A long-term spatiotemporal analysis of vegetation greenness over the himalayan region using google earth engine

The Himalayas constitute one of the richest and most diverse ecosystems in the Indian sub-continent. Vegetation greenness driven by climate in the Himalayan region is often overlooked as field-based studies are challenging due to high altitude and complex topography. Although the basic information about vegetation cover and its interactions with different hydroclimatic factors is vital, limited attention has been given to understanding the response of vegetation to different climatic factors. The main aim of the present study is to analyse the relationship between the spatio-temporal variability of vegetation greenness and associated climatic and hydrological drivers within the Upper Khoh River (UKR) Basin of the Himalayas at annual and seasonal scales. We analysed two vegetation indices, namely, normalised difference vegetation index (NDVI) and enhanced vegetation index (EVI) time-series data, for the last 20 years (2001–2020) using Google Earth Engine. We found that both the NDVI and EVI showed increasing trends in the vegetation greening during the period under consideration, with the NDVI being consistently higher than the EVI. The mean NDVI and EVI increased from 0.54 and 0.31 (2001), respectively, to 0.65 and 0.36 (2020). Further, the EVI tends to correlate better with the different hydroclimatic factors in comparison to the NDVI. The EVI is strongly correlated with ET with R2 = 0.73 whereas the NDVI showed satisfactory performance with R2 = 0.45. On the other hand, the relationship between the EVI and precipitation yielded R2 = 0.34, whereas there was no relationship was observed between the NDVI and precipi-tation. These findings show that there exists a strong correlation between the EVI and hydroclimatic factors, which shows that changes in vegetation phenology can be better captured using the EVI than the NDVI

Reply to comment by S. Ramachandran on "Surface changes in solar irradiance due to aerosols over central Himalayas"

[1] Dumka et al. [2006] (hereinafter referred as D06) made extensive measurements of radiative fluxes at the surface during December 2004 at Manora Peak, in the Shivalik ranges of the Central Himalayas during a comprehensive aerosol field campaign as a part of the Indian Space Research Organisation's Geosphere Biosphere Programme (ISRO-GBP). The surface radiative fluxes were used to estimate aerosol radiative forcing. Based on the data analysis D06 concluded that the anthropogenic aerosols (from valley below) transported upwards by atmospheric boundary layer (ABL) dynamics during daytime provide an atmosphere conducive for "mixed aerosols" and suggested that focused efforts are needed to address this issue

Short-Period Modulations in Aerosol Optical Depths over the Central Himalayas: Role of Mesoscale Processes

Multiyear measurements of spectral aerosol optical depths (AODs) were made at Manora Peak in the central Himalaya Range ($29^022^{\prime}N$, $79^027^{\prime}E$, \sim1950 m above mean sea level), using a 10-channel multiwavelength solar radiometer for 605 days during January 2002-December 2004. The AODs at $0.5 \mu m$ were very low ($\leq0.1)$ in winter and increased steeply to reach high values $(\sim0.5)$ in summer. It was observed that monthly mean AODs vary significantly (by more than a factor of 6) from January to June. Strong short-period fluctuations (within a daytime) were observed in the AODs.Further investigations of this aspect have revealed that boundary layer dynamics plays a key role in transporting aerosols from the polluted valley region to higher altitudes, causing large contrast in AODs between forenoon and afternoon. The seasonal variations in AODs, while examined in conjunction with synoptic-scale wind fields, have revealed that the transport of dust aerosols from arid regions to the valley regions adjacent to the observational site and their subsequent transport upward by boundary layer dynamics are responsible for the summer increases

Surface changes in solar irradiance due to aerosols over central Himalayas

During a comprehensive aerosol field campaign as a part of the ISRO-GBP, extensive measurements of radiative fluxes at the surface were made during December 2004 at Manora Peak, in the Shivalik ranges of the Central Himalayas. The surface radiative fluxes were used to estimate aerosol radiative forcing. Our analysis clearly shows that during the clean atmospheric conditions over Manora Peak, the observed aerosol radiative forcing is in good agreement to those of modeled ones, while for the higher aerosol optical depths (AODs), modeled values are significantly smaller than the observed ones. It was observed that at Manora Peak, the anthropogenic aerosols (from valley below) transported upwards by evolution of boundary layer during the daytime provide an atmosphere conducive for ‘mixed’ aerosols. Focused efforts are needed to address this issue for which simultaneous observations at high altitude site with those in nearby valley are essential

Altitude variation of aerosol properties over the Himalayan range inferred from spatial measurements

Altitude variations of the mass concentration of black carbon, number concentration of composite aerosols are examined along with the columnar spectral aerosol optical depths using state of the art instruments and the Angstrom parameters are inferred from the ground based measurements at several altitude levels, en route from Manora Peak, Nainital (similar to 1950 m above mean sea level) to a low altitude station Haldwani (similar to 330 m above mean sea level) at its foothill within an aerial distance of <10,000 m. The measurements were done during the winter months (November-February) of 2005, 2006 and 2007 under fair weather conditions. The results show a rapid decrease in all the measured parameters with increase in altitude, with >60% contribution to the AOD coming from the regions below 1000 m. The Angstrom wavelength exponent remained high in the well mixed region, and decreased above. The normalized AOD gradient was used to estimate aerosol mixing height, which was found to be in the altitude range 1000-1500 m, above which the particle concentrations are slowly varying as a function of altitude. The heating rate at the surface is found to be maximum but decreases sharply with increase in altitude. Analysis of the wavelength dependence of absorption aerosol optical depth (AAOD) showed that the aerosol absorption over the site is generally due to mixed aerosols. (C) 2011 Elsevier Ltd. All rights reserved

Atmospheric heating due to black carbon aerosol during the summer monsoon period over Ballia: A rural environment over Indo-Gangetic Plain

Black carbon (BC) aerosols are one of the most uncertain drivers of global climate change. The prevailing view is that BC mass concentrations are low in rural areas where industrialization and vehicular emissions are at a minimum. As part of a national research program called the “Ganga Basin Ground Based Experiment-2014 under the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Phase-III” of Ministry of Earth Sciences, Government of India, the continuous measurements of BC and particulate matter (PM) mass concentrations, were conducted in a rural environment in the highly-polluted Indo-Gangetic Plain region during 16th June to 15th August (monsoon period), 2014. The mean mass concentration of BC was 4.03 (± 0.85) μg m− 3 with a daily variability between 2.4 and 5.64 μg m− 3, however, the mean mass PM concentrations [near ultrafine (PM1.0), fine (PM2.5) and inhalable (PM10)] were 29.1(± 16.2), 34.7 (± 19.9) and 43.7 (± 28.3) μg m− 3, respectively. The contribution of BC in PM1.0 was approximately 13%, which is one of the highest being recorded. Diurnally, the BC mass concentrations were highest (mean: 5.89 μg m− 3) between 20:00 to 22:00 local time (LT) due to the burning of biofuels/biomass such as wood, dung, straw and crop residue mixed with dung by the local residents for cooking purposes. The atmospheric direct radiative forcing values due to the composite and BC aerosols were determined to be + 78.3, + 44.9, and + 45.0 W m− 2 and + 42.2, + 35.4 and + 34.3 W m− 2 during the months of June, July and August, respectively. The corresponding atmospheric heating rates (AHR) for composite and BC aerosols were 2.21, 1.26 and 1.26; and 1.19, 0.99 and 0.96 K day− 1 for the month of June, July and August, respectively, with a mean of 1.57 and 1.05 K day− 1 which was 33% lower AHR (BC) than for the composite particles during the study period. This high AHR underscores the importance of absorbing aerosols such as BC contributed by residential cooking using biofuels in India. Our study demonstrates the need for immediate, effective regulations and policies that mitigate the emission of BC particles from domestic cooking in rural areas of India

Aerosol optical properties over delhi and manora peak during a rare dust event in early april 2005

Dust storm events are annual phenomena observed over the Indo-Gangetic plain (IGP) during the pre-monsoon period (May-June). These dust storms affect the air quality, weather conditions and radiation budget of the region. In this paper we characterize the aerosol optical parameters associated with a rare dust storm event that hit the IGP during early April 2005. This event was considered rare as it occurred much earlier than the general occurrence of dust storms in India (May-June), and in the year 2005, the warmest year in the span of the previous hundred years. In this study we considered the optical aerosol parameters for two places in the IGP: Delhi (28.5° N, 77.2° E, 325 m asl) and the high altitude station, Manora Peak (29.4° N, 79.5° E, 1958 m asl). Of the two selected stations, Delhi represents a highly populated and polluted location whereas Manora Peak represents a cleaner location in the central Himalayan region. During this dust storm event, the aerosol optical depth (AOD) was observed to increase considerably. The increment was 2.6-4.6 times over Delhi and 1.6-3.2 times over Manora Peak at wavelengths 380 and 1020 nm, respectively, with respect to the background values, whereas the à ngström exponent (α) for both the stations remained close to zero during the event. The effect shows a considerable increase in direct dust radiative forcing in terms of a reduction in the broadband global irradiance for Delhi as well as for Manora Peak stations. The direct aerosol radiative forcing thus obtained was about 34 in the 400-1100 nm wavelength band at Manora Peak

Radiative effects of elevated aerosol layer in Central Himalayas

Systematic observations of light detection and ranging (LIDAR) to detect elevated aerosol layer were carried out at Manora Peak (29.4 degrees N, 79.5 degrees E, similar to 1960 m a.s.l), Nainital, in the Central Himalayas during January-May 2008. In spite of being a remote, high-altitude site, an elevated aerosol layer is observed quite frequently in the altitude range of 2460-4460 m a.s.l with a width of similar to 2 km during the observation period. We compare these profiles with the vertical profiles observed over Gadanki (13.5 degrees N, 79.2 degrees E, similar to 370 m a.s.l), a tropical station, where no such elevated aerosol layer was found. Further, there is a steady increase in aerosol optical depth (AOD) from January (winter) to May (summer) from 0.043 to 0.742, respectively, at Manora Peak, indicating aerosol loading in the atmosphere. Our observations show north-westerly winds indicating the convective lifting of aerosols from far-off regions followed by horizontal long-range transport. The presence of strongly absorbing and scattering aerosols in the elevated layer resulted in a relatively large diurnal mean aerosol surface radiative forcing efficiency (forcing per unit optical depth) of about -65 and -63 W m(-2) and the corresponding mean reduction in the observed net solar flux at the surface (cooling effect) is as high as -22 and -30 W m(-2). The reduction of radiation will heat the lower atmosphere by redistributing the radiation with heating rate of 1.13 and 1.31 K day(-1) for April and May 2008, respectively, in the lower atmosphere

Detection of long range transport of aerosols with elevated layers over high altitude station in the central Himalayas: A case study on 22 and 24 March 2012 at ARIES, Nainital

An advanced version of Boundary Layer LiDAR system, termed as LiDAR for atmospheric measurement and probing (LAMP) has been operational, at Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, a high altitude station (29.4°N, 79.5°E, ~1960 m above mean sea level), in central Himalayas, since October 2011. The site is at an altitude, which is well above the planetary boundary layer particularly during the night when observations are taken and thus, lies in the free troposphere. Also, there are no anthropogenic sources of aerosols nearby. However, from March to June, due to strong convection, the aerosols get transported to higher altitudes, up to 2 km or more, from the nearby urban and distant regions as well. Here, a case study of each long range transport and convectively driven elevated aerosol layers, observed with LAMP on 22 and 24 March 2012, has been presented. A normal profile observed on 28 March 2012 without any signature of elevated layer of aerosol is also discussed. The seven days back air mass trajectories over three altitude levels, viz. at 4.5, 3 and 1 km on 22 March; at 4.5, 2 km and 700 m on 24 March, and at 4.5, 2 km and 500 m on 28 March have been derived. The upper levels delineate that the possible origin of the multiple elevated aerosol layers on 22 and 24 March may be transported from far-off regions, such as the dry arid regions of North Africa and Saudi Arabia. To confirm the same, the observations were further substantiated with the TERRA satellite yielded aerosol optical depth (555 nm) obtained from the on board instrument Multi-angle Imaging Spectro Radiometer (MISR), which explicitly shows the high value of time averaged columnar aerosol optical depth (AOD) over Saudi Arabia and Red Sea during 18-23 March 2012 and an appreciable decrease during the period 25-29 March 2012, confirming the origin of long range transport. For the first time, such a high altitude aerosol layers (~4.5 km) are observed over this region. The lowest aerosol layer observed on 24 and 28 March 2012 in vertical aerosol backscatter profile is attributed to the transport from adjoining regions via boundary layer evolution and associated mixing