Scale interaction of the Diurnal Cycle of Rainfall: Influence of Large-scale Circulations

The influence of the large-scale circulations on the phase and amplitude of the diurnal cycle of rainfall during Australian Summer (‘December- February’; DJF) over the Maritime Continent (MC) and northern Australia was investigated using the TRMM 3B42 and 3G68 dataset. The gridded rainfalls were partitioned into MJO categories (active, suppressed and weak) and ENSO phases (La Nina, El Nino and neutral). The Real-time Multivariate MJO (RMM) Index of Wheeler and Hendon (2004) is used to stratify the rainfall into the three MJO phases. The diurnal cycles were composited and Empirical Orthogonal Analysis (EOF) was applied to extract the spatial and temporal variability. Distinct variations in the rainfall distribution pattern amongst categories of the MJO over land and ocean are seen. The result of the composite mean rainfall distribution shows that the average daily rainfall rate over islands is higher during suppressed MJO days, while for surrounding oceans and northern regions of Australia, more rainfall occurs during MJO active days. The normalized relative amplitude (NRA) of the diurnal cycle of rainfall shows that morning rainfall near coastal areas during active days of the MJO is one and a half times greater than the climatological mean rainfall, but is less than or equal to the climatological mean during other phases of the MJO. Similarly, during the suppressed phase of the MJO evening rainfall is greater over the islands than in other MJO phases. The first two modes of the EOF alone explain more than 88 (65) % of the variance for the 3B42 (3G68) rainfall and the corresponding principal component time series show a marked diurnal cycle. The results show that both the amplitude and phase of the diurnal cycle of rainfall are modulated by the categories of the MJO. In general, the peak in the diurnal cycle for active (suppressed/weak) days of the MJO lags (leads) the peak in the diurnal cycle for total rainfall by two hours. Over Darwin and its adjacent regions, the active phase of the MJO is responsible for the occurrence of maximum rainfall after midnight, which is unusual in this region. The influence of ENSO phases is currently being assessed.