Early morning peaks in the diurnal cycle of precipitation over the northern coast of West Java and possible influencing factors

Abstract

Abstract. The diurnal cycles of precipitation over the northern coast of West Java have been studied using the Tropical Rainfall Measuring Mission (TRMM) Real Time Multi-satellite Precipitation Analyses (TMPA-RT) products with records spanning from 2000 to 2016, with emphasis on the occurrences of early morning precipitation peaks. Diurnal precipitation over the study area during November to March is basically characterized by precipitation peaks that occur in the afternoon to evening time (15:00–21:00 LT), but secondary peaks in the night to morning time (01:00–07:00 LT) are also pronounced in January and February. The harmonic analysis method was then applied to data of January and February to objectively determine the diurnal phase and classify the timing of precipitation for each day into three categories, i.e. afternoon-to-evening precipitation (AEP), early morning precipitation (EMP), and late morning precipitation (LMP) with peaks that occur in the time windows of 13:00–24:00, 01:00–04:00, and 05:00–12:00 LT, respectively. In terms of frequency of occurrence, AEP, EMP, and LMP constitute 55 %, 18.9 %, and 26.1 % of total samples of precipitation events. In spite of the smallest percentage, EMP events are characterized by seaward (as well as landward) propagation, flat phase distribution, and large mean amplitudes. The propagating characteristics of EMP are more prominent, with indications of stronger connectivity between precipitation systems over land and ocean, when data are composited by taking the 99th percentile values in each grid to represent extreme precipitation events. The flat phase distribution of EMP events suggests that the timing of coastal precipitation is not necessarily locked to the phase of land–sea breezes, thus allowing precipitation to occur more randomly. Furthermore, the role of the South China Sea cold tongue (SCS-CT) and cross-equatorial northerly surge (CENS) as influencing factors for the occurrences of the EMP event have also been investigated. In agreement with previous studies, we confirmed that the SCS-CT generally prevails in January and February, and morning precipitation events over the northern coast of West Java mainly occurred when there was more enhanced sea surface temperature (SST) cooling in the South China Sea. Additionally, we found that CENS is the most differential factor with regard to the phase of coastal precipitation. In this case, CENS is positively correlated with SCS-CT and, when associated with EMP events, concurrent enhancement of CENS and SCS-CT is connected to a narrow channelling of strong surface northerly wind anomalies just offshore of Indochina and the Malay Peninsula.

Cross‐Equatorial Surges Boost MJO's Southward Detour Over the Maritime Continent

“…The resulting increase in column moisture over the SMC is characterized by positive total moisture flux convergence anomalies, indicative of positive net precipitation (Figure 1f). This finding is consistent with previous studies, showing that cold surges can propagate deep into the tropics, triggering convection and precipitation, which eventually lead to heavy rainfall and flood events (Hattori et al., 2011; Lim et al., 2017; Wu et al., 2013; Xavier et al., 2020; Yulihastin et al., 2020). In addition, the CES‐induced moist environment over the SMC suggests its potential role in providing additional moisture and energy for the intensification and propagation of the MJO, which will be investigated in the subsequent section.…”

Influences of MJO on the Diurnal Variation and Associated Offshore Propagation of Rainfall near Western Coast of Sumatra

“…As shown in Figures 3 and 4, the average phase speed of diurnal offshore precipitation propagation is 3-4 m s −1 near the coast, similar with the magnitudes in the land breeze or the cold pool outflow [28,30,32,61]. The phase speed of 8-9 m s −1 further offshore is noticeably distinguished from the magnitude of background wind, and indicates other possible factors such as gravity waves, with a similar speed of roughly 8-15 m s −1 , as generated by land-sea thermal contrast or convection [21,24,[62][63][64][65].…”

“…As one of the regions with most active convection and great influences by factors such as MJO or convectively coupled equatorial waves (CCEWs), MC generates frequent precipitation all over the years, and the associated latent heat from rainfall becomes one of the main energy sources driving global circulation [16], which plays nonnegligible roles in global weather and climate [17][18][19][20]. The diurnal variation of rainfall is a common phenomenon in coastal regions, including MC; more specifically, it is often characterized by offshore propagation near the coasts, whose mechanisms are closely related to gravity waves [21][22][23][24], background-wind modulations [21,23,25], land-sea breezes [26][27][28][29], and cold pool outflows [30][31][32].…”

Towards a Mechanistic Understanding of Precipitation Over the Far Eastern Tropical Pacific and Western Colombia, One of the Rainiest Spots on Earth

“…Wet days have a more intense and apparent offshore westward migration of precipitation with a phase speed of ∼10 ms −1 . The strong maritime diurnal cycle of precipitation with such long range has been documented in several locations around the world (Huang et al, 2018;Mapes et al, 2003;Yang & Slingo, 2001;Yokoi et al, 2019;Yulihastin et al, 2020). For example, Yokoi et al (2019) showed that long-lasting convective clusters over the west coast of Sumatra propagate westward at about ∼8-10 ms −1 .…”