The observational network for cyclone forecasting is aimed at continuous monitoring of the horizontal and vertical structure of the atmosphere. The latest advances in technology such as satellites, radar, and automated weather stations (AWS) provide invaluable support to the monitoring of tropical cyclones by the India Meteorological Department (IMD).
The observational system
Land and ocean based observations: Surface data is the foundation on which the edifice of synoptic meteorology rests. Horizontal coverage is generally good over populated land, and marginal to poor over oceanic or desert areas. Although oceanic buoys and AWSs are being deployed, yet observational data from ocean regions continues to remain sparse. Presently, there are about 675 AWS all over India, in addition to 1100 automated rain gauge stations (ARG’s). Winds and sea surface temperatures (SST) measured from ships, buoys, and island stations are important parameters in cyclone monitoring and forecasting.
Apart from the surface observations of voluntary observing fleet (VOF) ships, arrays of buoys and tide gauge network form an integral part of the ocean observing system.
Upper air observations: Wind and temperature data from the upper atmosphere is collected using pilot balloons, radiosonde/radiowind (RS/RW), wind profiler, radar and aircraft reconnaissance. Observations for measurement of wind speed and direction alone are made two to four times a day at pilot balloon observatories (PBO), which use optical theodolites. In India, there are 62 pilot balloon observatories spread all over the country.
Radiosondes serve as the main observing system for determination of vertical structure of the atmosphere and are essential in NWP analyses and model assessment. IMD presently uses GPS Sonde at 11 places and Mark IV radiosonde in other 28 places of its upper air network. There are 11 S-band radar stations in the country at Kolkata, Paradip, Visakhapatnam, Machhilipatnam, Chennai, Sriharikota, Karaikal, Kochi, Goa, Mumbai and Bhuj. Out of these 11 stations, five use Doppler weather radars (DWRs). S-band radars are good for tracking tropical cyclones over the sea. Although the maximum range for a cyclone detection radar is 500 km, the effective range is limited to 400 km mainly due to the earth’s curvature. DWRs provide vital information on radial velocity and rainfall, in addition to reflectivity that is also available in conventional S-band radars (Fig. 1). Reflectivity estimates obtained from these radars are more accurate in comparison to those from conventional radars.
Space based observations: Space based observations for TC monitoring is obtained from national and international geostationary and polar orbiting satellites. Presently, IMD receives and processes meteorological data from three Indian satellites, namely Kalpana-1, INSAT-3D and INSAT-3A. Cloud imageries are very useful for determining the position of a cyclone, its associated convection and intensity. In addition, microwave imageries help in predicting the structural characteristics and intensification of a tropical cyclone in short range. The sea surface wind as estimated by scatterometer- based satellites (ASCAT, Windsat and OSCAT) are very useful in locating the centre of a tropical cyclone. However, these satellites have their limitations, since they provide only two observations. These (Fig. 2) also suffer from rain contamination and are unable to measure wind speed beyond 50 knots (28 mps).
Standard operation procedure
The location and intensity of a TC is determined based on (a) Synoptic, (b) Satellite and, (c) Radar observations. When a system is far out at sea, satellite observation is given more weight. When it is within radar range, the radar is given greater weightage. Synoptic observations get maximum weightage, when the system is close to the coast. There can be small differences in the observations made by different methods. Hence, a consensus is arrived at based on all the data collected by the cyclone forecaster.
Intensity classification by satellite technique is based on V F Dvorak’s work. The intensity of the tropical system is indicated by a T number based on a pattern recognition technique. Based on the pattern of clouds associated with a tropical cyclone, the intensity of a it is defined on a T-scale ranging from 1 to 8. Another feature of the technique is a current intensity number (CI) which relates directly to the intensity (in term of wind speed) of the cyclone. The CI number may differ from the T number on some occasions to account for certain factors that are not directly related to cloud features.
The location estimation error has been about 55 km over sea, while errors in determining the tropical cyclone centre over the northwest Pacific Ocean can be up to 50 km by satellite fixes, 20-50 km by radar observations and by about 20 km by aircraft reconnaissance. The induction of DWR has reduced the error in fixing the centre of cyclones in radar range. The landfall point estimation error has been reduced to about 25 km by 2010 mainly due to the installation of coastal AWSs.