Comparison of cirrus optical depth from ground-based lidar and radiometer with data from CALIOP and MODIS over Amazon region (2.89° S 59.97° W)

The determination of optical characteristics of clouds is very important to evaluate the planetary radiative balance. For cirrus clouds, which are formed mainly by ice crystals and are found in the upper troposphere, a precisely determination of optical depth is crucial to evaluate its radiative impact. Depending on the shortwave albedo effect of the incoming solar radiation and the longwave absorption and reemission, cirrus clouds can act either as cooling or warming agents of the climate system. Its importance grows due to its lifetime that can go from hours to a few days and large coverage area. Since 2011, a UV Raman-Lidar system and a Multi Filter Rotating Shadowband Radiometer (MFRSR) are operational on the ACONVEX site located 30 km up-wind from Manaus-AM (2.89° S 59.97° W). As the results from the first two years of measurements using a ground-based lidar system, we found that the occurrence of high clouds with base altitude higher than 8 km (temperatures below -20°C) was approximately 71% of the total time of observation, varying between about 50% in the dry season (JJA) and about 80% in the wet season (DJF). In this work, we compared the cirrus clouds optical depth measured by these ground based instruments (lidar system and the MFRSR radiometer) and satellite measurements (CALIOP and MODIS). We found that the probability density function of the observed cirrus optical depth from both gound-based lidar and radiometer instruments were very similar.