School of Earth Sciences - Theses

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Start date: 2000 × End date: 2019 × Subject: Australia × Subject: El Niño × Subject: 3B42 ×

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    Investigating the diurnal variation of the water and energy cycle
    RAUNIYAR, SURENDRA ( 2015)
    Many weather forecasting models have fundamental difficulties in accurately predicting the intensity and timing of rainfall over the Maritime Continent (MC) just north of Australia, a region known as the weather “boiler box” of the globe. Here, we examine the spatio-temporal variability of the diurnal cycle of rainfall over the MC and northern Australia caused by the major modes of climate variability, namely the Madden Julian Oscillation (MJO) and the El Niño Southern Oscillation (ENSO) using the high resolution TRMM 3B42 and 3G68 rainfall datasets. In addition, using datasets of unprecedented spatial and temporal resolutions over Darwin, Australia and its vicinity, a life-cycle of rainfall building mechanisms is examined during an event of the eastward progressing MJO to determine the reasons behind occurrence of secondary maximum rainfall in the diurnal cycle of rainfall at Darwin. 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 many parts of islands is substantially higher compared to the climatology during the suppressed MJO. In contrast, over the surrounding oceans and northern regions of Australia more rainfall occurs during MJO active days. These differences are also well depicted in large-scale dynamical and thermodynamical fields derived from the NCEP reanalyses. Unlike previous studies, we found that the MJO modulates both the amplitude and phase of the diurnal cycle of rainfall. The amplitude of morning maximum rainfall near coastal areas during active days of the MJO is 1.5 times greater than the climatological mean rainfall, but is less than or equal to the climatological mean during other phases of the MJO and depends on the local geography and orography. Similarly, the peak in the diurnal cycle for active and suppressed/weak days of the MJO respectively, lags and leads the peak in the diurnal cycle for total rainfall by two hours. Despite the alternating patterns of widespread large-scale subsidence and ascent associated with the Walker circulation, which dominates the climate over the MC during the opposing phases of ENSO, many of the islands of the MC show localized differences in rainfall anomalies that depend on the local geography and orography. While ocean regions mostly experience positive rainfall anomalies during La Niña, some local regions over the islands have more rainfall during El Niño. These local features are also associated with anomalies in the amplitude and characteristics of the diurnal cycle in these regions. These differences are also well depicted in large-scale dynamical fields derived from the ERA-interim reanalyses. Detailed analysis of rainfall building mechanisms over Darwin and its vicinity showed that the large-scale forcing dominates during the MJO phase 4 - 7, starting by the reversal of low- to mid-level easterly winds to moist westerly winds, reaching a maximum in phase 5 and weakening through phases 6 to 7. During phases 4-6, most of the study domain experiences widespread rainfall, but with distinct spatial and temporal structures. In addition, during these phases, coastal areas near Darwin receive more rainfall in the early morning due to the spreading or expansion of rainfall from the Beagle Gulf, explaining the occurrence of a secondary peak over Darwin. Even with correct initialization of the MJO, the 12 km resolution ACCESS-A model fails to simulate this peak. In contrast, local-scale mechanisms (sea breezes) reinvigorate from phase 8, further strengthening through phases 1-3, when low-level easterly winds become established over Darwin producing rainfall predominately over land and island locations during the afternoon. During these phases, below average rainfall is observed over most of the radar domain, except over the Tiwi Islands in phase 2.