Intraseasonal Variability of the Diurnal Cycle of Precipitation in the Philippines

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.

南海周边陆地及沿海水域的降水呈现出强烈的日循环，这与海陆温度差异有关，该差异驱动了海风环流。北半球夏季季节内振荡（BSISO）是日降水模式的重要调节因素，对其的理解是“热带季节内振荡传播”（PISTON）外场观测活动的主要目标。利用菲律宾北部吕宋岛21年的CMORPH降水数据，结果表明，在与BSISO相关的更广泛的海洋对流包络到来前约1周，陆地的日循环振幅通常达到最大。在从非活跃期到活跃期的过渡阶段，沿海水域观测到强烈的日循环，这与日循环向海洋传播有关。棉兰老岛的日循环振幅与日平均降水同相，而在吕宋岛则几乎反相。BSISO对地形东侧日循环的影响与西侧几乎相反。利用ERA5再分析的风、水汽和辐射产品，提出在BSISO抑制阶段山脉以西日循环增强与日照增加以及较弱的盛行向岸风有关，与5 - 10月平均状态相比，这促进了更强的海风环流。在向BSISO活跃期过渡期间，直到周围海洋上空的中层环境水汽增加，向海传播才受到抑制。在BSISO增强阶段，强烈的低空风和云量增加抑制了海风环流。