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Abstract. Many global aerosol and climate models, including the widely used Community Atmosphere Model version 5 (CAM5), have large biases in predicting aerosols in remote regions such as the upper troposphere and high latitudes. In this study, we conduct CAM5 sensitivity simulations to understand the role of key processes associated with aerosol transformation and wet removal affecting the vertical and horizontal long-range transport of aerosols to the remote regions. Improvements are made to processes that are currently not well represented in CAM5, which are guided by surface and aircraft measurements together with results from a multi-scale aerosol-climate model that explicitly represents convection and aerosol-cloud interactions at cloud-resolving scales. We pay particular attention to black carbon (BC) due to its importance in the Earth system and the availability of measurements.We introduce into CAM5 a new unified scheme for convective transport and aerosol wet removal with explicit aerosol activation above convective cloud base. This new implementation reduces the excessive BC aloft to better simulate observed BC profiles that show decreasing mixing ratios in the mid-to upper-troposphere. After implementing this new unified convective scheme, we examine wet removal of submicron aerosols that occurs primarily through cloud processes. The wet removal depends strongly on the subgrid-scale liquid cloud fraction and the rate of conversion of liquid water to precipitation. These processes lead to very strong wet removal of BC and other aerosols over mid-to high latitudes during winter months. With our improvements, the Arctic BC burden has a 10-fold (5-fold) increase in the winter (summer) months, resulting in a muchbetter simulation of the BC seasonal cycle as well. Arctic sulphate and other aerosol species also increase but to a lesser extent. An explicit treatment of BC aging with slower aging assumptions produces an additional 30-fold (5-fold) increase in the Arctic winter (summer) BC burden. This BC aging treatment, however, has minimal effect on other underpredicted species. Interestingly, our modifications to CAM5 that aim at improving prediction of high-latitude and uppertropospheric aerosols also produce much-better aerosol optical depth (AOD) over various other regions globally when compared to multi-year AERONET retrievals. The improved aerosol distributions have impacts on other aspects of CAM5, improving the simulation of global mean liquid water path and cloud forcing.

Wide‐ranging multi‐platform data from a major field campaign conducted over Indian region was used to estimate the energy absorbed in ten layers of the atmosphere. We found that during pre‐monsoon season, most of Indian region is characterized by elevated aerosol layers. Three‐fold increase in aerosol extinction coefficient was observed at higher atmospheric layers (>2 km) compared to that near the surface and a substantial fraction (as much as 50 to 70%) of aerosol optical depth was found contributed by aerosols above (reflecting) clouds. Consequent absorption and hence strong warming above clouds was found larger by several degrees (K) compared to that near the surface. The aerosol‐induced elevated warming was mostly confined below 2 km over northern Indian Ocean while found up to 4 km over central India, thus exhibiting strong meridional gradients (∼4 K) at atmospheric levels above 2 km. Climate implications of the large elevated warming are discussed.

[1] Measurements of spectral aerosol optical depths (AODs) were made over the Bay of Bengal region (adjacent to the Indian landmass) on board the oceanographic research vessel Sagar Kanya during February 2003. Simultaneous measurements of spectral AODs and mass concentrations of the composite aerosols and aerosol black carbon (BC) were made at an island location, Port Blair (11.63°N, 92.71°E), also in the Bay of Bengal. At the cruise locations the AODs were in the range of $0.3-0.6 at 500 nm (with a mean value of 0.41 ± 0.14) and Angstrom wavelength exponent of $1.1 ± 0.1; while at Port Blair the AODs were in the range of 0.11-0.48 at 500 nm and Angstrom wavelength exponent of 0.98 ± 0.07. Aerosol BC constituted 5.8 ± 0.6% of the composite aerosol mass concentration with a single-scattering albedo of $0.

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[1] During an intense field campaign for generating a spatial composite of aerosol characteristics over peninsular India, collocated measurements of the mass concentration and size distribution of near-surface aerosols were made onboard instrumented vehicles along the road network during the dry, winter season (February-March) of 2004. The study regions covered coastal, industrial, urban, village, remote, semiarid, and vegetated forestlands. The results showed (1) comparatively high aerosol (mass) concentrations (exceeding 50 mg m À3 ), in general, along the coastal regions (east and west) and adjacent to urban locations, and (2) reduced mass concentration (<30 mg m À3) over the semiarid interior continental regions. Fine, accumulation-mode particles (<1 mm) contribute more than 50% to the total aerosol mass concentration in the coastal regions, which is more conspicuous along the east coast than the west coast, while the interior regions showed abundance (>50% of the total) of coarse-mode aerosols (>1 mm). The spatial composite of accumulation-mode share to the total aerosol mass concentration agreed very well with the monthly mean spatial composite of aerosol fine-mode fraction for February 2004, deduced from Moderate-Resolution Imaging Spectroradiometer data for the study region, while a point by point comparison yielded a linear association with a slope of 1.09 and correlation coefficient of 0.79 for 76 independent data pairs. Pockets of enhanced aerosol concentration were observed around the industrialized and urban centers along the coast as well as inland. Aerosol size distributions were parameterized using a power law. Spatial variation of the retrieved aerosol size index shows relatively high values (>4) along the coast compared to interior continental regions except at a few locations. Urban locations showed steeper size spectra than the remote locations.Citation: Moorthy, K. K., et al. (2005), Wintertime spatial characteristics of boundary layer aerosols over peninsular India,

Desert dust over the Indian region during pre-monsoon season is known to strengthen monsoon circulation, by modulating rainfall through the elevated heat pump (EHP) mechanism. In this context, an insight into long term trends of dust loading over this region is of significant importance in understanding monsoon variability. In this study, using long term (2000 to 2015) aerosol measurements from multiple satellites, ground stations and model based reanalysis, we show that dust loading in the atmosphere has decreased by 10 to 20% during the pre-monsoon season with respect to start of this century. Our analysis reveals that this decrease is a result of increasing pre-monsoon rainfall that in turn increases (decreases) wet scavenging (dust emissions) and slowing circulation pattern over the Northwestern part of the sub-continent.

Land use and land cover (LULC) change has been shown to have significant effect on climate through various pathways that modulate land surface temperature and rainfall. However, few studies have illustrated such a link over the Indian region using observations. Through a combination of ground, satellite remote sensing and reanalysis products, we investigate the recent changes to land surface temperature in the Eastern state of Odisha between 1981 and 2010 and assess its relation to LULC. Our analysis reveals that the mean temperature of the state has increased by ~0.3 °C during the past three decades with the most accelerated warming (~0.9 °C) occurring during the recent decade (2001 to 2010). Our study shows that 25 to 50% of this observed overall warming is associated with LULC. Further we observe that the spatial pattern of LULC changes matches well with the independently estimated warming associated with LULC suggesting a physical association between them. This study also reveals that the largest changes are linked to changing vegetation cover as evidenced by changes to both LULC classes and normalized difference vegetation index (NDVI). Our study shows that the state has undergone an LULC induced warming which accounts for a quarter of the overall temperature rise since 2001. With the expected expansion of urban landscape and concomitant increase in anthropogenic activities along with changing cropping patterns, LULC linked changes to surface temperature and hence regional climate feedback over this region necessitates additional investigations.