Variability and Trends of Atmospheric Moisture over the Indian Region
P. Mukhopadhyay, A.K. Jaswal and Medha Deshpande
Atmospheric water vapour is one of the most important factors in determining weather and climate as it plays a dominant role in the radiative budget of the troposphere. The main sources of water vapour in the lower atmosphere are evaporation from the Earth’s surface and transpiration by plants. Since surface humidity and temperature regulate evaporation and transpiration processes, these are connected to both the hydrological cycle and the surface energy budget. Water vapour is one of the most important greenhouse gasses and exceeds carbon dioxide several times in terms of its greenhouse contribution (Trenberth et al. 2007). As temperature rises, the atmosphere’s water holding capacity increases leading to increase in atmospheric water vapour (Philipona et al. 2005).
Long-term mean distributions of surface specific humidity show large seasonal and spatial variations (Oort 1983), while the variations in surface relative humidity are relatively small over the oceans (Peixoto and Oort 1996) but considerable over the USA (Gaffen and Ross 1999). Increased surface specific humidity and dew point temperature during the second half of the twentieth century have been reported over the contiguous USA by Robinson (2000), Sun et al. (2000) and Groisman et al. (2004). Increase in surface water vapour is also found over Europe (Schonwiese and Rapp 1997; New et al. 2000; Philipona et al. 2004), the former Soviet Union and China (Sun et al. 2000; Wang and Gaffen 2001), Canada (Vincent et al. 2007) and Japan (New et al. 2000). Investigating changes in global surface
P. Mukhopadhyay (H) • M. Deshpande
Indian Institute of Tropical Meteorology, Earth System Science Organization,
Pune 411008, India
India Meteorological Department, Earth System Science Organization,
Pune 411005, India
© Springer Science+Business Media Singapore 2017 129
M.N. Rajeevan and S. Nayak (eds.), Observed Climate Variability and Change Over the Indian Region, Springer Geology, DOI 10.1007/978-981-10-2531-0_8
humidity, Dai (2006) has reported increase in both specific and relative humidity over USA, India and China. A 4 % increase in atmospheric moisture has been observed, consistent with a warming climate (Trenberth et al. 2007). Dee et al. (2001) have found increasing trends in air temperature and relative humidity at most of the airport stations in India in winter season. Rao et al. (2004) have reported increasing trends in annual mean relative humidity at 11 out of 15 urban stations over India. Singh et al. (2008) have studied annual trends in relative humidity over nine river basins in north-west and central India and have found increasing trends in six river basins and decreasing trends in three river basins.
Along with atmospheric water vapour, surface soil moisture is also an important variable in the climate system. Understanding the changes and variability in temperature, precipitation and soil moisture is important for monitoring drought and floods, planning and management of water resources in crop-growing seasons and ensuring food security. In situ soil moisture measurements are costly and labour intensive to perform and are thus only available in limited regions of the world. Furthermore, historical records of soil moisture content (Robock et al. 2000) are sparse but still trends and dynamics in soil moisture have been studied from ground-based measurements (Robock et al. 2005). As in situ measurements commonly lack global coverage and representativeness, recent studies have mostly relied on model estimates (Sheffield and Wood 2008; Zhu and Lettenmaier 2007).
There is a close dynamical and thermos-dynamical link among the processes related to changes in precipitation, water vapour in the atmosphere, evapotranspi- ration and soil moisture, etc., which in turn manifests to produce the effect of global warming and climate change (Koster et al. 2004). In the context of changes in precipitation with climate change, Trenberth (2011) suggested a direct link between global warming and precipitation. Global warming leads to more evaporation and also increases the water holding capacity of air (by about 7 % per 1 °C warming). Increased water vapour increases intensity of weather systems such as thunderstorms or tropical cyclones.
There are other articles based on observations where the fact of changes in moisture content of the atmosphere has been reported (Albergel et al. 2013) and its link with other processes, namely the evapotranspiration (Jung et al. 2010). Jung et al. (2010) have attempted to test whether a soil moisture shortage could be the reason behind the declining evapotranspiration (ET) trend since 1998 and they found strong coherence between TRMM-based soil moisture data and ET trend (1998-2008) particularly for those regions where moisture supply controls the soil moisture. A detailed evaluation of global trends of surface soil moisture using multi satellite remote sensed data have been reported by Dorigo et al. (2012).
Recently, Calla et al. (2013) analysed surface soil moisture data from Soil Moisture and Ocean Salinity Satellite (SMOS) from 6 a.m. to the evening of the following day, i.e. 6 p.m., particularly to understand the subsurface soil water behaviour over arid region of Rajasthan. Using micro-meteorological tower data from four sites over the Indian region, Eswar et al. (2013) developed a model for disaggregation of evaporative fraction and evaluated the model based on available satellite data, e.g. MODIS. However, they mentioned that it will be worth to explore similar evaluation based on INSAT-3D and INSAT-3A NDVI data. Using SMOS soil moisture product, Calla et al. (2013) also mentioned the possibility of monitoring and analysing overnight surface soil moisture.
In a recent study by Jung et al. (2010), and references there in a significant negative trend is reported coherently in the soil moisture and evapotranspiration anomaly over the global tropics (28°S-38°N) and also over different land regions. Although this study has been carried out in a data-driven way using TRMM data for the period of 1998-2008, the limitation of remote-sensed data prompts towards a need of such analyses based on direct measurement over the global region in general and the Indian region in particular.
Although relatively fewer studies have been carried out over the Indian region in respect of analysing moisture/humidity trends over different parts, Jaswal and Koppar (2011) reported significant increasing trend of surface specific humidity, relative humidity and dry bulb temperature during summer, winter and monsoon months. The increasing trend of specific and relative humidity is particularly noted over north, north-west, central and south-east India. They have also mentioned that the increasing trend of surface moisture and temperature is contributing to enhanced human discomfort over the country.
Keeping above studies in mind and the gap areas that need to be addressed in the context of the Indian region, this article attempts to bring out the variability of atmospheric moisture over the Indian region.