A new perspective on the Ross Ice Shelf air stream (RAS)

A case study is presented illustrating cloud processes and other features associated with the Ross Ice Shelf air stream (RAS), Antarctica. The RAS is a semi-permanent low-level wind regime primarily over the western Ross Ice Shelf, linked to the midlatitude circulation and formed from terrain-induced and large-scale forcing effects. An integrated approach utilizing MODIS satellite imagery, automatic weather station (AWS) data, and Antarctic Mesoscale Prediction System (AMPS) forecast output is used to study the synoptic-scale and mesoscale phenomena involved in cloud formation over the Ross Ice Shelf during a RAS event. A synoptic-scale cyclone offshore from Marie Byrd Land draws moisture across West Antarctica to the southern base of the Ross Ice Shelf. Vertical lifting associated with flow around the Queen Maud Mountains leads to cloud formation that extends across the Ross Ice Shelf to the north. The low-level cloud has a warm signature in thermal infrared imagery, resembling a surface feature of turbulent katabatic flow typically ascribed to the RAS. Key meteorological features of this case study, in the context of previous studies on longer time scales, are inferred to be common occurrences. The assumption that warm thermal infrared signatures are surface features is found to be too restrictive.