Search Results

You are looking at 1 - 2 of 2 items for

  • Author or Editor: Michael B. Natoli x
  • Refine by Access: All Content x
Clear All Modify Search
Michael B. Natoli and Eric D. Maloney

Abstract

The impact of quasi-biweekly variability in the monsoon southwesterly winds on the precipitation diurnal cycle in the Philippines is examined using CMORPH precipitation, ERA5 data, and outgoing longwave radiation (OLR) fields. Both a case study during the 2018 Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign and a 23-yr composite analysis are used to understand the effect of the quasi-biweekly oscillation (QBWO) on the diurnal cycle. QBWO events in the west Pacific, identified with an extended EOF index, bring increases in moisture, cloudiness, and westerly winds to the Philippines. Such events are associated with significant variability in daily mean precipitation and the diurnal cycle. It is shown that the modulation of the diurnal cycle by the QBWO is remarkably similar to that by the boreal summer intraseasonal oscillation (BSISO). The diurnal cycle reaches maximum amplitude on the western side of the Philippines on days with average to above-average moisture, sufficient insolation, and weakly offshore prevailing wind. This occurs during the transition period from suppressed to active large-scale convection for both the QBWO and BSISO. Westerly monsoon surges associated with QBWO variability generally exhibit active precipitation over the South China Sea (SCS), but a depressed diurnal cycle. These results highlight that modes of large-scale convective variability in the tropics can have a similar impact on the diurnal cycle if they influence the local-scale environmental background state similarly.

Restricted access
Michael B. Natoli and Eric D. Maloney

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.

Free access