Annexure - III

General Policy of India Meteorological Department for clearance of tall buildings proposed to be constructed in the Major

Metropolitan Cities for operation of IMD’s Weather Radars

India Meteorological Department is the nodal agency of Govt. of India for providing weather services to the nation. It monitors weather systems and disseminates appropriate warnings on operational basis to the general public as well as to the specialized agencies related to civil administration, defense, aviation, agriculture, hydrology, water resource management and disaster management. Because of the unique geophysical nature of the Indian subcontinent, this region is influenced by various types of nature's vagaries in the form of severe local thunderstorms, cyclones and heavy precipitation, resulting in floods at many places. These events occur with frightening regularity causing loss of human life, property and livestock every year. One of the prime responsibilities of IMD is to forewarn all concerned on impending severe weather. IMD deploys a vast variety of equipment and instruments for detection and surveillance of various types of weather phenomena of varying scale with fast update of high resolution observations having temporal scale of 24 hours to tens of minutes. An important component of this chain is a network of weather radars covering most of the Indian coastal belt and significant tracts of its interior. Radar is considered to be a very potent tool for meteorological observations because of its ability to provide a continuum of pictorial and quantitative data over a large coverage area with fine resolution and high update rates. Its remote sensing capability enables observations deep into the sea area. Radar is an echo measuring device and operates in the microwave frequency that requires clear line of sight space between layers and the receiver. Any structure in between will hinder its operation. Recently, with the induction of Doppler Weather Radar in the IMD network and plan for more such radars to be inducted in future, these radars have various features to map the precipitation field within 200 Kms of range in quantitative manner. For full utilization of such capabilities of these systems, it is essential that the radar should have free line of sight.

Due to rapid urbanization of cities in India, cities are becoming mega cities and towns becoming cities. As a consequence, in most of the metros development is vertical increase in high rise building is the normal trend. Because of this it is essential that line of sight of radar all around its site is protected since the impact of severe weather phenomena is felt more in these mega cities. As mentioned earlier, the radar is a very potent tool for detecting severe weather systems in real time. Hence it is necessary to impose restrictions on high rise buildings that are coming up in the field of view of weather radars. Earlier, with the conventional radars, there used to be only one product based on reflectivity and it was only qualitative in nature. Simple geometrical calculation based on 0.5 degree elevation was used for calculations to consider cases for siting clearance of high rise buildings with respect to radar site. However, with induction of latest Doppler Weather radars and digital X-band weather radars, IMD has to reexamine the formulae for siting clearance, mainly in view of rapid increase in numbers of high rise building which naturally form a cluster. A more stringent approach is made for calculating the maximum permissible height of high rise building vis-à-vis an existing weather radar location of IMD.

Recently, IMD has adopted a formula based both on geometry and physics of propagation of phenomena for examining cases of siting clearance of all tall buildings from the point of view of getting proper quantitative radar products which envisage certain conditions/exposure of radar antenna. This formula is furnished below for calculation of maximum permissible height above mean sea level and height of existing radar antenna.

Max H = sqrt { r² + R' ² + 2rR'Sin(Ф) } - R' + H_o

Where:

r = Distance of the proposed site from the radar in meters

H = Max. Permissible height of the building in meters above mean sea level

H_o = Height of the radar antenna in meters above mean sea level

R' = 4/3 R (Curvature correction of the earth)

R = Earth's radius taken as 6374 Kms

Ф = Angle of Elevation in degrees

Updated Criteria

The new generation of Doppler weather radars being procured by IMD is of high cost. They impose strict requirements on environmental and observational criteria which are different from the conventional radars in the following aspects.

1. Their very high accuracy of rainfall observation provides a quantitative means of rain measurement. However, to achieve this accuracy it is necessary that the beam is not obstructed wholly or partially by any intervening structure

2. These radars are intended to use, out to the longest possible ranges (about 500 km), for observing cyclones well before their landfall. To achieve such long ranges, these radars must observe at elevation angles down to the horizontal level (i.e. 0° elevation). This is unlike the case of short-range observation (often performed with X-band radars) for which a minimum observation angle of about 0.5° is generally adequate. In the past, the use of such a criterion had resulted in higher permissible limits for tall structures than what is now dictated by the 0° elevation rule. However, with Doppler radars, the 0° elevation rule is now essential.

3. Low elevation observation is also of very high value for aviation operations. Since aircraft landing and takeoff operations occur at the ground level and at very low altitudes, it is essential to direct the radar beam horizontally in order to observe weather close to the ground, in particular high precipitation and wind shear which are very hazardous for takeoff and landing. Further the presence of permanently blanked out small sectors (Fig. 2) in the radar data field may at times coincide with critical stretches of aircraft operation, exposing them to unobserved danger in these areas. This assumes added emergency in the current scenario where rapidly increasing aviation density and frequency of takeoff and landing call for continuous airport operation, leaving very little freedom to reschedule flights because of uncertain weather observations.

4. Because of the need for continuous weather data (e.g. to support round the clock aviation activity, to assist local authorities in weather hazard mitigation and disaster management), and the ability to archive large amounts of data, it is now necessary and possible to operate weather radars (particularly the Doppler weather radar) continuously throughout the year. Thus the occupants of structures jutting into the radar beam are exposed to far more microwave radiation than earlier years in which radars were operated less frequently only in times of need.

In response to these recent developments and the changed scenario, policy for giving siting clearance for tall structure with regard to maximum permissible height for structures in the fields of view of IMD weather radars will be governed by following criteria:

1. The line of sight of Doppler weather Radar s to be kept clear of obstructions down to

0° elevation, the structures should not be allowed to rise above a horizontal plane drawn through the centre of the radar

2. In the case of X-band non-Doppler radars, which are used for relatively short ranges of observation (up to 50 km) to essentially cover urban/metropolitan areas, the minimum elevation may be assumed to 0.5°, i.e. the structure heights must be below a line drawn through the radar antenna centre and sloping upward at 0.5°.

3. In cases where a given observation area is covered by multiple radars, the height criterion due to each radar be calculated separately and the lowest of the permissible height limits among the radars be taken as the maximum permissible height of the structure.

4. Where non-Doppler radars are the only radars available to perform long-range

observations (>50 km) the minimum elevation requirement should be 0.2°.