An Easterly Wave Generated Heavy Rainfall Event over South India—A Case Study

Easterly waves are one of the rain-bearing systems of northeast monsoon and produce massive rainfall events over south India. In the present case study, an attempt is made to identify extreme heavy rainfall event over south India on 26th October, 2006 due to the passage of the easterly wave. Satellite images provide an inverted v-shape easterly wave. Next, circulation features at different levels clearly indicate the location, movement and speed of the easterly wave. Strong north-easterlies with a magnitude of 9.9 m/s are maintained at the surface. The convergence is mainly occupied between 12°N - 16°N, while the divergence is 5°N - 12°N on 26th October, 2006 at the surface level. On 25th, easterly wave is advected north of trough with a magnitude of 0.2 m/s and increased during the remaining days. There are two divergence cells along 5°N and 16°N before and after the event at 700 hPa level. Thus this study helps to bring out the 
essential characteristics of the easterly wave during northeast monsoon. The highlight of this study is that the easterly wave creates floods in the absence of tropical cyclones over south India.


Introduction
Northeast monsoon (NEM) is known to be more variable both in space and time caused by retreating monsoon winds and attain moisture from the Bay of Bengal on their way back to south from northeast India. This moisture provides rains in coastal and southern Andhra Pradesh, Tamil Nadu and parts of Karnataka between October through December [1] [2] [3]. Northeast monsoon winds and the International Journal of Geosciences sea breeze interact to produce strong offshore low-level convergence that favors organized mesoscale convection. [4] suggested that when upper-air divergence ahead of an approaching easterly wave trough superimposed upon the convergent area of low level monsoon trough, the trough would develop into a depression. The most important drivers of monsoon seasons are low pressures particularly those which form in the Bay of Bengal and move towards various parts of India [5] [6]. There is an increasing trend in the amount of NEM rainfall in the recent years and this rainfall possesses a lot of variability on the interannual scale [7]. The transient synoptic scale systems affect south peninsular India during NEM season [8].
An easterly wave is a trough with westward moving maximum speed 7 -8 m/s between 600 and 700 hPa; it has the wavelength of 2000 to 4000 km with a period of 3 -5 days about 6˚ -7˚ degrees longitude per day. The latitudinal extent is of 10˚ to 15˚ and the maximum amplitude in the lower to mid-troposphere.

Data and Methodology
European meteorological satellite images are considered for the identification of convection and movement of an easterly wave. India Meteorological Department (IMD) high resolution daily gridded rainfall is obtained for the study period [23].
The NCEP/NCAR reanalysis zonal and meridional wind components are also retrieved from the website to observe the circulation features at 1000, 850, 500 and 200 hPa levels [24]. Later potential vorticity and stream functions, divergence are also calculated from the above datasets.

Circulation Features at Different Levels
High-resolution images provide convection movement before, during and after

Dynamic and Thermodynamic Features
Hovmoller diagrams of wind indicate the vertical structure and its magnitude with latitude and time and are shown in Figure 3. At the surface, easterlies (5.5   The convergence is mainly occupied between 12˚ -16˚N, while the divergence is 5˚ -12˚N on 26 th October, 2006 at the surface ( Figure 5(a)). But at 850 hPa level the reverse pattern is observed when compared to the surface. The divergence area is shifted further northward and the convergence is intensified with a magnitude of −0.5 × 10 −5 /s ( Figure 5(b)). There are two divergence cells along 5˚N and 16˚N before and after the event at 700 hPa level ( Figure 5(c)). At upper level (200 hPa), convergence is mainly occupied at latitude belt 5˚ -15˚N ( Figure 5(d)). Thus the divergence area also reveals the propagation and intensity of the easterly wave. Next, the dynamic characteristics of the easterly wave at

Characteristics of Daily Rainfall
Easterly wave provides significant rainfall amount over southern peninsula during northeast monsoon season. The present case study shows that 110 mm rainfall is recorded along the Tamil Nadu and Coastal Andhra Pradesh by using IMD high resolution gridded rainfall data ( Figure 8). The easterly wave persists on 26 th October produced a good amount of rainfall over Tamil Nadu (90 mm).
Positive temperature advection and moisture incursion from anticyclonic flow close to the east coast of the south peninsular India provided the highest amount of rainfall. Hence, observing the waves with the internal mechanism makes the study used for operational forecasting and provides a better understanding of easterly waves.

Summary and Conclusions
Present attempt is based on the structure and properties of easterly waves and their impact on NEM rainfall activity. Circulation features of the easterly wave are analyzed the day before, on the day and after the day at different levels. On 25 th October 2006, the establishment of cyclonic circulation is mainly observed at 85˚E and its strength increased to 9.9 m/s at 850 hPa level. At 700 hPa level, an elongated trough appears over central India with an inverted "V" shape with diffluence near and ahead of the trough at 200 hPa. On the next day, the winds are further increasing its strength at the surface and a closed cyclonic circulation is further established at 850 hPa level. Next, vertical structure indicates surface easterlies (5.5 m/s) with cyclonic circulation are established along 9˚-16˚N. At 850 hPa level, wind circulation is further intensified and at 700 hPa level, the wind pattern is reversed during 26 th October, 2006. The subtropical westerly jet stream is located at 25˚ -28˚N with magnitude of 40.8 m/s (200 hPa level). The convective Available energy is very less on 25 th October, later the amount of energy increased in between 11˚ to 16˚N to a tune of 1500 J/Kg on 26 th October, 2006. The amount of energy varies in between 1500 to 2500 J/kg on 27 th October, 2006. Next, easterly wave influence is still continued up to 30 th October, 2006 where the amount of energy varies more than 2000 J/Kg. From this case study, convergence is mainly occupied between 12˚ -16˚N, while the divergence is 5˚ -12˚N on 26 th October, 2006 and reverse pattern is observed at 850 hPa level. The divergence area is shifted northward and the convergence is intensified with a magnitude of −0.5 × 10 −5 /s. There are two divergence cells along 5˚N and 16˚N before and after the event at