H. C. Upadhyaya

On Near-Diffusion-Free Advection over Spherical Geodesic Grids

Abstract A forward trajectory advection scheme has been designed for its use in an icosahedral–hexagonal grid model. The scheme has been evaluated with two-dimensional test cases: solid-body rotation and deformational flow; both depict important characteristics of atmospheric flows. The main motivation of this study is to achieve good accuracy without using higher-order interpolations in a numerical advection scheme, so that it may become viable in fine-resolution GCMs. The computation of the error norm shows its gradient as constant and the scheme is approximately first-order accurate. The other interesting feature of this study is that its downstream search algorithm reduces the complexity from O(n2) to O(n).

Sensitivity of radiative forcing to global carbonaceous emissions

ABSTRACT Direct radiative forcing at top of the atmosphere for black carbon aerosols from two inventories comes out to be +0.33 W m−2 for Global Emission Inventory Activity (GEIA) and +0.14 W m−2 for BOND (Bond et al., 2004). However, for organic matter aerosols, it is simulated as −0.44 W m−2 for GEIA and −0.11 W m−2 with BOND inventory. Simulated annual global burden and aerosol optical depth of carbonaceous aerosols from GEIA and BOND are also compared. Normalised differences plots show that model simulates generally higher values of carbonaceous aerosols with GEIA, which are far superior in some parts of the globe as compared to those simulated with BOND emission inventory. An evaluation of these quantities with the median of the response of the AeroCom models is considered here as a benchmark – shows that while simulations with GEIA inventory have closer agreement, values of radiative forcing with BOND inventory are comparatively of smaller magnitudes over most parts of the globe. The reasons for this disparity in results for the latter may possibly be attributed to key differences between the two inventories. The main conclusion of this study is that the radiative forcing appears to be highly sensitive to carbonaceous content in aerosol compositions.

Variations in sulphate aerosols concentration during winter monsoon season for two consecutive years using a general circulation model

During the field cruises of the Indian Ocean Experiment (INDOEX) extensive measurements on the atmo¬spheric chemical and aerosol composition are undertaken to study the long-range transport of air pollution from south and southeast Asia towards the Indian Ocean during the dry monsoon season in 1998 and 1999. The present paper discusses the temporal and spatial variations in aerosols and aerosol forcing during the winter monsoon season (January-March) for INDOEX first field phase (FFP) in 1998 and INDOEX intensive field phase (IFP) in 1999. An interactive chemistry/aerosol model (LMDZ.3.3) is used to investigate the variation in the spatial distribution of tropospheric sulphate aerosols during 1998 and 1999. The model results depict major enhancement in the sulphate aerosol concentrations, radiative forcing (RF) and optical depth over the Indian subcontinent and adjoining marine areas between INDOEX-FFP and IFP. A significant increase in transport of sulphate aerosols from the continents to the Indian Ocean region has also been simulated during the winter monsoon in 1999. The mean RF over INDOEX-FFP in 1998 is found to be –1.2 Wm -2 while it increased to –1.85 Wm -2 during INDOEX-IFP in 1999. Model results reveal a mean sulphate aerosol optical depth (AOD) of 0.08 and 0.14 over Indian subcontinent during 1998 and 1999, respectively. The model results suggest that elevated AOD downwind of source regions in India can significantly affect the regional air quality and adjoining marine environments.

Impact of carbonaceous aerosols on Indian monsoon rainfall

Aerosols - liquid or solid particles suspended in the air - are important constituents of the global atmosphere. Asia is the region of the great source of global carbon emission and this trend is expected to increase in the near future. There are two kinds of carbonaceous aerosols i.e. organic matter (OM) and black carbon (BC). Black carbon stands after only carbon dioxide (CO2) in the list of climate change contributors. BC can directly absorb solar radiation or mix with other aerosols to form atmospheric brown clouds which absorb incoming solar radiation and prevent it from reaching the surface, thereby warming the atmosphere. Thus, in this study, the Laboratoire de Meteorologie Dynamique model (LMD, version 3.3) is used to investigate the possible effect of carbonaceous aerosols over India for the monsoon periods on the atmospheric radiation transfer and over the precipitation. LMDZ.3.3 is integrated for different years for the Indian southwest monsoon periods (seasonal experiments) over the globe for the resolution 96x72x19 (approx. 300 km). Model simulated aerosol optical depth at 550 nm has been validated with satellite data (MODIS). The simulation results show that BC aerosol induce a positive radiative forcing, while organic matter show negative radiative forcing at the top of the atmosphere and a negative radiative forcing at the surface in this region. However, the impact of BC and OM over rainfall is different and complex for different places. The sensitivity studies for carbonaceous aerosols have been done for 21 years (1987-2007) for the monsoon period, and the rainfall is compared with GPCP (Global Precipitation Climatology Project) with the help of Principal component analysis.

Impact of moisture variations on the circulation of the south-west monsoon

The impact of moisture anomalies on the circulation of the south-west Indian monsoon has been studied with a general circulation model. Newtonian relaxation is adopted to subject the model atmosphere under sustained moisture anomalies. The impact of negative anomalies of moisture was seen as a divergent circulation anomaly, while the positive anomaly was a stronger convergent anomaly. Although the humidity fields display a resilient behaviour, and relax back to normal patterns 1–2 days after the forcing terms in humidity are withdrawn, the circulation anomalies created by the moisture variation keeps growing. A feedback between positive moisture anomalies and low level convergence exists, which is terminated in the absence of external forcings.