Abstract
Abstract. Moisture sources responsible for rains over Kolkata during the summer monsoon can be traced using backward air-mass trajectory analysis. A summary of such trajectories between June and September suggest that these moisture parcels originate from the Arabian Sea and travel over the dry continental region and over the Bay of Bengal (BoB) prior to their arrival at Kolkata. We use monthly satellite and ground-based observations of the hydrometeorological variables together with isotopic data of rainwater from Bangalore and Kakinada to quantify the contributions of advected continental and oceanic water vapour in the Kolkata rains. The vapour mass is modified during its transit from its original isotopic value due to addition of evaporated moisture from the BoB, and further modification occurs due to the process of rainout during transport. The evaporated component is estimated using the Craig–Gordon equation. The rainout process is simulated using a Rayleigh fractionation model. In this simulation we assume that the initial isotopic composition of vapour originating from the continent is similar to the rainwater composition measured at Bangalore. In order to explain the monthly isotopic composition in southwest monsoon rainwater at Kolkata, we invoke 65–75 % moisture contribution from the BoB; the remaining moisture is from the continental land mass.
Highlights
Indian land mass receives rainfall during the summer due to winds favouring moisture transport from the region of the Intertropical Converge Zone (ITCZ) located over the Indian Ocean
Studies have shown that the composition of surface seawater surrounding the Indian land mass can be isotopically distinguishable, i.e. heavier values for water in the Arabian Sea located in the west and dominated by strong evaporative forcing, whereas the Bay of Bengal (BoB) water in the east is characterized by lighter value due to the influence of rainfall and river runoff (Gupta and Deshpande, 2003; Rangarajan et al, 2013; Rahul et al, 2016a)
Isotopic composition measured in the shallow groundwater along the transect between Kolkata and New Delhi were used in a simple box model to estimate the percentage of moisture contribution in rain due to the process of evapotranspiration (ET) (Krishnamurthy and Bhattacharya, 1991)
Summary
Indian land mass receives rainfall during the summer due to winds favouring moisture transport from the region of the Intertropical Converge Zone (ITCZ) located over the Indian Ocean. In this study shallow groundwater over the Indo-Gangetic Plain was treated as the equivalent of rainwater due to its short residence time and the estimated presence of ∼ 40 % moisture due to recycling of shallow groundwater or rainwater This was further verified in a recent study where the average monthly (June– September) isotopic composition of rainwater measured at Kolkata and New Delhi was explained by supplementing ∼ 20 % moisture from the Arabian Sea to a parcel, modified due to Rayleigh-based fractionation of the original moisture parcel originating from the BoB, in addition to ∼ 45 % mois-. The Global Network of Isotopes in Precipitation (GNIP) (International Atomic Energy Agency; IAEA) study over India offers several years of rainwater isotopic values at monthly resolution for different stations along the travel path This database proves to be extremely useful to delineate the contribution of different moisture sources to the regional precipitation. We have used simultaneous satellite-based meteorological observations in a two-component mixing model to deduce the moisture contribution from continental land mass due to advection ( designated as land-based moisture) and supply from evaporation of the BoB surface water during the year 2004
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