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Shunya Koseki, Benjamin Pohl, Bhuwan Chandra Bhatt, Noel Keenlyside, and Arielle Stela Nkwinkwa Njouodo

Abstract

Adopting a state-of-the-art numerical model system, we investigate how the diurnal variations in precipitation and local breeze systems are characterized by lower-boundary conditions related to the Drakensberg highland and warm SST associated with the Agulhas Current. A control simulation can simulate the hydrometeorological climates in the region realistically, but the terrestrial rainfall is overestimated. During daytime, the precipitation is confined to the Drakensberg highland, and there is an onshore local breeze, while during midnight to morning, the rainfall is confined to the Agulhas Current, and the breeze is offshore. These variations are captured by the numerical simulation, although the timing of maximum rainfall is early over the land and delayed over the ocean. The sensitivity experiment in which the Drakensberg is absent shows a drastic modification in the diurnal variations over land and ocean. The terrestrial precipitation is largely decreased around the Drakensberg and is largest along the coast during daytime. The nocturnal marine precipitation along the Agulhas Current is also reduced. Although the daily residual breeze is still pronounced even without the Drakensberg, wind speed is weakened. We attribute this to the reduction of precipitation. In another sensitivity experiment with smoothened warm SST due to the Agulhas Current, the amplitudes of diurnal variations are not modified remarkably, but the coastal rainfall is diminished to some extent due to less evaporation along the Agulhas Current. This study concludes that the Drakensberg plays a crucial role for the diurnal cycle, and the impact of the Agulhas Current is limited on the diurnal cycle of the coastal precipitation in this region.

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Sunil Kumar Pariyar, Noel Keenlyside, Bhuwan Chandra Bhatt, and Nour-Eddine Omrani

Abstract

The space–time structure of intraseasonal (10–90 day) rainfall variability in the western tropical Pacific is studied using daily 3B42 TRMM and ERA-Interim reanalysis data for the period 1998–2014. Empirical orthogonal function (EOF) analysis of 10–90-day filtered daily rainfall anomalies identifies two leading modes in both May–October and November–April; together these modes explain about 11%–12% of the total intraseasonal variance over the domain in both seasons and up to 60% over large areas of the western Pacific in both climatological periods. The two leading modes in May–October are linearly related to each other and both are well correlated with the Madden–Julian oscillation (MJO) indices. Although the two leading EOF modes in November–April are linearly independent of each other, both show statistically significant correlations with the MJO. The phase composites of 30–80-day filtered data show that the two leading modes are associated with strong eastward and northward propagation of rainfall anomalies in May–October, and eastward and southward propagation of rainfall anomalies in November–April. The eastward propagation of rainfall anomalies in both seasons and southeastward propagation related with EOF2 in November–April is linked to the development of low-level moisture flux convergence ahead of the active convection. Similarly, the northward propagation in May–October is also connected with low-level moisture flux convergence, but surface wind and evaporation variations are also important. The wind–evaporation–SST feedback mechanism drives the southeastward propagation of rainfall anomalies associated with EOF1 in November–April. The different mechanisms for southeastward propagation associated with two leading modes in November–April suggest dynamically different relations with the MJO.

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Chee-Kiat Teo, Tieh-Yong Koh, Jeff Chun-Fung Lo, and Bhuwan Chandra Bhatt

Abstract

Principal component analysis (PCA) is able to diagnose the diurnal rain cycle in the Maritime Continent into two modes that explain most of the diurnal variability in the region. The first mode results from the differential variation in potential instability forced by surface heat flux, insolation, and longwave radiative cooling on land and sea. The second mode is associated with intrinsic mesoscale dynamics of convective systems and its interactions with gravity waves, density currents, and local circulations in coastal regions or mountainous terrain. The spatial phase relation between the two modes determines whether a diurnal signal is propagating or stationary. Thus, validating model simulations of diurnal rainfall using PCA provides insights on the representation of dynamics and physics. In this paper, the main modes of diurnal rain variability in the Maritime Continent from satellite observations are studied and are compared with those from Weather Research and Forecasting (WRF) model simulations. Hovmoeller analyses of the reconstructed rainfall from the first two PCA modes clarify the impact of coastlines and mountains as sources of propagating signals. Wave cavities are identified in the Straits of Malacca, Malay Peninsula, and north Sumatra where stationary signals are produced. WRF reproduces the first two modes but each with a phase lead of about 1–2 h or longer, depending on the satellite rainfall product used for comparison. The basic diurnal forcing in the model seems to be too strong and the model responds too strongly to small islands and small-scale topography. The phase speed of propagating signals over open sea is correctly modeled but that over land is too slow.

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