The Impact of Different Regimes in Estimating the Effects of Aerosols on Clouds. A Case Study over the Baltic Sea Countries.

Abstract:

The present study investigates the use of long-term satellite data to assess the influence of aerosols upon cloud parameters over the Baltic Sea region. This particular area offers the contrast of a very clean environment (Fennoscandia) against a more polluted one (Germany, Poland).

The datasets used in this study consist of Collection 6 Level 3 daily observations from 2002 to 2014 retrieved from observations by the NASA’s Moderate-Resolution Imaging Spectrometer (MODIS) instrument on-board the Aqua platform. The MODIS aerosol optical depth (AOD) and aerosol index (AI) products are used as a proxy for the number concentration of aerosol particles while the cloud effective radius (CER) and cloud optical thickness (COT) describe cloud microphysical and optical properties respectively. Through the analysis of a 12-years dataset, distribution maps provide information on a regional scale about the first aerosol indirect effect (AIE) by determining the aerosol-cloud interaction (ACI). The ACI is defined as the change in cloud optical depth or effective radius as a function of aerosol load, for which AI is used as a proxy, for a fixed liquid water path (LWP).

Reanalysis data from ECMWF, namely ERA-Interim, are used to estimate meteorological settings on a regional scale. The relative humidity (RH) and specific humidity (SH) are chosen at the pressure level of 950 hPa and they are linearly interpolated to match MODIS resolution of 1 x 1 deg. The Lower Tropospheric Stability (LTS) is computed from the ERA- Interim reanalysis data as the difference between the potential temperature at 700hPa and the surface.

In order to better identify and interpret the AIE, this study proposes a framework where the interactions between aerosols and clouds are estimated by dividing the dataset into different regimes. Regimes are defined by:

• Liquid Water Path (LWP). The discrimination by LWP allows assessing the Twomey effect. The AIE is more evident when the LWP is lower.
• Aerosol loading (both AOD and AI). Separated aerosol settings (AI/AOD <25th percentile versus AI/AOD > 75th percentile) provide information regarding the saturation effect.
• Meteorological environments. LTS determines an unstable thermodynamic environment (LTS <25th percentile) and a stable one ( LTS >75th percentile).