Intense rainfall events over the west coast of India

SummaryThe west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm day−1 at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment, was carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between 14° N–16° N and near 19° N. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.

[1]  G. Wahba,et al.  Some New Mathematical Methods for Variational Objective Analysis Using Splines and Cross Validation , 1980 .

[2]  K. K. Kumar,et al.  some aspects of daily rainfall distribution over India during the south-west monsoon season , 1990 .

[3]  Sulochana Gadgil,et al.  On breaks of the Indian monsoon , 2003 .

[4]  R. Sarker.  A DYNAMICAL MODEL OF OROGRAPHIC RAINFALL , 1966 .

[5]  R. Sarker.  Some Modifications in a Dynamical Model of Orographic Rainfall , 1967 .

[6]  Brian E. Mapes,et al.  Multiscale variability of deep convection in relation to large-scale circulation in TOGA COARE , 1996 .

[7]  S. Gadgil,et al.  Intraseasonal variation of the Indian summer monsoon. II. Theoretical aspects , 1992 .

[8]  Peter J. Webster,et al.  Coupled ocean–atmosphere dynamics in the Indian Ocean during 1997–98 , 1999, Nature.

[9]  Peter J. Webster,et al.  The Boreal Summer Intraseasonal Oscillation: Relationship between Northward and Eastward Movement of Convection , 2002 .

[10]  Bin Wang,et al.  Structures and Mechanisms of the Northward Propagating Boreal Summer Intraseasonal Oscillation , 2004 .

[11]  Sulochana Gadgil,et al.  On the Maximum Cloud Zone and the ITCZ over Indian, Longitudes during the Southwest Monsoon , 1980 .

[12]  Tetsuzo Yasunari,et al.  Cloudiness Fluctuations Associated with the Northern Hemisphere Summer Monsoon , 1979 .

[13]  B. Goswami,et al.  A dipole mode in the tropical Indian Ocean , 1999, Nature.

[14]  H. D. Orville,et al.  A Numerical Study of the Initiation of Cumulus Clouds over Mountainous Terrain , 1965 .

[15]  R. Keshavamurthy,et al.  Structure of an Arabian Sea Summer Monsoon System , 1968 .

[16]  R. Grossman,et al.  The Distribution of Deep Convection over Ocean and Land during the Asian Summer Monsoon , 1990 .

[17]  Mark Williams,et al.  Satellite-Observed Characteristics of Winter Monsoon Cloud Clusters , 1987 .

[18]  A. V. Gambheer,et al.  Life cycle characteristics of deep cloud systems over the indian region using INSAT-1B pixel data , 2000 .

[19]  P. N. Vinayachandran,et al.  Extremes of the Indian summer monsoon rainfall, ENSO and equatorial Indian Ocean oscillation , 2004 .

[20]  Veerabhadran Ramanathan,et al.  Scale Dependence of Monsoonal Convective Systems over the Indian Ocean , 2000 .

[21]  R. Banta.  The Role of Mountain Flows in Making Clouds , 1990 .

[22]  Robert A. Houze,et al.  Cloud clusters and superclusters over the oceanic warm pool , 1993 .

[23]  Y. Ogura,et al.  Numerical study of orographic-convective precipitation over the eastern Arabian Sea and the Ghat Mountains during the summer monsoon , 1988 .

[24]  J. Janowiak,et al.  Estimating the Satellite Equatorial Crossing Time Biases in the Daily, Global Outgoing Longwave Radiation Dataset , 2001 .

[25]  Robert A. Houze,et al.  Diurnal variation and life‐cycle of deep convective systems over the tropical pacific warm pool , 1997 .

[26]  Some Aspects of the Monsoon Circulation and Monsoon Rainfall , 1977 .

[27]  H. D. Orville.  Ambient Wind Effects on the Initiation and Development of Cumulus Clouds over Mountains , 1968 .

[28]  Ronald B. Smith.  The Influence of Mountains on the Atmosphere , 1979 .

[29]  D. Durran,et al.  Interaction of low-level flow with the Western Ghat mountains and offshore convection in the summer monsoon , 1984 .

[30]  Peter J. Webster,et al.  Mechanisms of Monsoon Low-Frequency Variability: Surface Hydrological Effects , 1983 .

[31]  S. Gadgil,et al.  An Objective Method for the Identification of the Intertropical Convergence Zone , 1990 .

[32]  P. Vinayachandran,et al.  Droughts of the Indian summer monsoon: Role of clouds over the Indian Ocean , 2003 .