Mesoscale Convective Systems Over the Tropical Western Pacific

Abstract

There exists considerable debate as to what extent convection is self-limiting due to the changes it induces in its immediate environment, and how to quantify these changes in a climatic sense without excessively masking the variability of individual convective systems. This study addresses these issues by looking at the thermodynamic changes that occur in the vicinity of Mesoscale Convective Systems (MCSs). A prelude to the main focus of the study involves quantifying how a MCS propagates and changes, as viewed with infrared (IR) satellite imagery. Convective systems show up clearly on IR images as patches whose brightness temperature (Tb) is significantly colder than their surroundings. The average Tb distribution of 138 MCSs is compiled, in order to obtain the criteria necessary for the main part of the study. Standalone results from this part of the study are: a) the maximum areal extent of the coldest cloud occurs before that of the less cold cloud, and b) the lifetime of a MCS is directly related to its size. Radiosonde soundings are composited together by the amount of Deep Convective Cloud (DCC) in their vicinity. In particular, consideration is given to the DCC amount before and after the sounding, not just at the sounding time. This allows an estimation of what stage of the convective lifecyle that the sounding has sampled. The following things are found from the composites: a) Convective Available Potential Energy (CAPE) is at a maximum just before the onset of convection. It then drops off and subsequently recovers as the convective event finishes. b) the boundary layer stability - expressed with Convective Inhibitive Negative Energy (CINE) - increases sharply as the convective event progresses c) the upper troposphere warms and stabilizes during the convective event, and then returns towards normal as convection finishes. The temperature rise is most pronounced at 280mb, where a warming of 0.8° occurs. d) a temperature drop occurs over the lower to mid troposphere, and also the very upper troposphere. The strongest cooling of 1°K occurs at the surface during convection. Another minimum of 0.8°K occurs at around 740mb, and propagates upward in the latter stages of the convective event. The upper tropospheric cooling occurs at about 130mb, primarily at the start and end of convection. e) relative humidity (U) and mixing ratio (r)increase over the majority of the free troposphere. The increase in r is most pronounced at around 720mb, of 2.8g kg-1. At around 960mb, r is slightly increased prior to convection, but decreases as convection progresses, and this level ends up slightly drier than before convection started. U increases by 40% at 500mb, and shows a general increase throughout the middle and upper troposphere. A major point raised is the importance of understanding the processes behind termination of individual MCSs, and also behind periods of more widespread convective supression. This must be regarded together with the sources of available convective energy, when considering the factors that regulates convection.