Fine vertical structure of the divergence in the upper troposphere of the tropics

The vertical structure of the (horizontal) divergence was investigated with NCEP reanalysis data, ECMWF analysis data and radiosonde data. The pair of the shallow divergence and convergence was frequently detected in the upper troposphere over the Indonesian region (90-120E along the equator) in the dry season (July - September). The divergent field in the tropical troposphere is very complicated and cannot be described easily with using only the rough image of Hadley and Walker circulation.

(1) The regional variability in the vertical structure of divergence does not correspond to the land-sea contrast directly. The longitudinal and latitudinal variability is particularly large in the Indian monsoon region and Indonesian region. (2) In the northern summer, the climatological divergence region at 150hPa extends mainly in the north of 5S where the cumulus activity is stronger than south of that latitude. On the other hand, the divergence at 300hPa is extended widely in the tropics of the both hemisphere in the Asian longitude. (3) During the rainy season, it is well known that the convergence is dominant in the lower troposphere and the divergence in the upper troposphere. However, besides that, the weak but long-lasting pair of divergence (below) and convergence (above) was detected around 5-7km levels. It may be related to the melting layer convergence near the 0C level indicated by some previous authors. (4) During the dry season in the Indonesian region (July - September), another divergence (above) and convergence (below) pair was frequently observed. This pair repeatedly appears and continues several days with a few day intervals. The divergence was detected around 150hPa and the convergence was around 250-300hPa in many cases. It can be well observed in 00Z data but not clear in 12Z data of ECMWF analysis. It is surely related to the diurnal convection cycle in this region (more active at 12Z than 00Z).

As the divergence field of the analysis data is much affected by the assimilation model, the original radiosonde dataset was also utilized to check the pattern found in analysis data. The fine structure of divergence detected in this study is surely related to the vertical structure of the equatorial wave and the vertical transport of the chemical species in the tropics.