Scale Dependence of Monsoonal Convective Systems over the Indian Ocean

Abstract

Deep convective clouds in the Tropics have long been recognized to occur on a wide spectrum of spatial scales, ranging from the individual cumulus to the meso- and synoptic-scale cloud systems. The objective of this paper is to examine the scale dependence of the properties of clouds embedded in the Intertropical Convergence Zone. The Indian Ocean during the winter and summer monsoons offers an ideal domain to undertake this study, which uses INSAT-1B infrared imagery. The cloud systems are retrieved using the detect and spread algorithm and classified according to their top temperatures. Their spatial extension spans a continuous spectrum of individual clouds ranging from 500 km2 to 106 km2. The spatial distribution of these convective clouds over the Indian Ocean exhibits an increase in convective activity during boreal winter compared to summer. Despite the drastic modification of the synoptic environment over the seasonal cycle, intrinsic cloud properties in January and July are shown to be very similar. The intrinsic cloud properties that are retrieved are the convective core area relative to the total cloud area, the area colder than 240 K (corresponding roughly to stratiform precipitation), the average cloud-top temperature of the entire cloud (core and anvil), and the minimum cloud-top temperature within a cloud that is assumed to denote the temperature of the overshooting cloud tops. The analysis reveals a critical scale of about 104 km2, which distinguishes two separate convective regimes of scale-dependent cloud properties. Below the critical scale, the cloud mean effective temperature increases with cloud size and the relative core area decreases with the size. The overshooting cloud-top temperature is invariant to the cloud scale. For scales larger than the critical value, the scale dependence is reversed: the mean cloud temperature decreases, the fractional core area increases, and the overshooting cloud top strongly decreases as the cloud size increases. Essentially, the area of undiluted deep convective core increases with the total area of the cloud system, in turn affecting the macroscale properties such as cloud greenhouse effect and tropopause temperature, to name a few. In particular, it is the larger-scale (>104 km2) organized system that penetrates to the tropopause and determines the tropopause altitude, while the smaller scales (<104 km2) hardly reach the upper troposphere. Diurnal variations of the convective cloud cover are also presented with respect to the cloud size. The diurnal cycle of these systems depends significantly on their scale and exhibits complex patterns. A discussion of these cloud statistics is then offered in the context of general circulation model parameterization.