Intraseasonal Variability of the Diurnal Cycle of Precipitation in the Philippines

Abstract

Precipitation in the region surrounding the South China Sea over land and coastal waters exhibits a strong diurnal cycle associated with a land–sea temperature contrast that drives a sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of diurnal precipitation patterns, an understanding of which is a primary goal of the field campaign Propagation of Intraseasonal Tropical Oscillations (PISTON). Using 21 years of CMORPH precipitation for Luzon Island in the northern Philippines, it is shown that the diurnal cycle amplitude is generally maximized over land roughly 1 week before the arrival of the broader oceanic convective envelope associated with the BSISO. A strong diurnal cycle in coastal waters is observed in the transition from the inactive to active phase, associated with offshore propagation of the diurnal cycle. The diurnal cycle amplitude is in phase with daily mean precipitation over Mindanao but is nearly out of phase over Luzon. The BSISO influence on the diurnal cycle on the eastern side of topography is nearly opposite to that on the western side. Using wind, moisture, and radiation products from the ERA5 reanalysis, it is proposed that the enhanced diurnal cycle west of the mountains during BSISO suppressed phases is related to increased insolation and weaker prevailing onshore winds that promote a stronger sea-breeze circulation when compared with the May–October mean state. Offshore propagation is suppressed until ambient midlevel moisture increases over the surrounding oceans during the transition to the active BSISO phase. In BSISO enhanced phases, strong low-level winds and increased cloudiness suppress the sea-breeze circulation.

Impact of the Boreal Summer Intraseasonal Oscillation on the Diurnal Cycle of Precipitation near and over the Island of Luzon

“…Mechanisms outlined previous studies for offshore precipitation involving convergence with a land/sea breeze (WMONEX, Houze et al 1981), katabatic winds (TiMREX, Ruppert et al 2013), or cold pool (TiMREX, Xu et al 2012) seem plausible, as the existence of a strong monsoon flow would provide the onshore winds to converge with any of these seaward features. This supports the finding in Natoli and Maloney (2019) that offshore movement of convection is maximized in the transition from inactive to active phases. During these periods, solar insolation is still pronounced (generating instability, sea-breeze circulations, and a stronger diurnal cycle over land); however, the influences of the active BSISO (increased moisture, stronger low-level winds) also begin to materialize.…”

“…The contrast in the relationship of landsea convection between active and inactive BSISO periods was explained in part by convective instability, which was found to be higher (lower) over the Philippines (SCS) during inactive periods compared to active periods. Natoli and Maloney (2019) examined the precipitation patterns in the Philippines, and how the diurnal cycle and offshore precipitation is impacted by the BSISO. Their findings show that the amplitude of the diurnal cycle is maximized at the end of inactive periods and minimized at the end of active periods.…”

Impact of multiple-scale circulation interactions on the spring diurnal precipitation over Luzon

“…This eastward propagation of diurnal precipitation is opposite of the prevailing low-level easterly wind (Fig. 2 a) and different from the well-known westward propagation of diurnal precipitation in summer 8 , 10 , 11 . Therefore, the question arises whether the observed eastward propagation of diurnal precipitation in Fig.…”

“…The reason for the east–west asymmetry in the low-level wind convergence over Luzon at 14 h must be determined. In general, the local circulation over Luzon, induced by the land–sea thermal contrast, is characterized by island-scale sea breezes during daytime 8 , 12 , 13 . However, the formation of island-scale sea breezes alone, without considering the effect of topography, should be symmetrical.…”

“…5 , we suggest that the eastward propagation of diurnal precipitation over Luzon is a result of the interaction between the topography and multiple-scale circulation changes (including island-scale LSB, mountain–valley breeze, large-scale diurnal wind anomalies, and mid-to-upper-level prevailing wind) during the day. Among these mechanisms, the impact of island-scale LSB, mountain–valley breeze, and upper-level prevailing wind are also suggested to be important factors that drive the westward propagation of diurnal precipitation over Luzon in summer 7 , 8 , 10 , 11 . However, unlike the above-mentioned upper-level prevailing westerly wind in spring, the upper-level prevailing wind in summer is easterly 7 .…”