Retrieval of cirrus properties by Sun photometry: A new perspective on an old issue. Issue 10 (20th May 2013)

• Record Type: Journal Article
• Authors: Segal‐Rosenheimer, Michal; Russell, Philip B.; Livingston, John M.; Ramachandran, S.; Redemann, Jens; Baum, Bryan A.
• Is Part Of: Journal of geophysical research. Volume 118:Issue 10(2013:Oct.)
• Page Start: 4503
• Page End: 4520
• Publication Date: 2013-05-20
• Subjects: cirrus microphysical properties -- cirrus optical depth -- Sun photometers -- cloud optics -- aerosols and clouds; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/jgrd.50185
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract : [1] Cirrus clouds are important modulators of the Earth radiation budget and continue to be one of the most uncertain components in weather and climate modeling. Sun photometers are widely accepted as one of the most accurate platforms for measuring clear sky aerosol optical depth (AOD). However, interpretation of their measurements is ambiguous in the presence of cirrus. Derivation of a valid AOD under cirrus conditions was focused previously on correction factors, rather than on derivation of cirrus cloud optical thickness (COT). In the present work, we propose a new approach that uses the total measured irradiance to derive cirrus COT and ice particle effective diameter ( D eff ). For this approach, we generate lookup tables (LUTs) of total transmittance for the Sun photometer field of view (FOV) due to the direct and scattered irradiance over the spectral range of 400–2200 nm, for a range of cirrus COT (0–4), and a range of ice cloud effective diameters (10–120 µm) by using explicit cirrus optical property models for (a) cirrus only and (b) a two‐component model including cirrus and aerosols. The new approach is tested on two cases (airborne and ground‐based) using measured transmittances from the 14‐channel NASA Ames Airborne Tracking Sun photometer. We find that relative uncertainties in COT are much smaller than those for D eff . This study shows that for optically thin cirrus cases (COT < 1.0), the aerosol layer between the instrument and the cloud plays an important role, especially in derivation of D eff . Additionally, the choice of the cirrus model may introduce large differences in derived D eff . Key Points: New approach to retrieve cirrus properties from sunphotometers Utilization and comparison of several explicit cirrus optical properties models Investigating the effect of aerosol below cloud on cirrus property retrievals