Strongly Absorbing Aerosols Affect Retrievals of Cloud Optical Depth

Science

Surface-based and satellite remote sensing techniques have proven indispensable for learning more about cloud temporal and spatial variations. Comparisons between cloud properties retrieved from surface and spaceborne platforms are commonly done as a way of validating satellite retrievals. Observations made at the surface are often treated as the “ground-truth” with respect to satellite retrievals. Both retrieval methods suffer from uncertainties, though. It is important to understand and reconcile any systematic differences that arise before attributing any discrepancies to satellite retrieval errors. This research identifies a common cause of retrieval errors based upon a set of cloud properties obtained from 17 May 2008 to 12 May 2009 over the Taihu Lake station located in the center of the heavily-polluted Yangtze River Delta. The culprit is the heavy loading of strongly absorbing aerosols endemic to the region.

Impact

Aerosol optical depth (AOD) is high and single scattering albedo is moderately low over the site. The mean AOD during the study period was 0.87, and a typical value for single-scattering albedo at the site was 0.913. The region around Taihu hosts one of the densest concentrations of factories in the world with many coal-based power plants. Air quality is also affected by the seasonal influx of mineral dust from the remote western desert parts of China as well as thick layers of biomass-burning aerosols, presumably from the local burning of agriculture waste.

To explore how the turbidity of the atmosphere around Taihu affects the retrieval of cloud properties, an iterative algorithm for the surface-based simultaneous retrieval of cloud optical depth (τc) and effective radius (re) was first developed, which combines downwelling radiative fluxes at 415 nm, measured by a multi-filter rotating shadowband radiometer and liquid water path retrievals from a microwave radiometer (MWR). Comparisons between surface-retrieved and satellite-retrieved cloud properties were then done. One finding is the effect of absorbing aerosols on downwelling surface solar fluxes at 415 nm can result in significant errors in the retrieved τc. The errors increase with increasing τc and AOD. These aerosols have a significant impact on the top-of-the-atmosphere albedo, and thus lead to serious errors in the retrieved τc from satellite as well. Both errors increase with τc in opposite directions: positive biases for surface retrievals and negative biases for satellite retrievals. Regarding other cloud properties, MODIS retrievals of re are larger (smaller) than ground retrievals for low (high) liquid water paths (LWP). Biases in the retrieval of τc and re lead to biases in LWP. For LWP less than about 200 g/m2, MODIS systematically overestimates LWP relative to ground-based MWR retrievals.<.p>

Summary

Such a significant bias may be misconstrued as the semi-direct effect of aerosols, namely, COD decreases with increasing AOD for cloud droplets undergoing evaporation by aerosol-induced absorption. Caution is thus called for when dealing with aerosol-cloud interactions using both satellite and ground retrievals over regions characterized by strongly absorbing aerosols.