On Droplet-Mediated Sensible and Latent Heat Transfer in the Marine Atmospheric Boundary Layer: “Polar Low” Versus “Tropical Cyclone” Conditions

Abstract

We investigate sensible and latent heat exchange between evaporating saline droplets and carrier air by performing direct numerical simulation (DNS) of an idealized flow modelling the marine atmospheric boundary layer. Turbulent, droplet-laden Couette airflow over a waved water surface is considered in the DNS under two different sets of bulk air and water temperatures, $T_{a} = -\, 10\,^{\circ}{\hbox{C}}$, $T_{s} = 0\,^{\circ} {\hbox{C}}$, and $T_{a} = 27\,^{\circ} {\hbox{C}}$, $T_{s} = 28\,^{\circ} {\hbox{C}}$, conditionally termed as “polar low” and “tropical cyclone” conditions, respectively. Coupled equations of the airflow and droplet transport are solved, and diffusive sensible and latent heat fluxes from droplets to the air are evaluated, and their distributions over droplet size are obtained. The DNS results show a qualitative difference in the droplet-mediated heat transfer of the two cases. Under polar low conditions, both the sensible heat, Q_S, and the latent heat, Q_L, fluxes are positive (directed upwards), and the Q_L contribution is significantly reduced as compared to the Q_S contribution. In this case, droplets mostly warm up the air. On the other hand, under tropical cyclone conditions, Q_S is negative and Q_L is positive, and thus droplets cool and moisturize the air. In both cases, the droplet-mediated enthalpy flux, Q_S + Q_L, is positive, vanishes for sufficiently small droplets (with diameters d ≤ 150 μm), and further increases with d. The net droplet-mediated enthalpy transfer is most pronounced in the buffer region and negligible in the logarithmic region of the boundary layer, and reduced with increasing surface-wave slope.