Using additional sounding observations during the Intensive Observing Period (IOP) of the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) in 2006, the characteristics of weather systems and associated environmental conditions are further analyzed during four temporal regimes (active wet monsoon, suppressed dry monsoon, clear day, and monsoon break). Monsoon low-pressure systems are predominant during the active wet monsoon and monsoon break periods. During the active monsoon period, heavy rainfall (> 100 mm day-1) is produced over the nearby tropical ocean of north Australia and the maritime continent centered on the Tiwi Islands, where the apparent southerly-to-southwesterly winds dominate at low levels over the ocean under the influence of the monsoon low in contrast to the westerly winds during a typical active north Australia summer monsoon regime. In the monsoon break, limited scattered rainfall is distributed over coastal regions of northern Australia and inland.
During the suppressed monsoon period and clear days, the environment is very dry under the influence of a monsoon ridge. This is not favorable for the development of convection and the production of the rainfall. In a suppressed monsoon regime, the dry air intrusion in the mid-troposphere with the driest center around the 600-hPa level is related to dry air advection from the subtropical regions and the subsidence associated with the approaching of a monsoon ridge. Typical westerly winds prevail between 850 and 700 hPa in Darwin. During clear days, the subsidence related to the monsoon ridge is the major reason for the driest environmental condition during 2006 TWP-ICE. With the help of additional sounding data, dominant weather systems during four periods are clearly identified when compared with NCEP/FNL global analysis. Also the daily rainfall predictions are improved during the IOP, especially in the active monsoon period.