Mass flux characteristics of tropical cumulus clouds: A wind profiler perspective.

Description

Most cumulus parameterizations use mass flux schemes with assumptions which are yet to be fully tested. Some assumptions in the mass flux schemes have been evaluated using cloud-resolving models. But, few assumptions in the mass flux schemes have been tested with actual observations. In this study, mass flux associated with cumulus clouds are extracted at minute resolution over two wet seasons, using direct observations of vertical velocities estimated from a 50-MHz wind profiler located near Darwin, Australia. These 1-min vertical measurements cover a small cross-sectional area of a few square kilometres and are converted into mass flux estimates representing model grid boxes of hundreds of square kilometres using a time-to-space conversion technique. This conversion is based on an objective method and was necessary in order to achieve direct comparison with model mass flux. It is found that the mass flux is primarily regulated by the occurrence frequency of updraft cores, with vertical velocity intensities playing a secondary role. The mass flux profiles show clear sensitivity to humidity, with a sharp peak near 4 km in dry conditions and the peak shifted to a higher altitude of 8 km in moist conditions. Another interesting contrast between dry and moist conditions was that the vertical velocity intensities of cumulus clouds growing in dry environment were larger than those growing in moist environment. The occurrence frequency and velocity intensities of downdraft core were much less than updrafts, with very different characteristics compared to updrafts. It was noticed a cumulus cell could have one continuous updraft core, but three-four different types of short-lived downdraft cores occurring in the different layers of the troposphere.