Study of deep clouds over the indian region using insat-1B pixel data

Abstract

In Earth’s atmosphere, a variety of clouds exist, and the present thesis is about the study of deep convective clouds, whose tops extend up to and beyond the mid troposphere. Deep convective clouds are often organized in clusters having horizontal length scales of a few tens of kilometres to several hundred kilometres. They play an important role in the dynamics and energetics of the atmosphere. A large number of studies on deep clouds have been carried out using both geostationary and polar orbiting satellite data covering various parts of the globe. However, studies over the Indian subcontinent and surrounding oceanic regions using pixel level data are very few. The present study uses INSAT 1B pixel data (22 km × 22 km) and covers the area 30°S–30°N and 40°E–110°E. The main objective is to investigate the characteristics of deep clouds, including:

frequency of occurrence, size distribution, lifetime, preferred regions of formation and dissipation, propagation speeds, cloud tracks, rate of area growth, and relationships with ambient wind, land–sea contrasts, and diurnal variation.

In this study, a cloud system is defined as a set of contiguous pixels having brightness temperature equal to or below a specified threshold. No restriction is imposed on cloud shape. A new automatic cloud tracking algorithm has been developed that accounts for mergers and splits. The algorithm uses maximum allowable displacement of cloud systems in 3 hours along with area overlap criteria. It is observed that cloud systems with areas below 10 pixels are extremely numerous, but their total area coverage is less than 1%. Therefore, the minimum size threshold is fixed at 10 standard pixels ( 4840 km²). This corresponds to a mesoscale system as per Orlanski’s (1975) classification. The temperature threshold is varied from 201 K–261 K. This is the first detailed study of deep cloud systems over the Indian region using INSAT 1B pixel data. As in other parts of the world, the study region is dominated by a large number of small cloud systems and a small number of large cloud systems, with the latter contributing the most to the total cloudy area. The number of cloud systems increases approximately linearly with threshold temperature up to 241 K. For cloud systems defined using the 261 K threshold that also contain systems at 201 K, the relative areas occupied by pixels at different thresholds vary linearly when plotted against corresponding cloud top pressure. Cloud tracking results are not highly sensitive to the chosen successor identification criterion, except in very long lived cloud systems. On average, the number of growing and decaying cloud systems is nearly equal. Propagation speeds range from 6–14 m/s. A majority of cloud systems dissipate within a 2.5° × 2.5° box surrounding their formation region. Long lived systems, however, travel considerable distances during their lifetime. An important observation regarding diurnal variation is that, during monsoon months, the variability of cloud systems over land is significantly lower compared to other months.