Estimation of pressure drop and storm surge height associated to tropical cyclone using Doppler velocity

Transcription

1 Indian Journal of Radio & Space Physics Vol 41, June 2012, pp Estimation of pressure drop and storm surge height associated to tropical cyclone using Doppler velocity Devendra Pradhan 1,$,*, Anasuya Mitra 2 & U K De 3 1 Doppler Weather Radar, India Meteorological Department, New Secretariat Building, Kolkata , India 2 India Meteorological Department, Mausam Bhawan, Lodi Road, New Delhi , India 3 School of Environmental Studies, Jadavpur University, Kolkata , India $ pradhandev1960@gmail.com Received 3 June 2011; revised 13 March 2012; accepted 11 May 2012 A study of five tropical cyclones in the Bay of Bengal has been conducted to estimate the pressure drop in the eye of the cyclone and associated storm surge height using Doppler velocity data. A new value of constant K (13.637) has been found in the empirical relation V max = K (P-Pc) between maximum velocity (V max ) and central pressure drop (P-P c ) in terms of maximum radial velocity measured by the Doppler Weather Radar (DWR) for coastal region of India. The present study provides an alternate method for estimating central pressure drop and expected storm surge height associated to a tropical cyclone. The study also reveals that the storm surge height estimated from Doppler velocity measurements for these cyclones is very close to the actual occurrence. The results of pressure drop estimates from Doppler velocity are in close agreement with the satellite estimates. It is also observed that DWR estimates are sometimes better than those from satellite. However, the limitation of DWR is the limited range of observation (400 km) and shorter duration of cyclone tracking. Keywords: Tropical cyclone, Storm surge height, Central pressure drop, Radial velocity, Cyclonic storm PACS Nos: hv; Qx 1 Introduction Tropical cyclones are warm core low pressure systems having a large vortex in the atmosphere, which is maintained by the release of latent heat by convective clouds that form over warm oceans. Frank 1 concluded that although only 7% of the global tropical cyclones occur in the North Indian Ocean (Bay of Bengal), they are the most deadly in the world. The shallow water of the Bay of Bengal, the low flat bathymetry of the coastal zone and funneling shape of the coast line can lead to devastating losses of life and property due to the high storm surge. Frank also estimated that the average rainfall within the inner 200 km radius of a cyclone averages about 10 cm per day. In addition to the storm surge, a high speed wind also contributes to the casualties. Gray 2 documented the initial detection points of each storm for a period of 27 years during and concluded that about two-thirds of all storms form in the northern hemisphere. A detailed study over the tropical cyclones has been compiled by India Meteorological Department (IMD) 3 regarding damage potential, structure, climatology, the causes of damage and damages associated with tropical cyclones. A record of tropical cyclones tracks in the Bay of Bengal and Arabian Sea during is well documented by IMD. The available records help in predicting the cyclone tracks during pre-monsoon and post monsoon seasons. Prediction of cyclone movement is very important in estimating the landfall point of cyclone, in deciding areas to be warned on a real time basis. Studies on cyclone show that their diameter at the surface lies between 100 and 1000 km, with the axis tilting towards colder region. In the northern hemisphere, the winds in a cyclone blow anticlockwise in the lower troposphere and clockwise in the upper troposphere. Tropical cyclones become more dangerous natural hazard because of their high wind speed of 180 to 300 km h -1, high tidal surge and high rainfall intensity. Very low atmospheric pressure at the centre causes unusual rise in sea level and their persistence for several days. Total cumulative effect of high velocity winds, torrential rainfall and transgression of sea water on the coastal land become so enormous that the cyclones cause havoc in the

2 PRADHAN et al.: PRESSURE DROP AND STORM SURGE HEIGHT ESTIMATION 349 affected areas and thus tremendous loss of human lives and properties occurs. The most spectacular part of a matured cyclonic storm is its cloud free central part termed as eye due to its shape like an eye when viewed in satellite pictures. The eye forms a central dense overcast (CDO) region at the centre of the storm and its diameter ranges km as observed from the records of hundreds of tropical cyclones in this region. The eye part is surrounded by thick wall clouds around and it is a region of no winds with practically no rain and warmer than the surrounding region. The eye is generally seen when the storm is severe and the surface pressure falls below 980 HPa in the Indian Ocean areas. A geo-stationary satellite provides an estimate of the pressure drop within the eye of the cyclone and the maximum wind velocity in the eye wall region is estimated using an empirical relation given by Fletcher 4. The empirical relation for estimating the maximum wind speed associated with the wall cloud region of the cyclone is given by V max = K (P-P c ) (1) where, (P-P c ), is the central pressure drop; and V max, the maximum wind in the wall cloud region of the cyclone. Several attempts were made in the past to estimate the value of K following both empirical estimation and analytical models. Fletcher studied one hurricane over Atlantic and assigned a value of 16.0 to the proportionality constant. But after studying another 63 hurricanes in the same zone, he derived a lower value of While a tropical cyclone is characterized by strong winds of the order of 100 to 200 km h -1, the entire system moves only at about km h -1 along with the large-scale atmospheric flow around it. The eye of a fully matured cyclone is surrounded by a km thick wall of convective clouds where the maximum wind occurs. The height of the wall goes up to 15 km and is the most dangerous part of a cyclonic storm. The intense convection in this wall cloud region produces torrential rain, sometimes of the order of 50 cm per day. The storm surge associated with a cyclonic storm, responsible for 80% loss of human lives, depends on the wind speed in the eye wall region. The analog radars were unable to provide the quantitative estimates of the severity of the tropical cyclones and no information was available for the velocity of the winds associated with the cyclone. However, after the introduction of Doppler Weather Radar (DWR), analysis of tropical cyclones structure, movement and wind velocity associated may be measured with very high accuracy. Brantley & Barczys 5 suggested that Doppler radar would be valuable in weather studies because, in principle, it can separate fast-moving targets from slow-moving targets. Lhermitte & Atlas 6, Doviak et al. 7 and Wilson & Wilk 8 established that the radial wind information obtained from a single DWR can be used to derive and estimate the horizontal wind, wind shear and the vertical profile of horizontal uniform wind useful for the study of a cyclone. Banerjee et al. 9 discussed the typical features of tropical cyclone formed in the Bay of Bengal during Nov 2002 and hit the coastal area of south West Bengal on 12 Nov Pradhan et al. 10 performed a structural and wind shear analysis of the same cyclone using DWR Kolkata data and concluded that the eye of the cyclone remains circular for few km height and then its shape gets distorted with height. Also the tilting of the upper part of the cyclone shows the expected direction of the movement of the cyclone. Lee et al. 11 used the mesoscale vorticity method (MVM) in conjunction with the ground-based velocity track display (GBVTD) to derive the inner-core vertical velocity from Doppler radar observations of tropical cyclone (TC) Danny during the year The method MVM derives the vertical velocity from vorticity variations in space and time based on the mesoscale vorticity equation. The combined use of MVM and GBVTD provided good correlations among the eyewall maximum wind, bow-shaped updraft and echo east of the eye-wall in hurricane Danny. In this study, a realistic horizontal and vertical structure of the vertical velocity and the induced radial flow in Danny s inner core was found. This study demonstrated that the MVM can use high temporal resolution data observed by Doppler radars to derive realistic vertical velocity and the radial flow of TCs. Liou 12 extended the GBVTD technique to two Doppler radars to retrieve the structure of tropical cyclones circulation. With this extension, it is found that the asymmetric part of the TC radial wind component can be derived up to its angular wavenumber-1 structure, and the accuracy of the retrieved TC tangential wind component can be further improved. Although two radar systems are used, a comparison with the traditional dual Doppler synthesis indicates that this extended GBVTD (EGBVTD) approach is able to estimate more of

3 350 INDIAN J RADIO & SPACE PHYS, JUNE 2012 the TC circulation when there are missing data. Lewis et al. 13 analyzed the potential usefulness of space borne Doppler radar as a tropical cyclone observing tool by conducting a high resolution simulation of an intense hurricane and generating synthetic observations of reflectivity and radial velocity. The ground-based velocity track display (GBVTD) technique was used to process the radial velocity observations and generate retrievals of meteorologically relevant metrics such as the maximum wind (MW), radius of maximum wind (RMW) and radius of 64-kt wind (R64). The results indicate that the performance of the retrieved metrics compares favourably with the satellite methods for intensity estimation and somewhat better than current methods for structure (i.e. wind radii). The advent of meteorological satellites in the 1960 s made an immediate impact on the detection and monitoring of tropical cyclones and their track prediction 14,15. Soon thereafter, the technique formulated by D Vorak 16 for intensity estimation of tropical cyclones from satellite derived visible imagery available once a day gained widespread acceptance. The scope of the technique was later enlarged to include IR imagery which became available at night-time also. D Vorak 17 used satellite picture pair technique to estimate the intensity of cyclonic storms of Atlantic and Pacific origin and used to estimate maximum sustained wind speed and pressure drop very accurately. Both visible and IR pictures were used in the picture pair technique. Takahashi 18 derived the value of K as 13.4 by studying the typhoons over Pacific Ocean but later on redefined the value in 1952 as Natarajan & Rammurthy 19 worked with the data from cyclones over Indian sea and derived the value of K as Mishra & Gupta 20 derived the value of K as 14.2 based on wind speed and pressure data of 29 cyclones over North Indian basin. They also found a linear correlation coefficient of 0.8 between maximum velocity (V max ) and pressure deficit [ (P-P c )] by studying 35 pairs of observations. This value of K is widely used in India Meteorological Department 21 (IMD) for the estimation of the maximum velocity from satellite observations. Raj et al. 22 estimated the value of K based on an idealized surface pressure model and found to vary in the range Doppler weather radar provides a good velocity estimate of the winds associated with a tropical cyclone and therefore, has been utilized for measuring high speed winds in the wall cloud region of the cyclone. In the present study, a comparison has been made between the satellite estimated cyclone intensity in terms of T no and that estimated by Doppler velocity. 2 Data India Meteorological Department (IMD) has installed four numbers of S-band DWRs along the east coast of India for tracking tropical cyclones and other severe weather events at Chennai and Kolkata (2002), Machilipatnam and Visakhapatnam (2005), which were procured from M/s Gematronik, Germany. Technical characteristics of this radar are given in Table 1. The radial velocity data from these radars has been used in the present study. Doppler velocity data for the following cyclonic storms have been analyzed: (i) Severe cyclonic storm during 8-12 Nov 2002 (No name) DWR Kolkata (ii) Severe cyclonic storm during Oct 2006 (OGNI) DWR Chennai (iii) Very Severe cyclonic storm during Nov 2007 (SIDR) DWR Kolkata (iv) Severe cyclonic storm during Oct 2008 (RASHMI) DWR Kolkata (v) Severe cyclonic storm during May 2009 (AILA) DWR Kolkata In fact before 2005, there was no nomenclature used for the tropical cyclones formed in the Indian oceans and therefore, the cyclone during Nov 2002 had no name, whereas all remaining cyclones occurred after 2005 have nomenclature. To maintain the uniformity of the data for analysis, tropical Parameter Table 1 Characteristics of DWRs used in the study Description Type Pulsed Doppler Wavelength 10 cm (S-band) Spot frequency 2875 MHz Peak power 750 KW Amplifying device Klystron Pulse widths Short pulse - 1 µsec (for radial velocity) Long pulse 2 µsec (for radar reflectivity) Pulse recurrence frequency (PRF) Hz Max unambiguous measurements Reflectivity 500 km; Radial velocity - 64 m s -1 Beam width (horizontal and 1 vertical) Doppler processing Pulse pair processing

4 PRADHAN et al.: PRESSURE DROP AND STORM SURGE HEIGHT ESTIMATION 351 cyclones developed in the east coast of India (Bay of Bengal) only have been analyzed. Moreover, DWRs tracking the cyclones were also identical (S-Band with same technical specifications). The radial velocity data derived from Plan Position Indicator for velocity (PPI_V) at an elevation of 0.2 deg and maximum velocity (MAX_V) have been utilized for estimating the radial winds associated with the cyclonic storms. Details of these products and their algorithms are described in Rainbow 3.4 software manual by M/s Gematronik 23. This is well known fact that during cyclonic circulation, the DWR measured radial winds are same as that of actual winds because during rotation, at least once, the wind direction is along the radar beam and hence, the measured radial wind velocity is same as actual. 3 Methodology 3.1 Estimation of the pressure drop In case of DWR, radial velocity of the winds associated with the wall cloud region of the cyclone is measured accurately as the radial velocity is same as that of the actual velocity and therefore, the central pressure drop calculated by above relation is accurate. However, a new value of K has to be evaluated for calculating central pressure drop from radial velocity. The methodology adopted in this study is: (i) Initialization of the analysis is done from the tropical cyclone occurred during Nov (ii) Doppler velocity (V max ) is evaluated from DWR observations. (iii) Central pressure drop (P-P c ) from satellite observations corresponding to DWR observations are taken. (iv) Constant K is determined for different sets of V max and (P-P c ) using Eq. (1) and their average value K av is found. (v) The value of K, thus so obtained (K av ), is taken as standard for evaluating the central pressure drop for other tropical cyclones formed in the Bay of Bengal. 3.2 Height of the storm surge Very high speed rotating winds, in the wall cloud region surrounding the eye of the cyclone, are mainly responsible for the storm surge. The larger the velocity of the winds, the greater is the storm surge height. Several studies have been conducted by various authors to relate these two parameters and it has been found that a co-relationship exists between them. A regression analysis for calculating storm surge height from estimated and recorded maximum wind speed for eastern coast of India and Bangladesh has been performed by SAARC Meteorological Research Centre (SMRC) using records available for cyclones during According to SMRC publication 25, the maximum wind speed and the storm surge height have been found to be linearly correlated with correlation coefficient of The regression equation for eastern coast of India is given by: H= V max (2) In the present study, same equation has been used to calculate the storm surge height by radial velocity measurements taken as V max. 3.3 Percentage error in the storm surge height Percentage error between DWR estimated and actually observed storm surge heights is evaluated by the following formula: (DWR estimatedsurge height % Error = Actuallyobservedsurge height) 100 (Actuallyobservedsurge height (3) The error is taken as positive if the storm surge height estimated by DWR is more than that actually occurred and negative in the alternate way. 4 Results and Discussion Technical characteristics of Doppler Weather Radars used in the present study are given in Table 1 indicating that short pulse and dual pulse recurrence frequency (PRF) technique has been adopted for the measurement of radial velocity. Table 2 shows relation between maximum wind speeds and central pressure drop in a storm field used in IMD. So far, the Table 2 Radial pressure drop and T number in a storm field (IMD standards for low pressure systems) T number (P-P c ), HPa Type of system Depression (D) Deep Depression (DD) Cyclonic Storm (CS) Cyclonic Storm (CS) Severe Cyclonic Storm (SCS) Severe Cyclonic Storm (SCS) Very Severe Cyclonic Storm (VSCS-I) Very Severe Cyclonic Storm (VSCS-I) Very Severe Cyclonic Storm (VSCS-II) Very Severe Cyclonic Storm (VSCS-II) 6.5 >76 Super Cyclone

5 352 INDIAN J RADIO & SPACE PHYS, JUNE 2012 technique adopted by IMD for the estimation of cyclone intensity (in terms of T number) follows the same relation using satellite estimated central pressure drop and maximum velocity associated with cyclone using K=14.2 in the relation (1). 4.1 Estimation of the value of K using Doppler velocity Satellite estimated pressure drop and DWR estimated radial velocity for the tropical cyclone during Nov 2002 have been taken for initialization to evaluate a new value of K (the constant in Fletcher s relation) for Bay of Bengal coast in Indian region. Table 3 indicates different sets of DWR and satellite observations for evaluation of the constant K applied to the cyclone during Nov Since the criterion for T number estimation is also chosen on the basis of maximum velocity and central pressure drop (Table 2), the T number may be estimated from DWR measured radial velocity independently and may be compared with the satellite estimates. This approach prompted the authors to proceed for further investigations of tropical cyclones in the Bay of Bengal to find a new value of K for cyclone intensity estimations using Doppler velocity measurements. Table 3 shows only few observations before the landfall of the cyclone as the system hit the southern part of the Kolkata at 100 km distance at Sagar Island (21.5 N, 88.5 E) on 12 Nov 2002 at 0700 hrs UTC (as per DWR observations) and its intensity rapidly degraded. The average value of K has been found to be This value of K is utilized for the estimation of cyclone intensity of other tropical cyclones in the Bay of Bengal as mentioned earlier. On the basis of above calculations, the following empirical relation is proposed for calculation of central pressure drop within the tropical cyclone: V rad = (P-P c ) (4) where, V rad is the maximum radial velocity measured from DWR observations in the wall cloud region Time, hrs UTC and (P-P c ) is the pressure deficit at the centre of the cyclone. 4.2 Analysis of severe cyclonic storm (OGNI) (28-30 October 2006) The system was observed by the Kalpana satellite when it was in the initial stages of depression and deep depressions and far from the coast. However, when the system arrived within the range of Doppler Weather Radar, Chennai and Machlipattanam, tracking of the cyclone was done by reflectivity and velocity observations. Table 4 shows that satellite and DWR observations differ very much specially for the velocity estimations. In fact, there was a big discussion over this issue between the satellite and DWR experts of IMD regarding authenticity of velocity estimation. After analyzing the synoptic charts, surface wind observations and damage caused by the system, it was later concluded by IMD scientists that DWR observations were correct and to document the authenticity of DWR observations, a special monogram was released on OGNI cyclone in the year 2007 confirming the results provided by DWR Chennai about the intensity of the cyclone. In the original observations from DWR Chennai, only the radial velocity was described and not the intensity as T number, however, the radial velocity derived from DWR have been converted into T number using value of K as as shown in Table 4. The table shows clear discrepancy between the satellite observed T number and the calculated one using radial velocity from DWR. Initially on 28 October, when the satellite observations were showing the system as depression, DWR Machilipatnam observed the system as cyclone with intensity T 2.5 (CS) up to 0918 hrs UTC but the DWR observed intensity became T 3.0 at 1018 hrs UTC and it was maintained till 0300 hrs UTC on 29 October. However, during this period, satellite indicated the intensity only T 1.5, which was much less than that Table 3 Estimation of the value of K for severe cyclonic storm on 12 Nov 2002 Radial velocity, m s -1 Radial velocity, knots Pressure drop from satellite (P-P c ) T number from satellite K=V/ (P-P c ) K av =13.637

6 PRADHAN et al.: PRESSURE DROP AND STORM SURGE HEIGHT ESTIMATION 353 Date Time, hrs UTC Table 4 Estimation of pressure drop and T number associated with OGNI Max radial velocity, knots observed by DWR. At 0600 hrs UTC, when satellite based intensity became T 2.0 as DD, the DWR showed the intensity T 3.5 as CS. At 1200 hrs UTC on 29 October, the satellite observations revealed the intensity as T 2.5 whereas DWR measured intensity was T 3.5. At 1500 hrs UTC of the day, when the satellite observations declared the system at T 2.5 (CS), DWR measured the intensity as T 3.0. The cyclonic storm crossed coast at 0600 hrs UTC on 30 October 2006 when the satellite reported the T number as 2.0, however, the DWR velocity observations showed that the system was in the cyclonic stage. 4.3 Analysis of very severe cyclonic storm (SIDR) (11-15 November 2007) The system was observed by the satellite during its initial phase when the system was in the deep sea and away from coast and subsequently by Doppler Weather Radar located at Kolkata when it reached within its range. The system was first observed by the satellite at 0900 hrs UTC on 11 November over southwest Bay of Bengal near 10 0 N and 92 0 E about 200 km south-southwest of Portblair. The P=(V rad /K) 2 (K=13.637) T number calculated from DWR T number from satellite (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (CS) 1.5 (D) (SCS) 2.0 (DD) (SCS) 2.0 (DD) (SCS) 2.5 (CS) (SCS) 2.5 (CS) (SCS) 2.5 (CS) (SCS) 2.5 (CS) (CS) 2.0 (DD) 09: (DD) 1.5 (DD) system entered within the range of DWR Kolkata from 0500 hrs UTC on 15 November, when its centre was located about 380 km south of the Kolkata. DWR estimated the velocity m s -1 from 0800 to 1600 hrs UTC during the movement of the cyclone. Table 5 indicates that the T number estimated by DWR during hrs UTC was 5.0, whereas from satellite observations the T number was 4.5 and both were indicating a very severe cyclonic storm (VSCS). The system, further, intensified after 1000 hrs UTC and it may be seen from Table 5 that both the estimates from DWR and satellite during hrs UTC were exactly identical (T number 5.5). The radial velocity measured by DWR during hrs UTC was 57 m s -1 equivalent to the T number 6, whereas satellite estimation of the T number was maintained at 5.5. In general, it has been observed that the DWR estimated T number of the cyclone was almost same as that estimated by satellite. The system crossed the Bangladesh coast near Baleswar river at 1700 hrs UTC on 15 November (89.5 E, 21.7 N) with wind velocity km h -1 measured by Bangladesh Meteorological Department.

7 354 INDIAN J RADIO & SPACE PHYS, JUNE Severe cyclonic storm (RASHMI) (25-27 October 2008) The system was first observed by the Kalpana I satellite on 25 October 2008 with centre at 16.5 N, 86.5 E with T number 1.5 and the estimated maximum surface wind was 25 knots. From 0300 hrs UTC on 26 October 2008, the system showed north easterly movement and remained as deep depression (DD) up to 0900 hrs UTC. According to the observation at 1200 hrs UTC, the system reported a cyclonic storm lying at 19.5 N, 88.0 E with 40 knots of surface wind and T number 2.5. At 1800 hrs UTC, the system reported an associated maximum surface wind of 40 knots. At 2100 hrs UTC, the system was reported as situated at 21.5 N 89.5 E with associated T number of 3.0 and maximum surface wind of 45 knots. Satellite observations showed that the system crossed Bangladesh coast near 89.5 E (about 50 km West of Khepupara) between 2200 and 2300 hrs UTC on 26 Oct 2008, as a cyclonic storm. DWR Kolkata observed the system round the clock since 0000 hrs UTC on 26 October. The signatures of the depression were first observed at 0844 hrs UTC on 26 October, as spiral bands could be seen at a distance of km south-southwest of Kolkata. However, better organized spiral bands were observed at 0934 hrs UTC. At 1209 hrs UTC when the system became cyclone, a clear circular eye was seen at a distance 280 km south-southeast of DWR, Kolkata. The estimated diameter was about km. The shape of the circular eye was maintained till 1239 hrs UTC. The circular eye was seen till 1400 hrs UTC and after that its shape got disorganized. No eye was seen at 1500 hrs UTC and it appeared that system got weakened gradually. From DWR pictures, it was found that the system entered Bangladesh coast at around 1800 hrs UTC and it took around 3 h to cross the coast completely. The system advanced towards Bangladesh and weakened gradually. The maximum radial wind recorded by DWR Kolkata from PPI_V pictures was about 30 m s -1 (60 knots) at 1300 hrs UTC. Table 6 shows that DWR observations were available only from 1200 hrs UTC on 26 October and Table 5 Estimation of pressure drop and T number associated with SIDR Date Time, hrs UTC Max radial velocity, knots P=(V rad /K) 2 (K=13.637) T number calculated from DWR T number from satellite (VSCS) 4.5 (VSCS) (VSCS) 4.5 (VSCS) (VSCS) 4.5 (VSCS) (VSCS) 5.5 (VSCS) (VSCS) 5.5 (VSCS) (VSCS) 5.5 (VSCS) (VSCS) 5.5 (VSCS) (VSCS) 5.5 (VSCS) (VSCS) 5.5 (VSCS) (VSCS) 5.5 (VSCS) Table 6 Estimation of pressure drop and T number associated with RASHMI Date Time, hrs UTC Max radial velocity, knots P=(V rad /K) 2 (K=13.637) T number calculated from DWR T number from satellite N/A N/A N/A 2.0 (DD) N/A N/A N/A 2.0 (DD) N/A N/A N/A 2.0 (DD) N/A N/A N/A 2.0 (DD) (CS) 2.5 (CS) (SCS) 2.5 (CS) (CS) 2.5 (CS) (SCS) 2.5 (CS) (CS) 3.0 (CS) (CS) 3.0 (CS) (CS) 3.0 (CS) (CS) 3.0 (CS) (CS) 2.5 (CS)

8 PRADHAN et al.: PRESSURE DROP AND STORM SURGE HEIGHT ESTIMATION 355 the maximum radial velocity observed was 50 knots that time. The T number calculated from DWR velocity estimate was 3.0 (CS), whereas that by satellite was 2.5 (CS). DWR observations were continuously available at an interval of 15 minutes but satellite observations were reported at hourly interval only. Therefore, Table 6 shows few observations for comparison between DWR and satellite estimates. 4.5 Severe cyclonic storm (AILA) (23-25 May 2009) At a very initial stage, the system was observed by the satellite at 16.5 N and 88 E (about 470 km southsoutheast of Paradip) as a depression at 0600 hrs UTC on 23 May. The T number was reported by satellite as 1.5 at this stage. The system remained as a depression up to 1200 hrs UTC on 23 May. On 24 May at 0600 hrs UTC, the system was reported as deep depression lying at 18 N, 88 E with T number 2.0. Gradually, it intensified as a cyclonic storm with a T number 2.5 and located at 300 km of southeast of Paradip. The system entered within 400 km from Kolkata and DWR could observe some spiral bands by 1800 hrs UTC on 24 May. The system was declared as cyclonic storm by 0000 hrs UTC on 25 May and DWR Kolkata could also measure the wind velocity of the magnitude 60 knots associated with the cyclone. Table 7 shows that the corresponding pressure drop HPa indicating a severe cyclonic storm (SCS) of T number 3.5, however, till this time satellite observations indicated T number 3.0 (CS). As per DWR observations, the intensity of system further increased and by 0300 hrs UTC on 25 May, maximum radial velocity observed was 72 knots equivalent to pressure drop of HPa (T 4.0) showing a very severe cyclonic system (VSCS), but the satellite still had the intensity maintained at T 3.0. DWR observations at 0600 hrs UTC found the radial velocity at 72 knots and the intensity same as T 4.0, but the satellite based intensity became T 3.5. The surface observatory at Alipore (Kolkata) reported maximum sustained winds at 0600 hrs UTC of magnitude 130 km h -1 (72 knots) and confirms DWR Kolkata observations. AILA was a large core system of diameter about 150 km and no clear eye was observed in the system but the strong winds were associated with the system. The system crossed Sagar Island between 0800 and 0900 hrs UTC and moved in northerly direction towards Kolkata. DWR observed the system very closely and measured the radial velocity at every 15 min interval. Gradually, the system was cyclonic storm and depression and lastly depression at 0600 hrs UTC on 26 May. The system last reported as a well marked depression at 0900 hrs UTC on 26 May. 4.6 Storm surge height analysis Table 8 shows the height of the storm surge estimated from DWR velocity measurements using empirical relation (4) and that actually observed after the landfall of the cyclones. It is observed that in case of very severe cyclonic storm on 12 Nov 2002, OGNI and SIDR cyclones, the DWR estimated storm surge height was less than that actual, whereas in the case of RASHMI and AILA, DWR estimated storm surge height was more. This is to be noted that first three cyclones were severe cyclonic storms with small core systems [Figs (1-3)] associated with very high wind Table 7 Estimation of pressure drop and T number associated with AILA Date Time, hrs UTC Max radial velocity, knots P=(V rad /K) 2 (K=13.637) T number calculated from DWR T number from satellite (SCS) 3.0 (CS) (VSCS) 3.0 (CS) (VSCS) 3.5 (SCS) (SCS) 2.5 (CS) (CS) 2.5 (CS) Name of cyclone Table 8 Evaluation of storm surge height from DWR measured radial winds Max radial velocity observed Storm surge height, m by DWR, km h -1 DWR estimated Actually observed % error (No name) (2002) OGNI SIDR RASHMI AILA

9 356 INDIAN J RADIO & SPACE PHYS, JUNE 2012 Fig. 1 Max_Z image of cyclone on 12 November 2002 Fig. 3 Max_Z image of SIDR on 15 November 2007 Fig. 2 Max_Z image of OGNI on 29 October 2006 speeds whereas remaining two were large core systems [Figs (4 and 5)]. Figure 6 shows the radial velocity image of the tropical cyclone SIDR. The percentage error between these observations is found to be in the range of (-5.69) (+11.36)%. The cause of such variations may be due to one or more factors as given below:- Fig. 4 Max_Z image of AILA on 25 May 2009 (i) manual measurement of the storm surge height after the landfall; (ii) assumption of radial winds as maximum winds during DWR velocity estimates; and (iii) variation in the intensity of the cyclone in terms of the wind speeds associated.

10 PRADHAN et al.: PRESSURE DROP AND STORM SURGE HEIGHT ESTIMATION 357 Fig. 5 Max_Z image of RASHMI on 26 October 2008 calculations by DWR are either same or slightly on the higher side compared to the satellite observations. The storm surge height estimates from radial velocity are also very close to the actual ones in all the cyclones except AILA. This is to be mentioned that observations of storm surge height is a subjective matter and normally it is measured from the marks of the surge on the coast after crossing the cyclone and therefore, it is very likely that manual observation may vary significantly and may be different from DWR estimated storm surge height. Any ways, this study has concluded that DWR radial velocity observations may be very useful and indicative of expected destruction from the high wind speed and the storm surge. Although, the period of cyclone observation is only few hours using single radar data, with the installation of more DWRs by India Meteorological Department along east and west coasts and their networking, more exhaustive studies and long period observations may become possible in near future. Multiple radar data will be another phase for evaluation of many other parameters for cyclone analysis in addition to the estimation of the central pressure drop, intensity of the cyclone as T number, wind speed and storm surge height. 6 Future scope of the work The study will be continued with more number of cases in this region and their validation with the surface observations. As DWR observes the system very closely, the internal structure may also be studied using radial velocity and radar reflectivity data. Expected direction of movement and the landfall time of the cyclone may be estimated more precisely than satellite at least 12 h in advance. More such studies are required by the scientists working with DWRs regarding cyclone analysis as very few studies have been conducted in India so far. Fig. 6 PPI_V image of SIDR on 15 November Conclusions The study concludes that DWR estimation of pressure drop at the centre of the cyclone is very close to that observed from satellite. Sometimes, the radar estimates are much better than satellite as in the case of OGNI cyclone when the satellite had under estimations. For other cyclones like November 2002, SIDR, RASHMI and AILA, the T number Acknowledgements The authors are grateful to India Meteorological Department for providing an opportunity to work with Doppler Weather Radar at Kolkata in conducting this study. They also express their sincere thanks to Director, Cyclone Detection Radar, Chennai and Machlipattanam for providing raw data and other velocity information for OGNI cyclone during October The authors are also thankful to SMRC, Bangladesh for providing necessary data on

11 358 INDIAN J RADIO & SPACE PHYS, JUNE 2012 the storm surge height and other details about cyclone SIDR during November References 1 Frank M W, Tropical cyclone formation - A global view of tropical cyclones, Proc First WMO International Workshop on Tropical Cyclones, Bangkok, Thailand (WMO Tropical Meteorology Research Programme, Geneva, Switzerland), 1985, pp Gray W M, Global view of the origin of tropical disturbances and storms, Mon Weather Rev (USA), 96 (1968) pp India Meteorological Department, Damage Potential of Tropical Cyclones (Office of Additional Director General of Meteorology (Research), Pune, India), Fletcher D R, Computation of maximum surface winds in hurricanes, Bull Am Meteorol Soc (USA), 36 (1955) pp Brantley J Q & Barczys D A, Some weather observations with continuous wave Doppler radar, Proc of sixth Weather Radar Conference (American Meteorological Society, Boston), 1957, pp Lhermitte R M & Atlas D, Precipitation motion by the Pulsed Doppler Radar, Proc 9th Weather Radar Conference (American Meteorological Society, Boston), 1961, pp Doviak R J & Zrnic D S, Doppler weather radar, Proc IEEE (USA), 67 (1979) pp Wilson J W & Wilk E W, Nowcasting applications of Doppler radar, in Nowcasting applications of Doppler radar by K A Browning (Academic Press, London), 1982, pp Banerjee S K, Kundu S K & Singh H A K, A study of 12th Nov 2002 cyclonic storm in the Bay of Bengal using Doppler Weather Radar, Mausam (India), 55 (4) (2004) pp Pradhan D & Singh H A K, Structural and wind shear analysis of Nov tropical cyclone at the time of landfall using Doppler Weather Radar Kolkata (India), Vatavaran (India), 30 (2) (2006) pp Lee J L, Lee W C & Macdonald A E, Estimating vertical velocity and radial flow from Doppler radar observations of tropical cyclones, Q J R Meteorol Soc (UK), 132 (2006) pp Liou Y C, Wang T C C, Lee W C & Chang Ya-Ju, The retrieval of asymmetric tropical cyclone structure using Doppler Radar simulations and observations with the Extended GBVTD Technique, Mon Weather Rev (USA), 134 (2006) pp Lewis William E, Eastwood I, Simone T, Ziad H, Gregory J T & Eric A S, Geostationary Doppler radar and tropical cyclone surveillance, J Atmos Ocean Technol (USA), 28 (2011) pp Sikka D R, Development of tropical cyclones in the Indian seas as revealed by satellite radiation and television data, Indian J Meteorol Geophys, 22 (1971a) pp Sikka D R, Evaluation of the use of satellite photography in determining the location and intensity changes of tropical cyclones in Bay of Bengal, Indian J Meteorol Geophys, 22 (1971 b) pp Dvorak V, Tropical cyclone intensity analysis and forecasting from satellite imagery, Mon Weather Rev (USA), 103 (1975) pp Dvorak V, Tropical cyclone intensity analysis using satellite data, NOAA Tech Rep (USA), 11 (1984) pp Takahashi K, Distribution of pressure and wind in a typhoon, J Meteorol Soc (Japan), 17 (1939) pp Natarajan R & Ramamurthy K M, Estimation of central pressure of cyclonic storms in Indian seas, Indian J Meteorol Geophys, 26 (1975) pp Mishra D K & Gupta G R, Estimation of maximum wind speeds in tropical cyclones, Indian J Meteorol Geophys, 27 (1976) pp India Meteorological Department, Cyclone manual (IMD, Delhi), Raj Y E A, Relation between pressure defect and maximum wind in the field of a tropical cyclone - Theoretical derivation of proportionality constant based on an idealised surface pressure model, Mausam (India), 61 (3) (2010) pp Gematronik, Rainbow 3.4 Technical Manual (Neuss, Germany), SAARC Meteorological Research Centre, The impact of tropical cyclones on the coastal regions of SAARC countries and their influence in the region, SMRC Publication 1 (SMRC, Dhaka), 1998.