Climatological characteristics of diurnal variations in summer precipitation over the Asian monsoon region are comprehensively investigated based on the Tropical Rainfall Measuring Mission(TRMM) satellite data during 1998-2008.The topographic influence on the diurnal variations and phase propagations of maximum precipitation are identified according to spatiotemporal distributions of the amplitude and peak time of the diurnal precipitation.The amplitude and phase of diurnal precipitation show a distinct geographical pattern.Significant diurnal variations occur over most of continental and coastal areas including the Maritime Continent,with the relative amplitude exceeding 40%,indicating that the precipitation peak is 1.4 times the 24-h mean.Over the landside coasts such as southeastern China and Indochina Peninsula,the relative amplitude is even greater than 100%.Although the diurnal variations of summer precipitation over the continental areas are characterized by an afternoon peak(1500-1800 Local Solar Time(LST)),over the central Indochina Peninsula and central and southern Indian Peninsula the diurnal phase is delayed to after 2100 LST,suggesting the diurnal behaviors over these areas different from the general continental areas.The weak diurnal variations with relative amplitudes less than 40% exist mainly over oceanic areas in the western Pacific and most of Indian Ocean,with the rainfall peak mainly occurring from midnight to early morning(0000-0600 LST),indicating a typical oceanic regime characterized by an early morning peak.However,apparent exceptions occur over the South China Sea(SCS),Bay of Bengal(BOB),and eastern Arabian Sea,with the rainfall peak occurring in daytime(0900-1500 LST).Prominent meridional propagations of the diurnal phase exist in South Asia and East Asia.Along the eastern Indian Peninsula,there is not only the southward phase propagation with the peak occurring around 25°N but also the northward phase propagation with the peak beginning with the southernmost continent,and both reach
Within the frame of the Zebiak-Cane model,the impact of the uncertainties of the Madden-Julian Oscillation(MJO) on ENSO predictability was studied using a parameterized stochastic representation of intraseasonal forcing.The results show that the uncertainties of MJO have little effect on the maximum prediction error for ENSO events caused by conditional nonlinear optimal perturbation(CNOP);compared to CNOP-type initial error,the model error caused by the uncertainties of MJO led to a smaller prediction uncertainty of ENSO,and its influence over the ENSO predictability was not significant.This result suggests that the initial error might be the main error source that produces uncertainty in ENSO prediction,which could provide a theoretical foundation for the data assimilation of the ENSO forecast.
Based on 6-hourly sensible heat flux and latent heat flux from the NCEP Climate Forecast System Reanalysis(CFSR) and circulation data from the Japanese 25-year Reanalysis(JRA-25),the initial developing process of tropical cyclone Mindulle(1005) in 2010 has been diagnosed to reveal the impact of air-sea interaction over the South China Sea(SCS) on the genesis of its incipient vortex.The results show that the incipient vortex first occurred east of the Luzon Island on 0000 UTC 20 August,suggesting that the topographic forcing of the Luzon Island for easterly winds over the western Pacific might be one of the factors responsible for the formation of the incipient vortex.During the formation stage of the incipient vortex,strong southeasterlies over the SCS caused warm water of the middle and eastern SCS to flow toward the Luzon Island due to Ekman transport resulting from wind stress,leading to an increase of the sea surface temperature and sensible heat flux into the atmosphere.Although the anomalous sensible heating favored surface pressure to reduce,it was not conducive to the increase of local vorticity associated with the vortex above the heating area because,according to the atmospheric thermal adaptation theory,the anticyclonic vorticity would be created in the lower troposphere due to the decreased vertical gradient of the sensible heating.However,the ascending motions occurred over the eastern area of the anomalous sensible heating due to the augmentation of the vorticity advection with increasing height,causing water vapor to condense in the middle and upper troposphere.In turn,cyclonic vorticity was generated in the lower troposphere due to the increased vertical gradient of the condensation latent heating,resulting in the formation and further growth of the incipient vortex.Therefore,the vorticity creation due to the condensation heating played a dominant role during the subsequent enhancing stage of the incipient vortex.
Recent progress in the study of nonlinear atmospheric dynamics and related predictability of weather and climate in China (2007-2011) are briefly introduced in this article. Major achievements in the study of nonlinear atmospheric dynamics have been classified into two types:(1) progress based on the analysis of solutions of simplified control equations, such as the dynamics of NAO, the optimal precursors for blocking onset, and the behavior of nonlinear waves, and (2) progress based on data analyses, such as the nonlinear analyses of fluctuations and recording-breaking temperature events, the long-range correlation of extreme events, and new methods of detecting abrupt dynamical change. Major achievements in the study of predictability include the following:(1) the application of nonlinear local Lyapunov exponents (NLLE) to weather and climate predictability; (2) the application of condition nonlinear optimal perturbation (CNOP) to the studies of El Nin o-Southern Oscillation (ENSO) predictions, ensemble forecasting, targeted observation, and sensitivity analysis of the ecosystem; and (3) new strategies proposed for predictability studies. The results of these studies have provided greater understanding of the dynamics and nonlinear mechanisms of atmospheric motion, and they represent new ideas for developing numerical models and improving the forecast skill of weather and climate events.
The interannual variability of wintertime snow depth over the Tibetan Plateau(TP) and related atmospheric circulation anomalies were investigated based on observed snow depth measurements and NCEP/NCAR reanalysis data.Empirical orthogonal function(EOF) analysis was applied to identify the spatio-temporal variability of wintertime TP snow depth.Snow depth anomalies were dominated by a monopole pattern over the TP and a dipole structure with opposite anomalies over the southeastern and northwestern TP.The atmospheric circulation conditions responsible for the interannual variability of TP snow depth were examined via regression analyses against the principal component of the most dominant EOF mode.In the upper troposphere,negative zonal wind anomalies over the TP with extensively positive anomalies to the south indicated that the southwestward shift of the westerly jet may favor the development of surface cyclones over the TP.An anomalous cyclone centered over the southeastern TP was associated with the anomalous westerly jet,which is conducive to heavier snowfall and results in positive snow depth anomalies.An anomalous cyclone was observed at 500 hPa over the TP,with an anomalous anticyclone immediately to the north,suggesting that the TP is frequently affected by surface cyclones.Regression analyses revealed that significant negative thickness anomalies exist around the TP from March to May,with a meridional dipole anomaly in March.The persistent negative anomalies due to more winter TP snow are not conducive to earlier reversal of the meridional temperature gradient,leading to a possible delay in the onset of the Asian summer monsoon.
Simulated outgoing longwave radiation(OLR) outputs by two versions of the grid-point atmospheric general circulation model(GAMIL) were analyzed to assess the influences of improvements in cloud microphysics and convective parameterization schemes on the simulation of the Madden-Julian oscillation(MJO) and other tropical waves.The wavenumber-frequency spectral analysis was applied to isolate dominant modes of convectively coupled equatorial waves,including the MJO,Kelvin,equatorial Rossby(ER),mixed Rossby-gravity(MRG),and inertio-gravity(IG) waves.The performances of different versions of the GAMIL model(version 1.0(GAMIL1.0) and version 2.0(GAMIL2.0)) were evaluated by comparing the power spectrum distributions of these waves among GAMIL1.0,GAMIL2.0,and observational data.GAMIL1.0 shows a weak MJO signal,with the maximum variability occurring separately at wavenumbers 1 and 4 rather than being concentrated on wavenumbers 1-3,suggesting that GAMIL1.0 could not effectively capture the intraseasonal variability.However,GAMIL2.0 is able to effectively reproduce both the symmetric and anti-symmetric waves,and the significant spectra of the MJO,Kelvin,and MRG waves are in agreement with observational data,indicating that the ability of GAMIL2.0 to simulate the MJO and other tropical waves is enhanced by improving the cloud microphysics and convective parameterization schemes and implying that such improvements are crucial to further improving this model's performance.
