Oceanic single-layer warm clouds missed by the cloud profiling radar as inferred from MODIS and CALIOP measurements

Abstract

Attributed to its unique advantage of cloud vertical resolving, Cloud Profiling Radar (CPR) measurements have been used as the primary component in synthetic cloud data for relevant studies. However, due to surface clutter and sensitivity limitation, considerable warm clouds over global oceans are missed by CPR, which causes severe sampling biases and problematic statistics of cloud properties. By using independent cloud mask data jointly from Moderate Resolution Imaging Spectroradiometer and Cloud-Aerosol Lidar with Orthogonal Polarization measurements, this study gives an evaluation on these issues and focuses on single-layer clouds. By excluding effects from CPR’s spatial resolution, the CPR detection failures are found to be frequent over global oceans, with an overall miss rate at ~0.39. For each cloud type, altocumulus, stratus, stratocumulus, and cumulus, its miss rate decreases monotonically with height, and it is only at 2.5-3.0 km altitude that the miss rate is negligible for the ensemble of oceanic single-layer warm clouds. The miss rates are different among cloud types, which are attributed to their distinct microphysical properties. Clouds with droplet effective radius below 12 µm or cloud optical depth below 4 are very likely to be missed by CPR, resulting the globally averaged overestimation of 10-24% and 24-36%, respectively. The miss rate has a strong negative correlation with cloud water path (CWP) and decreases below 0.1 only for CWP exceeding 200 gm-2. The resulting overestimation on globally averaged CWP is 36.6 gm-2 (44.3%). Throughout the globe, the biases are mostly positive and have notable regional variations. Especially in the typical oceans that have abundant warm clouds, the CWP is overestimated by 20%-80%.