Synoptic Applications In

The major application of satellite data has been the monitoring of Synoptic weather systems ranging from thunderstorms to cyclones and planetary scale phenomena such as monsoon. The dynamic nature of weather systems could be captured through the time series of satellite observations leading to better understanding of the process of genesis, growth and decay. This has led to satellite based technique (Dvorak technique) to assess the intensity of TC accurately and estimate the growth potential. The specific applications include identification of primary weather system such as low pressure, depression, troughs/ridges, jet streams regions of intensive convection, inter-tropical convergence zones etc. and onset and progress of monsoon system.

Following are the major applications of satellites images in operational weather forecasting:

i. Watch and monitor growth of weather phenomena like cumulonimbus cells, thunderstorm, fog etc. and their decay.

ii. Identify and locate primary synoptic systems like troughs/ridges, jet streams, regions of intense convection, inter tropical convergence zones etc.

iii. Monitor onset and progress of monsoon systems.

iv. Detect genesis and growth of TC and monitor their intensification and movement till landfall. This application is included in the next section.

Satellite imagery is being extensively used by synoptic network in conjunction with other available conventional meteorological data for analysis and weather forecasting. Zones of cloudiness are identified from the satellite imagery as regions of upward velocity and hence potential areas for occurrence of rainfall. Visible, infra red and water vapour images have distinctive uses and are complementary to each other.

We shall summarize below very briefly some of the very important applications in the operational synoptic analysis and weather forecasting.

Satellite imagery is very handy for remote and inaccessible areas such as Himalayas where heavy precipitation usually builds up. Though the characteristic cloud patterns of cold and warm fronts are not seen over India, the Western disturbances giving rise to heavy snow fall are well captured in the Satellite imagery (Kalsi and Mishra, 1983). The Cloud band ahead of well marked westerly trough is clearly seen in the Satellite imagery. The characteristic structure of snow is easily identified and its areal extent is monitored for estimating run-off and also for long range prediction of monsoon.

Deep penetrative CB clouds and thunderstorm complexes (Kalsi and Bhatia, 1992) are rather easy to be identified in visible and infrared imagery. Squall lines are clearly seen in the satellite loops. Satellite imagery provides powerful signals for forecasting severe weather (Purdom, 2003).

The rain bearing Southwest monsoon system advances northward usually as an intermittent band of cloudiness called inter-tropical convergence zone (ITCZ). It comprises of numerous rain showers and thunderstorms associated with the convergence in the shear zone. One of the earliest studies (Sikka &

Gadgil, 1980) showed the 30-40 day oscillatory nature of monsoon flow. The INSAT and NOAA sounding data have brought out the unique nature of monsoon onset with large scale changes in wind and moisture profiles in lower troposphere prior to monsoon onset. Using satellite data Joshi et al. (1990) have also noted a spectacular rise in the 300 mb temperature over the western central and eastern Tibetan Plateau and over the region of the heat low over Pakistan. They noted this rise commencing almost 2 weeks prior to the onset of monsoon rains over southwestern India. This appears to be an important parameter for monitoring the onset of monsoons and requires to be monitored in connection with forecast of the onset of monsoon operationally. Joseph et al. (2003) have identified conditions leading to onset of monsoon over Kerala using SST, OLR and winds obtained from satellite systems.

The monsoon depressions are the principal rain bearing systems of the southwest monsoon period over India. Substantial amounts of rainfall are generated by the westward passage of monsoon depressions forming in the bay. Monsoon depressions usually develop from innocuous looking cloud systems and from diffuse pressure fields over the head Bay of Bengal. Satellite imagery shows heavy overcast cloud mass in the southern sector with low level cumulus clouds determining the Low-Level Circulation Centre (LLCC) to the northeast. The LLCC is often free of deep convection. The widespread and heavy rainfall in the southwest sector is often accompanied with deep convection in that sector. Kalsi et al. (1996) have shown from satellite imagery that a few of these depressions acquire structure of marginal cyclones with almost vertical structure upto mid-tropospheric levels. Following Scoffield and Oliver (1977), Mishra et al. (1988) also used the enhanced infrared satellite imagery to compute satellite derived rainfall estimates which were found to be realistic. These signatures provide a lot of insight into physical and dynamical processes at work in the case of monsoon depressions and are extremely useful for short range forecasting.

The 16 parameter statistical model used by India Meteorological Department has several parameters that are provided by Satellite data such as the SST, Snow Cover, E1 Nino event etc. Several recent modeling studies show that a significant fraction of the inter-annual variability of monsoon is governed by internal chaotic dynamics (Goswami, 1998). The numerical weather prediction of monsoon received impetus from the satellite observations. The parameters of SST, cloud motion vector, OLR are found to have impact on model results.

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