Cloud type statistics and shallow convective cloud processes over complex terrain in central Argentina

Abstract:

Data collected from October 2018 to April 2019 during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign over the Sierras de Córdoba range in central Argentina provide the unique opportunity to study cloud’s characteristics over a region of the world where convective clouds of all scales have a high frequency of occurrence. Classifying time periods by cloud type facilitates the study of the primary mechanisms that drive their evolution. For this purpose, we developed an objective and automated cloud type classification algorithm based on vertically pointing Ka-band radar and ceilometer data. We analyze several remotely sensed and in-situ observations to link cloud macrophysical and microphysical evolution to atmospheric thermodynamics, aerosols, and circulations that affect cloud evolution.

During the 6.5 months observational period, numerous extremely variable cloud types were sampled with 130 deep convective cloud periods identified. These clouds have two preferred initiation times in the afternoon and late at night, are likely to have a short lifetime over the observing site (< 2 hours), and have depths that typically range from 4 to 10 km. On the other hand, shallow convective clouds were the most frequently observed cloud type, with over 1300 detected separate periods. Their initiation times occur most frequently in the late afternoon, likely in connection with orographic thermal upslope flow forcing and a deepening boundary layer. Most shallow cloud periods are relatively short (< 1 hour) but lifetimes associated with stratocumulus conditions extend to longer 10 hours and depths greater than 3 km. Long-lived shallow clouds with a lifetime > 30min are deeper than short-lived clouds and have evening and late morning initiation peaks suggesting further controls on occurrence than thermal upslope flow alone that are analyzed in further detail. We explore differing mechanisms for onset of precipitation in stratocumulus periods by analyzing characteristic cases in more detail. One mechanism includes decoupling from the boundary layer and key roles for limited CCN concentration and cloud top turbulence. Another involves mesoscale lift associated with orographic circulations.