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Speaker: Dr. Faycal Lamraoui from City College of New York
Title: "Post-Cold Frontal Clouds: Relationship Between Upper-level and Boundary Layer Properties"
Abstract: Using cloud and environment observations from the Atmospheric Radiation Measurement program (DOE-ARM) Eastern North Atlantic (ENA) site in the Azores Islands and Weather Research and Forecasting Model (WRF) simulations, cloud properties in post‐cold front (PCF) periods are examined and compared to similar conditions of subsidence (non‐PCF). Also, we investigate how distinct pairs of planetary boundary layer (PBL) and convection parameterization schemes represent cloud fraction in the PCF and their interaction with upper-level. Low‐level clouds are predominant and are found to have higher cloud‐base and top heights, colder cloud‐top temperature, as well as greater vertical extent and liquid water path during PCF than non‐PCF periods. The environmental metric that is best correlated with cloud boundaries for both PCF and non‐PCF periods is the difference in potential temperature between the sea surface and 800 hPa, a parameter used to locate cold air outbreak conditions. A greater sensitivity of PCF clouds to convection than PBL schemes was found in the PCF region. The key processes that are found to significantly impact the cloud fraction distribution in the PCF region are the strength of the PBL decoupling, the vertical wind shear and the occurrence of drizzle. The results suggest both the properties of the boundary layer and the presence of an upper‐level cyclone associated with the cold front determine PCF cloud properties.[Background reading]