Summertime surface mass balance and atmospheric processes on the McMurdo Ice Shelf, Antarctica.

The aim of this research was to demonstrate the relationship between variations in summertime surface mass balance of the McMurdo Ice Shelf and atmospheric processes. The approach encompassed a broad range of techniques. An existing energy balance mass balance model was adapted to deal with debris-covered ice surfaces and modified to produce distributed output. Point based surface energy and mass balance for two key surfaces of the ice shelf were linked to different synoptic types that were identified using a manual synoptic classification. The distributed model was initialised with distributed parameters derived from satellite remote sensing and forced with data from a regional climate model. Patterns of summertime surface mass balance produced by the distributed model were assessed against stake measurements and with respect to atmospheric processes. During the summers of 2003-2004 and 2004-2005 an automatic weather station (AWS) was operated on bare and debris-covered ice surfaces of the McMurdo Ice shelf, Antarctica. Surface mass balance was calculated using the energy balance model driven by the data from the AWS and additional data from permanent climate stations. Net mass balance for the measurement period was reproduced reasonably well when validated against directly measured turbulent fluxes, stake measurements, and continuously measured surface height at the AWS. For the bare ice surface net radiation provided the major energy input for ablation, whereas sensible heat flux was a second heat source. Ablation was by both melt (70%) and sublimation (30%). At the debris-covered ice site investigated, it is inferred that the debris cover is sufficient to insulate the underlying ice from ablation. Synoptic weather situations were analysed based on AVHRR composite images and surface pressure charts. Three distinct synoptic situations were found to occur during the summers, these were defined as Type A, low pressure system residing in the Ross Sea Embayment; Type B, anticyclonic conditions across region; and Type C, a trough of low pressure extending into the Ross Sea Embayment. A dependence of surface energy fluxes and mass balance on synoptic situation was identified for the bare ice surface. The distributed model was found to produce spatial patterns of mass balance which compared well with stake measurements. Mass balance patterns show that the McMurdo Ice Shelf was generally ablating in the west, and accumulating in the east during summer. Areas of enhanced ablation were found which were likely to be caused by the surface conditions and topographic effects on the wind field. The mean summertime surface mass balance across the entire ice shelf for the 2003-2004 and 2004-2005 summers were –2.5 mm w.e. and –6.7 mm w.e. respectively. The differences between the two summers are inferred to be a result of more frequent type A conditions occurring during the summer of 2004-2005