We present results from sun/sky radiometer measurements of aerosol optical characteristics carried out in New Delhi during March–June, 2006, as part of the Indian Space Research Organization's Integrated Campaign for Aerosol Radiation Budget. For the first time at this site, derived are parameters such as aerosol optical depth (AOD), single scattering albedo (SSA), asymmetry parameter, Ångstrom exponent, and real and imaginary refractive indices in five spectral channels. During the campaign, a consistent increase in aerosol loading from March to June with monthly average AOD values at 0.5μm of 0.55, 0.75, 1.22 and 1.18, respectively, was observed. Ångstrom exponent gradually decreases from 1.28 (March) to 0.47 (June), indicating an increased abundance of coarse particles due to dust storms that transport desert dust from the Thar desert and adjoining regions. SSA at 0.5 μm is found to be in the range of 0.84 to 0.74 from March to June, indicating an increasing contribution from the mixture of anthropogenic and desert dust absorbing aerosols. Optical properties derived during the campaign are used in a radiative‐transfer model to estimate aerosol radiative forcing at the surface and at the top‐of‐the atmosphere. A consistent increase in surface cooling is evident, ranging from −39 W m−2 (March) to −99 W m−2 (June) and an increase in heating of the atmosphere from 27 W m−2 (March) to 123 W m−2 (June). Heating rates in the lower atmosphere (up to 5 km) are 0.6, 1.3, 2.1, and 2.5K/d from March, April, May, and June 2006, respectively. Higher aerosol induced heating in the premonsoon period has been shown to have an impact on the regional monsoon climate.

Analysis of the microphysical structure of deep convective clouds using in situ measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the Indian peninsular region is presented. It is shown that droplet size distributions (DSDs) in highly polluted premonsoon clouds are substantially narrower than DSDs in less polluted monsoon clouds. High values of DSD dispersion (0.3–0.6) and its vertical variation in the transient and monsoon clouds are related largely to the existence of small cloud droplets with diameters less than 10 μm, which were found at nearly all levels. This finding indicates the existence of a continuous generation of the smallest droplets at different heights. In some cases this generation of small droplets leads to the formation of bimodal and even multimodal DSDs. The formation of bimodal DSDs is especially pronounced in monsoon clouds. Observational evidence is presented to suggest that in-cloud nucleation at elevated layers is a fundamental mechanism for producing multimodal drop size distribution in monsoon clouds as well as in most deep convective clouds. These findings indicate that inclusion of continued nucleation away from the cloud base into numerical models should be considered to predict microphysics and precipitation of clouds in monsoons and other cloud-related phenomena.

Abstract. In order to reduce uncertainty in the estimation of Direct Aerosol Radiative Forcing (DARF), it is important to improve the estimation of the single scattering albedo (SSA). In this study, we propose a new data processing method to improve SSA retrievals for the SKYNET sky radiometer network, which is one of the growing number of networks of sun-sky photometers, such as NASA AERONET and others. There are several reports that SSA values from SKYNET have a bias compared to those from AERONET, which is regarded to be the most accurate due to its rigorous calibration routines and data quality and cloud screening algorithms. We investigated possible causes of errors in SSA that might explain the known biases through sensitivity experiments using a numerical model, and also using real data at the SKYNET sites at Pune (18.616 • N/73.800 • E) in India and Beijing (39.586 • N/116.229 • E) in China. Sensitivity experiments showed that an uncertainty of the order of ±0.03 in the SSA value can be caused by a possible error in the ground surface albedo or solid view angle assumed for each observation site. Another candidate for possible error in the SSA was found in cirrus contamination generated by imperfect cloud screening in the SKYNET data processing. Therefore, we developed a new data quality control method to get rid of low quality or cloud contamination data, and we applied this method to the real observation data at the Pune site in SKYNET. After applying this method to the observation data, we were able to screen out a large amount of cirruscontaminated data and to reduce the deviation in the SSA value from that of AERONET. We then estimated DARF using data screened by our new method. The result showed that the method significantly reduced the difference of 5 W m −2 that existed between the SKYNET and AERONET values of DARF before screening. The present study also suggests the necessity of preparing suitable a priori information on the distribution of coarse particles ranging in radius between 10 µm and 30 µm for the analysis of heavily dust-laden atmospheric cases.

[1] In situ aircraft measurements of cloud microphysical properties and aerosol during the 1st phase of the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX-I) over the Indian sub-continent provided initial opportunities to investigate the dispersion effect and its implications for estimating aerosol indirect effects in continental cumuli. In contrast to earlier studies on continental shallow cumuli, it is found that not only the cloud droplet number concentration but also the relative dispersion increases with the aerosol number concentration in continental cumuli. The first aerosol indirect effect estimated from the relative changes in droplet concentration and effective radius with aerosol number concentration are 0.13 and 0.07, respectively. In-depth analysis reveals that the dispersion effect could offset the cooling by enhanced droplet concentration by 39% in these continental cumuli. Adiabaticity analysis revealed aerosol indirect effect is lesser in subadiabatic clouds possibly due to inhomogeneous mixing processes. This study shows that adequate representation of the dispersion effect would help in accurately estimating the cloud albedo effect for continental cumuli and can reduce uncertainty in aerosol indirect effect estimates.

[1] The formation of first raindrops in deep convective clouds is investigated. A combination of observational data analysis and 2D and 3D simulations of deep convective clouds suggests that the first raindrops form at the top of undiluted or slightly diluted cores. It is shown that droplet size distributions in these regions are wider and contain more large droplets than in diluted volumes. The results of the study suggest that the initial raindrop formation is determined by the basic microphysical processes within ascending adiabatic volumes. It allows one to predict the height of the formation of first raindrops considering the processes of cloud condensation nuclei activation, droplet diffusion growth, and coalescence growth. The results obtained in the study explain observational results through which the in-cloud height of first raindrop formation depends linearly on the droplet number concentration at cloud base. The results also explain why a simple adiabatic parcel model can reproduce this dependence. The present study provides a physical basis for retrieval algorithms of cloud microphysical properties and aerosol properties using satellites. The study indicates that the role of mixing and entrainment in the formation of the first raindrops is not of crucial importance. It is also shown that low variability of effective and mean volume radii along horizontal traverses, as regularly observed by in situ measurements, can be simulated by high-resolution cloud models in which mixing is parameterized by a traditional 1.5 order turbulence closure scheme.