Abstract:
During the Northeast monsoon (NEM) in Sri Lanka thunderstorm activities are concentrated
to the southwest comer of the island. The lightning data measured by Lightning Locating
Project (LLP) in February 1999 and by Lightning Imaging Sensor (LIS) project (NASA) give
sufficient evidence on this. Though the heaviest precipitation in NEM is reported in the
windward side, the eastern and northeastern regions, lightning flashes appear as banners over
lee side of the central mountain area. The flashes are much more frequent over the land, but
appear mostly in the late afternoon with a maximum around 18 LST. Numerical simulations
by a higher-order closure numerical model show the evidence of lightning flashes occurrence
within the lee-side wake. A counter rotating vortex pair is often evident in the wake. The
formation of positive and negative vortices shows a diurnal variation. The incoming trade
wind splits at the upstream region, tends to flow around the mountain massif and forms a
convergence area over the lee side. A reversed flow is visible in between the vortex pair at
some hours enhancing the convergence. This convergence area is clearly visible over satellite
images. According to the model simulations significant precipitation occurred over the
convergence area. Two case studies for day-time and night-time show the relationship
between the lightning banners and the convergence area of the wake. During the NEM, the
stability of the lowest part of the atmospheric boundary layer (BL) over the sea is statically
neutral or weakly stable. Friction in combination with vorticity causes vertical winds. The
effect of the combination of vorticity and vertical velocity can be expressed by the helicity. It
is shown that potential vorticity and the helicity calculated from positive vorticity and related
positive vertical wind agrees in time, space and strength with the lightning flash occurrence
both out over the sea and the land. Further the possibility of severe thunderstorm environment
is examined through weather indices, K-index and SWEAT index. Dispersion of air
pollutants in NEM is analyzed and the effect of wake and vortices on pollutant concentration
is examined in the same case studies.
