An assessment of cirrus heights from MISR oblique stereo using ground‐based radar and lidar at the Tropical Western Pacific ARM sites

Abstract

We compare cirrus presence and heights (CTHs) using oblique stereo by the Multiangle Imaging SpectroRadiometer (MISR) with measurements from ground‐based cloud radar and lidar sensors at the Tropical Western Pacific (TWP) sites operated by the U.S. Department of Energy Atmospheric Radiation Measurement Program. Precise point‐wise comparisons, limited to only 195 coincident cases, showed that the total number of cirrus retrieved using oblique‐stereo analysis improved to 70% from 39% using the standard‐stereo technique. The stereo technique detects cloud with the highest contrast, which is often at lower altitude. The oblique‐stereo technique's efficiency depends on the thickness and number of underlying cloud layers. A histogram approach allowed similar regions to be compared statistically with many more samples and showed three distinct peaks at ≈13 km, 15 km, and 19 km related to deep convective clouds, tropical tropopause layer (TTL) cirrus, and overshooting convective clouds, respectively. Most differences between the satellite and ground‐based measurements resulted from a number of cases of invalid cloud comparisons (14%), blunders from edges and broken clouds (7%), low contrast stereo mismatches (4%), and under‐estimation of CTHs (3%). Overall, the oblique‐stereo analysis detected a cirrus‐top layer in 65% of all the valid coincident cases, mostly <1 km in thickness. The oblique‐stereo derived cirrus CTHs differed from the heights of cirrus‐top layers from ground‐based cloud radar and lidar by −0.5 ± 1.0 km, validating the MISR retrievals. This suggests global thin cirrus retrievals are possible with the oblique‐stereo technique after the screening of occasional blunders.