Comparison of POLDER cloud phase retrievals to active remote sensors measurements at the ARM SGP site.

In our present study, cloud boundaries derived from a combination of active remote sensors at the ARM SGP site are compared to POLDER cloud top phase index which is derived from polarimetric measurements using an innovative m ethod. This approach shows the viability of the POLDER phase retrieval algo rithm, and also leads to interesting results. In particular, the analysis d emonstrates the sensitivity of polarization measurements to ice crystal sha pe and indicates that occurrence of polycrystalline ice clouds has to be ta ken into account in order to improve the POLDER phase retrieval algorithm a ccuracy. Secondly, the results show that a temperature threshold of 240 K c ould serve for cloud top particle phase classification. Considering the lim itations of the analysis, the temperature threshold could be biased high, b ut not by more than about 5 degrees.