Evaluating the LCL-Updraft Width Hypothesis Using Ground-Based Observations

Several studies have proposed a relationship between the lifting condensation level (LCL) and the width of convective updrafts. Most recently, Mulholland et al (2021) proposed a mechanism by which updrafts with a higher LCL entrain moist air below the LCL, resulting in a wider updraft at cloud base. This study, and others that support this hypothesis, have been performed using convective-resolving model simulations. The purpose of this project was to determine whether the same relationship was visible in empirical observations of shallow convective cumulous clouds at the ARM Southern Great Plains site in Lamont, Oklahoma. Acquiring measurements of cloud width from surface instrumentation is a challenge due to the one-dimensional nature of most active sensors. Thus, cloud chord length is the primary measurement used as a proxy for cloud width. Ceilometer data, radar wind profilers, and LiDAR planetary boundary layer data products were used for this analysis. A dataset of cloud objects was collected over a ten-year period. For each cloud object, the lowest ceilometer range measurement was taken as the cloud base height. In cases where the average minimum range of the entire cloud object did not closely match the measured base height, the assumption was two clouds passed over the ceilometer simultaneously, and these objects were discarded. The time the cloud object persisted over the ceilometer as well as radar wind profiler measurements at cloud base were used to calculate cloud chord length. This analysis showed a positive relationship between cloud base height and chord length up to cloud bases of one kilometer. Above one kilometer, the correlation between cloud base height and chord length becomes decoupled, and a strong relationship is no longer present. The initial analysis appears to support the LCL-updraft width hypothesis, up to 1,000 meters. The results raise further questions about clouds forming at higher LCLs. Further analysis on planetary boundary layer conditions, such as layer height and state, will be incorporated into this project.