Characteristics of Low Pressure Systems Associated with Intraseasonal Oscillation of Rainfall over Bangladesh during Boreal Summer

Abstract

Characteristics of low pressure systems (LPSs) responsible for submonthly-scale (7-25 days) intraseasonal oscillation (ISO) in rainfall over Bangladesh and their impact on the amplitude of active peaks are investigated for 29 summer monsoon seasons. Extreme and moderate active peaks are obtained based on the amplitude of 7-25-day-filtered rainfall series. By detecting the LPSs that formed over the Indian monsoon region, it was found that about 59% (62%) of extreme (moderate) active peaks of rainfall are related to LPSs. These LPSs have horizontal scale of about 600 km and vertical scale of about 9 km. For the extreme active peak, the locations of the LPS centers are clustered significantly over and around Bangladesh, accompanied by the maximum convergence in the southeast sector of the LPSs. After their formation, they tend to remain almost stationary over and around Bangladesh. In contrast, for the moderate active peak, the LPS centers are located over the Ganges Plain around 858E, and the maximum convergence of the LPSs occurs around their centers. This difference in the convergence fields is closely associated with the geographical features to the north and east of Bangladesh and the horizontal scale of the LPSs. These features suggest that the amplitude of the active peaks in the submonthly-scale ISO is modulated by small differences in the locations of the LPS centers. These findings suggest that improved predictions of both genesis location and the tracks of the LPSs are crucial to forecasting seasonal rainfall over Bangladesh.

Trends in Precipitation Characteristics in Bangladesh from 1950 to 2008

“…APHRODITE gridded precipitation dataset (Yatagai et al 2012) has been used by several papers (e.g., Hatsuzuka et al 2014). Precipitation indices obtained from BMD data and that from APHRODITE were different in climatologically (Fig.…”

“…During the summer monsoon season (JuneSeptember), much of the precipitation was brought by the monsoon low (pressure system) originated from the Bay of Bengal and moved along the Ganges. Precipitation in Bangladesh shows clear intraseasonal variation with time scales of 7−25 days during the monsoon season (Fujinami et al 2011;Hatsuzuka et al 2014). Precipitation caused by severe local convective storms is predominant in the pre-monsoon (March-May) season (Rafiuddin et al 2010;Yamane et al 2010).…”

Effects of the South Asian Monsoon Intraseasonal Modes on Genesis of Low Pressure Systems over Bangladesh

“…The area around Bangladesh exhibits a distinct QBW during summer monsoon rainfall (e.g., Ohsawa et al 2000;Murata et al 2008;Fujinami et al 2011Fujinami et al , 2014Sato 2013;Hatsuzuka et al 2014). Fujinami et al (2011) found that the rainfall spectrum in Bangladesh has a pronounced peak at about 14 days, while no statistically significant peak appears within the 30-60-day period.…”

“…They also reported that the interannual variability of the total summer monsoon rainfall over Bangladesh is correlated significantly with the 7-25-day rainfall variance, suggesting that the QBW controls the interannual variability of the total seasonal rainfall in the country. Recently, Hatsuzuka et al (2014) found that a vortex-type low pressure system (LPS) is often observed in the active phase of the QBW over Bangladesh. The typical horizontal scale of these LPSs is ;600 km, which is considerably smaller than the socalled Indian monsoon depressions (e.g., Krishnamurti et al 1975;Godbole 1977), and most of these systems occur over and around Bangladesh.…”

“…The typical horizontal scale of these LPSs is ;600 km, which is considerably smaller than the socalled Indian monsoon depressions (e.g., Krishnamurti et al 1975;Godbole 1977), and most of these systems occur over and around Bangladesh. During the 29 summer monsoon seasons considered by Hatsuzuka et al (2014), it was revealed that about 60% of the extreme and moderate active peaks were related to such vortex-type LPSs and that small differences in their central locations caused modulation of the amplitude of the active peaks. Furthermore, they reported that heavy rainfall over the lowland area of Bangladesh was more likely when the LPS was present (LPS case) than during its absence (non-LPS case), under extreme active peaks.…”