LAKE-INDUCED ATMOSPHERIC CIRCULATIONS DURING BOREAS

Lake-induced atmospheric circulations over three lakes ranging from 3 to 10 km width are analyzed using data from three aircraft during the 1994 Boreal Ecosystem-Atmosphere Study (BOREAS), A well-defined diverg ent lake breeze circulation is observed over all three lakes during th e day, Under light wind conditions, the lake breeze is not very sensit ive to the water temperature, and the strength of the divergence over the lake decreases with increasing lake size. The boundary-layer devel opment over the surrounding land can be very important for generating a horizontal pressure difference which drives the lake breeze, Diurnal and seasonal variations of lake breezes are investigated on the basis of repeated passes from the different aircraft at different altitudes from late spring to early fall of 1994. The lake breeze divergence in creases with time during the day and reaches a maximum around 1300 LST . The latent heat flux over 10-km-wide Candle Lake increases steadily from spring to fall as the lake temperature increases, The latent heat flux over the land reaches a maximum during the summer due to evapotr anspiration. The lake effect on area-averaged fluxes sometimes leads t o a negative heat transfer coefficient for an averaging scale of sever al times the lake width.