequent forecast. Generally, the impact of lidar backscatter from clouds is smaller than that of cloud radar reflectivity. © 2015 Royal Meteorological Society.

Lang S.T.K.,European Center for Medium Range Weather Forecasts Reading | Bonavita M.,European Center for Medium Range Weather Forecasts Reading | Leutbecher M.,European Center for Medium Range Weather Forecasts Reading
Quarterly Journal of the Royal Meteorological Society | Year: 2015

Many weather forecasting centres generate the initial conditions for their ensemble prediction system by adding a set of perturbations to a single analysis. At the European Centre for Medium-Range Weather Forecasts (ECMWF), initial conditions from an ensemble of data assimilations (EDA) are re-centred on the deterministic high-resolution analysis. This study assesses how the re-centring step affects the ensemble forecasts and what benefits starting directly from the perturbed EDA members may bring. Numerical experiments are based on suitably modified configurations of the ECMWF medium-range ensemble. Re-centring leads to an artificial increase of the climatological variance of the initial conditions for variables and spatial scales that are not well constrained in the analysis. For those variables and spatial scales, the EDA variance can reach a similar magnitude as the climatological variance. Re-centring on the unperturbed member of the EDA leads to a degradation of the ensemble skill in comparison to starting directly from the perturbed EDA members. Depending on the amplitude of the EDA spread and which scales are less constrained, different lead times are affected. Furthermore, the re-centring step increases the jumpiness of the ensemble forecasts in general and alters the precipitation frequency during the first 12 h of the forecasts. It also becomes apparent from our results that the benefit for the ECMWF ensemble of starting directly from EDA initial conditions is sensitive to several factors: the difference in analysis accuracy between the unperturbed EDA control and the high-resolution analysis, the number of EDA members, and the EDA spread. At present, the forecast skill still benefits from centring the ensemble initial conditions on the high-resolution analysis, but future changes to the EDA may change the balance. © 2015 Royal Meteorological Society.

Simmons A.J.,European Center for Medium Range Weather Forecasts Reading | Poli P.,European Center for Medium Range Weather Forecasts Reading
Quarterly Journal of the Royal Meteorological Society | Year: 2014

Near-surface and lower-tropospheric warming of the Arctic over the past 35 years is examined for several datasets. The new estimate for the near surface reported by Cowtan and Way in 2014 agrees reasonably well with the ERA-Interim reanalysis for this region. Both provide global averages with a little more warming over recent years than indicated by the widely used HadCRUT4 dataset, which has sparse coverage of the high Arctic. ERA-Interim is more sensitive than the Cowtan and Way estimate to the state of the underlying Arctic Ocean. Observational coverage of the Arctic varies considerably over the period. Surface air-temperature data of identified types are generally fitted well by ERA-Interim, especially data from ice stations, which appear of excellent quality. ERA-Interim nevertheless has a warm wintertime bias over sea-ice. Mean fits vary in magnitude as coverage varies, but their overall changes are much smaller than analysed temperature changes. This is also largely the case for fits to data for the free troposphere. Much of the information on trends and low-frequency variability provided by ERA-Interim comes from its background forecast, which carries forward information assimilated from a rich variety of earlier observations, rather than from its analysis of surface air-temperature observations. ERA-Interim agrees quite well with the new JRA-55 reanalysis, and with the MERRA reanalysis until recent years when MERRA exhibits weaker surface warming. Temperatures vary coherently between the surface and middle troposphere, with largest amplitude at the surface except in summer, when air temperatures are constrained by sea-ice and open-sea temperatures that differ little from 0°C. Much of the recent near-surface warming of the Arctic is associated with reduced cold-season sea-ice cover, with low temperatures over ice replaced by much higher ones over open sea. This occurs primarily in a relatively well-observed region around the northernmost islands of Europe and western Asia. © 2014 Royal Meteorological Society.

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