School of Earth Sciences - Research Publications
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ItemNo Preview AvailableDiurnal Cycle of Surface Winds in the Maritime Continent Observed through Satellite Scatterometry(AMER METEOROLOGICAL SOC, 2019-06)Abstract The diurnal cycle of surface winds throughout the Maritime Continent plays a significant role in the formation of precipitation over the islands of the region and over the surrounding seas. This study investigates the connection between the diurnal cycles of surface wind and offshore precipitation using data from four satellite scatterometer instruments and two satellite precipitation radar instruments. For the first time, data from three scatterometer instruments are combined to yield a more temporally complete picture of the surface wind diurnal cycles over the Maritime Continent’s surrounding seas. The results indicate that land–sea breezes typically propagate over 400 km offshore, produce mean wind perturbations of between 1 and 5 m s−1, and propagate as gravity waves at 25–30 m s−1. Diurnal precipitation cycles are affected through gravity wave propagation processes associated with the land–sea breezes, and through the convergence of land breezes from nearby islands. These observational results are then compared with previous mesoscale modeling results. It is shown that land–sea breezes occur too early, and are too intense in these modeling results, and this may partly explain why these modeling results also exhibit an early, overly intense diurnal precipitation cycle. This study also investigates variations in the diurnal cycle of surface winds at seasonal and intraseasonal time scales. Previous work has suggested that seasonal and intraseasonal variations in surface heating affect the land–sea breeze circulation and diurnal precipitation cycles; we argue that variations in background winds also play a defining role in modulating coastally influenced local winds.
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ItemNo Preview AvailableMesoscale Variation in Diabatic Heating around Sumatra, and Its Modulation with the Madden-Julian Oscillation(AMER METEOROLOGICAL SOC, 2018-08-01)Diabatic heating in the Maritime Continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variability associated with propagating planetary-scale disturbances. In this study, the diabatic heating from a 10-yr austral summer simulation over the Maritime Continent with a 4-km horizontal grid length is analyzed with respect to diurnal, spatial, and intraseasonal variations. Results are compared, where possible, to analogous estimates from the TRMM precipitation radar. We show that the heating budget is largely a balance between latent heating and vertical advection, with rays of heating and cooling extending upward and outward from the coast evident in the advection terms, consistent with the gravity wave representation of the tropical sea breeze. By classifying rainfall into convective and stratiform components, it is shown that simulated convective heating over Sumatra peaks in MJO phases 2 and 3, while simulated stratiform heating peaks in phase 4. Similarly, spectral latent heating estimates from the TRMM Precipitation Radar show that stratiform heating peaks in phases 3 and 4, while convective heating peaks in phases 2 and 3. It is also shown that stratiform precipitation plays a greater role in offshore precipitation during the night, albeit with embedded convective cores, than over the land during the day. These results emphasize the importance of achieving a realistic representation of convective and stratiform processes in high-resolution simulations in the tropics, both for total rainfall estimates and for realistic latent heating
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ItemScatterometer estimates of the tropical sea-breeze circulation near Darwin, with comparison to regional models(WILEY, 2017-10)In tropical coastal environments, simulating the diurnal cycle of wind and precipitation in numerical weather and climate models presents unique challenges due to the interaction of intraseasonal and mesoscale dynamics. This can lead not only to incorrect short‐term weather forecasts but also to unphysical energy and momentum transport by convective processes. In particular, the sea/land‐breeze circulation and its role in initiating convection has been identified as a possible source of errors in the timing and offshore extent of coastal precipitation in the Tropics. In this study, the offshore land/sea breeze around Darwin, Australia, is examined using scatterometer wind observations and two regional atmospheric models. Although the comparison is limited by satellite swath times which cluster around two times of day, useful results are obtained by sub‐sampling the simulated data to match the coverage of the scatterometer data. We find that offshore surface sea‐breeze characteristics (intensity and horizontal spatial extent) from the models and satellite estimates are generally in good agreement, with intensity differences less than 2 m s−1, and offshore extents not varying by more than approximately 150 km. The variation in offshore extent and amplitude of the land/sea‐breeze wind perturbations with monsoon regime is well simulated. Furthermore, despite the simplifying assumptions of linear sea‐breeze theory, the model and scatterometer results are in broad agreement with theoretical values, particularly in the presence of the weak background winds during the monsoon break period.
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ItemEvolution of the Diurnal Precipitation Cycle with the Passage of a Madden-Julian Oscillation Event through the Maritime Continent(AMER METEOROLOGICAL SOC, 2016-05)Changes in the diurnal precipitation cycle as the Madden–Julian oscillation (MJO) propagates through the Maritime Continent are investigated to explore the processes behind seaward-propagating precipitation northeast of New Guinea. Satellite rainfall estimates from TRMM 3B42 and the Climate Prediction Center morphing technique (CMORPH) are combined with simulations from the Weather Research and Forecasting (WRF) Model with a horizontal resolution of 4 km. Comparison with 24-h rain gauge measurements indicates that both satellite estimates and the WRF Model exhibit systematic biases. Despite these biases, the changing patterns of offshore precipitation with the passage of the MJO show good consistency between satellite estimates and the WRF Model. In the few days prior to the main MJO envelope, light background wind, relatively clear skies, and an increasingly moist environment promote favorable conditions for the diurnal precipitation cycle. Two distinct processes are identified: 100–200 km from the coast, precipitation moves offshore as a squall line with a propagation speed of 3–5 m s−1. Farther offshore, precipitation propagates with a speed close to 18 m s−1and is associated with an inertia–gravity wave generated by diurnally oscillating heating from radiative and moist convective processes over the land. A gravity wave signature is evident even after the MJO active period when there is little precipitation. By correcting for the background flow perpendicular to the coast, potential temperature anomalies for the lead-up, active, and follow-on MJO periods are shown to collapse to a remarkably invariant shape for a given time of day.
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ItemA local index of Maritime Continent intraseasonal variability based on rain rates over the land and sea(AMER GEOPHYSICAL UNION, 2016-09)Abstract A local index for describing intraseasonal variability over the Maritime Continent is developed. The index is based on the ratio of area‐averaged rain rate over the land to that over the sea. It takes advantage of the fact that the main convective envelope of intraseasonal variability events tends to modulate the diurnal precipitation cycle over the land over the entire Maritime Continent. Lagged analysis is used to create composite intraseasonal variability events, where “day 0” is chosen according to when the normalized rain rate over the sea becomes greater than that over the land. The index identifies intraseasonal variability events associated with the Madden Julian Oscillation as well as equatorial Kelvin waves and westward propagating equatorial Rossby waves. The results suggest a similar local impact of all such events in suppressing the rain rate over land relative to that over the sea when the main convective envelope approaches.
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ItemA 10-Year Austral Summer Climatology of Observed and Modeled Intraseasonal, Mesoscale, and Diurnal Variations over the Maritime Continent(American Meteorological Society, 2017-05-15)The Maritime Continent is one of the wettest regions on the planet and has been shown to be important for global budgets of heat and moisture. Convection in the region, however, varies on several interrelated scales, making it difficult to quantify the precipitation climate and understand the key processes. For example, the diurnal cycle in precipitation over the land varies substantially according to the phase of the Madden–Julian oscillation (MJO), and the diurnal precipitation cycle over the water is coupled to that over the land, in some cases for distances of over 1000 km from the coast. Here, a 10-yr austral summer climatology of diurnal and MJO-scale variations in rain rate over the land and sea over the Maritime Continent is presented. The climatology is based on mesoscale model simulations with a horizontal grid length of 4 km and satellite precipitation estimates. The amplitude of the observed diurnal precipitation cycle is shown to reach a maximum just prior to the MJO active phase, with a weaker secondary maximum after the MJO active phase. Although these two maxima also exist in the modeled diurnal precipitation cycle, there is less difference between the maxima before and after the MJO active phase than in the observations. The modeled sea-breeze circulation is also shown to possess approximately equal maxima just before and just after the MJO active period, suggesting that the asymmetry of the diurnal precipitation cycle about the MJO active period is related more to moisture availability than kinematic forcing.