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    Current Issue
      24 September 2017 , Volume 34 Issue 11   
    A Step Forward toward Effectively Using Hyperspectral IR Sounding Information in NWP
    Jun LI, Wei HAN
    2017 , 34 (11 ): 1263 -1264.   DOI: 10.1007/s00376-017-7167-2
    Abstract ( 60 )   HTML PDF (108 KB) ( 109 )
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    A New Infrared Atmospheric Sounding Interferometer Channel Selection and Assessment of Its Impact on Met Office NWP Forecasts
    Young-Chan NOH, Byung-Ju SOHN, Yoonjae KIM, Sangwon JOO, William BELL, Roger SAUNDERS
    2017 , 34 (11 ): 1265 -1281.   DOI: 10.1007/s00376-017-6299-8
    Abstract ( 63 )   HTML PDF (15834 KB) ( 47 )

    A new set of Infrared Atmospheric Sounding Interferometer (IASI) channels was re-selected from 314 EUMETSAT channels. In selecting channels, we calculated the impact of the individually added channel on the improvement in the analysis outputs from a one-dimensional variational analysis (1D-Var) for the Unified Model (UM) data assimilation system at the Met Office, using the channel score index (CSI) as a figure of merit. Then, 200 channels were selected in order by counting each individual channel's CSI contribution. Compared with the operationally used 183 channels for the UM at the Met Office, the new set shares 149 channels, while the other 51 channels are new. Also examined is the selection from the entropy reduction method with the same 1D-Var approach. Results suggest that channel selection can be made in a more objective fashion using the proposed CSI method. This is because the most important channels can be selected across the whole IASI observation spectrum. In the experimental trial runs using the UM global assimilation system, the new channels had an overall neutral impact in terms of improvement in forecasts, as compared with results from the operational channels. However, upper-tropospheric moist biases shown in the control run with operational channels were significantly reduced in the experimental trial with the newly selected channels. The reduction of moist biases was mainly due to the additional water vapor channels, which are sensitive to the upper-tropospheric water vapor.

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    Decadal Indian Ocean Dipolar Variability and Its Relationship with the Tropical Pacific
    Yun YANG, Jianping LI, Lixin WU, Yu KOSAKA, Yan DU, Cheng SUN, Fei XIE, Juan FENG
    2017 , 34 (11 ): 1282 -1289.   DOI: 10.1007/s00376-017-7009-2
    Abstract ( 55 )   HTML PDF (3162 KB) ( 107 )

    A robust decadal Indian Ocean dipolar variability (DIOD) is identified in observations and found to be related to tropical Pacific decadal variability (TPDV). A Pacific Ocean-global atmosphere (POGA) experiment, with fixed radiative forcing, is conducted to evaluate the DIOD variability and its relationship with the TPDV. In this experiment, the sea surface temperature anomalies are restored to observations over the tropical Pacific, but left as interactive with the atmosphere elsewhere. The TPDV-forced DIOD, represented as the ensemble mean of 10 simulations in POGA, accounts for one third of the total variance. The forced DIOD is triggered by anomalous Walker circulation in response to the TPDV and develops following Bjerknes feedback. Thermocline anomalies do not exhibit a propagating signal, indicating an absence of oceanic planetary wave adjustment in the subtropical Indian Ocean. The DIOD-TPDV correlation differs among the 10 simulations, with a low correlation corresponding to a strong internal DIOD independent of the TPDV. The variance of this internal DIOD depends on the background state in the Indian Ocean, modulated by the thermocline depth off Sumatra/Java.

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    Why Was the Strengthening of Rainfall in Summer over the Yangtze River Valley in 2016 Less Pronounced than that in 1998 under Similar Preceding El Niño Events?——Role of Midlatitude Circulation in August
    Chaofan LI, Wei CHEN, Xiaowei HONG, Riyu LU
    2017 , 34 (11 ): 1290 -1300.   DOI: 10.1007/s00376-017-7003-8
    Abstract ( 109 )   HTML PDF (3854 KB) ( 132 )

    It is widely recognized that rainfall over the Yangtze River valley (YRV) strengthens considerably during the decaying summer of El Niño, as demonstrated by the catastrophic flooding suffered in the summer of 1998. Nevertheless, the rainfall over the YRV in the summer of 2016 was much weaker than that in 1998, despite the intensity of the 2016 El Niño having been as strong as that in 1998. A thorough comparison of the YRV summer rainfall anomaly between 2016 and 1998 suggests that the difference was caused by the sub-seasonal variation in the YRV rainfall anomaly between these two years, principally in August. The precipitation anomaly was negative in August 2016——different to the positive anomaly of 1998. Further analysis suggests that the weaker YRV rainfall in August 2016 could be attributable to the distinct circulation anomalies over the midlatitudes. The intensified "Silk Road Pattern" and upper-tropospheric geopotential height over the Urals region, both at their strongest since 1980, resulted in an anticyclonic circulation anomaly over midlatitude East Asia with anomalous easterly flow over the middle-to-lower reaches of the YRV in the lower troposphere. This easterly flow reduced the climatological wind, weakened the water vapor transport, and induced the weaker YRV rainfall in August 2016, as compared to that in 1998. Given the unique sub-seasonal variation of the YRV rainfall in summer 2016, more attention should be paid to midlatitude circulation——besides the signal in the tropics——to further our understanding of the predictability and variation of YRV summer rainfall.

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    Modification of the SUNFLUX Solar Radiation Scheme with a New Aerosol Parameterization and Its Validation Using Observation Network Data
    Yongjian HE, Zhi'an SUN, Guoping SHI, Jingmiao LIU, Jiandong LI
    2017 , 34 (11 ): 1301 -1315.   DOI: 10.1007/s00376-016-6262-0
    Abstract ( 34 )   HTML PDF (1862 KB) ( 36 )

    SUNFLUX is a fast parameterization scheme for determination of the solar radiation at the Earth's surface. In this paper, SUNFLUX is further modified in the treatment of aerosols. A new aerosol parameterization scheme is developed for five aerosol species. Observational data from Baseline Surface Radiation Network (BSRN), Surface Radiation Budget Network (SURFRAD) and Aerosol Robotic Network (AERONET) stations are used to evaluate the accuracy of the original and modified SUNFLUX schemes. General meteorological data are available at SURFRAD stations, but not at BSRN stations. Therefore, the total precipitable water content and aerosol data are obtained from AERONET stations. Fourteen stations are selected from both BSRN and AERONET. Cloud fraction data from MODIS are further used to screen the cloud. Ten-year average aerosol mixing ratios simulated by the CAM-chem system are used to calculate the fractions of aerosol optical depth for each aerosol species, and these fractions are further used to convert the observed total aerosol optical depth into the components of individual species for use in the evaluations. The proper treatment of multiple aerosol types in the model is discussed. The evaluation results using SUNFLUX with the new aerosol scheme, in terms of the BSRN dataset, are better than those using the original aerosol scheme under clear-sky conditions. However, the results using the SURFRAD dataset are slightly worse, attributable to the differences in the input water vapor and aerosol optical depth. Sensitivity tests are conducted to investigate the error response of the SUNFLUX scheme to the errors in the input variables.

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  • CN 11-1925/04

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