Pre-monsoon/monsoon thunderstorm characteristics over Pune—An investigation using Doppler Sodar observations

Doppler sodar observations of three dimensional (3D) wind fields and thermal structure of convective boundary layer (CBL) on a few thunderstorm days of 2009 during pre-monsoon (May and June; June due to delayed arrival of monsoon over Pune) and monsoon (July and August) are analyzed. They reveal the typical signatures of wind fields for the late afternoon thunderstorm (TS) such as deceleration of winds with or without change in direction leading to convergence a few minutes (∼15–30 min) prior to the onset of TS. Pre-monsoon TS are characterized by broad updrafts and narrow downdrafts in CBL in contrast to the narrow updrafts and broad downdrafts of a normal day (i.e. No-TS day). Mean vertical velocity averaged over CBL period shows net updraft on TS days and net downdraft on No-TS day for the pre-monsoon cases. Similarly calm winds are observed in the CBL on TS-days that support enhanced free convection. During the monsoon period updrafts are observed on both TS and No-TS days with higher values on TS days in comparison, due to the dominance of large-scale monsoon flow over local convection. Relatively higher turbulence kinetic energy (TKE) in CBL is observed on all TS days. Analysis shows that TKE maximum for the day is attained about 1.5–2.0 h prior to the onset of afternoon TS. Mixed-layer depth, determined from TKE profile, is higher than lifting condensation level (LCL) on TS days in May and June indicating saturation of air parcels in updrafts.

► During pre-monsoon period mean CBL vertical velocity indicates updrafts (downdrafts) on a TS (No-TS) day.
► During monsoon period updrafts prevail in CBL and they are higher on TS day than on No-TS day.
► Wind convergence and/or deceleration occurs about 15–30 min prior to afternoon thunderstorm (TS).
► TKE in monsoon drops to about 1/3 of its pre-monsoon value.
► TKE is relatively higher on TS day and it exhibits a peak roughly about 1–2 h before the onset of TS.