INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: 793–807 (2014) Published online 16 May 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3719
Modulation of the diurnal cycle of rainfall over India by intraseasonal variations of Indian summer monsoon N. R. Deshpande* and B. N. Goswami Indian Institute of Tropical Meteorology, Pune, India
ABSTRACT: Changes in the amplitude and phase of the diurnal cycle of rainfall due to changes in the background circulation and thermodynamics associated with the intraseasonal variability of Indian summer monsoon have important implications for weather prediction over the region. Hourly rainfall data available at 91 Indian stations for 30 years of observations are examined to determine the role of intraseasonal variations of Indian summer monsoon in modulating the diurnal cycle of rainfall over four homogenous regions. Harmonic analysis of the diurnal cycle of rainfall shows predominance of principal harmonic during the break phase of monsoon. During an active phase first two harmonics contribute substantially to the total variance in central parts of the country. It is observed that peak rainfall occurs in the morning hours during both the active and break phases along the West Coast with small diurnal variation in rainfall. Two peaks in the diurnal cycle are observed during active phases over central India. This region shows delay in the occurrence of afternoon peak and rise in rainfall intensity during the break phase of recent years, while, increase in rainfall intensity at all hours during break phases is observed along the West Coast of India in recent years. Further analysis of meteorological parameters indicates that lower-level convergence during late afternoon hours, reduction in geopotential height and increase in specific humidity (850 hPa) in central parts of India during morning and evening hours are in phase with the two maxima observed in the diurnal cycle of rainfall in this region. Therefore, spatial variations in daily rainfall pattern during active/break phases can be attributed to orographic effect and heterogeneous convective development in different parts of the country. KEY WORDS
diurnal cycle of rainfall; Indian summer monsoon rainfall; active/break spells; harmonic analysis
Received 13 August 2012; Accepted 13 April 2013
1. Introduction The diurnal cycle is one of the important characteristics of rainfall over a region and is dominated by the physical processes governed by the topographical features and synoptic systems controlling the rainfall over that area. The pattern of diurnal cycle varies with season and place. The summer monsoon season (June to September) accounting for 75% of the annual rainfall, is the most crucial period to study the diurnal variation of rainfall over India. Basu (2007) stated that mesoscale circulations like land–sea breezes, katabatic–anabatic winds, mountain valley winds, so on can modulate the rainfall pattern and produce a typical diurnal cycle of rainfall at a place. He further commented that ground heating caused by solar radiation is the dominant force for diurnal variation of rainfall as major part of India lies in the tropical belt. Several investigations on the diurnal variation of rainfall during summer monsoon season in India showed a significant role of land/sea breezes in determining the diurnal cycle of rainfall along the coast (Ramage, 1964;
* Correspondence to: N. R. Deshpande, Indian Institute of tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune-411008, India. E-mail:
[email protected]
2013 Royal Meteorological Society
Prasad, 1970, 1974; Haldar et al., 1991; Pathan, 1994; SenRoy and Balling, 2005; SenRoy, 2007; Deshpande et al., 2011). In general, a breeze blowing opposite to the synoptic wind produces low-level convergence leading to the enhancement in convection and in turn increases the rainfall, while, the breeze in the same direction as that of the synoptic wind produces the opposite effect on the rainfall rate. Therefore, westerly winds prevailing along the West Coast of India during monsoon season interact with the land breeze and causes maximum rainfall at late night/early morning hours. Prasad (1974) reported the maximum in rainfall in the early morning hours and minimum in the afternoon, along the foothills of the eastern Himalayas. This is mainly due to an increase in convection by the low-level convergence produced by katabatic mountain winds. Haldar et al. (1991) reported a detailed spatial and temporal analysis of the diurnal variation of rainfall and effect of the mesoscale system on the diurnal cycle of rainfall over the Central India. He concluded that central part of India records a maximum rainfall in the afternoon hours when the ground temperature is high. Similar observations have also been reported by Pathan (1994). The diurnal cycle of rainfall has also been studied in other parts of the world (Hamilton, 1981; Oki and Musiake, 1994; Dai et al., 1999; Ho et al., 2008; Li et al. 2008; Liu et al.,
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2009). In recent years, weather satellites provide rainfall estimates at smaller spatial resolutions to study the diurnal cycle of different weather parameters (Rao and Rao, 1993; Yang and Slingo, 2001; Slingo et al., 2004; Basu, 2007). Liu et al. (2009) examined diurnal variability in summer precipitation over the Tibetan Plateau using the Tropical Rain Measuring Mission (TRMM) precipitation analysis product for five summer seasons in relation to the diurnal variability in the atmospheric circulation and thermodynamics in this region using 6-hourly National Centre for Environmental Prediction (NCEP) data. Relationship between intraseasonal oscillation and diurnal variation of summer rainfall over South China Sea has been studied by Ho et al. (2008). Yuan et al. (2010) showed that long-duration (>7 h) early-morning rainfall dominates the active period, while short-duration (
