Abstract
Abstract. This study illustrates the convectively generated gravity wave generation mechanisms during the passage of thunderstorms and squall line using Indian MST radar. For the first time, it has been shown that all three generation mechanisms have been involved in the generation of gravity waves during the passage of squall line event. It is observed that the periodicities in the range of 8–80 min in the tropospheric and 8–32 min in the lower stratospheric regions and vertical wavelengths in the range of 3.2–4.8 km in the tropospheric and 1.2–1.92 km in the lower stratospheric regions are found to be dominant in the present study and are distinctly different during initial, mature and dissipative phases of convection. Amplitude of vertical wind has been weakened (from ~ 4–6 m s−1 to ~ 1 m s−1) considerably after 10–30 min of a convection event. It appears that the wind shear associated with the convective clouds acted like an obstacle to the mean background flow during the squall line passage generated gravity waves. The phase profiles corresponding to the dominant period show both downward and upward propagation of waves. The vertical extent of heating is found to be deeper during squall line event compared with thunderstorm event. From the phase profiles, during 27 September 2004, two peaks of constant phase region are observed. One is due to convective elements and the other is due to strong background wind shear; however, only one peak is observed on 29 September 2004, which is only due to convective processes.
Highlights
It is well known that gravity waves significantly affect various dynamical processes in the upper and middle atmosphere (Fritts and Alexander, 2003; Mohankumar, 2008)
We adopted the criteria from TBB of less than 235 K to demarcate deep convective clouds (Chen et al, 1996; Arkin and Meisner, 1987)
Even though cumulonimbus (Cb) clouds were going through Gadanki (GDK) towards Chennai (MDS) on September at 09:12 LT (03:42 UTC) and on September at 17:42 LT (12:12 UTC, as in Fig. 2), no significant rainfall had been recorded at GDK
Summary
It is well known that gravity waves significantly affect various dynamical processes in the upper and middle atmosphere (Fritts and Alexander, 2003; Mohankumar, 2008). Convection plays a major role in generating GWs by releasing enormous amounts of latent heat and contributes to the wave mean flow interactions in the tropical middle atmosphere (Alexander, 1995; Pandya and Alexander, 1999). Convective instability creates oscillatory displacements in the isentropic/isobaric surfaces and generates GWs that propagate vertically as a harmonic oscillator (Clark et al, 1986; Fovell et al, 1992; Alexander, 1995; Alexander and Holton, 1997; Lane and Reeder, 2001; Dhaka et al, 2002, 2003; Kumar, 2006, 2007; Dutta et al, 2009). Alexander (1995) simulated squall lines, observing harmonics with a period of around 23–28 min in the troposphere and 12.8–7.8 min in the stratosphere, Published by Copernicus Publications on behalf of the European Geosciences Union. From Indian MST radar, the ECWF model and satellite data an attempt to find the dominant generation mechanism of CGWs during the passage of thunderstorms and squall lines over Gadanki is made
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