Numerical simulations of the formation of melting-layer cloud

A number of previously published observational studies have reported the common occurrence of cloudy layers at around 5 km elevation in the tropics. There are two candidate processes that are able to explain the common occurrence of cloudy layers in the middle level: cloud detrainment promoted by the stable layer and enhanced condensation to compensate melting cooling. In the present study, we used a cloud-resolving nonhydrostatic model and conducted numerical simulations of a squall-line in order to clarify the process responsible for the formation of mid-level thin cloud, especially the cloud at the 0°C level.

In a two-dimensional control experiment, cloud coverage showed a notable peak just below the 0°C level for environments without a stable layer in the initial temperature profile. Enhanced and weakened stability layers simultaneously appeared above and below the peak level of the cloud coverage. The formation of mid-level thin cloud is associated with intensified condensation to compensate strong cooling due to melting of ice particles. The enhancement of condensation continues until ice is no longer provided to the cloud at the melting level. This means that the cloud survives for a longer period than cloud at other levels.

To investigate the influence of the commonly observed tropical stable layer on the occurrence of mid-level thin cloud, we performed three sensitivity tests in which a warm rain microphysics scheme was employed and/or the initial temperature profile had enhanced and weakened stability layers in the middle level. Comparisons among the control and sensitivity experiments revealed that intensified condensation related to melting cooling also plays a critical role in the formation of mid-level thin cloud, although the stable layer is associated with the inhibition of convection growth in the middle level. A three-dimensional experiment under more realistic conditions simulates cloud formation at the 0°C level, although the peak of the cloud coverage is less prominent than those in the two-dimensional experiments.