CALIPSO Observed Low Clouds Over the Subtropical Northeast Pacific: Summer Climatology and Interannual Variability

Abstract

This study examines the spatial distribution and temporal variations of low‐cloud top over the subtropical northeast Pacific (NEP), using observations from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). A comparison between CALIPSO observations and in‐situ soundings reveals a distinct bimodal distribution of low‐cloud tops, characterized by peaks near the inversion base and transition layer, corresponding to shallow cumulus clouds below the stratocumulus deck and boundary‐layer decoupling phenomena. Climatologically, while traversing along a great circle from Los Angeles to Honolulu, the frequency of low cloud occurrence experiences a gradual reduction from 80% to 30%. Meanwhile, cloud top height increases from 0.6 to 1.6 km, and surface roughness varies from 50 to 300 m. This study marks the first presentation of long‐term characteristics of low clouds over a vast, remote area, expanding beyond discrete field campaign observations. Accurately characterizing the marine atmospheric boundary layer (MABL) including its depth and degree of decoupling, enables further investigation into the interannual variations of low clouds, using 14 years of CALIPSO observations. Significant interannual variability in areal fraction and vertical bimodality of low clouds emerges downstream of the anomalously cold water; increased cloud cover with weakened bimodality is associated with a shallow weakly decoupled MABL. This research emphasizes the need to consider the downstream effects in studying low cloud‐sea surface temperature feedback within climate models.