Diurnal Variations during the Australian Monsoon Experiment (AMEX) Phase II

Abstract

The diurnal variations in tropical cloudiness and tropospheric winds during the Australian Monsoon Experiment (AMEX) Phase II are documented and compared to those observed elsewhere. A diurnal variation in tropical cloudiness was found to be a consistent feature of both disturbed and undisturbed conditions. The tropical cloudiness, as inferred from satellite and radar data, averaged over the entire period of AMEX Phase II, was at a maximum during the morning over the ocean and during the late afternoon over the Arnhem Land region of northern Australia. The diurnal variation in high cloud, as measured by satellite was 3:2 over the ocean and 4:1 over Arnhem Land. Radar data indicated a 1 0: 1 variation in convection over Arnhem Land, a 2:1 variation over the neighboring ocean and a 3:2 variation in the stratiform echoes over both Ambem land and the neighboring mean. Interaction between local circulations and the large scale flow was found to he associated with the observed diurnal variations in tropical cloudiness. The large scale monsoon circulation exhibited a diurnal oscillation with maxima in both the low-level easterly and equatorial westerly flow during the early morning. Variations in the vertical motion fields were in phase with the inferred cloudiness changes, but the midlevel maximum in vertical motion did not correspond with the strongest boundary layer convergence. The precise timing upward vertical motion over oceanic regions within the primary AMEX domain and the less reliably observed region to the north of Australia varied according to the degree of convective activity; consistent features were a maximum in vertical motion at 0830 LST during disturbed conditions and an 0230 LST maximum during suppressed conditions.