Heterogeneous initial conditions affect the pathway of moisture growth and the radiative effects of trade cumuli

The role of patterning of clouds in climate sensitivity is unknown. Smaller clouds grow due to the coupling with a shallow circulation, where the cloud resides on the ascending branch of the circulation. However, there seems to be a certain maximum size in large-eddy simulations (LESs) to this scale growth, which is smaller compared to observations. Therefore, we have conducted two 500 km-domain LESs: One (Control) with homogeneous initial conditions and one (“Fish”) with a 250 km initial moisture perturbation, created such that the domain-mean moisture is kept constant. The Fish simulation emulates the advection of large-scale moisture structures into the trades, and thus reminds us of “Fish-clouds”, which appear to form this way. This simulation indeed leads to a larger cloud, which has an elongated shape and develops a bimodal Total Water Path (TWP) distribution. Moreover, the moisture growth behaves differently as compared to previous research and the Control simulation. The upward motion of the circulation does not lead to moisture growth, but balances the moisture sink (rain). The moisture growth is caused by horizontal growth in the cloud top. Eventually, the Fish has a larger cloud fraction (26%) and a larger albedo (11%) than the Control simulation, leading to a larger daily short-wave cloud radiative effect (SW CRE) of 40%. In absolute numbers, this SW CRE is up to 40 W/m^2 larger in the Fish simulation compared to the Control simulation. Thus, the initial conditions seem to trigger a different pathway for moisture growth, which affects the cloud radiative effect. This study is a first step in unravelling this process, which might affect the climate sensitivity of patterning of clouds.