The radiative properties of soot particles depend on their morphology and mixing state, but their evolution during transport is still elusive. Here we report observations from an electron microscopy analysis of individual particles transported in the free troposphere over long distances to the remote Pico Mountain Observatory in the Azores in the North Atlantic. Approximately 70% of the soot particles were highly compact and of those 26% were thinly coated. Discrete dipole approximation simulations indicate that this compaction results in an increase in soot single scattering albedo by a factor of ≤2.17. The top of the atmosphere direct radiative forcing is typically smaller for highly compact than mass-equivalent lacy soot. The forcing estimated using Mie theory is within 12% of the forcing estimated using the discrete dipole approximation for a high surface albedo, implying that Mie calculations may provide a reasonable approximation for compact soot above remote marine clouds.

[1] This paper reports the first observational results of aerosol characteristics over Pune, India (an AERONET site), using CIMEL Sun-sky radiometer. Here 5 years of observational data have been analyzed. The aerosol loading, in terms of the aerosol type and optical depth, undergoes strong seasonal variation over Pune with coarse-mode aerosols dominating during premonsoon season while fine-mode anthropogenic pollution particles result in boundary layer haze during postmonsoon and winter months. The daily mean AOD 440 at this location ranged between 0.10 and 1.12, with the most frequently occurring value in the range 0.45-0.55. The overall mean for the 5 year period is 0.49, with a day-to-day variability of about 36.7%. The annual average of CWV over the region shows a fluctuating trend with a marginal increase in the last 2 years. CWV shows strong annual cycle with two maxima, one in the postmonsoon and the other in the premonsoon season. Ångström exponent (a) varies in the range 0.3-1.7, with large seasonal variability. The spectral variation of SSA has been found to be large during the postmonsoon season as compared to that in the other seasons. Real n(l) and imaginary k(l) parts of complex refractive index show contrasting spectral behavior during the observational period. The average asymmetry factor (g) representative of the total aerosol layer exhibits a distinct wavelength dependence with g = 0.70 (0.02) at 440 nm and decreasing to 0.64 (0.04) at 1020 nm.

This study illustrates the validation of Aeolus Horizontal Line-Of-Sight (HLOS) winds, both Rayleigh-clear and Mie-cloudy, using in situ satellite wind (Atmospheric Motion Vectors, AMVs) observations, and NWP equivalents for three months, June-August 2020, covering the Indian summer monsoon season. Estimated errors in the Mie-cloudy (Rayleigh-clear) winds are clustered around 0.5-4 m⋅s −1 (3-8 m⋅s −1 ), and the differences between Aeolus Mie-cloudy (Rayleigh-clear) and sonde winds are within ±5 m⋅s −1 (± 8 m⋅s −1 ), but the systematic error is close to zero over the Northern Hemisphere where there are more sonde reports. Validation shows the quality of Mie-cloudy winds is better than Rayleigh-clear winds. Though the comparison against the observations like sonde (radiosonde and pilot balloons together) and aircraft indicate the quality of the Aeolus winds, their sparse spatial and temporal coverage limits the validation. Validation of Aeolus winds against AMVs provides similar results but with a better and nearly complete picture of the quality and quantity, with more information over the data-sparse and remote regions. Statistical scores suggest the characteristics of the Aeolus winds at different vertical levels and geographical regions remain the same irrespective of the validation reference datasets. The Indian summer monsoon features like Low-Level Jet (LLJ) and Tropical Easterly Jet (TEJ) are well represented in the Aeolus winds. This study also investigated the impact of the Aeolus HLOS winds over the Indian region through the collocated radiosonde and ALADIN wind profile assimilation experiments. Observing System Experiments (OSEs) suggest assimilation of Aeolus winds produced marginal improvement in the simulation of north Indian Ocean cyclones.

No abstract