High-resolution images provide convection movement before, during and after the easterly wave at every six-hour interval (Figure 1). Meteosat images are clearly depicting the organized pattern of convection with an inverted “V” shape around 26^th October 2006. It is necessary to understand the evaluation of easterlies with circulation preceding, during and succeeding the wave at different levels. The streamlines analysis shows closed circulations at 850 and 700 hPa (Figure 2). It has a northeast-southwest oriented axis near the coast. Figure 2 shows the cyclonic circulation is mainly observed at 85˚E with a magnitude of 2 m/s on 25^th October 2006. The magnitude of northeasterlies is increased to 9.9 m/s at 850 hPa level. The elongated trough over central India shows that wind is shifting from northerlies to southerlies over the Arabian Sea at 700 hPa level. In contrast,

strong anticyclonic circulation prevails over the Bay of Bengal with a maximum wind speed of 12 m/s at 15˚N. At the same time diffluence is noted near and ahead of the trough at 200 hPa. On the next day, the strength of the northeasterlies is further increased at the surface. A closed cyclonic circulation is further established at 850 hPa level.

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

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, where the wind pattern is reversed during 26^th October, 2006. Figure 3 represents the subtropical westerly jet stream at 200 hPa level with magnitude of 40.8 m/s at 25˚-28˚N. The vertical structure shows that northeasterlies are intensifying when an easterly wave carry moisture from the Bay of Bengal towards south India. Figure 4 provides the convective available potential energy during the easterly wave from 25^th October to 1^st November, 2006. The convective available energy is very less on 25^th October, later the amount of energy is increased between 11˚N to 16˚N to a tune of 1500 J/Kg on next day. 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 greater than 2000 J/Kg.

Tracking the development and movement of easterly waves requires identification of a reference point, usually the trough axis, using a variety of methods. The prominent method is low-level convergence (“C” Figure 5) occurs ahead of the trough and divergence (D) behind. This pattern is reversed at 700 and 200 hPa, where areas of divergence and convergence are observed above the regions of low-level convergence and divergence. Corresponding maxima in cloudiness and rainfall are observed ahead of the trough coincident with the low-level convergence.

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 upper level using potential vorticity and stream function are also studied and the results revealed a very good information.

Isentropic PV is conserved for adiabatic and frictionless flow is a useful tracer of synoptic-scale disturbances. Especially maximum PV on the 315˚K potential temperature is one of the best tools to identify the waves. The PV of the low-level vortex has been identified as part of the synoptic scale structure and it had a polar vortex that moved by the trough. These waves cause severe weather via heavy rainfall and deadly floods over south India. The climatology of sign reversals of mid-level meridional PV gradients over south India showed heating from deep convection associated with the active monsoon. Coincident with the negative mid-troposphere PV gradient is a strong gradient of potential temperature in the lower-troposphere. Figure 6 provides PV variation during the easterly

wave from 25^th to 27^th October, 2006. Potential vorticity maxima shifted towards south where the value is 2 units (10⁻⁶ m⁻² s⁻¹ K kg⁻¹) on 26^th October, 2006. In general, the greater the number of PV units is, the deeper dry, stratospheric air is expected to penetrate into the atmosphere.

The stream function at the 700 hPa is another method for finding the easterly wave generated troughs and ridges. The stream function is analogous to geo-potential height, which is used to find troughs and ridges in the mid-latitudes. The troughs, the stream function minima, have an inverted “V” shape. In the objective method, developed by Berry et al. (2007), defines easterly wave troughs and ridges where the advection of the stream function-vorticity by wind is equal to zero. The trough has positive vorticity advection ahead and negative vorticity advection behind. The trough position is distinguished from the ridge by finding the zero contour of the advection in regions where stream function curvature vorticity exceeds 0.25 × 10⁻⁵ s⁻¹ and flow is easterly. On 25^th October 2006, the easterly wave advected north of trough with a magnitude of 0.2 m/s and increased in their magnitude in remaining days (Figure 7).

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.