Intraseasonal Variability in the Diurnal Cycle of Precipitation in the South China Sea and Maritime Continent

Precipitation in the region surrounding the South China Sea (SCS) over land and coastal waters exhibits a strong diurnal cycle due to land-sea temperature contrast driving the sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of the daily mean precipitation rate and the amplitude of the diurnal cycle, an understanding of which is a primary goal of the PISTON field program. Using 19 years of the TRMM 3B42HQ (high-quality) precipitation product to minimize interference from infrared measurements, it is shown that near the equator, the daily mean precipitation and diurnal cycle amplitude over land are maximized before the arrival of the broader oceanic convective envelope associated with the BSISO.

However, the off-equator portions of the SCS region do not see the same response in advance of the northward propagating envelope of the BSISO. In the case of Luzon, on the north end of the Philippine archipelago, there is little coherent variation in diurnal cycle amplitude as a function of BSISO phase, although a strong modulation of daily mean precipitation occurs as the BSISO evolves. Unlike near the equator, over both land and coastal waters, Luzon’s maximum daily mean precipitation rate occurs in phase with the minimum in outgoing longwave radiation (OLR). In coastal portions of southeast Asia, the diurnal cycle amplitude is maximized before BSISO onset, but the daily mean is still at its highest during the phase with minimum OLR.

We further examine the tendency for offshore propagation of convective systems over the various landmasses of of the Maritime Continent and surrounding the SCS in different BSISO regimes, including a comparison of different TRMM precipitation products. The TRMM precipitation radar alone tends to show an earlier peak in the diurnal cycle compared to products such as 3B42, which relies on microwave measurements from other satellites and infrared readings. Results are interpreted in the context of stratiform and convective heating modes, including variations in such modes as a function of phase of the diurnal cycle and BSISO as well as large scale phenomena, including equatorial wave dynamics, BSISO propagation, and monsoon patterns.