Convective cloud parameterization

Abstract

In the context of climate modelling, convective clouds in tropical regions play a major role in determining the climate sensitivity. The vertical transport of mass and energy associated with this type of clouds is often represented with so called mass-flux parameterization schemes. In this work the aim is to evaluate the relationship between mass-flux and large scale environmental conditions using observations in a tropical region, over a period of 13 wet seasons. A uniquely comprehensive data set from the C-band polarimetric radar (CPOL) in Darwin, Australia, is used to estimate vertical velocity inside precipitating convective clouds. Ultimately, mass-flux is derived over a domain size similar to that of a general circulation model (GCM) grid box. Five parameters (RH_500, CAPE, CIN, ­Omega_500 and Chi_crit) are selected to describe environmental conditions and with these, the magnitude and shape of mass-flux is analysed. Chi_crit appears to be the most valid parameter to represent both the shape and magnitude of mass-flux. All other selected parameters strongly influence only one of the two aspects of the profile. Additionally, fractional entrainment is retrieved from mass-flux profiles and partitioned into two terms: one dependent on area fraction and the other on vertical velocity. Under all environmental conditions, the layer between 4.5 and 7 km experiences detrainment. It can be inferred that a stable layer, known as the freezing level, is present at 4.5 km. Below the freezing level, the vertical velocity and the mass-flux shape are most relevant to determine entrainment rates leading to the conclusion that the vertical velocity should not be disregarded when parameterizing convection.