Synoptic controls over katabatic layer characteristics above a melting glacier

The objective was to demonstrate the degree of control exercised by synoptic‐scale meteorological conditions upon the development and characteristics of a katabatic layer above a melting glacier. Field measurements were undertaken at a point on the snout of a temperate‐alpine type valley glacier in Alberta during part of the 1979 ablation season. At this site vertical wind velocity profiles were recorded within a 7 m layer above the glacier surface, together with vertical air temperature difference over 1‐4 m and wind direction at 2 m. An hourly sampling period was chosen to facilitate investigation of the variation of near‐ice wind and temperature conditions under varying large‐scale synoptic weather patterns. Five synoptic types were identified for the study period using a subjective classification. The greatest number of significant differences between pairs of types occurred for 700 mbar temperature and relative humidity, 500 mbar temperature, and local cloud‐cover and barometric pressure at the glacier site. Representative estimates of the glacier katabatic force and synoptic pressure‐gradient force indicated the former exceeded the latter by about 12:1 under strongly developed warm anticyclones and by about 3:1 during cool cyclonic situations. Consequently under anticyclonic conditions a well developed katabatic layer was often observed with a wind velocity maximum below 5 m, whereas weaker katabatic layer development characterized cyclonic conditions. During anticyclonic influence increased deviation of ambient (700 mbar) winds from the down‐glacier direction produced significantly stronger katabatic layer inversion strength but had little impact on katabatic wind velocity or direction. Under cyclonic control across‐slope ambient winds were associated with weaker katabatic wind velocity and inversion strength, together with deflection of surface wind direction. An inverse relationship between katabatic layer wind velocity and inversion strength was significant for two synoptic types.