Åke Johansson
Ph.D.
Research areas
Research and development aimed at improving and refining the methodology on which ensemble forecasting is based. The work includes
- Development of methodologies to construct equally probable ensemble members. This work is closely related to the research area data assimilation.
- Development of methodologies aimed at more accurately estimating and specifying the desirable spread of an ensemble prediction system at the initial time.
- Development of verifications which more accurately and realistically measures the quality of an ensemble prediction system.
- Development of verification methodologies which produces similar results regardless of whether the verification is done against observations or analysis.
The work is done within the international HIRLAM-ALADIN cooperation for Limited Area Weather Forecasting.
Follows the development – research and practical applications – in seasonal forecasting, especially regarding the predictability of the NAO (North Atlantic Oscillation) och ENSO (El Niño-Southern Oscillation).
Research interests and special competence
- Dynamic meteorology and its practical application in short and medium-range Numerical Weather Prediction (NWP)
- Predictability, non-linear dynamical systems and chaos theory
- Low-frequency phenomena such as NAO and ENSO and its importance and relevance for longer range forecasting (sub-seasonal and seasonal forecasting)
- Empirical and statistical methods in meteorology and climatology
Latest publications
Curved flight procedure construction with site-specific statistical meteorological data
Xin Zhao, Ulrika Ziverts, Henrik Ekstrand, Maria Ullvetter, Peter Lukic, Anette Nas, Esbjörn Olsson, Martin Ridal, Åke Johansson, Martin Wall, Olivier Petit, Tomas Gronstedt
The ETKF rescaling scheme in HIRLAM
Jelena Bojarova, Nils Gustafsson, Åke Johansson, Ole Vignes
In: Tellus. Series A, Dynamic meteorology and oceanography, Vol. 63, No. 3
2011
Abstract
The ETKF rescaling scheme has been implemented into the HIRLAM forecasting system in order to estimate the uncertainty of the model state. The main purpose is to utilize this uncertainty information for modelling of flow-dependent background error covariances within the framework of a hybrid variational ensemble data assimilation scheme. The effects of rank-deficiency in the ETKF formulation is explained and the need for variance inflation as a way to compensate for these effects is justified. A filter spin-up algorithm is proposed as a refinement of the variance inflation. The proposed spin-up algorithm will also act to prevent ensemble collapse since the ensemble will receive 'fresh blood' in the form of additional perturbation components, generated on the basis of a static background error covariance matrix. The resulting ETKF-based ensemble perturbations are compared with ensemble perturbations based on targeted singular vectors and are shown to have more realistic spectral characteristics.
Seasonal-to-decadal predictability and prediction of North American climate - The Atlantic influence
H. M. Van den Dool, Peitao Peng, Åke Johansson, Muthuvel Chelliah, Amir Shabbar, Suranjana Saha
Abstract
The question of the impact of the Atlantic on North American (NA) seasonal prediction skill and predictability is examined. Basic material is collected from the literature, a review of seasonal forecast procedures in Canada and the United States, and some fresh calculations using the NCEP-NCAR reanalysis data. The general impression is one of low predictability (due to the Atlantic) for seasonal mean surface temperature and precipitation over NA. Predictability may be slightly better in the Caribbean and the (sub) tropical Americas, even for precipitation. The NAO is widely seen as an agent making the Atlantic influence felt in NA. While the NAO is well established in most months, its prediction skill is limited. Year-round evidence for an equatorially displaced version of the NAO (named ED_NAO) carrying a good fraction of the variance is also found. In general the predictability from the Pacific is thought to dominate over that from the Atlantic sector, which explains the minimal number of reported Atmospheric Model Intercomparison Project (AMIP) runs that explore Atlantic-only impacts. Caveats are noted as to the question of the influence of a single predictor in a nonlinear environment with many predictors. Skill of a new one-tier global coupled atmosphere-ocean model system at NCEP is reviewed; limited skill is found in midlatitudes and there is modest predictability to look forward to. There are several signs of enthusiasm in the community about using "trends" (low-frequency variations): (a) seasonal forecast tools include persistence of last 10 years' averaged anomaly (relative to the official 30-yr climatology), (b) hurricane forecasts are based largely on recognizing a global multidecadal mode (which is similar to an Atlantic trend mode in SST), and (c) two recent papers, one empirical and one modeling, giving equal roles to the (North) Pacific and Atlantic in "explaining" variations in drought frequency over NA on a 20 yr or longer time scale during the twentieth century.
