| Surface observations, lake evaporation data and upper-air soundings were collected in a special data-collection effort during the months of August and September of 2004 and 2005 on an island in the middle of Great Bear Lake, Northwest Territories. For this period of the year, the late summer, when increasing variability in surface pressure starts occurring, the effect of different synoptic-scale regimes on the lake-atmosphere interface is investigated to understand the regional specificities, with comparisons to nearby stations and a high spatial and temporal resolution reanalysis model (North American Regional Reanalysis). The planetary boundary layer over the lake systematically shows typical surface and mixed layer structures but with varying depths depending on the mean lower tropospheric temperature. Inversions in the surface layer, linked with warm regimes, support deep mixed layers. Within the surface layer, during warm regimes, a superadiabatic lapse rate can support (or not) a higher-stability sublayer on top of it, in which case the mixed layer happens to be much deeper. The first mechanism for lake evaporation includes the occasional passage of synoptic-scale storm systems with both warm and cold frontal features creating strong, short-lived latent heat interactions between the lake and the atmosphere, but relatively low total amounts of evaporation. The main mechanism for extensive evaporation is a cold anticyclone allowing for significant daytime insolation. A lake-breeze circulation can then develop and provide strong-enough mesoscale winds for diurnal upward latent heat exchanges, with wind channelling into Keith Arm of the lake for strong lake-breeze onshore winds. |
