Diagnosis of multilayer clouds using photon path length distributions

Abstract

Photon path length distribution is sensitive to 3‐D cloud structures. A detection method for multilayer clouds has been developed, by utilizing the information of photon path length distribution. The photon path length method estimates photon path length information from the low level, single‐layer cloud structure that can be accurately observed by a millimeter‐wave cloud radar (MMCR) combined with a micropulse lidar (MPL). As multiple scattering within the cloud layers and between layers would substantially enhance the photon path length, the multilayer clouds can be diagnosed by evaluating the estimated photon path information against observed photon path length information from a co‐located rotating shadowband spectrometer (RSS). The measurements of MMCR‐MPL and RSS at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site have been processed for the year 2000. Cases studies illustrate the consistency between MMCR‐MPL detection and the photon path length method under most conditions. However, the photon path length method detected some multilayer clouds that were classified by the MMCR‐MPL as single‐layer clouds. From 1 year statistics at the ARM SGP site, about 27.7% of single‐layer clouds detected by the MMCR‐MPL with solar zenith angle less than 70° and optical depth greater than 10 could be multilayer clouds. It suggests that a substantial portion of single‐layer clouds detected by the MMCR‐MPL could also be influenced by some “missed” clouds or by the 3‐D effects of clouds.