Climatic Equilibrium of the Atmospheric Convective Boundary Layer over a Tropical Ocean

Abstract

A one-dimensional thermodynamic model for a partially mixed, partly cloudy, convective boundary layer (CBL) is coupled to a radiation model to compute equilibrium solutions for a tropical CBL and troposphere in energy balance over the ocean. For a sea surface temperature (SST) of 300 K, the model gives an equilibrium cloud base ≈ 950 mb, a CBL top ≈ 800 mb and a low level θe ≈ 347 K, close to climatic values. The CBL deepens and low level θe rises with increasing wind speed and SST. We explore the change in CBL structure and surface fluxes with external parameters on three timescales; namely, the CBL (∼1 day); the tropospheric radiative equilibrium (∼10 days); and the oceanic thermal equilibrium (>100 days). The variation in cloud top decreases with greater coupling to atmosphere and ocean. The slope of the latent heat flux with increasing SST decreases with more tropospheric coupling, and reverse sign with a coupled ocean. This simplified model gives an increase of tropical SST with a doubling of CO2 on climatic timscales of 2–3°K, increasing with upper tropospheric moisture.