A parameterization of soil freezing-thawing physics for use in the land-surface model of the National Center for Atmospheric Research(NCAR) Community Climate Model(CCM3) is developed and evaluated.The new parameterization scheme has improved the representation of physical processes in the existing land surface model.Numerical simulations using CCM3 with improved land-surface processes and with the original land-surface processes are compared against the NCEP reanalysis.It is found that the CCM3 version using the improved land surface model shows significant improvements in simulating precipitation in China during the summer season,the general circulation over East Asia,and wind fields over the Tibet Plateau.For the summer season,the improved model was able to better simulate the Indian summer monsoon components,including the mean northerly wind in the upper troposphere and mean southerly wind in the lower troposphere.
In this paper, the possible reason of Tibetan Plateau （TP） temperature increasing was investigated. An increase in T min （minimum temperature） plays a robust role in increased TP temperature, which is strongly related to SST over the warm pool of the western Pacific Ocean, the subtropical westerly jet stream （SWJ）, and the tropical easterly upper jet stream （TEJ）, and the 200hPa zonal wind in East Asia. Composite analysis of the effects of SST, SWJ, and TEJ on pre and postabrupt changes in T a （annual temperature） and T min over the TP shows remarkable differences in SST, SWJ, and TEJ. A lag correlation between T a /T min , SST, and SWJ/TEJ shows that changes in SST occur ahead of changes in T a /T min by approximately one to three seasons. Partial correlations between T a /T min , SST, and SWJ/TEJ show that the effect of SWJ on T a /T min is more significant than the effect of SST. Furthermore, simulations with a community atmospheric model （CAM3.0） were performed, showing a remarkable increase in T a over the TP when the SST increased by 0.5 ？ C. The main increase in T a and T min in the TP can be attributed to changes in SWJ. A possible mechanism is that changes in SST force the TEJ to weaken, move south, and lead to increased SWJ and movement of SWJ northward. Finally, changes in the intensity and location of the SWJ cause an increase in T a /T min . It appears that TP warming is governed primarily by coherent TEJ and SWJ variations that act as the atmospheric bridges to remote SSTs in warmpool forcing.
The interaction between the low-frequency atmospheric oscillation(Madden-Julian Oscillation,MJO) and the diabatic heating over the Qinghai-Xizang Plateau(QXP) from March to June is analyzed.The results show that there are respectively two and one wave trains around the QXP during the onset of the South China Sea monsoon in strong and weak monsoon years.The locations and strength of the wave train propagation differ between the strong and weak monsoon years.Because diabatic heating of the QXP prevents the low-frequency oscillation,the wave train of interaction between the diabatic heating and the zonal wind MJO propagates along the west and east of the QXP in the strong monsoon years.The distribution of the wave train interaction between the diabatic heating and the zonal wind MJO traverses the QXP and coincides with the location of the southern and northern upper-level jet streams,showing that they are remarkably correlated.An interesting and notable phenomenon is that the interaction between diabatic heating and the zonal wind MJO over the QXP suddenly disappears during the monsoon onset in weak monsoon years.
Using the Global Energy and Water Cycle Experiment-Asian Monsoon-Tibet Plateau Experiment (GAME-Tibet) observa-tional data-from October 2002 through September 2003-of Gaize in the western Qinghai-Xizang Plateau (QXP),the land-surface characteristics of the Qinghai-Xizang Plateau are simulated by the improved land-surface model Common Land Model (CoLM).The results show that CoLM can reproduce the land-surface characteristics of plateau areas well.In the sur-face-energy balance of the western QXP in the winter half year,the sensible heat (SH) flux constituted the dominant energy,and the latent heat (LH,here and after) flux is very small.Although the LH flux nearly equals zero in freezing season,it cannot be ig-nored during the period of freezing-thawing in QXP.In the transition season from midto late-May,the frequent phase change of soil water that is caused by the freezing-thawing process leads to the increase of LH flux and decrease of the Bowen ratio.The simulation results also indicate that the changes of surface effective heat fluxes (SH and LH flux) are associated with precipitation and the frequent change between freezing and thawing processes in soil surface.
ChengHai Wang,Rui Shi,Yang Cui,HongChao Zuo College of Atmospheric Sciences,Lanzhou University.No.222,South Tianshui Road,Lanzhou,Gansu 730000,China.
