Modulation of the diurnal cycle of rainfall over India by intraseasonal variations of Indian summer monsoon

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: (2014) Published online 16 May 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: /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 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 , India. nrdesh@tropmet.res.in 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., 2013 Royal Meteorological Society

2 794 N. R. DESHPANDE AND B. N. GOSWAMI 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 (<6h) rainfall with late-afternoon diurnal peaks is common during the break period in central-east China region. Singh and Nakamura (2010), using TRMM data for the period , showed that the precipitation over central India during wet period is characterized by a large amount of rainfall with a high frequency of rain and a secondary morning peak. Li et al. (2011) noticed increasing trend in the intensity of short duration rainfall of late afternoon hours in north and south China region. As large scale circulation and thermodynamic regimes influence the convective activity and rainfall, the diurnal cycle of rainfall is expected to be different during different monsoon regimes such as the active and break conditions within the season. During active monsoon conditions prevailing over central parts of the country, rainfall is scanty along foothills of Himalayas and vice versa. Due to such spatial structure of monsoon intraseasonal oscillations (MISO), the diurnal cycle from coastal areas or foothills of Himalayas may differ from central land portion of India (Goswami, 2005). Hence, we may expect substantial changes in the characteristics of the diurnal cycle (amplitude as well as the phase) during active and break conditions. Thus, the active/break phases of MISO are the main modulators of the diurnal cycle of rainfall. Therefore, the average diurnal cycle during the monsoon season without segregating into active/break Figure 1. Location of stations in India (showing topography and four regions used in the study).

3 DIURNAL CYCLE OF RAINFALL IN ACTIVE/BREAK PHASES OF MONSOON SEASON 795 phases may not show any interannual variability. This may be one of the reasons why we did not notice any significant interannual variability in our earlier study (Deshpande et al., 2011). This study is taken up because there is a need to re-examine the issue by segregating the data into active and break phases of monsoon. It has also been observed by many researchers that the frequency as well as the amplitude of extreme rainfall events is increasing during the last 50 years in the central parts of India (Goswami et al., 2006). Deshpande et al. (2011) have reported an increase in short duration rainfall amounts (<12 h of duration) in recent years at many places in India. Hence, it motivates us also to examine the long-term temporal changes in the diurnal cycle of rainfall during active/break phases of monsoon in four different parts of the country. Four regions used in this study are namely, monsoon core region or central parts of India (20 25 N, 7585 E), foothills of Himalayas Figure 2. Amplitude, phase and variance explained (%) by first two harmonics in rainfall intensity during active phase of monsoon.

4 796 N. R. DESHPANDE AND B. N. GOSWAMI (25 30 N, E), West Coast of India, i.e. the region between Arabian Sea and Western Ghats running almost parallel to the coast and lastly Peninsular India or Deccan plateau region of India. These regions are defined by other researchers (Dhar et al., 1984; Kothawale et al., 2010; Singh and Nakamura, 2010). Region defined as foothills of Himalayas is restricted to eastern part of foothills only, as it is mentioned by Dhar et al. (1984) that during the break phase of monsoon, western part of foothills experiences general decrease of rainfall as seen in central parts of India while the areas located in the eastern parts of foothills show positive departures during break phase. With this background, this study proposes; 1. To examine the modulation of the diurnal cycle of rainfall across India during active/break phases of monsoon and to study the changes in amplitude, phase and variance explained (%) by different harmonics. 2. To analyze the temporal changes in the diurnal cycles of rainfall over four homogeneous regions Figure 3. Amplitude, phase and variance explained (%) by first two harmonics in rainfall intensity during breaks in monsoon season.

5 DIURNAL CYCLE OF RAINFALL IN ACTIVE/BREAK PHASES OF MONSOON SEASON 797 Diurnal Variation of Rainfall (Jul-Aug) Active days Break days 1 Central India 0.8 Central India Foothills of Himalayas Foothills of Himalayas Rainfall Intensity (mm h -1 ) Peninsular India 0.4 Peninsular India West Coast of India 1.2 West Coast of India Time (Hours) Figure 4. Diurnal cycle of rainfall during active/break phases of monsoon at four regions. as mentioned above, during active/break phases of monsoon. 3. To examine the role of various meteorological parameters such as wind field (850 hpa), specific humidity and geopotential height of 850 hpa level in modulating the diurnal variations of rainfall over India during different phases of monsoon. It is assumed that daily four values of these meteorological parameters are sufficient to explain the average diurnal pattern in these parameters. 2. Data used 2.1. Self recording rain gauge dataset Hourly rainfall data of 91 stations are considered for the analysis. Figure 1 shows the map of India showing the locations of stations along with the four homogeneous regions used in the study. Data period extends from 1969 to 2005 with variable data length (with a minimum of 30 years of observations) NCEP reanalysis derived data sets Six-hourly NCEP reanalysis data at pressure level 850 hpa on various parameters such as zonal/meridional wind (m s 1 ), geopotential height (gpm) and specific humidity (g kg 1 ) at the resolution of 2.5 latitude 2.5 longitude for the period , retrieved from the web site: are used in the analysis. 3. Methodology Active (break) days as defined by Rajeevan et al. (2010), are the spells of minimum 3 d when normalized rainfall anomaly on a particular day during July to August months is greater than its long-term mean + standard deviation (less than mean standard deviation).

6 798 N. R. DESHPANDE AND B. N. GOSWAMI (a) Figure 5. (a) Spatial patterns of 6-hourly rainfall during active phase of July to August. (b) Spatial patterns of 6-hourly rainfall during break condition of July to August. Harmonic analysis of diurnal cycle of rainfall is carried out to study the characteristic features of diurnal cycle of rainfall during active/break phases of monsoon. Three important derived parameters in the harmonic analysis namely, amplitude, phase and variance explained (%) by different harmonics have been studied. Amplitude gives the peak intensity in the respective harmonics while phase angle indicates the hour of maximum intensity of that particular harmonic (SenRoy and Balling, 2005). First harmonic indicates the full diurnal cycle of rainfall intensity while the second harmonic gives the semidiurnal cycle. Spatial and temporal changes in the regional diurnal cycle of rainfall have been studied by computing first, average diurnal cycles at 91 stations, during active/break phases of July to August months of available data period. These composite diurnal cycles are then spatially averaged for the four homogeneous regions considered in the study. Temporal changes in the diurnal cycle of rainfall at four regions have been examined by making composite diurnal cycle of rainfall for each of the two time periods (former period) and (recent period), separately. These diurnal cycles are then smoothed out by passing through a 3-h moving average filter. Composite spatial patterns (with 6 h interval) of low-level zonal/meridional winds and relative vorticity

7 DIURNAL CYCLE OF RAINFALL IN ACTIVE/BREAK PHASES OF MONSOON SEASON 799 (b) Figure 5. (continued). computed from wind field, specific humidity and geopotetial height (850 hpa) are made to examine their coherence with diurnal variations of rainfall during active/break phases of July to August months. Results are discussed in the following section. 4. Results and discussion 4.1. Harmonic analysis of diurnal cycle of rainfall intensity during active/break phases of monsoon across India Mao and Wu (2012) studied the topographic influence on the diurnal variations of rainfall and phase propagations of maximum rainfall during summer monsoon season of Asian monsoon region using TRMM data for the period However, intraseasonal variations were not taken into account by the authors. This study focuses spatio-temporal variations in the amplitude and phase during different phases of Indian summer monsoon rainfall. Harmonic analysis of the diurnal cycle of rainfall intensity at each of the 91 stations has been carried out for active/break periods during July to August monsoon months using available data. Analysis indicates that only the first two harmonics have significant contribution (explaining nearly 75 80%) to the total variance of the rainfall series irrespective of active/break phases. Therefore parameters, such as amplitude, phase and variance explained have been discussed for the first two harmonics only. Figure 2 shows spatial patterns of these parameters for the first two harmonics during the active phase, while Figure 3 depicts the same parameters but for the break phase of the two monsoon months. It is seen that the amplitudes of first two harmonics are higher

8 800 N. R. DESHPANDE AND B. N. GOSWAMI (a) Figure 6. (a) Spatial patterns of 6-hourly relative vorticity and streamlines at 850 hpa during active phase of July to August. (b) Spatial patterns of 6-hourly relative vorticity and streamlines at 850 hpa during break condition of July to August. (approximately 1.2 mm h 1 ) and comparable to each other in monsoon core region of central India and along West Coast of India during active phase of monsoon, while along foothills of Himalayas, they are higher (0.6 mm h 1 ) and comparable to each other during break situations. First harmonic has its maximum amplitude during afternoon hours in active monsoon conditions along foothills of Himalayas while during break phase, it is observed in the morning hours. Peninsular India shows maximum amplitude of first harmonic of diurnal cycle around 21 h IST during break phase and by few hours earlier in active conditions. Mao and Wu (2012) noticed diurnal phase of maximum amplitude in the central and southern Indian peninsular region during late evening midnight hours ( LST) irrespective of active/break phases of monsoon. Almost all parts of India show maximum amplitude of second harmonic during early morning hours irrespective of active/break situation. First harmonic shows phase difference of nearly 3 h during the active and break phases in many parts of the country. In monsoon core region, second harmonic contributes more to the total variance (40%) than the first harmonic (20%) during active phase of monsoon, while, during break periods, first harmonic itself contributes 50% to the total variance of the diurnal variability in rainfall. Contribution from second harmonic is less than 20% over entire India during break period. In short, second harmonic contributes substantially to the total

9 DIURNAL CYCLE OF RAINFALL IN ACTIVE/BREAK PHASES OF MONSOON SEASON 801 (b) Figure 6. (continued). variance of the rainfall series during the active phase of monsoon. The rest of the cycles have small amplitudes contributing to a small quantity to the total variance compared with first two (not shown) Temporal changes in diurnal cycle of rainfall across India during active/break phases of July to August monsoon season Intraseasonal and interannual variability of rainfall over India have been discussed by several investigators (Krishnamurthy and Shukla, 2000 and Goswami and Mohan, 2001). They reported that the nature of intraseasonal variability remains constant irrespective of major flood or drought years and that both intraseasonal and interannual variations are governed by a common mode of spatial variability. It is also shown by Sahany et al. (2010) that there is no significant interannual variation associated with the summer season diurnal cycle of rainfall over the Indian land and its neighbouring oceans. Presuming that it is sufficient to investigate the variability of diurnal cycle of rainfall during active/break phases without segregating them into flood/drought years, this study examines the role of intraseasonal variations in the diurnal cycle of rainfall during July to August months in India. Keeping in mind the active/break condition as defined by Rajeevan et al. (2010) which primarily refers to central parts and West Coast of India. In general, the amplitude of the diurnal cycle is significantly enhanced during the active condition as compared with that during the break condition in monsoon core region. While along the foothills of Himalayas, rainfall activity is enhanced in break situation. Active and break days, as defined above, during July

10 802 N. R. DESHPANDE AND B. N. GOSWAMI (a) Figure 7. (a) Spatial patterns of 6-hourly specific humidity during active phase of July to August. (b) Spatial patterns of 6-hourly specific humidity during break conditions of July to August. to August months for the period have been used for computing diurnal cycle of rainfall in 91 stations. Composite diurnal cycles of rainfall for four homogeneous regions are computed separately for active and break phases of monsoon. These diurnal cycles are then smoothed out by passing through a 3-h moving average filter. Figure 4 shows these composite diurnal cycles of rainfall for four homogeneous regions. It is seen that diurnal variations in the rainfall patterns during active/break situations is very small along the West Coast of India, except that, rainfall intensity is high during the active period. Two peaks in the diurnal cycle of rainfall have been observed over monsoon core region during active phase of monsoon. It may be due to semidiurnal pressure oscillation which affects the cloudiness and rainfall at a place (Brier and Simpson, 1969). The fact is supported by examining the association between diurnal variations of some of the meteorological parameters and that of diurnal rainfall pattern. Foothills of Himalayas show morning maxima during break phase of monsoon which is the wet period in this region. These observations are similar to that noticed by Singh & Nakamura (2010). Peninsular India receives rainfall during both the active/break phases of monsoon with morning maximum in active period and afternoon peak in rainfall activity during break situations. Temporal changes in the diurnal cycles indicate that afternoon maximum in central India during break period and morning maximum along foothills of Himalaya during active phase of monsoon seem to be enhanced in recent period. No substantial temporal changes in the

11 DIURNAL CYCLE OF RAINFALL IN ACTIVE/BREAK PHASES OF MONSOON SEASON 803 (b) Figure 7. (continued). diurnal cycle of rainfall has been observed in peninsular India and along West Coast of India, except that rainfall activity at all 24 h during break situation seems to be increasing in recent years ( ) along the West Coast. Enhanced rainfall activity may be due to an increase in convective available potential energy (CAPE) in tropical region in recent years (Mani et al. 2009) Diurnal variations of various meteorological parameters during active/break phases of July to August months The active (break) conditions are generally associated with an increase (decrease) of cyclonic vorticity and decrease (increase) of surface pressure over the central Indian monsoon trough region and strengthening (weakening) of the low level jet (LLJ) (Goswami, 2005). This study investigates the meteorological conditions prevailing during active/break phases of July to August months to explain the impact of intraseasonal variability on diurnal pattern of rainfall and also to investigate the bimodal nature of diurnal cycle of rainfall in central parts of India. Diurnal variations of rainfall in different parts of India are examined by analyzing 6-hourly rainfall composites made from available data set of 91 stations during active/break phases of two monsoon months. In all, there are 236 active days and 278 break days observed based on the criterion suggested by Rajeevan et al. (2010) during the period Composite spatial patterns of these 6-hourly rainfall during active/break phases are presented in Figure 5(a) and (b), respectively. Two rainfall maxima (first in morning hours around 6 h and another in evening hours around 18 h) are clearly seen from the Figure 5(a). Figure 5(b) represents the spatial patterns of 6-hourly rainfall during the break situations of July to August months. It is seen from the figure that

12 804 N. R. DESHPANDE AND B. N. GOSWAMI (a) Figure 8. (a) Spatial patterns of 6-hourly geopotential height of 850 hpa level during active phase of July to August. (b) Spatial patterns of 6-hourly geopotential height of 850 hpa level during break condition of July to August. central India is devoid of rainfall during such conditions. Only afternoon hours show some rainfall activity owing to the convection that prevails over this region Diurnal variation in relative vorticity and streamlines at 850 hpa during active/break phases of July to August months It has been observed that cross-equatorial flow enhances during the active phase of monsoon. Such conditions are favourable for good rainfall activity over the central parts of the country. Figure 6(a) shows spatial patterns of relative vorticity computed from u and v wind field (850 hpa) at four representative hours of a day during active phase of monsoon. Streamlines are displayed on the same figure to examine the convergence of winds. Strong south-westerlies are prevailing to the south of monsoon trough region in the lower troposphere, while, easterlies are seen over northern India at representative 4 h. Though the significant change is not seen in 6- hourly wind patterns at 850 hpa, convergence of the westerlies and easterlies along the latitude of 25 N is more prominent in afternoon or evening hours. In general, positive values of relative vorticity (cyclonic) at 850 hpa during afternoon hours are observed leading to the intensification of the instability causing heavy rainfall activity. Figure 6(b) shows the spatial pattern of relative vorticity and streamlines at 850 hpa during the break situations. Westerlies are seen all over India

13 DIURNAL CYCLE OF RAINFALL IN ACTIVE/BREAK PHASES OF MONSOON SEASON 805 (b) Figure 8. (continued). and negative vorticity (anticyclonic) is seen over the central parts of India indicating unfavourable conditions for rainfall activity at all hours Diurnal variation in specific humidity at 850 hpa during active/break phases Figure 7(a) and (b) shows the spatial patterns of specific humidity at representative 4 h during active and break days. Clear signal is seen from the diurnal variation of the moisture field (Figure 7(a)). The enhanced specific humidity in the afternoon hours is attributed to cumulative impact of increased surface water fluxes due to convective development and resulting into increase in rainfall intensity during afternoon hours. Whereas, increase in specific humidity in lower troposphere during morning hours can be attributed to radiative cooling which helps to increase the moisture in lower atmosphere and thus resulting into increase in the rainfall activity during early morning hours. Sahany et al. (2010) mentioned that strong moisture convergence at the mid-troposphere during night hours leads to the rainfall peak seen during early morning hours over central India. They further concluded that during both active and break phases of summer monsoon, mid-level moisture convergence seems to be one of the primary factors governing the phase of the diurnal cycle of rainfall Diurnal variation in geopotential height of 850 hpa during active phases Composites of 6-hourly geopotential height of 850 hpa level during active/break phases of July to August months are made and their spatial patterns are shown in Figure 8(a) and (b). Figure 8(a) clearly indicates that during morning and evening hours (6 and 18 h) geopotential

14 806 N. R. DESHPANDE AND B. N. GOSWAMI height at 850 gpm level reduces to 1375 gpm over central India, while for another 2 h, i.e. 12 and 24 h (noon and midnight) it remains around 1400 gpm. Reduction in geopotential height indicates the instability in the atmosphere and hence increase in the rainfall activity during these hours. During break situations, at all hours goepotential height remains above 1425 gpm, indicating stability of the atmosphere which is unfavourable for rainfall processes (Figure 8(b)) Summary and conclusions The maximum rainfall intensity over central parts of India corresponds to intense solar radiation during afternoon hour. Along the foothills of the Himalayas, the katabatic wind leads to enhanced convection and maximum rain occurs in the early morning hours. Along the West Coast of India, the interaction of the synoptic-scale westerly wind with the opposing land breeze produces low-level convergence, enhancing the convection and thus causing maximum rain in the early morning hours. Along the east coast also the maximum rainfall activity occurs in the early morning hours as the synoptic-scale westerly is weak over this region and the diurnal variation is dominated by the land sea breeze. From this analysis, we have drawn some important conclusions: 1. The maximum amplitude of second harmonic, in both active and break situations, is seen in early morning hours almost all over India. Phase of first harmonics shifts earlier by nearly 3 h during active situations in many parts of the country compared with break period. Contribution of second harmonic to the total variance of the rainfall series is more than that of first harmonic in central parts of the country during the active phase of monsoon. 2. Along the West Coast and foothills of Himalayas, peak rainfall during 24 h occurs in the early morning hours in both active/break phases while over central India it occurs in the afternoon or evening hours. Two maxima are observed in diurnal cycle of rainfall during active phases over this region. Diurnal variation in rainfall intensity is very small along West Coast during both active and break phases. 3. In recent years, enhanced rainfall activity is seen during afternoon hours of active period in central India and during morning hours of break days along foothills of the Himalayas. This may be due to the effect of increasing number of aerosols in the atmosphere in central parts of India. The influence of aerosol on the monsoon rainfall mainly depends on the size, concentration and hygroscopic nature which make them compatible as cloud condensation nuclei. It is reported that in contrasting monsoon years, aerosols play a significant role in modulating the rainfall activity (Panicker, 2010). 4. Diurnal variations in meteorological parameters such as increase in specific humidity and reduction in geopotential height of 850 hpa level twice a day are in phase with the two maxima seen in the diurnal cycle of rainfall during active conditions of July to August months over monsoon core region or central India. Acknowledgements The authors are thankful to the NDC, IMD, Pune, India for providing valuable datasets used for the analysis. Authors are also grateful to websites providing NCEP reanalysis online data sets on different meteorological parameters. Thanks are also due to anonymous referees for providing useful suggestions to improve the manuscript. IITM is fully funded by Ministry of Earth Sciences, New Delhi. References Basu BK Diurnal variation in precipitation over India during the summer monsoon season: observed and model. 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