Ritesh Gautam

Enhanced pre-monsoon warming over the Himalayan-Gangetic region from 1979 to 2007

[1] Fundamental to the onset of the Indian Summer Monsoon is the land-sea thermal gradient from the Indian Ocean to the Himalayas-Tibetan Plateau (HTP). The timing of the onset is strongly controlled by the meridional tropospheric temperature gradient due to the rapid premonsoon heating of the HTP compared to the relatively cooler Indian Ocean. Analysis of tropospheric temperatures from the longest available record of microwave satellite measurements reveals widespread warming over the Himalayan-Gangetic region and consequent strengthening of the land-sea thermal gradient. This trend is most pronounced in the pre-monsoon season, resulting in a warming of 2.7C in the 29-year record (1979–2007), when this region is strongly influenced by dust aerosols at elevated altitudes. The enhanced tropospheric warming is accompanied by increased atmospheric loading of absorbing aerosols, particularly vertically extended dust aerosols, raising the possibility that aerosol solar heating has amplified the seasonal warming and in turn strengthened the land-sea gradient. Citation: Gautam, R., N. C. Hsu, K.-M. Lau, S.-C. Tsay, and M. Kafatos (2009), Enhanced pre-monsoon warming over the Himalayan-Gangetic region from 1979 to 2007, Geophys. Res. Lett., 36, L07704, doi:10.1029/2009GL037641.

Variability and Trends of Aerosol Properties over Kanpur, Northern India using AERONET Data (2001-10)

Natural and anthropogenic aerosols over northern India play an important role in influencing the regional radiation budget, causing climate implications to the overall hydrological cycle of South Asia. In the context of regional climate change and air quality, we discuss aerosol loading variability and trends at Kanpur AERONET station located in the central part of the Indo-Gangetic plains (IGP), during the last decade (2001‐10). Ground-based radiometric measurements show an overall increase in column-integrated aerosol optical depth (AOD) on a yearly basis. This upward trend is mainly due to a sustained increase in the seasonal/monthly averaged AOD during the winter (Dec‐Feb) and post-monsoon (Oct‐Nov) seasons (dominated by anthropogenic emissions). In contrast, a neutral to weak declining trend is observed during late pre-monsoon (Mar‐May) and monsoon (Jun‐Sep) months, mainly influenced by inter-annual variations of dust outbreaks. A general decrease in coarse-mode aerosols associated with variable dust activity is observed, whereas the statistically significant increasing post-monsoon/winter AOD is reflected in a shift of the columnar size distribution towards relatively larger particles in the accumulation mode. Overall, the present study provides an insight into the pronounced seasonal behavior in aerosol loading trends and, in general, is in agreement with that associating the findings with those recently reported by satellite observations (MODIS and MISR) over northern India. Our results further suggest that anthropogenic emissions (due mainly to fossil-fuel and biomass combustion) over the IGP have continued to increase in the last decade.

Precursory signals using satellite and ground data associated with the Wenchuan Earthquake of 12 May 2008

Multi-satellite sensor and ground observation data were analysed soon after the Wenchuan Earthquake of magnitude 7.9 (according to the USGS) or 8.0 (according to Xinhua, the Chinese News Agency) that occurred on 12 May 2008. This earthquake was felt by millions of people living in a number of countries; it took 65 000 lives and made millions of people homeless, damaged small and large infrastructure, and large surface manifestations were observed on the surface. Soon after the earthquake multi-satellite sensor and ground observatory data were analysed to study changes in ground, meteorological and atmospheric parameters. A detailed analysis of these data shows pronounced anomalous changes prior to the Wenchuan Earthquake event. Analysis of Atmospheric InfraRed Sounder (AIRS) data at different pressure levels clearly shows characteristic behaviour of the air temperature and relative humidity compared to other days. Such changes are observed up to 500 hPa, with maximal change at the lower levels. Changes in the surface, atmosphere, meteorological and ionospheric parameters prior to the Wenchuan Earthquake are complementary to one another and show the existence of a strong coupling between land, atmosphere and ionosphere, associated with the Wenchuan Earthquake.

Rising and falling river flows: contrasting signals of climate change and glacier mass balance from the eastern and western Karakoram

Abstract Field observations and geodetic measurements suggest that in the Karakoram Mountains, glaciers are either stable or have expanded since 1990, in sharp contrast to glacier retreats that are prevalently observed in the Himalayas and adjoining high-altitude terrains of central Asia. Decreased discharge in the rivers originating from this region is cited as a supporting evidence for this somewhat anomalous phenomenon. Here, we show that river discharge during the melting season of the glaciers in the eastern and western Karakoram, respectively, exhibits rising and falling trends. We have implemented a statistical procedure involving non-parametric tests combined with a benchmark smoothing technique that has proven to be a powerful method for separating the stochastic component from the trend component in a time series. Precipitation patterns determined from ERA-40 and GPCP data indicate that summer-monsoonal precipitation has increased over the Karakoram Mountains in recent decades. Increasing flows in June and July in the eastern Karakoram are due to an increase in summer-monsoonal precipitation. The rising trend of August discharge is due to an increase in the loss of glacier storage at an approximate average rate of 0.186–0.217 mm d-1 year-1 during the period 1973–2010. Moreover, this rate is higher than the rate of increase in monsoonal snowfall during the months of August and September. Therefore, most plausibly, glacier mass balance in the eastern Karakoram is negative. In the western Karakoram, river flows show declining trends for all summer months for the period 1966–2010, corresponding to a rate of increase of glacier storage by approximately 0.552–0.644 mm d-1 year-1, which is also higher than the rate of increase in summer-monsoonal precipitation. The gain of the cryospheric mass in the western Karakoram is in the form of increased thickness of the glaciers and perennial snowpacks instead of areal expansion. This investigation shows two contrasting patterns of trends of river flows that signify both negative and positive mass balance of the Karakoram glaciers. Trends of river flows are spatially and temporally integrated responses of a watershed to changing climate and thereby are important signals of the conditions of the cryospheric component of a watershed where it is highly significant. However, they cannot unequivocally provide indications of the state and fate of the glaciers in the complex hydrometeorological setting of the Karakoram. Extreme caution and care must be exercised in interpreting trends of river discharge in conjunction with climatic data.

Challenges in Early Warning of the Persistent and Widespread Winter Fog over the Indo-Gangetic Plains: A Satellite Perspective

Each year during the season (December–January), dense fog engulfs the Indo-Gangetic Plains (IGP) in Southern Asia, extending over a stretch of 1,500 km, for more than a month disrupting day-to-day life of millions of people inhabiting in the IGP. Increasing atmospheric pollution combined with sufficient moisture available due to the passage of frequent north-westerlies favor fog formation in this region. Trends in poor visibility suggest a significant increase in worsening air quality and foggy days over the IGP. The persistent and widespread nature of the winter fog is strongly influenced by the regional meteorology during winter-time, i.e., a stable boundary layer, cold temperatures, high relative humidity, and light winds. The valley-type topography of the IGP, adjacent to the towering Himalaya, and high concentrations of pollution aerosols/particulates, further favors the persistence of hazy/fog conditions. A satellite- and surface-based observational portrayal is presented here, using various cloud, aerosol, and meteorological datasets, to characterize the widespread nature of winter fog. While the understanding of fog formation is known in the literature, detailed surface observations are needed to help build sophisticated fog forecast and early warning systems to minimize its impact on public safety. Additionally, linkages between the seasonal fog cover, regional meteorology, and increasing air pollution should be further investigated against the backdrop of a changing climate scenario.