Role of Coastal Convection to Moisture Buildup during the South China Sea Summer Monsoon Onset

Abstract

In this study, the climatological characteristics of object-based precipitation systems (OPSs) and moisture development are analyzed over the South China Sea (SCS) during the sharp transition of the summer monsoon onset. The satellite-observed statistics of the OPSs showed that over the 20-day pre-onset period, OPSs of small (< 100 km) to medium size (100-300 km) are active over the lands surrounding the SCS. The pre-onset composite mean shows a basin-scale (~ 1000 km) local circulation with anomalous subsidence over the ocean, and ocean convection is mostly suppressed. Over the 20-day post-onset period, large (> 300 km) OPSs develop over the coastal ocean and contribute to over 60 % of the total precipitation. The number of large OPSs observed significantly increases along with the sharp moisture buildup within 10 days after the onset. The moisture budget suggests that the local contribution from convective vertical mixing is the major moisture source during the first pentad after the onset. The relationship between moisture buildup and convection organization is then examined using a set of idealized cloud-resolving model (CRM) experiments, with a land-ocean configuration approximating the SCS basin. The CRM appropriately represents the observed development of coastal convection. In the noshear environment, a strong basin-scale circulation is formed, which suppresses the ocean moisture development. When large-scale vertical wind shear is imposed to represent the changes of large-scale circulation during the onset pentad, organized convection systems are increased over the coastal ocean and propagate toward the open ocean, accompanied by fast ocean moistening within 5-10 days.

Convective Cloud Regimes From a Classification of Object‐Based CloudSat Observations Over Asian‐Australian Monsoon Areas

“…Recent studies have found that the cloud vertical structure and the associated diabatic heating are tightly connected to the modulation of the large‐scale environment during the transition of intraseasonal variability (e.g., Ciesielski et al., 2017; Del Genio et al., 2012; DelGenio & Chen, 2015; Hung et al., 2020; Riley et al., 2011) and monsoon onset (e.g., Chen, Wu, & Ma, 2019a, Chen, Wu, Tsai et al., 2019b). In the future, collocating the other satellite products from the A‐train, the Global Precipitation Measurement, or the geostationary Himawari‐8 / 9 satellites can be carried out to provide additional properties to the convective systems.…”

“…CloudSat cloud mask observations were first processed by the 4‐way connection to identify cloud objects (Chen, Wu, Tsai et al., 2019; Tsai & Wu, 2017; Su et al., 2019). The connecting algorithm is provided in the supporting material .…”

A Framework to Evaluate Convective Aggregation: Examples With Different Microphysics Schemes

“…[2008]), the land surface model (Noah LSM, F. Chen & Dudhia [2001]), the first‐order turbulence closure (Shutts & Gray, 1994), the predicted particle properties microphysical scheme (Huang & Wu, 2020; Morrison & Milbrandt, 2015), and the block topography (Chien & Wu, 2016; Wu & Arakawa, 2011) has been implemented in the VVM for studying complicated interactions associated with convection, such as the unified parameterization of deep convection (Arakawa & Wu, 2013; Wu & Arakawa, 2014), stratocumulus transition (Tsai & Wu, 2016), the aggregated deep convection (Tsai & Wu, 2017), afternoon thunderstorms (Kuo & Wu, 2019), land‐atmosphere interactions (Wu et al., 2015, 2019; Wu & Chen, 2021), cloud‐aerosol interactions (Chang et al., 2021), and the coastal convection during summer monsoon onset (W.‐T. Chen et al., 2019).…”