Cloud-aerosol transition zone radiative effects from modeling and observational perspectives

Zusammenfassung

Aerosols and clouds, as two particular cases of a single phenomenon (i.e., a suspension of particles in the air), are important components in the climate system. They play a crucial role in determination of Earth’s energy budget, as they strongly affect the balance between the incoming shortwave solar radiation absorbed by Earth’s atmosphere and surface, and the thermal longwave radiation emitted from the Earth. Although aerosols and clouds interact and affect each other's properties, their radiative properties and effects are usually treated separately in climate, meteorological, and weather forecasting studies and models. Thus, a discrimination between the cloudy and noncloudy skies is often required in such contexts. Traditionally, the algorithms used for performing this discrimination assume that the state of the sky is either cloudy or noncloudy (but containing a certain aerosol load), leaving no space for an intermediate phase. However, the change in the state of sky from cloudy to cloudless (or vice versa) occurs gradually, and it comprises an additional phase called “transition zone” (or “twilight zone”), which may represent a variety of atmospheric processes: hydration/dehydration of aerosols, cloud fragments shearing off from the adjacent clouds, decaying and incipient clouds, etc. As a result of this simplified assumption about the state of sky, the area corresponding to the transition zone is often labeled as an area containing optically thin layers of cloud or aerosol. However, the microphysical and radiative characteristics of the transition zone are expected to lay on the border between those corresponding to a cloud and those corresponding to an atmospheric aerosol. In other words, radiative and optical properties corresponding to clear (noncloudy) or cloudy skies are misleadingly used to characterize such transition zone conditions. In the present thesis we contribute to the knowledge available about the transition zone from an energy balance perspective. First, we investigated the uncertainties which may arise from neglection of the transition zone (assuming it as cloud or aerosol) in the radiative processes simulated in the models. To this aim, we isolated some of the shortwave and longwave radiative schemes included in the Advanced Research - Weather Research and Forecasting model (WRF-ARW) version 4.0, which allow users to consider different treatments of aerosols and clouds (RRTMG, NewGoddard and FLG) and then utilized them to perform a number of simulations under ideal “cloud” and “aerosol” modes, for different values of (i) cloud optical thicknesses resulting from different sizes of ice crystals or liquid droplets, cloud height, mixing ratios; and (ii) different aerosol optical thicknesses combined with various aerosol types. We found that assuming a situation corresponding to the transition zone as optically thin layers of cloud and aerosol by the radiative parameterizations can indeed introduce substantial uncertainties to the radiative processes simulated by the parameterizations in both shortwave and longwave bands. Based on these simulations we showed that assigning the properties of clouds and aerosol to a transition zone condition producing an optical depth of 0.1 (at 0.550 μm wavelength) can introduce uncertainties up to 27.0 W/m2 and 7.2 W/m2 to the simulated surface shortwave and longwave irradiances, respectively. Furthermore, with the aim of understanding the role that the transition zone plays in the determination of the Earth’s energy budget, we developed a method for quantifying the transition zone broadband longwave radiative effects at the top of the atmosphere. This method quantifies the transition zone radiative effects over the oceans based on the combination of instantaneous radiative measurements made by MODIS and CERES spaceborne radiometers and radiative transfer simulations. We tested this method using the daytime data recorded by MODIS and CERES instruments onboard Aqua spacecraft during August 2010 over South-East Atlantic Ocean. The results obtained from this analysis showed that this method is capable of detecting the transition zone broadband longwave radiative signature with an accuracy of +- 3.7 W/m2 at a spatial resolution of 20 km at nadir. Worth mentioning that the transition zone broadband longwave radiative effect for the studied domain and time was on average equal to 8.0 W/m2 (heating effect), although cases with radiative effects as large as 50 W/m2 were also found.