@misc{AcevedoFallahReichetal.2017,
  author    = {Acevedo, Walter and Fallah, Bijan and Reich, Sebastian and Cubasch, Ulrich},
  title     = {Assimilation of pseudo-tree-ring-width observations into an atmospheric general circulation model},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  number    = {627},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-41874},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-418743},
  pages     = {545 -- 557},
  year      = {2017},
  abstract  = {Paleoclimate data assimilation (DA) is a promising technique to systematically combine the information from climate model simulations and proxy records. Here, we investigate the assimilation of tree-ring-width (TRW) chronologies into an atmospheric global climate model using ensemble Kalman filter (EnKF) techniques and a process-based tree-growth forward model as an observation operator. Our results, within a perfect-model experiment setting, indicate that the "online DA" approach did not outperform the "off-line" one, despite its considerable additional implementation complexity. On the other hand, it was observed that the nonlinear response of tree growth to surface temperature and soil moisture does deteriorate the operation of the time-averaged EnKF methodology. Moreover, for the first time we show that this skill loss appears significantly sensitive to the structure of the growth rate function, used to represent the principle of limiting factors (PLF) within the forward model. In general, our experiments showed that the error reduction achieved by assimilating pseudo-TRW chronologies is modulated by the magnitude of the yearly internal variability in themodel. This result might help the dendrochronology community to optimize their sampling efforts.},
  language  = {en}
}
@article{AcevedoFallahReichetal.2017,
  author    = {Acevedo, Walter and Fallah, Bijan and Reich, Sebastian and Cubasch, Ulrich},
  title     = {Assimilation of pseudo-tree-ring-width observations into an atmospheric general circulation model},
  series = {Climate of the past : an interactive open access journal of the European Geosciences Union},
  volume    = {13},
  journal   = {Climate of the past : an interactive open access journal of the European Geosciences Union},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {1814-9324},
  doi       = {10.5194/cp-13-545-2017},
  pages     = {545 -- 557},
  year      = {2017},
  abstract  = {Paleoclimate data assimilation (DA) is a promising technique to systematically combine the information from climate model simulations and proxy records. Here, we investigate the assimilation of tree-ring-width (TRW) chronologies into an atmospheric global climate model using ensemble Kalman filter (EnKF) techniques and a process-based tree-growth forward model as an observation operator. Our results, within a perfect-model experiment setting, indicate that the "online DA" approach did not outperform the "off-line" one, despite its considerable additional implementation complexity. On the other hand, it was observed that the nonlinear response of tree growth to surface temperature and soil moisture does deteriorate the operation of the time-averaged EnKF methodology. Moreover, for the first time we show that this skill loss appears significantly sensitive to the structure of the growth rate function, used to represent the principle of limiting factors (PLF) within the forward model. In general, our experiments showed that the error reduction achieved by assimilating pseudo-TRW chronologies is modulated by the magnitude of the yearly internal variability in themodel. This result might help the dendrochronology community to optimize their sampling efforts.},
  language  = {en}
}
@article{PolanskiFallahBefortetal.2014,
  author    = {Polanski, Stefan and Fallah, Bijan and Befort, Daniel J. and Prasad, Sushma and Cubasch, Ulrich},
  title     = {Regional moisture change over India during the past millennium: A comparison of multi-proxy reconstructions and climate model simulations},
  series = {Global and planetary change},
  volume    = {122},
  journal   = {Global and planetary change},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0921-8181},
  doi       = {10.1016/j.gloplacha.2014.08.016},
  pages     = {176 -- 185},
  year      = {2014},
  abstract  = {The Indian Monsoon Variability during the past Millennium has been simulated with the ECHAM5 model in two different time slices: Medieval Climate Anomaly and the Little Ice Age. The simulations are compared with new centennial-resolving paleo-reconstructions inferred from various well-dated multi-proxies from two core regions, the Himalaya and Central India. A qualitative moisture index is derived from the proxies and compared with simulated moisture anomalies. The reconstructed paleo-hydrological changes between the Little Ice Age and the Medieval Climate Anomaly depict a dipole pattern between Himalaya and Central India, which is also captured by the model. In the Medieval Climate Anomaly the model exhibits stronger (weaker) dipole signals during summer (winter) compared to Little Ice Age. In summer (winter) months of "Medieval Climate Anomaly minus Little Ice Age" the model simulates wetter conditions over eastern (western and central) Himalaya. Over Central India, a simulated weakening of Indian Summer Monsoon during warmer climate is coincident with reconstructed drying signal in the Lonar Lake record. Based on the model simulations, we can differentiate three physical mechanisms which can lead to the moisture anomalies: (i) the western and central Himalaya are influenced by extra-tropical Westerlies during winter, (ii) the eastern Himalaya is affected by summer variations of temperature gradient between Bay of Bengal and Indian subcontinent and by a zonal band of intensified Indian-East Asian monsoon link north of 25 degrees N, and (iii) Central India is dominated by summer sea surface temperature anomalies in the northern Arabian Sea which have an effect on the large-scale advection of moist air masses. The temperatures in the Arabian Sea are linked to the Ind Pacific Warm Pool, which modulates the Indian monsoon strength. (C) 2014 The Authors. Published by Elsevier B.V.},
  language  = {en}
}