The Diurnal Cycle of Convection over the Northern South China Sea

Abstract

The diurnal cycle of convection over the northern South China Sea (SCS) during the onset of the summer monsoon is documented using data from the May−June 1998 South China Sea Monsoon Experiment (SCSMEX) and from ten years of observations from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar.Results both during SCSMEX and from TRMM observations show a prominent signal of propagating convection over the northern SCS. In particular, convection is found to initiate just off the southern coast of mainland China near sunrise and propagate southeastward at about 10−15 m s −1 to the central part of the northern SCS the early afternoon, finally dissipating about 500 km offshore in the late evening. Both indirect measures, such as infrared brightness temperatures and areal cold (< 208 K) convective cloud coverage, and direct measures, i.e., radar reflectivities from the Bureau of Meteorology Research Centre (BMRC) and TRMM radars, show this behavior. The radar data indicate an increasing stratiform contribution to total precipitation as the convective systems move farther offshore, indicative of maturing mesoscale convective systems. Neither advection nor gravity current processes appear to explain the offshore propagation of the envelope of convection, suggesting gravity wave dynamics, possibly associated with discrete propagation, as a factor in the system motion.

Diurnal Cycle of Monsoon Convection

“…Radar data from the R/V Ron Brown in the Bay of Bengal indicate that the convection associated with the diurnal signal has characteristics of squall lines with trailing stratiform precipitation. A southward propagation of convective systems over the South China Sea was also observed during the 1998 South China Sea Monsoon Experiment (SCSMEX), similar to that over the Bay of Bengal (Aves and Johnson 2008). The monsoon onset over the northern South China Sea (near 20°N) occurred around mid-May and is characterized by a regular signal of southward propagation of convection (low values of IR brightness temperature) at an approximate speed of 15 m s -1 (Fig.…”

“…Advective effects can be ruled out because these systems propagate approximately at right angles to the low-level southwesterly monsoon flow. Also, gravity current dynamics appear to be an unlikely explanation since the speeds of propagation (~15 m s -1 ) in both regions (Webster et al 2002;Aves and Johnson 2008) are too fast to be accounted for by the relatively weak cold pools observed in these regions. Gravity waves, however, may play some role.…”

“…Some of these exceptions are associated with propagating signals of convection. For example, eastward propagation of convection from the Rocky Mountains, Andes, and Tibetan Plateau has been reported by Carbone et al (2002), Velasco and Fritsch (1987), Laing and Fritsch (1997), and Wang et al (2004), whereas southward propagation of convection over the Bay of Bengal and the South China Sea has been documented by Yang and Slingo (2001), Webster et al (2002), Zuidema (2003), and Aves and Johnson (2008). To better view the global patterns of propagation, a plot of the time of maximum rainfall has been prepared from the TRMM 3B42 data set (Fig.…”

“…They argued that thermally forced gravity waves (produced by elevated terrain and propagating at about 15 m s -1 ) are an essential part of the process, and that they produce a warm anomaly offshore during the daytime, thereby capping convection, while a cooling is produced at night, thus allowing convection to develop. Propagation of convection away from coastlines, typically commencing in the early morning and continuing into the afternoon, has been observed in many regions of the Asian monsoon: the Bay of Bengal (Yang and Slingo 2001;Webster et al 2002;Zuidema 2003), the northern South China Sea (Aves and Johnson 2008), off the coast of New Guinea (Liberti et al 2001;Zhou and Wang 2006), off Borneo Johnson and Priegnitz 1981;Ichikawa and Yasunari 2006), and off Sumatra (Mori et al 2004;Sakurai et al 2005). While gravity waves may play a role in the propagation of convection in these regions, details of the mechanisms are still not well understood.…”

Regimes of Diurnal Variation of Summer Rainfall over Subtropical East Asia

“…Studies are usually confined to areas such as the South China Sea (Ohsawa et al 2001;Lu and Xu 2007;Aves and Johnson 2008;Li et al 2010) or to the several islands of China and Japan (Ramage 1952). Most of the studies reported a dominant early-morning diurnal peak in the oceanic areas and a propagating signal from the southern coastline of mainland China to the northern South China Sea southeastwardly.…”