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    Current Issue
      30 July 2018 , Volume 35 Issue 10   
    Value-added Impact of Geostationary Hyperspectral Infrared Sounders on Local Severe Storm Forecasts——via a Quick Regional OSSE
    Zhenglong LI, Jun LI, Pei WANG, Agnes LIM, Jinlong LI, Timothy J. SCHMIT, Robert ATLAS, Sid-Ahmed BOUKABARA, Ross N. HOFFMAN
    2018 , 35 (10): 1217 -1230.   DOI: 10.1007/s00376-018-8036-3
    Abstract ( 185 )   HTML PDF (3069 KB) ( 71 )

    Accurate atmospheric temperature and moisture information with high temporal/spatial resolutions are two of the key parameters needed in regional numerical weather prediction (NWP) models to reliably predict high-impact weather events such as local severe storms (LSSs). High spectral resolution or hyperspectral infrared (HIR) sounders from geostationary orbit (GEO) provide an unprecedented source of near time-continuous, three-dimensional information on the dynamic and thermodynamic atmospheric fields——an important benefit for nowcasting and NWP-based forecasting. In order to demonstrate the value of GEO HIR sounder radiances on LSS forecasts, a quick regional OSSE (Observing System Simulation Experiment) framework has been developed, including high-resolution nature run generation, synthetic observation simulation and validation, and impact study on LSS forecasts. Results show that, on top of the existing LEO (low earth orbit) sounders, a GEO HIR sounder may provide value-added impact [a reduction of 3.56% in normalized root-mean-square difference (RMSD)] on LSS forecasts due to large spatial coverage and high temporal resolution, even though the data are assimilated every 6 h with a thinning of 60 km. Additionally, more frequent assimilations and smaller thinning distances allow more observations to be assimilated, and may further increase the positive impact from a GEO HIR sounder. On the other hand, with denser and more frequent observations assimilated, it becomes more difficult to handle the spatial error correlation in observations and gravity waves due to the limitations of current assimilation and forecast systems (such as a static background error covariance). The peak reduction of 4.6% in normalized RMSD is found when observations are assimilated every 3 h with a thinning distance of 30 km.

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    Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the Tropical Western North Pacific during Summer
    Xinyu LI, Riyu LU
    2018 , 35 (10): 1231 -1242.   DOI: 10.1007/s00376-018-7304-6
    Abstract ( 101 )   HTML PDF (2149 KB) ( 47 )

    There is a well-known seesaw pattern of precipitation between the tropical western North Pacific (WNP) and the Yangtze River basin (YRB) during summer. This study identified that this out-of-phase relationship experiences a subseasonal change; that is, the relationship is strong during early summer but much weaker during mid-summer. We investigated the large-scale circulation anomalies responsible for the YRB rainfall anomalies on the subseasonal timescale. It was found that the YRB rainfall is mainly affected by the tropical circulation anomalies during early summer, i.e., the anticyclonic or cyclonic anomaly over the subtropical WNP associated with the precipitation anomalies over the tropical WNP. During mid-summer, the YRB rainfall is mainly affected by the extratropical circulation anomalies in both the lower and upper troposphere. In the lower troposphere, the northeasterly anomaly north of the YRB favors heavier rainfall over the YRB by intensifying the meridional gradient of the equivalent potential temperature over the YRB. In the upper troposphere, the meridional displacement of the Asian westerly jet and the zonally oriented teleconnection pattern along the jet also affect the YRB rainfall. The subseasonal change in the WNP-YRB precipitation relationship illustrated by this study has important implications for the subseasonal-to-seasonal forecasting of the YRB rainfall.

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    A High-Resolution Modeling Study of the 19 June 2002 Convective Initiation Case during IHOP_2002: Localized Forcing by Horizontal Convective Rolls
    Qi-Wei WANG, Ming XUE
    2018 , 35 (10): 1243 -12543.   DOI: 10.1007/s00376-018-7218-3
    Abstract ( 73 )   HTML PDF (10043 KB) ( 44 )

    The initiation processes of one of the initial convective cells near and on the east side of a dryline on 19 June 2002 during the IHOP_2002 field experiment in the central United States is analyzed in detail based on a high-resolution numerical simulation. Prominent horizontal convective rolls and associated near-surface moisture convergence bands [called roll convergence bands (RCBs) here] develop within the convective boundary layer (CBL) due to surface heating, in the hours leading to convective initiation (CI). The RCBs east of the dryline are advected toward the primary dryline convergence boundary (PDCB) by the southerly moist flow as the CBL deepens with time. Backward trajectories of air parcels forming the initial precipitating updraft of the convective cell are found to primarily originate at about 1-1.5 km above ground, within the upper portion of the shallower CBL earlier on. The representative air parcel is found to follow and stay on top of a surface RCB as the RCB moves toward the PDCB, but the RCB forcing alone is not enough to initiate convection. As this RCB gets close to the PDCB, it moves into a zone of mesoscale convergence and a deeper CBL that exhibits an upward moisture bulge associated with the PDCB. The combined upward forcing of the RCB and the mesoscale PDCB convergence quickly lifts the representative air parcel above its level of free convection to initiate convection. A conceptual model summarizing the CI processes is proposed.

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    Impact of Global Oceanic Warming on Winter Eurasian Climate
    Xin HAO, Shengping HE, Tingting HAN, Huijun WANG
    2018 , 35 (10): 1254 -1264.   DOI: 10.1007/s00376-018-7216-5
    Abstract ( 162 )   HTML PDF (3343 KB) ( 66 )

    In the 20th century, Eurasian warming was observed and was closely related to global oceanic warming (the first leading rotated empirical orthogonal function of annual mean sea surface temperature over the period 1901-2004). Here, large-scale patterns of covariability between global oceanic warming and circulation anomalies are investigated based on NCEP-NCAR reanalysis data. In winter, certain dominant features are found, such as a positive pattern of the North Atlantic Oscillation (NAO), low-pressure anomalies over northern Eurasia, and a weakened East Asian trough. Numerical experiments with the CAM3.5, CCM3 and GFDL models are used to explore the contribution of global oceanic warming to the winter Eurasian climate. Results show that a positive NAO anomaly, low-pressure anomalies in northern Eurasia, and a weaker-than-normal East Asian trough are induced by global oceanic warming. Consequently, there are warmer winters in Europe and the northern part of East Asia. However, the Eurasian climate changes differ slightly among the three models. Eddy forcing and convective heating from those models may be the reason for the different responses of Eurasian climate.

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    High-Order Statistics of Temperature Fluctuations in an Unstable Atmospheric Surface Layer over Grassland
    Rui LYU, Fei HU, Lei LIU, Jingjing XU, Xueling CHENG
    2018 , 35 (10): 1265 -1276.   DOI: 10.1007/s00376-018-7248-x
    Abstract ( 23 )   HTML PDF (5022 KB) ( 7 )

    Skewness (S) and kurtosis (K) of temperature in the surface layer over a grassland are investigated under unstable thermal stratifications. We find that both skewness and kurtosis generally obey Monin-Obukhov similarity theory and tend to be constant values (1.5 and 5.3, respectively) when the stability parameter z/L<-2. Quantitative formulas of the similarity functions are proposed. The temperature probability density function (PDF) is close to Gaussian in near neutral stratification and non-Gaussian in unstable stratification. The influence of coherent motions on the PDF behavior is analyzed using the quadrant analysis technique. It shows that PDF behaviors are controlled by ejections and sweeps. The results also indicate that the PDF type of the ejections always follows a Gaussian distribution, while the PDF of the sweeps changes with stability.

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