A regional climate model is configured to study boundary layer clouds over the eastern Pacific off South America. The model captures major features of the marine boundary layer structure in the eastern Pacific, including a surface mixed layer, a capping temperature inversion, stratocumulus (Sc) clouds, and their diurnal cycle. Sc clouds develop in the lower half and below the temperature inversion layer that increases its height westward off the Pacific coast of South America. The strength of the capping inversion is determined not only by large-scale subsidence and local sea surface temperature (SST), but also by cloud-radiation feedback. While of secondary importance on the seasonal mean, solar radiation drives a pronounced diurnal cycle in the model boundary layer clouds. Consistent with observations, boundary-layer clouds thicken after the sunset and cloud liquid water content reaches a maximum at 6 am of local time just before the sunset.
The boundary layer clouds over the Southeast Pacific induce a net diabatic cooling within and just above the cloud layer, which is much smaller than the net cloud-induced radiative cooling due to the compensation by condensational heating in clouds. In the lower troposphere, a meridional circulation forms in response to the net cloud-induced diabatic cooling over the Southeast Pacific. This circulation enhances the subsidence over the Southeast Pacific above the cloud layer, helping maintain boundary layer clouds and temperature inversion there. Meanwhile, the lower branch of the meridional circulation strengthens the cross-equatorial flow near the surface, enhancing moisture convergence and convection in the ITCZ north of the equator. The latter can in turn enhance the local Hadley circulation and hence the large-scale subsidence and boundary layer clouds over the Southeast Pacific. This positive feedback therefore enhances the inter-hemispheric climate asymmetry over the tropical eastern Pacific.