Advection in the Coastal Hudson Bay Lowlands, Canada. II. Impact of Atmospheric Divergence on the Surface Energy Balance

Abstract

A box model, contiguous with the coastline, 250 m in height, and with variable dimensions is employed to study the storage and the divergence/convergence of heat, water vapor, and mass in the internal boundary layer and to identify their impacts on the surface energy balance. Simultaneous measurements with tethersondes were made at the upwind and downwind boundaries of the box to facilitate these calculations. The analysis includes 2 d of onshore winds and 2 d of offshore winds. Storage terms are negligible and are ignored. The absolute magnitude of the horizontal divergence of latent heat was less than 5% that of sensible heat and the vertical divergence was less than 2% that of sensible heat. Thus the sensible heat flux predominates in divergence within the internal boundary layer. It always coincides in direction with the vertical flux of mass, whereas the latent heat flux is strongly influenced by the vapor pressure gradient and behaves more independently. During onshore winds, a well-developed thermal inversion of marine origin promotes a strong downward flux of mass and sensible heat near the coast. This entrains cold, moist air of marine origin into the surface boundary layer which leads to larger Bowen ratios by enhancing the sensible heat flux and diminishing the latent heat flux. This becomes particularly pronounced during the high sun period around solar noon and helps explain the exponential increase in Bowen ratios toward the coast during onshore winds. During offshore winds, there may be either vertical divergence or convergence near the coast. Divergence is due to backing up of the horizontal wind by denser marine air at the coast. Convergence is due to synoptic scale subsidence and it is hypothesized that, near the coast, it entrains cold marine air into the surface boundary layer. This can explain the observed increase in Bowen ratios toward the coast during offshore winds.