Convectively Forced Diurnal Gravity Waves in the Maritime Continent

Long-lived, zonally propagating diurnal rainfall disturbances are a highly pronounced and common feature in the Maritime Continent (MC). A recent study argues that these disturbances can be explained as diurnally phase-locked gravity waves. Here we explore the origins of these waves through regional cloud-permitting numerical model experiments. The gravity waves are reproduced and isolated in the model framework through the combined use of realistic geography and diurnally cyclic lateral boundary conditions representative of both characteristic easterly and westerly background zonal flow regimes. These flow regimes are characteristic of the Madden–Julian oscillation (MJO) suppressed and active phase in the MC, respectively. Tests are conducted wherein Borneo, Sumatra, or both islands and/or their orography are removed. These tests imply that the diurnal gravity waves are excited and maintained directly by latent heating from the vigorous mesoscale convective systems (MCSs) that form nocturnally in both Borneo and Sumatra. Removing orography has only a secondary impact on both the MCSs and the gravity waves, implying that it is not critical to these waves. We therefore hypothesize that diurnal gravity waves are fundamentally driven by mesoscale organized deep convection, and are only sensitive to orography to the measure that the convection is affected by the orography and its mesoscale flows. Factor separation further reveals that the nonlinear interaction of synchronized diurnal cycles in Sumatra and Borneo slightly amplifies this gravity wave mode compared to if either island existed in isolation. This nonlinear feedback appears most prominently at longitudes directly between the two islands.

长期存在、呈纬向传播的日降雨扰动是海洋大陆（MC）一个非常显著且常见的特征。最近一项研究认为，这些扰动可以解释为日锁相重力波。在此我们通过区域允许云存在的数值模式实验来探究这些波的起源。通过综合使用真实的地理情况以及代表东风和西风背景纬向流型特征的日循环侧向边界条件，重力波在模式框架中得以重现和分离。这些流型分别是海洋大陆中 Madden - Julian振荡（MJO）抑制相和活跃相的特征。进行了一些测试，其中移除了婆罗洲、苏门答腊，或者这两个岛屿以及/或者它们的地形。这些测试表明，日重力波是由婆罗洲和苏门答腊夜间形成的强烈中尺度对流系统（MCSs）的潜热直接激发和维持的。移除地形对中尺度对流系统和重力波只有次要影响，这意味着地形对这些波并不关键。因此我们假设，日重力波从根本上是由中尺度有组织的深对流驱动的，并且只有在对流受地形及其中尺度气流影响的程度上才对地形敏感。因子分离进一步揭示，与单独存在一个岛屿相比，苏门答腊和婆罗洲同步日循环的非线性相互作用略微放大了这种重力波模式。这种非线性反馈在两岛之间的经线上最为显著。