New AERONET product: Cloud Optical Depth

January 1, 2012 — Click on image to enlarge.

The posture of Cimel radiometer in cloud mode. The map shows the distribution of the 271 AERONET stations used to retrieve cloud optical depth.

A snapshot of regional cloud optical depth from AERONET and MODIS. The underlying map is the COD measurements of MODIS/Terra on 16:20 GMT, Aug. 03, 2011. Based on a 10 min. time window centered at the MODIS overpass time, 20 AERONET sites provided COD measurements indicated by circle.

Clouds are the largest, yet least understood regulators of climate. Accurate measurements of key cloud-climate variables like cloud optical depth are critical to predicting climate change.

Figure 1 shows the operational cloud mode sites which are the first ground-based global network for cloud observations.  Having collocated aerosol and cloud measurements is an important step in understanding the links between these two components of our climate system.

Figure 2 reveals that AERONET cloud-mode optical depth retrievals are well consistent with those retrieved from MODIS. As a result of monitoring clouds at many AERONET sites, the retrieved cloud optical depth will be used not only for validation of satellite retrieved cloud properties, but also for studying local climatology and the diurnal cycle not available from satellites. Monitoring clouds under different climatological conditions provides a reliable database for comparison with cloud models output in support of cloud system modeling.

The Aerosol Robotic Network (AERONET) is a ground‐based network designed to measure microphysical and optical properties of aerosols. In addition to measuring aerosols, we proposed a method for monitoring cloud optical properties by using the Cimel sunphotometer's "idle time", which is inappropriate for aerosol study, by taking measurements of zenith radiance at 0.44 and 0.87 µm. Cimel zenith radiance measurements combined with surface reflectance properties are used to determine the overlying cloud properties. Working in a new "cloud" mode every 15 min, if the Sun is blocked by clouds, the Cimel radiometer points straight up and takes 10 measurements with a 9 sec. time interval. Surface reflectance properties are updated twice a month using standard Terra and Aqua atmospherically-corrected surface reflectance products. The latest AERONET product, 'Cloud Optical Depth' is available on the AERONET website (