Work

Disentangling Bimodal and Unimodal Patterns in the Diurnal Cycle of Low Clouds and Their Meteorological Drivers at SACOL

Year: 2025

Type: article

Abstract: Diurnal cycle of low clouds (DCLC) is strongly modulated by regional meteorological conditions, leading to complex cloud diurnal patterns particularly in semi‐arid regions. However, the mechanisms governing DCLC behavior in these regions remain poorly understood due to sparse high‐precision observations and intricate meteorological conditions. In this study, we analyze 6 years (2014–2019) of ground‐based Ka‐band Zenith Radar (KAZR) observations from the Semi‐Arid Climate and Environment Observatory of Lanzhou University (SACOL) to investigate DCLC variability. Using Fourier decomposition, we propose a Bimodal Prominence Index (BPI) to quantify DCLC features and examine their seasonal transitions linked with key meteorological factors. Our results show that low clouds exhibit prominent bimodal structure with greater coverage and amplitude in cold seasons, transitioning toward unimodal pattern during warm months. Key meteorological factors, including estimated inversion strength (EIS), mid‐to‐lower tropospheric wind shear (MLWS), 700 hPa relative humidity (RH700) and 700 hPa vertical velocity (ω700), exhibit strong relationships with the DCLC variability. A multiple regression model incorporating these variables effectively reproduces the DCLC, achieving a high correlation coefficient of 0.71 with observations. RH700 and EIS primarily control low cloud formation while ω700 and MLWS modulate seasonal patterns. We found noon‐peaked RH700 and ω700 drive unimodal cloud in summer while strong inversions and enhanced nocturnal MLWS support bimodal clouds in winter. Autumn pattern reflect enhanced nighttime humidity and weakened daytime EIS peak favoring bimodality. These findings provide an insight into the meteorological controls of DCLC in semi‐arid environments and their role in affecting regional weather and climate.

Source: Journal of Geophysical Research Atmospheres

DOI: 10.1029/2025jd044125