Earth, Ocean and Atomospheric Sciences - Publications

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Browsing Earth, Ocean and Atomospheric Sciences - Publications by Author "Ahn, Joong Bae"
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    Assessment of the APCC coupled MME suite in predicting the distinctive climate impacts of two flavors of ENSO during boreal winter
    ( 2012-07-01) Jeong, Hye In ; Lee, Doo Young ; Ashok, Karumuri ; Ahn, Joong Bae ; Lee, June Yi ; Luo, Jing Jia ; Schemm, Jae Kyung E. ; Hendon, Harry H. ; Braganza, Karl ; Ham, Yoo Geun
    Forecast skill of the APEC Climate Center (APCC) Multi-Model Ensemble (MME) seasonal forecast system in predicting two main types of El Niño-Southern Oscillation (ENSO), namely canonical (or cold tongue) and Modoki ENSO, and their regional climate impacts is assessed for boreal winter. The APCC MME is constructed by simple composite of ensemble forecasts from five independent coupled ocean-atmosphere climate models. Based on a hindcast set targeting boreal winter prediction for the period 1982-2004, we show that the MME can predict and discern the important differences in the patterns of tropical Pacific sea surface temperature anomaly between the canonical and Modoki ENSO one and four month ahead. Importantly, the four month lead MME beats the persistent forecast. The MME reasonably predicts the distinct impacts of the canonical ENSO, including the strong winter monsoon rainfall over East Asia, the below normal rainfall and above normal temperature over Australia, the anomalously wet conditions across the south and cold conditions over the whole area of USA, and the anomalously dry conditions over South America. However, there are some limitations in capturing its regional impacts, especially, over Australasia and tropical South America at a lead time of one and four months. Nonetheless, forecast skills for rainfall and temperature over East Asia and North America during ENSO Modoki are comparable to or slightly higher than those during canonical ENSO events. © 2012 Springer-Verlag.
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    Improvement of grand multi-model ensemble prediction skills for the coupled models of APCC/ENSEMBLES using a climate filter
    ( 2013-07-01) Lee, Doo Young ; Ahn, Joong Bae ; Ashok, Karumuri ; Alessandri, Andrea
    Twelve coupled model simulations of two multi-model ensemble (MME) systems for boreal winters from 1983 to 2005 are used to improve the climate prediction. From grading the relative capability of each simulation in reproducing the observed link between the tropical El Niño-Southern Oscillation (ENSO)-related Walker circulation and the Pacific rainfall, we find an optimal MME suite with improved prediction skills. This study demonstrates that the climate filter concept, proposed by us in a recent work, is not only useful in improving the MME prediction skills as compared to a single MME system, but also the skills of a grand MME that encompasses two well-performing MMEs. © 2013 Royal Meteorological Society.
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    Quantifying the reliability of precipitation datasets for monitoring large-scale East Asian precipitation variations
    ( 2012-01-01) Sohn, Soo Jin ; Tam, Chi Yung ; Ashok, Karumuri ; Ahn, Joong Bae
    Early detection of extreme drought and flood events either over the whole globe or a broad geographical region, and timely dissemination of this information, is indispensable for mitigation and disaster preparedness. Recently, the APEC Climate Center (APCC) has launched a global precipitation variation monitoring product based on the Climate Anomaly Monitoring System-Outgoing Longwave Radiation Precipitation Index (CAMS-OPI) data. Here we quantify the reliability of CAMS-OPI, as well as other gauge-satellite-merged and reanalysis precipitation datasets, for the purpose of monitoring large-scale precipitation variability in East Asia. The ground truth is the newly available gauge-based data from the project titled 'Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation (APHRODITE) of the Water Resources'. It is found that the seasonal-to-interannual rainfall deficit and surplus given by various reanalysis systems sometimes do not match the spatial patterns seen in the APHRODITE data. Moreover, maps showing the Standardized Precipitation Index (SPI) become less and less reliable as the time scale based on which values are calculated increases. In contrast, the performance of gauge-satellite-based rainfall datasets is satisfactory and the quality of SPI maps does not decay as the time scale increases. Overall, CAMS-OPI is found to be reliable for monitoring large-scale precipitation variations over the East Asian sector. © 2011 Royal Meteorological Society.
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    Toward enhancement of prediction skills of multimodel ensemble seasonal prediction: A climate filter concept
    ( 2011-01-01) Lee, Doo Young ; Ashok, Karumuri ; Ahn, Joong Bae
    Using the APEC Climate Center (APCC) operational multimodel ensemble (MME) hindcasts of precipitation and temperature at 850 hPa for boreal winters for the period 1981-2003, along with those of the individual models as well as corresponding observed and reanalyzed data, we propose the use of a "climate" filter to diagnose and improve the prediction skills. The "filter" is based on the observed strong association between the El Niño-Southern Oscillation (ENSO)-associated Walker circulation and the tropical Pacific rainfall. The reproducibility of this relationship is utilized to evaluate the fidelity of the models. It is found that the retrospective forecast skill of a newer type of MME that contains only the "more skillful" models is superior to that of the all-inclusive operational MME. The difference of the prediction skills between the "more skillful" and "less skillful" MMEs varies with the region and is significant in subtropics such as East Asia, while most of the models perform well in tropics adjacent to the Pacific. Our pilot forecast with the proposed MME for two boreal winter seasons indicates that the method generally works better than the all-inclusive MME in many of the target regions. Copyright 2011 by the American Geophysical Union.